1

AUTOMATED

HIGH

RESISTANCE

MEASUREMENT

BRIDGE

Setup Guide & Operator’s Manual

2

Letter of Transmittal

University of Maryland

College Park, MD. 20742

April 21, 2015

Dear Scientists and Engineers,

We are pleased to present you the manual for an automated high resistance measurement

system. This manual was composed in partial fulfillment of the requirements for ENGL393

course during spring 2015 Semester. Specifically, this manual includes contributions of both

Jacob and myself. Being students of the University of Maryland majoring in Electrical

Engineering, Jacob and I gained the basic knowledge of electronic circuit theory and

applications. The technique described in this manual is based on the Wheatstone bridge

principle, which was introduced to us during our sophomore level circuit theory courses.

Furthermore, I have been employed at the National Institute of Standards and Technology

(NIST) since September 2014. During my first few months of employment, I was given the

opportunity to conduct research on advancing the existing high resistance measurement system

and to develop a new system that includes latest equipment and software. Similarly, Jacob has

gained a thorough understanding of the basic theory of the high resistance measurement system

and its applications from his course work. Based on our experience, we believe that performing

accurate, high resistance measurements are challenging and time consuming. Therefore, we are

taking this as an opportunity to present a detailed and simplified manual that describes the

equipment selection and the setup procedure for an automated high resistance measurement

system.

3

As we mentioned above, accurate high resistance measurements require a great effort. It

is important to consider the barriers ordinary scientists and engineers face while attaining

accurate high resistance measurements. Usually, engineers or scientists working on large scale

projects request measurement services at NIST. However, this requires time for shipping and

additional costs. Similarly, the second option would be to purchase a commercially available

measurement device. This option isn’t viable because it would cost more money and would

affect the accuracy of the measurement result. Therefore, from our experience, we believe that it

is more feasible to set up a measurement system in the lab if your project relies on accurate high

resistance measurements. The manual we completed focuses on engineers and scientists who

relies on high resistance measurements in a laboratory environment. Furthermore, the manual

will guide the reader through the proper equipment selection, assembly, software design, and

troubleshooting.

As emerging engineers, we recognize the importance of accuracy and the efficiency of

physical measurements for engineering projects. Therefore, we hope that this manual will ease

the complications linked with high resistance measurements and will make a positive impact on

your projects.

Sincerely,

Enclosure: Manual for an automated high resistance measurement system

4

Limitation of Liability

This manual was produced in partial fulfillment of the requirements for a University professional

writing course. The information contained in this user manual is provided "as is". The author(s),

though well-informed and experienced, are not professional or experts in the topic, so [they]

make[s] no representations or warranties of any kind, express or implied, about the completeness

of the information for a particular use, or merchantability. Nor do [they] make any

representations of freedom from infringement of any intellectual property or proprietary rights of

any third party. You use the information at your risk. You are advised to consult an appropriate

professional if you have any doubts related to the advice or procedures in this manual.

Identification of commercial equipment does not imply any endorsement or that the equipment is

the best available for the purpose.

5

Table of Contents

Section Title Page

1 Safety Information 8

Proper safety measures 9

Symbols used in this manual 10

First Aid for electric shock 11

2 Introduction 14

Specifications of the System 14

3 Background Information & Basic Theory 16

Wheatstone Bridge 17

Modified Wheatstone Bridge 18

4 Measurement Algorithm 20

5 Equipment Selection 22

Programmable High Precision Voltage Supply 22

Alternative Options for the Calibrator 23

Current Detector 24

Alternative Options for the Current Detector 25

Cables 25

Air Bath 27

Alternative Options for the Air Bath 28

Computer with GPIB support 28

6 Assembly Procedure 29

6

7 Creating the Automation Software 33

Visual Basic 33

LabVIEW 33

Developing the Automation Software 34

Overview of a sample Automation Software 36

8 Measurement Procedure 38

9 Testing the System for Accuracy 43

Using two standard resistors with adequate calibration data 43

Using a Hamon Transfer Standard 43

10 Troubleshoot Guide 45

11 Appendix 47

Glossary of Terms 47

Metric Prefixes 48

References 49

Interview Write-up 50

7

List of Figures

Figure Title Page

1.1 Warning Sign [1] 8

3.1 Conventional Wheatstone Bridge [14] 17

3.2 Modified Wheatstone Bridge with Guard Resistors [14] 18

4.1 Modified Wheatstone Bridge with Currents Labeled. 20

5.1 Overview of the Fluke 5730A Voltage Calibrator. 23

5.2 Overview of the Keithley 6430 Current Detector. 24

5.3 Polyethylene Cable and Teflon Cable. 25

5.4 Cable Connectors 26

5.5 Junction Boxes for the Voltage Calibrator 26

5.6 Node Box 26

5.7 Overview of the MI Model 9300 Air Bath 27

5.8 GPIB Cable 28

7.2 Overview of a sample automation software 36

9.1 Internal Diagram of a Hamon Transfer Standard [3] 43

8

1 Safety Information

WARNING

HIGH VOLTAGE is used during the operation of this measurement system.

High voltage is present during the operation, observe all safety precautions

mandated by your laboratory safety guide.

To prevent possible electric shock, when the measurement system is plugged in,

do not touch terminal connections. Do not attempt to change wires while the

system is running.

Figure 1.1 [1]

9

1.1 Proper Safety Measures

To prevent possible electrical shock, equipment damage, or personal injury:

Adhere to safety instructions mandated by your laboratory safety guide when assembling

and using the measurement system [7].

Carefully read all instructions.

Do not implement the measurement system around explosive gas, vapor, or in damp or wet

environments [8].

Do not use damaged cables.

Implement the system indoors only.

Do not let unauthorized personal use the measurement system [7].

When the measurement system is running, make sure to isolate the environment by either a

caution tape or a rope [7].

Do not set up the measurement system where access to the main power cord is blocked.

Use only the mains power cord and connector approved for the voltage and plug

configuration in your country and rated for the each equipment of the measurement system

[9].

Always program the software so that the output current is limited by the voltage supply.

Replace the power cords if the insulation is damaged or if the insulation shows signs of

wear.

Make sure the ground conductor in the main power cord is connected to a protective earth

ground. Disruption of the protective earth could put voltage on the chassis that could cause

death [8].

Avoid using an extension cord or adapter plug.

Do not apply more than the rated voltage, between the terminals or between each terminal

and earth ground [8].

Use only cables with correct voltage ratings [9].

If it is possible use a caution light when the system is running to alert other personnel.

10

1.2 Symbols Used In This Manual

The following symbols are used throughout the manual to alert readers about safety concerns.

Table I: Symbols marked on this manual [10].

Symbol Definition

Danger is used to alert readers about an

immediate and serious hazard that will likely

be fatal.

Warning is used to alert readers about the

potential for serious injury, death, or serious

damage to equipment used.

Caution is used to alert readers about the

potential for anything from s moderate injury

to serious equipment damage or destruction.

Note: Note is used for a tip or suggestion to help

readers carry out the procedure successfully.

11

1.3 First Aid for Electric Shock

The following procedures must be followed in the case of an electric shock. Adapted from U.S.

National Library of Medicine [11].

Symptoms

Symptoms depend on several factors, including:

Type and strength of voltage

How long you were in contact with the electricity

How the electricity moved through your body

Your overall health

● Symptoms may include:

Changes in alertness (consciousness)

Broken bones

Heart attack (chest, arm, neck, jaw, or back pain)

Headache

Problems with swallowing, vision, or hearing

Irregular heartbeat

Muscle spasms and pain

Numbness or tingling

Breathing problems or lung failure

Seizures

Skin burns

12

First Aid

1. If you can do so safely, turn off the electrical current. Unplug the power cord, remove the fuse

from the fuse box, or turn off the circuit breakers. Simply turning off an appliance may NOT

stop the flow of electricity. Do NOT attempt to rescue a person near active high-voltage lines.

2. Call your local emergency number, such as 911.

3. If the current can't be turned off, use a non-conducting object, such as a broom, chair, rug, or

rubber doormat to push the person away from the source of the current. Do not use a wet or

metal object. If possible, stand on something dry and that doesn't conduct electricity, such as a

rubber mat or folded newspapers.

4. Once the person is away from the source of electricity, check the person's airway, breathing,

and pulse. If either has stopped or seems dangerously slow or shallow, start first aid.

5. If the person has a burn, remove any clothing that comes off easily, and rinse the burned area

in cool running water until the pain subsides. Give first aid for burns.

6. If the person is faint, pale, or shows other signs of shock, lay him or her down, with the head

slightly lower than the trunk of the body and the legs elevated, and cover him or her with a warm

blanket or a coat.

7. Stay with the person until medical help arrives.

8. Electrical injury is frequently associated with explosions or falls that can cause additional

severe injuries. You may not be able to notice all of them. Do not move the person's head or neck

if the spine may be injured.

13

DO NOT

● Do NOT touch the person with your bare hands if the body is still touching the source of

electricity

● Do NOT apply ice, butter, ointments, medications, fluffy cotton dressings, or adhesive

bandages to a burn

● Do NOT remove dead skin or break blisters if the person has been burned

● After the power is shut off, do NOT move the person unless there is a risk of fire or

explosion

When to Contact a Medical Professional

Call your local emergency number, such as 911 if a person has received an electrical burn.

14

2. Introduction

Precise electrical measurements have been a key ingredient in groundbreaking

engineering projects and research developments. Resistance measurements up to and beyond 100

Tera-Ohms requires expensive equipment and additional resources. However, resistance

measurements in the above mentioned scale can be performed in a laboratory environment with a

low uncertainty using an automated resistance measurement bridge. An automated high

resistance measurement bridge as described in this manual, consist of two resistors, two high

precision voltage supplies (Calibrators) and a current detector. This manual describes the best

equipment, the setup procedure and the measurement process of an automated high resistance

measurement bridge. Furthermore, this manual is primarily based on the automated resistance

measurement bridge that was implemented at the National Institute of Standards and Technology

(NIST). The measurement technique described in this manual is based on the Wheatstone bridge

principle. A Wheatstone bridge is a four resistor bridge that consists of three known resistors and

one unknown resistor [12]. However, large uncertainties and inaccessibility of standard resistors

makes the ordinary Wheatstone bridge unfeasible in high resistance measurements [7].

Therefore, a modified version of the Wheatstone bridge is used to precisely measure high

resistance values. A complete description of the Wheatstone bridge and the modified Wheatstone

bridge, is included in the following section of the manual.

2.1 Specifications

The basic specifications of the measurement system described in this manual are included in the

table below

Table II: Specifications of the measurement system described on this manual

Voltage Source Fluke 5730A Calibrator

Current Detector Keithley 6430 Detector

Air Bath Used Measurement International Model 9300

Connectivity Method Polyethylene Cables

Minimum Applied Voltage 0 V

Maximum Applied Voltage +1100 V or -1100 V

15

Voltage Source Uncertainty approximately ~2 ppm [8]

Current Limitation 3 mA (set by the measurement software)

Uncertainty in Resistance Range 1 Tera-Ohm ~10 ppm1

100 Tera-Ohm ~ 50 ppm

Settling Time Average 120 Seconds

Temperature Performance 0 °C to 50 °C

Relative Humidity 30% - 50%

Standard Interfaces GPIB (General Purpose Interface Bus)

Connectors to communicate with the

computer

Programming Platform Used LabVIEW Platform

Additional Options Available -Voltage Calibrator (voltage source)

Correction.

-Null Current Measurement for offset

correction

-Acquiring resistor data from a standard

resistor data excel sheet

-Save measurement data onto a readable text

file

1 Parts per Million (ppm) is a value that represents the part of a whole number in units of 1/1000000.

1 PPM = 0.0001%

16

3. Background Information & Basic Theory

3.1 Background

High resistance measurements are an important part of the precision measurement

industry. Particularly, several commercial and government industries depend on accurate high

resistance measurements. For example, the department of energy and the nuclear power industry

uses Dosimeters to measure background radiation [7]. These Dosimeters uses a very small

current, usually in the Femto-Amp range. Therefore, in order to measure the small current, these

industries have to rely on precise high resistance standards. Similarly, high resistance

measurements are used to test the purity of materials. For example, in the textile industry, high

resistance measurements are used to judge the purity of the textile material and to determine the

use of the textile material [7].

Even though accuracy in these high resistance measurements is crucial for the above

mentioned applications, due to constraints such as longer settling times and inefficiencies in

manual operation, achieving accurate high resistance measurements has always been

challenging. There are several commercially available products that can be used to achieve high

resistance measurements with a higher uncertainty. The Guideline 6535 Automated High

Resistance Measurement System can produce high resistance measurements with approximately

50 parts per million (ppm) in the 1Tera Ohm range [13]. However, some industries require the

need of precise measurements up to 100 Tera ohms with an uncertainty of 5-10 ppm. As an

example, a common customer of the National Institute of Standards and Technology (NIST),

Sandia national labs, supports the nuclear weapons program at U.S. Army. According to an

interview with Mr. Dean Jarrett at NIST, Sandia national labs requested NIST to measure the

resistivity of a ceramic coating that was well above the 1Tera Ohm range. [7]. Therefore

measurement capabilities of the commercially available products are insufficient to serve the

needs of the industry.

A measurement system that can measure high resistance values with high precision can

be implemented using the Wheatstone bridge principle.

17

3.2 Wheatstone Bridge.

A conventional Wheatstone bridge is the most used method for high resistance measurements.

Circuit diagram of the conventional measurement bridge is shown below.

The conventional Wheatstone bridge used for high resistance measurements requires

three resistors to measure a single unknown resistor [12]. In the figure 3.1, Rx is the unknown

resistor that we are interested in. If the current of the detector is at zero, ratios between R1 and

R2 and Rx and Rs should be equal to each other based on Kirchhoff's Current laws. Therefore

the equation for the unknown resistor value can be derived as follows.

𝑅2

𝑅1=

𝑅𝑠

𝑅𝑋 → 𝑅𝑋 =

𝑅1

𝑅2∙ 𝑅𝑠

Please note that the complete derivation for the case of the modified Wheatstone bridge is

included in the following section. The derivation for the conventional Wheatstone bridge is

analogous to the modified Wheatstone bridge.

As shown on figure 3.1, resistors R1 and R2, typically in the range of 104 ohms and 107

ohms form the reference arms of the resistance measurement bridge [15]. When the bridge is

balanced, the ratio of these is equal to the ratio of the unknown resistor to the other resistor Rs

which form the other arms of the bridge as shown on figure 3.1. Wheatstone bridges used in the

measurement industry are often manually operated [15]. As an example, manually operated

Figure 3.1: Conventional Wheatstone bridge [14]

18

guarded Wheatstone bridge at the national institute of standards and technology required an

operator to manually change the resistance ratios while paying close attention to the current

detector [7].

Drawbacks of the Wheatstone bridge method are evidently as follows. Due to the nature

of the bridge, three resistors with precision and enough calibration data are required to perform a

measurement. Therefore, the resistance value of the unknown resistor will always depend on the

uncertainties of the other three resistors. Also, leakage in resistors is a possible source of error in

measurements [15]. Similarly, manual operation of the resistance bridge leads to human errors.

According to Mr. Jarrett at NIST, “patience of a manual operator is very limited. Therefore, we

always rely on two different individuals to operate the system in two different times” [7].

Therefore, the measurement industry showed the need of an improved resistance measurement

system.

3.3 Modified Wheatstone Bridge.

A modified Wheatstone bridge uses the same basic theory as the conventional

Wheatstone bridge. However, as shown in figure 3.2, modified Wheatstone bridge uses two

highly precise voltage supplies on the left side of the bridge replacing the R1 and R2 resistors in

the conventional Wheatstone bridge [14].

ID A B

IX

Is

Figure 3.2: Modified Wheatstone bridge with Guard Resistors [14]

19

The low output impedance of the high precision voltage sources minimizes the errors caused by

the leakage currents [14]. Also, by choosing programmable voltage sources, the system can be

automated to set the voltage precisely up to 2 ppm or lower [8]. Automating the system

eliminates the human factor from the measurement and helps to further refine the measurement.

Furthermore, automating the measurement process improves the repeatability and provides the

opportunity to perform measurements overnight without relying on a human operator.

A complete derivation of the equation for the unknown resistance value using Kirchhoff’s

current laws is as follows,

First, using the Kirchhoff’s current law at node ‘B’ on figure 3.2 results,

𝐼𝑋 – 𝐼𝑆 + 𝐼𝐷 = 0

Similarly, using Kirchhoff’s voltage laws at the top loop and the bottom loop,

(𝐼𝑋𝑅𝑋) − (𝐼𝐷𝑅𝐷) − 𝑉1 = 0

(𝐼𝑆𝑅𝑆) − 𝑉2 + (𝐼𝐷𝑅𝐷) = 0

When the bridge is balanced, 𝐼𝐷 = 0 since no current is flowing through the detector. Therefore

the above equations can be simplified as,

(𝐼𝑋𝑅𝑋) = 𝑉1

(𝐼𝑆𝑅𝑆) = 𝑉2

Rearranging, obtaining the ratios and solving for 𝑅𝑋 gives,

𝑅𝑋 = 𝑉1𝐼𝑠𝑅𝑠

𝑉2𝐼𝑋 When, 𝐼𝐷 = 0 𝐼𝑠 = 𝐼𝑋

Therefore, 𝑅𝑋 equals to,

𝑅𝑋 = 𝑉1

𝑉2𝑅𝑠

This equation can be used to calculate the unknown resistor value using the voltage of the

Calibrators and the reference resistor.

Equation 3.1: Unknown resistance based on other parameters

20

4. Measurement Algorithm

The measurement algorithm for the measurement system is based in the modified Wheatstone

bridge principle explained on section 3 of this manual.

First, the voltage on V1 and V2 will be set according to the ratio of the nominal values of

the two resistors. For example, if the RS value is 100Giga ohm and the RX value is 10 Giga

ohms, the Ratio will be 10:1. Then, V1 and V2 will be set using the software based on the ratio.

After the voltage is being set, the detector current will be measured based on the settling time of

the resistor. Settle down time of the resistors, also known as the RC time constant, will depend

on the resistor value. Therefore, the software will have the functionality to manually enter the

settle time before starting the measurement. After the settling time, current detector will acquire

200 data points of the current (ID). The average value of the above mentioned current will be

used in the calculations shown below.

Using Kirchhoff’s current and voltage laws, derivation of the value for the V2 voltage at the

balance point is as follows.

Applying Kirchhoff’s current law at node B as shown

in figure 4.1 results,

𝐼𝑋 = 𝐼𝑆 + 𝐼𝐷

Converting the above equations to voltages;

𝑉1

𝑅𝑋 =

𝑉2

𝑅𝑆 + 𝐼𝐷 − (1)

At the Balance point, current 𝐼𝐷 = 0; Therefore,

𝑉1

𝑅𝑋 =

𝑉2′

𝑅𝑆 − (2)

Where 𝑉2′ is the voltage of the calibrator (V2) at the balance point.

Figure 4.1: Modified Wheatstone bridge with labeled currents

21

Subtracting (2) from (1) and solving gives the 𝑉2′ value to be set, in order to balance the bridge.

𝑉2′ = 𝑉2 + 𝐼𝐷𝑅𝑆

The above calculated voltage will be set on the V2 detector and the current is measured again

after the settling time. Furthermore, in order to make a precise measurement, the whole process

will loop two times. Therefore, a finer value of 𝑉2′ voltage can be calculated.

Using the 𝑉1and 𝑉2′ at the balance point, unknown resistance value can be calculated using the

equation 3.1 derived in section 3 as shown below.

𝑅𝑋 (𝑢𝑛𝑘𝑛𝑜𝑤𝑛) =𝑉1

𝑉2′ × 𝑅𝑆

Equation 4.1: Recalculating V2 voltage at balance point.

Equation 4.2: Unknown resistance based on V1 voltage corrected V2 voltage and the reference resistor Rs.

22

5. Equipment Selection

Choosing the right equipment is crucial to achieve the highest possible accuracy of an

automated high resistance measurement system. Specifically, certain evaluation factors are being

used to determine the best equipment for the measurement system. The selection process of the

equipment is based on the user’s budget, level of uncertainty required in the measurement, and

the amount of measurement time. For example, if the measurement system is being developed in

a laboratory where a high resistance measurement system is used for testing the effectiveness of

an insulator, a higher accuracy may not be required. Therefore, the necessity of the equipment is

based on the user's needs.

Resistance laboratory at the National Institute of Standards and technology is constantly

in the forefront of research in high accuracy resistance measurements. Therefore, the equipment

described below are based on the research that has been conducted at NIST for the highest level

of accuracy [7]. According to Mr. Jarrett at NIST, an accuracy of 5 ppm was achieved using the

equipment described below [7]. However, based on the user’s expectations, different equipment

with the same functionality can be used. A brief description of alternative options is included in

the end of each section.

5.1 Programmable High Precision Voltage Supply (Voltage Calibrator).

The elimination of the R1 and R2 resistors of the conventional Wheatstone bridge as

described in section 3 of this manual, resulted the system to rely on high precision voltage

supplies. Similarly, as described in section 3, using a voltage source instead of two resistors

eliminates the possible leakage current that could occur between the resistors [15]. Furthermore,

choosing a programmable voltage source will provide the opportunity to control the supplied

voltages using an automation software such as LabVIEW or Microsoft Visual basic.

Examining these reasons, it is evident that the voltage sources are a crucial part of the

measurement system. Accuracy on these voltage supplies will ultimately affect the accuracy of

the final measurement. There are several commercially available voltage supplies on the market

that can produce accurate constant voltages. Namely, industrial testing equipment manufacturer

Fluke Corporation, manufactures several programmable voltage supplies that can produce a high

precision voltage. From Fluke’s collection, Fluke 5730A is capable of producing a voltage

output with an uncertainty of 1.5 ppm [8]. Therefore, the best possible accuracy can be achieved

using the Fluke 5730A voltage source.

23

Furthermore, the Fluke 5730A can be programmed using a computer software through

the GPIB interface. This functionality is extremely useful to eliminate the human operator during

the balancing stage of the bridge.

Overview of the front panel of the voltage calibrator is shown below.

Figure 5.1: Overview of the Fluke 5730A Voltage Calibrator

(1) Standby / Operate indicator

(4) Power Switch

(2) Output Terminals

(5) Number Keys for User Input

(3) Information Display

(6) Operate Key

5.2 Alternative options for the voltage calibrator

Based on the user’s budget and the expected uncertainty level, a different voltage source

than the above mentioned fluke 5730A can be used as the two voltage source for the

measurement bridge. Based on his experience, Mr. Jarrett recommends buying a used voltage

source from the used equipment market [7]. However, he also mentioned that old equipment may

no longer be supported by the manufacturer and would not be repaired [7]. Furthermore, if an

absolute precision is not necessary, any programmable voltage source can be used as the two

voltage supplies for the measurement system.

1

2

3

5

4

6

1

24

5.3 Current Detector (Current Meter)

The current detector is another crucial part of the automated high resistance measurement

system. The basic use of the current detector on the measurement bridge is to measure the

current at the balance point. Specifically, after the voltages V1 and V2 are set based on the two

resistor ratio, current (ID) as shown in figure 3.2 is measured using the current detector.

Therefore, the accuracy of the current detector is vastly important to produce accurate

measurement results from the measurement bridge.

There are several current detectors available on the commercial measurement equipment

market that can produce high precision measurement results. Specifically, the Keithley 6430

current detector can measure current less than a femto-Amp with an accuracy of 1% of the

nominal value [16]. Therefore, for the maximum possible accuracy of the measurement, the

Keithley 6430 current detector is the best option available on the market at the time this manual

was composed. Furthermore, Keithley 6430 can be programmed remotely. Remote programming

functionality is useful to automate the measurement process and eliminate human errors.

Overview of the front panel of the current detector is shown below.

Figure 5.2: Overview of the Keithley 6430 Current Detector

(1) Measured Current Display

(3) Measurement ON/OFF Key

(2) Main Power Switch

(4) Device Status Indicator

1

2

4

3

25

5.4 Alternative options for the current detector.

Analogous to the case of the voltage calibrator, based on the user’s intended budget and

the level of uncertainty, different equipment can be used. However, using a current detector that

can measure current up to nano-Amp range is recommended because, at the balance point of the

measurement, the detector current (ID) as shown in figure 3.2 is significantly lower. Also, a

current detector with the remote programing functionality is recommended in order to automate

the measurement system.

5.5 Cables

For the measurement system described in this manual, connection cables are used to

connect the unknown resistor (Rx), the reference resistor (RS), voltage sources and the current

detector. Choosing the correct type of connection cable is important to minimize noise during

measurement and improve the measurement time. In the measurement industry, Teflon and

Polyethylene are the two types of connection cable that are used for resistance measurements [7].

At NIST, Teflon cables are widely used for resistance measurements due to its high

insulation characteristics. However, experiments conducted at NIST prove that Teflon cables

retains charge at high resistance levels [7]. This charge retention during a measurement results in

a noisy measurement result. Therefore, for high resistance measurements, Polyethylene cables

are used to reduce errors in the measurement result. Polyethylene cables are specially made to

reduce the charge retention and to operate with low noise [7]. Additionally, Polyethylene cables

have same insulation characteristics as Teflon cables. Therefore, for high resistance

measurements, Polyethylene is the most suitable type of wire for highest precision of the

measurement. Additionally, junction boxes and node boxes, as shown in figure 5.5 and figure

5.6, are used to connect cables to the calibrator and to wire the bridge circuit.

Figure 5.3: Polyethylene Cable shown left and Teflon Cable shown right.

26

Figure 5.6: Node Box

Figure 5.4: Cable Connectors

Figure 5.5: Junction Boxes for the voltage Calibrator.

27

5.6 Air Bath

Standard resistors are highly susceptible to variations in temperature [7]. Therefore, a

temperature controlled enclosure is used to minimize errors from temperature fluctuations. In the

resistance measurement industry, standard resistors are either stored in oil or air baths to prevent

temperature fluctuations. According to Mr. Jarrett at NIST, Oil Baths provide better temperature

control than Air baths. However, Mr. Jarrett also mentions that, for high resistance values,

especially resistors that are higher than 10 Mega Ohm, using Oil Baths could create leakage

problems due to the lower insulation of the Oil [7]. Therefore, for high resistance standards, a

high precision temperature controlled Air Bath should be used.

For the measurement system described in this manual, Measurement International (MI)

Model 9300 Air Bath will be used for this purpose. Specifically, the MI Model 9300 Air Bath is

capable of controlling its temperature with an accuracy of ± 50mK for ± 1°C change in ambient

temperature [5].

Figure 5.7 Overview of the MI Model 9300 Air Bath

Temperature

Display

Temperature

Controller

Resistor

Storage Area

28

5.7 Alternatives options for the Air Bath.

Similar to the case of the Voltage Calibrator or the Current Detector, a different Air Bath

can be used based on the user’s budget and expected level of uncertainty. Furthermore, there are

several different temperature controlled Air Baths available in the industry that are relatively

lower in price with lower performance and functionality. Another alternative for the above

mentioned Air Bath is to build your own Air Bath. However, similar to any build your own

project, this process would require additional resources such as time, money and knowledge.

5.8 Computer with GPIB Support.

The measurement system described on this manual can be automated using a

programming platform with the aid of a GPIB interface. General Purpose Information Bus, also

known as GPIB, is a widely used communication interface in the measurement industry.

Moreover, GPIB connectors provide the ability to attach more than one component onto the

same cable providing the ability to control every equipment on the measurement system using a

single computer.

Figure 5.8: GPIB Cable [4]

Furthermore, in order to automate the measurement system, a computer that meets the

minimum system requirements for the LabVIEW software or the Visual Basic software, is

required.

29

6. Assembly Procedure

The following instructions elaborate the hardware setup procedure before operating the

measurement system.

The calibrators are capable of producing lethal voltages up to 1100V. Do not

touch output terminals when the device operates.

Step 1: Power off the Calibrator.

Before attempting to

make any connections

ensure the device is

powered off.

Figure 6.1 illustrates the front layout of

the Fluke 5730A voltage calibrator.

Power off the calibrator by switching

the power button ‘off’ as shown in figure

6.1

Step 2: Attaching the Junction Boxes to the Calibrators

Figure 6.1 POWER SWITCH

OUTPUT TERMINALS

BACK TERMINALS

FRONT TERMINALS

Figure 6.2

30

Figure 6.2 illustrates the front terminals and the back terminals of the junction boxes. The

back terminals of the junction boxes should align with the output terminals of the

calibrator as shown in figure 6.3. Similarly, the front terminals are used to connect other

components to the calibrator using cables.

Align the back terminals of the junction box with

the output terminals of the calibrator as shown in

figure 6.4.

Ensure that connectors are properly aligned when

attaching the junction box to the

calibrator. Improper alignment will

damage the terminals of the

junction box and the calibrator.

Repeat the above process for the second calibrator.

Figure 6.3

Figure 6.4

31

Step 3: Attaching front cables to the Calibrator

As shown on Figure 6.5, connect the

Positive wire to the top terminal and the Ground

wire to the second terminal.

Ensure that the open ends of

the wires are not connected to

any equipment while

performing this step.

Repeat the above step for the second calibrator.

Step 4: Connecting the Node Boxes

Figure 6.6 Illustrates a 3 terminal Node Box.

Connect the two ground wires from the two

calibrators (step 3), to any two of the three

terminals of the Node Box.

Connect a third wire to the remaining terminal

of the Node box. Figure 6.7 illustrates the final

result.

Note: the three terminals of the Node Box are

internally shorted together. Therefore, the wires

can be connected to any of the terminals.

Figure 6.5

(+) POSITIVE

GROUND

Figure 6.6

32

After completing the steps above the wiring

assembly should look similar to Figure 6.7.

Step 5: Current Detector

The current detector does not require additional

wire connections because, the Remote PreAmp as

shown in Figure 6.9, comes pre-assembled with

the current detector.

The Remote PreAmp, as shown in Figure 6.9, is

used to measure the current of the bridge during

runtime.

Note: instructions on attaching resistors to the

measurement system are included in the

measurement procedure section of the manual.

Figure 6.7

Figure 6.8

Figure 6.9

33

7. Creating the Automation Software

The implemented measurement system can be automated using a programming platform

to perform the balancing algorithm with minimum human interaction. Voltage calibrator and the

current detector used in this system provides the functionality to remote control over a GPIB

interface. Therefore, using the GPIB interface as the main communication protocol for the

measurement system makes it feasible to automate the measurement process. There are a wide

variety of programming platforms available in the industry. A comparison between two major

programming platforms used in laboratory environments is included below.

7.1 Visual Basic

Microsoft Visual Basic is widely used in laboratory environments due to its ease of use

and compatibility. According to Microsoft, Visual Basic is engineered for productively building

type-safe and object-oriented applications. Furthermore, Visual Basic enables developers to

target Windows, Web, and mobile devices [6]. Main programming style of Visual Basic is text

based. Therefore, creating a software requires fundamental knowledge in programming and

syntaxes. Furthermore, Visual Basic is vastly compatible with measurement equipment and

components. However, due to its code based nature, upgrading an equipment or an operating

system could create compatibility issues.

7.2 LabVIEW

LabVIEW is also widely used in laboratory environments and measurement industry due

to its simplicity and ease of use. LabVIEW program platform consists of a graphics based

programming structure. Therefore, in order to make a software program, a user can select several

graphical components from a LabVIEW library and attach using graphical wires. According to

National Instruments, LabVIEW is a platform that helps engineers scale from design to test and

from small to large systems. It offers unprecedented integration with existing legacy software,

IP, and hardware while capitalizing on the latest computing technologies [2]. Therefore, in

comparison, LabVIEW programming platform is more suitable for this measurement system.

34

7.3 Developing the automation software.

The voltage Calibrators are capable of producing lethal voltages up to

1100V. While testing the software, keep away from the output terminals.

Key aspects of the automation software

1. Safe operation of the system.

The Calibrators used on this measurement system are capable of producing

voltages up to 1100 volts. In order to ensure safe operation, output current of the

voltage Calibrators must be remotely limited to 3 mA using the software [7].

2. Controlling the voltage sources/calibrators

The software will control the outputs of the voltage Calibrators based on the value

of the resistor and the balance point calculations.

3. Controlling the current source/detector

The software will control the functionality of the current detector based on the

amount of settling time and the number of data points of the detector current

required.

4. Acquiring resistor data from a resistor database

When the user selects the resistors on the measurement software, the software will

search for the relevant data and use the search results during the measurement.

5. Applying voltage corrections

Corrections for the voltage supplies based on the calibration data will be applied

during the runtime.

6. Measurement algorithm and Data acquisition

The software will automate the measurement algorithm based on the input

parameters of the user. Furthermore, the software will acquire measurement data

and save them accordingly.

7. Performing calculations

Calculations such as the voltage at the balance point and the unknown resistor

value will be calculated using the measurement software.

35

The following flowchart shows a simplified procedure to be used on the measurement software.

Figure 7.1: Flow chart of the measurement procedure [15]

36

7.4 Overview of a sample automation software.

The following program was created at NIST using the LabVIEW programming platform.

(1) GPIB address for the V1 Calibrator

(7) Directory path for the measurement

results

(2) GPIB address for the V2 Calibrator

(8) Measurement voltage (V1 calibrator

voltage)

(3) GPIB address for the Current Detector

(9) Number of tests to be performed.

(4) Directory path for the resistor database

file

(10) Button to test the resistor in reverse

polarity

(5) Resistor to be tested (RX)

(11) Settle time for the test

(6) Reference resistor (RS)

(12) Test control Start/Stop.

1 2 3

4

7

5 6

8 9 10 11

13 14 15

16

19

18 17

Figure 7.2: Overview of a sample automation software.

12

37

(13) R1 (RX) Nominal value display

(17) V1 Voltage real-time graph

(14) R2 resistor information display

(18) V2 Voltage real-time graph

(15) Voltage ratio of the bridge display

(19) Detector Current real-time graph

(16) Real-time information of the ongoing test

38

8. Measurement Procedure

The following instructions will guide you to measure a resistor using the measurement system.

Additional Things You Will Need:

Resistors.

o One Standard Resistor with accurate calibration data.

o Unknown Resistor. (Resistor you would like to measure.)

Figure 8.1 illustrates a 10 Tera-Ohm Standard

Resistor used at NIST.

The calibrators are capable of producing lethal voltages up to 1100V.

Do not touch output terminals when the device operates. Use a caution

tape to isolate the system during operation.

Figure 8.1

39

Step 1: Attaching Resistors

Referring to the set-up procedure described in section 6 of this manual, connect the

positive terminal wire 1 from the first calibrator to one of the terminals of the standard

resistor as shown in Figure 8.2 and 8.3.

Similarly, connect the positive terminal wire 2 to one of the terminals of the unknown

resistor.

Step 2: Connecting the Current Detector

Connect the two open ends of the two resistors to

the Remote PreAmp of the current detector using

two wires.

Figure 8.4 illustrates the connection terminals of

the Remote PreAmp of the current detector.

Figure 8.2 Figure 8.3

POSITIVE TERMINAL WIRE 1

POSITIVE TERMINAL WIRE 2

Figure 8.4

40

Step 3: Storing in the Air Bath

Resistors must be kept in an Air Bath during the

measurement to minimize errors from temperature

fluctuations.

Place the two resistors and the Remote PreAmp of

the current detector as shown in figure 8.5.

Set the desired temperature on the Air Bath

controller.

Wait at least 2 hours before measurement.

Note: approximately 2 hours of settling time is required

before the measurement to ensure temperature

equilibrium inside the Air Bath.

Step 4: Setting up the software and making a measurement.

Before running the software, make sure that all the cables are attached.

Use a caution tape to warn or catch the attention of passersby of the area

Figure 8.5 TEMPERATURE CONTROLLER

41

Follow steps 1 through 12 to set up the software for the measurement (refer to figure 8.6).

(1) Select the GPIB address for the V1

Calibrator

(7) Set the directory path for the

measurement results

(2) Select the GPIB address for the V2

Calibrator

(8) Set the measurement voltage (V1

calibrator voltage)

(3) Select the GPIB address for the Current

Detector

(9) Set the number of tests to be performed.

(4) Set the directory path for the resistor

database file

(10) Select to test the resistor in reverse

polarity

(5) Select the resistor to be tested (RX)

(11) Set the settle time for the test

(6) Select the reference resistor (RS)

(12) Press ‘Start’ to begin the measurement

and press ‘Stop’ to abort.

1 2 3

4

7

5 6

8 9 10 11

13 14 15

16

19

18 17

Figure 8.6

12

42

Progress of the measurement can be monitored using 13 through 19 (refer to figure 8.6).

(13) R1 (RX) Nominal value display

(17) V1 Voltage real-time graph

(14) R2 resistor information display

(18) V2 Voltage real-time graph

(15) Voltage ratio of the bridge display

(19) Detector Current real-time graph

(16) Real-time information of the ongoing test

43

9. Testing the System for Accuracy

After the implementation, it is crucial to test the system for accuracy before using the

system for resistance measurements. Testing of the system for accuracy can be conducted within

the laboratory environment using two methods.

9.1 Using two standard resistors with adequate calibration data.

In order to verify the accuracy of the measurement system, two standard resistors with

adequate calibration data can be used as the known resistor and the unknown resistor of

the measurement system. For this test, a standard resistor with adequate calibration data,

such as, the measured resistance value, temperature coefficient and the resistance drift,

must be used to judge the accuracy of the measurement system. Procedure for the test is

analogous to the resistance measurement procedure described in section 8 of this manual.

After conducting the test using the resistors with adequate calibration data, produced

results can be compared with the previously measured data of the resistor to verify the

accuracy of the system.

9.2 Using a Hamon Transfer Standard.

If a resistor with adequate calibration data is unavailable, a Hamon transfer standard can

be used to verify the accuracy of the measurement system. A Hamon transfer standard is

a resistor that contains 10 or more resistors in series. Specifically, the Hamon transfer

standard provides the functionality to measure resistance in all 10 of these resistors in

series or in parallel [3].

Series

Connectors

Parallel

Connectors

Figure 9.1: Internal Diagram of a Hamon Transfer Standard [3]

44

By switching the relevant terminals as shown in figure 9.1, Hamon transfer standard can

be used either as 10 resistors in series or 10 resistors in parallel.

In order to test the system, a ratio measurement using the Hamon transfer standard can be

conducted. Specifically, two measurements must be made in all the resistors in series and

resistors in parallel. Afterwards, the two measurements can be compared with each other

to verify the accuracy of the system.

45

10. Troubleshoot Guide

To avoid electrical shock, personal injury or damage to equipment:

Do not perform any troubleshooting described below unless you are authorized

and qualified to do so.

Please follow electrical safety precautions carefully while performing these

procedures.

Problem Probable Cause Solution

No voltage output on the

Voltage Calibrator.

Calibrator in ‘standby’ mode

when the voltage is set

beyond 22V.

No input power to the

calibrator

Disable the automatic

‘standby’ of the voltage

calibrator using the

automation software. (refer to

the user’s manual of the

calibrator)

Ensure the calibrator is

plugged in to the main power

and verify the main power is

good.

Voltage calibrator shows

‘Calibration due’ message on

the display.

Fluke calibrator must be

calibrated periodically to

ensure an accurate voltage

output.

Refer to calibrator’s user

manual to perform the

calibration.

Voltage Calibrator won’t

power on.

Blown fuse of the voltage

calibrator.

No input power to the

calibrator.

Refer to the user’s manual of

the calibrator and check the

fuse. If the fuse is blown,

replace it with an appropriate

replacement fuse.

Ensure the calibrator is

plugged in to the main power

and verify the main power is

good.

46

Current detector shows no

output during run-time

Remote PreAmp of the

current detector is not

properly plugged in.

Verify the connections of the

Remote PreAmp of the

current detector and the

resistor.

Current detector won’t power

on.

Blown fuse of the current

detector.

No input power to the current

detector.

Refer to the user’s manual of

the current detector and check

the fuse. If the fuse is blown,

replace it with an appropriate

replacement fuse.

Ensure the current detector is

plugged in to the main power

and verify the main power is

good.

Signal of the current detector

on the real-time display of the

automation software appears

to be noisy.

Inadequate settling time for

the measurement.

Improper temperature control

of the Air Bath.

Input voltage of the calibrator

is too low.

Ensure that the settle time for

the measurement is adequate

based on the magnitude of the

resistor.

Check the Air Bath is

properly closed and the

temperature is set.

If the voltage of the calibrator

is too low, the current of the

detector could be noisy. For

higher resistance values, use

higher voltages.

Resulting resistor value is

significantly different than

the theoretical predictions.

Incorrect nominal value of

the unknown resistor.

Incorrect corrected value of

the standard resistor

(reference resistor).

Improper cable attachments.

Ensure that the nominal value

of the unknown resistor is

valid.

Ensure that the corrected

value of the standard resistor

is calculated to today’s date

and used during the

measurement.

Verify the cable attachments.

Refer to setup guide and the

measurement guide sections

of this manual.

47

11. Appendix

I. Glossary of Terms

Air Bath

High precise temperature controlled enclosure

filled with air that can be used to minimize

the effects of temperature fluctuations during

measurements.

Calibrator

Electronic device that can produce a precise

voltage supply that can be used to calibrate

other voltage source. During this manual, the

Calibrator is used as a voltage supply.

Current

The electric current. Current is a flow of

electric charge. SI unit of current is Ampere

and symbolized as ‘A’

Current Detector

Current detector, also known as the Amp-

Meter, is a device that detects electrical

current.

Current Leakage

Current leakage is the loss of useful electric

current in a circuit. For example, if the circuit

has a bad insulator, the current could flow in a

direction that is unintended by the user.

Dosimeters

Dosimeter is a device that can measure

radiation absorbed by an object.

GPIB Interface

GPIB (General Purpose Interface Bus), also

known as IEEE-488 Interface, is a

communication interface often used on

measurement instruments.

Hamon Transfer Standard

Hamon Transfer Standard is a resistor that

contains 10 or more resistors in series within

the resistor. Using different terminals, the

value of the resistor can be changed based on

user’s intention.

Kirchhoff’s Laws

Kirchhoff’s laws in current: at any node of a

circuit, sum of current in and out should be

equal to zero.

Kirchhoff’s voltage law: sum of the voltages

around a closed loop should be zero.

Nominal Value

The value stated on the face of the object. For

example, a resistor nominal value is the value

the resistor is built for. The actual value may

differ from the nominal value.

48

Oil Bath

High precise temperature controlled enclosure

filled with oil that can be used to minimize

the effects of temperature fluctuations during

measurements.

Parts Per Million (PPM)

Parts per Million (ppm) is a value that

represents the part of a whole number in units

of 1/1000000.

1 PPM = 0.0001%

Resistor

A resistor is a device that is designed to resist

the current flow on a circuit. The SI unit for

resistance is Ohms.

Signal Noise

Random fluctuations of a signal due to

external factors.

Standard Resistor

Standard resistor is a resistor that is used for

precise calibrations. Usually standard resistors

are very stable and contains adequate data

about its performance.

Voltage

Voltage, also known as electromotive force, is

the potential difference between two points of

a circuit.

II. Metric Prefixes

Table I I I

METRIC PREFIXES – SOURCE: U.S. DEPARTMENT OF COMMERCE, NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, THE

INTERNATIONAL SYSTEM OF UNITS (SI), NIST SPECIAL PUBLICATION 330, 1991 EDITION (WASHINGTON, DC, AUGUST 1991), P.10.

49

III. References

[1] K. Barnett, 'Electrician’s assistant sues over alleged electric shock received at New Orleans

Riverwalk project | Louisiana Record', Louisianarecord.com, 2015. [Online]. [Accessed: 04-

May- 2015].

[2] Ni.com, 'NI LabVIEW - Improving the Productivity of Engineers and Scientists - National

Instruments', 2015. [Online]. Available: http://www.ni.com/labview/. [Accessed: 04- May-

2015].

[3] D. Jarrett, 'Evaluation of guarded high-resistance Hamon transfer standards', IEEE Trans.

Instrum. Meas., vol. 48, no. 2, pp. 324-328, 1999.

[4] National Instruments, 'Shielded GPIB Cables', 2015. [Online]. Available:

http://sine.ni.com/nips/cds/view/p/lang/en/nid/1281. [Accessed: 04- May- 2015].

[5] MI Model 9300 Temperature Controlled Air Bath, 1st ed. Ontario, Canada: Measurement

International, 2015, p. 2.

[6] Msdn.microsoft.com, 'Visual Basic', 2015. [Online]. Available:

https://msdn.microsoft.com/en-us/library/2x7h1hfk.aspx. [Accessed: 04- May- 2015].

[7] D. Jarrett, 'ENGL393 - Expert Interview on high resistance measurement bridge', Panera

Bread, Gaithersburg, MD., 2015. [Summary attached].

[8] FLUKE 5730A Multifunction Calibrator Operators Manual, 1st ed. Fluke Coperation, 2013,

pp. 5-7, 13-25.

[9] P. Sutherland, Principles of Electrical Safety. Hoboken: Wiley, 2014.

[10] M. Markel, Technical communication. Boston, Mass.: Bedford/St Martins, 2012. Pp. 587.

[11] Jacob L. Heller, 'Electrical injury: MedlinePlus Medical Encyclopedia', Nlm.nih.gov, 2015.

[Online]. Available: http://www.nlm.nih.gov/medlineplus/ency/article/000053.htm. [Accessed:

05- May- 2015].

[12] E. Oberg, F. Jones, H. Horton and H. Ryffel, Machinery's handbook. New York: Industrial

Press, 1988.

50

[13] 6535 Automated High Resistance 6535 Automated High Resistance Measurement System,

1st ed. Ontario, Canada: Guildline Instruments Limited, 2015, pp. 1-5.

[14] D. Jarrett, 'Automated guarded bridge for calibration of multimegohm standard resistors

from 10 MΩ to 1 TΩ', IEEE Trans. Instrum. Meas., vol. 46, no. 2, pp. 325-328, 1997.

[15] L. Henderson, 'A new technique for the automatic measurement of high value resistors',

Journal of Physics E: Scientific Instruments, vol. 20, no. 5, pp. 492-495, 1987.

[16] Sub-femtoamp Remote SourceMeter® SMU Instrument Data Sheet, 1st ed. USA:

KEITHLEY Corporation, 2015, pp. 1-4.

51

IV. Interview Summary

We interviewed Mr. Dean Jarrett, project leader of the metrology of the ohm project at

NIST, to obtain an expert’s opinion and feedback on our manual. When we asked why he

thought the Dual source bridge is the best measurement system compared to a regular

Wheatstone bridge, he mentioned that it depends on the user’s intentions of the measurement

system. However, for the lowest uncertainty, the Wheatstone works better. Since there was no

human operator, he mentioned that the automated modified Wheatstone bridge is more feasible.

He also said that the optimal equipment selection depends on user’s goals. He states that if users

do not have the money to buy the best Calibrators on the market, then they can buy used ones for

cheaper. He also says to use polyethylene cables because it does not have charge retention issues.

Furthermore, for high resistances he also recommends to use air baths because oil baths have

leakage currents at high resistance ranges. He believes that the manual will be useful to

companies such as NASA and defense contractors because they do not specialize in one area like

NIST. The smallest uncertainty he has obtained is .08 ppm from a measurement bridge. Mr.

Jarrett believes that anyone who would be around this system should have basic electronics

training along with knowledge of where the high voltage areas are. The limitations of the system

include the settling time of RC constant and issues with drift current if you wait too long to take

a measurement.

A complete transcript of the interview is included below.

Interview Questions and Responses

1. You have worked with high resistance bridges for over 20 years. Based on your experience,

why do you think the Dual source bridge is the best measurement system compared to a regular

Wheatstone bridge?

It depends on the situation, what your goals are. Overall, the dual source bridge, if you

want to have the lowest uncertainty, it is the best system. If you don’t need much accuracy there

are other systems. But I’d say, because of the demands of our customers and the work we do, we

have gone to the lowest uncertainty as possible that’s why we used the Wheatstone bridge.

When I first started, we had a guarded Wheatstone bridge and the Tera-ohm meter

system, the limitations were that the guarded Wheatstone bridge was manually operated. So, we

had someone looking up the detector and turning knobs. It was very inefficient. It was subjective

depends on the patient of the operator, we used to have two operators one operator in the

morning and operator at night. Automating will prevent leakage problems. The old bridges were

only able to use up to 10 Giga ohms. Because we replaced two resistors with the programmable

52

DC calibrators so we don’t have leakage problems and also its automated and it can go to higher

voltages. Plus it’s less labor. Before we used battery supply, but then we had to check the

voltages on the batteries.

2. Do you think an automated high resistance bridge system is more efficient and effective

compared to other systems in the industry? If so, why?

Certainly anything automated is more efficient than anything that is manually operated.

The main thing that we developed in our measurement bridges is that we have automated

guarded switching so we can attach a number of resistors and then automated software can

measure a lot of resistors at the same time. The measurement can run over night without an

operator. It eliminated the labor and the human factors to minimize systematic errors. The

timings are consistent and it’s not based on someone's judgements. There are also some other

systems in the industry, they don’t operate in the same wide range, and they don’t have the same

uncertainty range. There is another system that is widely used that is the binary voltage divider

bridge. Where a binary voltage bridge is used on two of the arms and that system is automated,

but it only works up to a 100 mega ohms. It has comparable uncertainties.

3. What are your recommendations for equipment to be used in an automated resistance

bridge?

There are some other Calibrators that are available and that don’t cost a lot. You could

build a bridge that wouldn’t be as accurate, but you can still make measurements. The other

option that some people have done is that they have bought used calibrators on the used

equipment market. The downside is that they are not supported by the manufacturer. We do have

two of those bridges in our oldest bridge. Those are probably 30 something years old, so at one

point there won't be repairable. That’s why we are building the new bridge to make the bridge

very robust and try to address some of the shortcomings, especially from 1 Tera-Ohm to 1 Kilo-

Ohm

4. Teflon and Polyvinyl cable are widely used in the measurement industry. In order to

minimize noise during a measurement, what cable would you recommend for the measurement

system? And why?

So, we use a lot of Teflon cable, we use that because Teflon is a high insulator. However,

we have also found that Teflon at the high resistance range Teflon retain charge so we looked at

some other alternatives. There is some cable that has. I forgot the name the insulator. Its poly

something. But not polyvinyl. That’s a low noise cable that’s the best one for the higher

resistance levels. It doesn’t have the charge retention issues and it’s a pretty good insulator.

53

5. Standard resistors are kept in either oil or air baths. For high resistance standards, what is

the ideal resistance storage method? And why?

At the resistance lab we use oil baths for basically anything from one mega ohm to

anything under less than an ohm. We use the oil because the oil is a pretty good insulator

because it’s clean and it’s not contaminated, it will have an insulation of 1014 ohms. Some of the

resistors have temperature coefficients that is they are susceptible to the changes of the

temperature. We can control oil better than air. The oil that we have in big baths in the labs, we

can control them with the covers on up to a one mili degree Celsius. We can't use oil after 10

mega ohm range because we get to the point where the resistance of the oil will create a leakage

path and create a leakage problem therefore we don’t use it for the high resistance range. We

could possibly use at 10 mega ohm then we would have to have a separate area.

6. What industries/work environments do you think, will benefit from this manual? And what

are some applications of high Resistance standards?

Often we have a lot of customers that will send us equipment and standard resistors for

calibration. Some of them are very capable of and outstanding experts in the field and some are

not. We have some customers that calibrate their resistors on the bench and wonder why the

value change by 50 ppm. I think a manual like this would be beneficial to people in the industry.

We are specialized in our labs because we are working with resistances. But some other labs like

in military labs and NASA and a lot of the defense contractors can't go into one discipline like

we can. So, they will be doing resistance, voltage, impedance, pressure, and temperature

measurements so a manual like this would be helpful to give them some things to consider. For

example the effect of temperature, the type of cable that they are using or the guarding. So I

think they will be useful for them.

Lot of the applications have to do with people who need to measure low currents. Some

of the applications that I have encountered are customers that calibrate standards for other labs.

In the department of energy and the nuclear power industry, they have equipment for measuring

background radiation dosimeters and the output is a really small current. In the pico-amp or in

the femto-amp range. To calibrate those dosimeters, they use a standard resistors to compare the

output of the dosimeters and calibrate them. Also, people making connectors use high resistance

standards because, that they want to check the purity of the material they are using. Another

54

application is that, we did a special test for Boeing where they wanted a resistor calibrated at 10

Tera-ohms and what they were using that for was to calibrate their fuel test sets. So that they can

grade the purity of the fuel going into their jet engines. A lot of the applications are dealing with

the purity. Also I have talked to someone who wanted to have some resistors Calibrators in the

10 Giga-ohm and 100 Giga-ohm and they were using the measurements to grade the purity of

raw textile material and to determine what it can be used for in the textile industry. One of the

other applications is for one of our most common customers, which is the Sandia national labs

and one of the things that they do at Sandia is that they support the nuclear weapons program.

They wanted to measure the resistivity of a ceramic coating on a neutron tube part of a nuclear

system.

7. You have implemented several automated high resistance measurement systems at NIST.

What is the highest level of accuracy that you were able to achieve from your systems?

One of the systems I have worked on, which is an automated double Kelvin bridge which

can measure up to 0.08 parts per million and those are using special resistors that are designed in

the 1970’s by a couple of manufacturers and they are stable and has low temperature

coefficients. Those are probably the lowest uncertainties that I have worked on. Also in high

resistance, we were able to achieve few PPM with the dual source bridge.

8. What safety measures would you recommend to improve the safe operation of the system?

One of the issues that we have with the high resistance is that sometimes we have to go

high voltages like 1000 volts. One of the things that we implemented in our dual sources bridges

is that, we limit the current. We build that into the code so that the Calibrators will limit the

current to 3 mA. Also we rope off the area. Another thing that I recommend is to have some kind

of a visual indicator when the system is running with a bright light. That will be useful to alert

people and also it will be good if the system gets stuck at one point. If the person is not

intimately familiar with the system a warning light will be very useful to know that the system

isn’t functioning properly. Another thing would be to have a display running off of a computer

saying the actual status of the measurement.

9. What type of training do you think people should receive, in order to operate this system?

Why?

The first thing that we do require is that basic electrical safety training and to go over our

standard operating procedures that we have in place and to be familiar with the instruments that

make up the bridge. For example, how the calibrators work, how the detector work, and how the

55

cables are connected. Also people should be aware of where the high voltage is at and where the

ground is when the system is operating.

10. What are the limitations of a bridge measurement system of this nature?

One of the limitation is the settling time of the RC time constant. Also if you wait too long what

happens is that it will take a long time to take a single measurement and then will have errors in

other components from drift.