Current Loops

8/4/2019 Current Loops

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Current Loops

Current loop signals (i.e. 0-1 mA DC, 4-20 mA, 10-50 mA etc) ride on the supply orsignal voltage supplied by a power supply. This can be a separate device, such as theback of panel mounting Deltron 112A, which will convert a standard 120VAC or 240VACline input into a stable 24V DC output @ 1.2 amps (1200 milliamps). Or the powersupply might come from a smart digital panel meter, such as the Red Lion PAXP0000.

The meter is powered by 120/240 and outputs a 24 VDC @ 50 milliamps. The Deltronexample could power up to 60 separate 4-20 mA signals (1200 milliamps divided by 20milliamps = 60); the Red Lion however only two. Sizing should really be based on atleast 25 milliamps per device.

The power supply's 24 Volt DC output will be reduced by the voltage drop of eachinstrument in the series connection of the loop, and by the length and gauge of the wireutilized. Care must be taken that the voltage is not dropped below the minimumoperating range of the instruments used in the loop. The signal voltage in the loop is thedifference between the voltage at one terminal (referenced to ground) and anotherterminal (see diagram above).

Fortunately, most Digital (about 20 ohms) and Analog Instruments (about 3 ohms) havea very low resistance. Therefore, their utilization of voltage in the series circuit is usuallyminimal. One exception is if a receiver, such as a chart recorder or other such device, isreally a 1-5 VDC device, and takes the 4-20 mA signal only through a 250 ohm resistormounted on the signal input terminals of the device. According to Ohm's Law, themaximum drop for such a device is 5VDC (250 ohms x 20 milliamps or .02 amps = 5)

Ohm's Law: Voltage = Current x Resistance or E=I x R

Current =Voltage Divided by Resistance or I=(E/R)

Also fortunately, the length and gauge of instrument wire utilized has also usually aminimal effect on the voltage drop. Let us take a common situation with a single sensorand readout, and a distance between the two of 1000 feet:

A common instrument signal wire is 22 Gauge. The resistance of this wire is only0.0165 per foot (each leg must be considered). A thousand foot run from the sensor tothe power supply would provide the following resistance: 1000 x 2 x 0.0165 = 33 ohms.


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By Ohm's Law, Current times Resistance (I x R) = Voltage (E).

20 milliamps (.02 amps) x 33 ohms = 0.66 volts.

If you have a 24 volt DC power supply, you would still have 23.34 volts left over. Even

10,000 feet would only take 6.6 volts away.

Caution: If you remove an instrument from the ioop, the entire ioop goes down. Anotherconsideration is the prevention ofGROUND LOOPS. Please click on the underlined fora discussion on this topic.
http://www.branom.com/literature/groundloop.htmlhttp://www.branom.com/literature/groundloop.html


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RESISTANCE TEMPERATUREDETECTORS

INTRODUCTION

A resistance temperature detector (commonly called RTD, resistance bulb, etc.)assembly consists of (1) an element (2) a support or bobbin for the element (3) aprotection tube or sheath (4) connecting wires which extend from the element to thetermination end (5) a means of securing the connecting wires to the termination end,and (6) a means of connecting it to the resistance-measuring equipment.

Resistance temperature detectors (R.T.D.) or resistance thermometry is based on awell-known principle that most metals increase in resistivity when their temperature isincreased, and on cooling to the original temperature, will return to the originalresistivity. The resistance-temperature curves of pure metals, e.g., platinum and nickel,over definite spans makes them ideal materials for the elements in resistancethermometers.

Laboratory resistance temperature detectors of pure platinum, fully annealed and strainfree, have been chosen as the International Standard of Temperature Measurement

from liquid oxygen [(LO2)-182.97 deg C] to the melting point of antimony [(sb) + 630.5deg C]. Range -250 to +500 deg C for platinum. Temperature coefficient: .003915ohms/ohm/ deg C and .00385 ohms/ohm/ deg C. (0.00385 or din standard has beenadopted as the world and USA standard.)

Pure nickel has been widely used as a temperature-sensitive element over the range of-700 deg C to +3000 deg C principally because of its low cost and high temperaturecoefficient of resistivity.


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ADVANTAGES

Absolute Measurement- Resistance thermometers, unlike thermocouples, do not

require a reference point. No ice baths or compensation circuits.

High Output- With an output of 50 to 200 times that of a thermocouple, resistancethermometers permit the use of simpler indication and control instruments. No amplifiersare needed and the resulting system is less expensive and more reliable.

Greatest Accuracy- The pre-eminent position of the resistance thermometer as aprecision temperature measuring instrument is demonstrated by its selection to definethe International Temperature scale from -260 deg C to +660 deg C. The main reasonsfor its selection are: 1) the exceptional stability and 2) the repeatability of the resistancethermometer.

USES

Resistance Thermometers can be used for a wide variety of industrial applications. Ahigh electrical output can be obtained by using the RTD with many types of simpleresistance bridges. This high output can then be fed directly into recorders, temperaturecontrollers, transmitters, or digital readouts which can be calibrated to read very preciseincrements of temperature over wide dynamic ranges. RTD's can also be read out onprecision laboratory bridges and digital ohmmeters.

BASIC INSTRUMENTATION

A simple Wheatstone bridge circuit with a reasonable high impedance detector isrecommended for reading out RTD probes.

If the detector impedance is assumed infinite

Eo Rx Rs-- = ------ - ------E Rx - R Rs + R

where R = Ratio ArmsRx = Probe resistance (at temperature x)Rs = Balancing arm (equal to Rx at lowest

temperature which may be variablefor zero set.)


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Such a bridge is non-linear, when the probe undergoes any reasonable temperatureexcursion. In the case of platinum wire, the ratio arms (R) should be as large aspossible (at least 10 times Rs) to minimize bridge non-linearity. To protect the probeand minimize the errors due to self-heating, an operating current of 1 MA isrecommended. This current can be controlled by choice of R or L.

To measure temperature difference, two identical probes can be used in adjacent armsof the bridge (second one replaces Rs). In this case, provisions for zero setting (ifdesired) should be moved to one of the R arms.

CONSIDERATIONS FOR RTD SELECTION

1. How the point of measurement can be made. Whether in a small area, whichwould necessitate a tip sensitive, or a large area which would make a stemsensitive more desirable.

2. The O.D. of the tube.3. The temperature and/or temperature range of the media to be measured

accuracy excellent at room temperature.4. What length of immersion would be required for your application.

5. How the R.T.D. is to be inserted, and how best it can be supported or mounted.6. If pressure or vacuum has to be maintained, then the R.T.D. has to be supplied

with either a compression fitting, fixed fitting, head with connector, or athermowell.

TYPICAL APPLICATIONS


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PRECISION PROCESS TEMPERATURE CONTROL

Textile Chemical Food

Brewing

AUTOMATIC TEMPERATURE CONTROL

Test Chambers Oven Temperature Plastic Extruders Injection Molders Solder Pots Bearing Temperature

READILY AVAILABLE RTD INSTRUMENTS:

Digital Temperature Indicators 12-Inch Round Chart Recorders Branom Steam Control Systems and Multipoint Rtd Indicators Crompton and Jewell Rtd Analog Meters and Setpoint Controllers Red Lion digital Indicators and Controllers R.I.S. Transmitters and Trips and 36 Point Alarm Monitors Rustrak Miniature Recorders West Rtd Controllers: On-Off, Hi-Lo Limit, or PID

GLOSSARY OF TERMS

RTD - Denotes resistance temperature detector, a device which provides a useablechange in resistance to a specified temperature change.

SENSING ELEMENT - The electrical portion of an RTD (Resistance winding) in whichthe change originates.

CALIBRATION ACCURACY (INTERCHANGEABILITY) - The conformance of the RTD's

measured output to a standard calibration curve calibrated by a governmental standardsagency such as NBS or calibrated on equipment directly traceable to NBS.

REPEATABILITY - The ability of the RTD to reproduce consecutive readings when thesame temperature is applied to it consecutively under the same conditions, and in thesame direction.


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STABILITY - The ability of an RTD to retain its repeatability (and other specifiedperformance characteristics) for a relatively long period of time.

SELF-HEATING - internal heating resulting from electrical energy dissipated within theresistance sensor. This is usually specified in watts or millivolts/ deg C. This is

determined by the amount of power it takes to raise the output of the sensor 10C undercertain conditions such as air, water or oil flowing at a specified velocity.

RESPONSE TIME - The length of time required for the output of an RTD to respond to63.2% of a step change in temperature. This is usually specified in air, oil or waterflowing at a specified velocity.

MAXIMUM SAFE CURRENT - The maximum current recommended to be applied to aparticular RTD to prevent burn out or open circuiting. This is determined by the sensorwire diameter and the configuration.

INSULATION RESISTANCE - The resistance measured between specified in-insulatedportions of an RTD (such as between sensing element and outer case) when aspecified DC voltage is applied.

PRACTICAL PRECAUTIONS

1. Use shielding and twisted-pair wire, avoid stress and steep gradients, use largeextension wire. Use 3 wire or 4 wire cable.

2. Due to its construction, the RTD is somewhat more fragile than a thermocouple

and some care should be taken to protect it3. A current is and must be passed througout the RTD to provide a voltage that canbe measured. This current causes joule (I2R) heating within the RTD. This self-heating does appear as a temperature error. To reduce self heating errors, usethe minimum current possible, and use the largest rtd you can that will still giveyou the response you need. A typical value for self-heating error is 1/2 deg c permilliwatt in free air. If you immerse the RTD in a liquid, or any other thermallyconductive medium, you will dissipate the self-heating aspect to a negligibleerror.

MOST COMMON RTD TYPES

1. 1/8 inch x 2 inch encapsulated 100 ohm platinum (.00385) RTD for surfacemounting.

2. 1/4 inch od x any practical length ss sheath 100 ohm plt RTD (.00385)--availablewith 1/8 inch, 1/4 inch or 1/2 inch brass or ss fitting or with standard wells and


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with aluminum or cast iron heads, or high temperature plastic (450 or 850 deg f)heads.


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THERMOWELL CONVERSION KIT

THE ADAPTER SET CONSISTS OF:1. An adapter nut.2. A metal liner.

SELECTION OF BIMETAL THERMOMETER:

1. Measure well depth by instering a pencil orsmall diamter rod into well until it reaches the

bottom (Figure 1).2. Using thumb as index, measure distancefrom end of rod to index point (Figure 2).

3. Refer to Selection Table (below) to selectproper thermometer stem length.Thermometer stem length must match welldepth as indicated on the Selection Table.

INSTALLATION OF ADAPTER ANDTHERMOMETER

1. Drop or push metal liner into well.2. Thread adapter nut into well and tighten.3. Install Reotemp Bimetal Thermometer into

well. (Note: a small amount of graphite andgrease or other heat transfer compound onthe lower 2" of stem and liner will improveresponse time.)

Well Depth in Inches Bimetal Stem Length

3 - 3-1/4 4"

5 - 5-1/4 6"

7 - 7-1/4 8"


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THERMOWELLS (DRY WELLS)

A. Usage & Disadvantages

The use of thermowells (also known as dry wells) is common in industrial applications

involving the need to remove a temperature sensor from a tank or line without shuttingdown the system. Thermowells, however, have some basic disadvantages which mustalso be taken into consideration for any temperature application:

1. Transmission Time: The added mass, and the type of material has a slowing effectupon how quickly the actual temperature reading will show up on the indicator (brasshas fastest transmission time).

2. Fit: Proper, very tight fit is essential, as air gaps create insulation, and thereforeinaccurate readings. Unfortunately, there are few standards in the industry for RemoteReading Gas and vapor Filled Thermometers, with every brand, every style and every

range with a different diameter and length and connection. Bulb lengths anddimensions, internal & external thread requirements etc must be carefully measured forthe specially ordered well, so that the internal diameters and lengths and connectionsmatch the sensing bulb. Heat transfer compounds should be used whenever anabsolutely tight fit is not possible; an inexpensive compound consists of a pastecontaining 1/3 water and 2/3rds magnesium hydroxide (available from us, or fromchemical suppliers).

Thermocouples and RTD's can also come in any size and shape; a common size,however, is 1/4" OD, and with a 1/2" NPT Spring Loaded Male Fitting, these can fit intoinexpensive and commonly found bimetal thermometer wells, which have a .260 bore.

3. "Lagging" thermowells take into account the insulation, pipe fittings, or walls etc.through which a sensor might have to pass.

B. Installation Considerations


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The most common method of installing a well is to purchase and install a "tee" from aplumbing supply house and use a standard threaded well; ASA 150#, 300# and 600#Flanged Wells, Van Stone Wells, and Socket Weld types are also readily available. Allthreaded wells are made in easily welded or brazed materials. This is important forinstallations requiring sealing; the pipe thread provides the mechanical strength, while

the brazing or welding provides the seal.

The object is to measure the temperature of the medium, so the insertion should be tothe point in the pipe where the measurement is desired, usually in the middle of thepipe. However, the sensing portion and range of the instrument will often determine theminimum insertion length of the well. The "U" dimension of a well is the insertion lengthof the sensing bulb (the distance from the tip of the internal bottom of the well to the firstthread or other connection means) should be entirely immersed in medium beingmeasured. A properly installed element will project into the liquid an amount equal to itssensitive length plus at least one inch. In air or gas, the element should be immersed itssensitive length plus at least three inches. Some low range bi-metal thermometers, for

example, are not available without at least a 4" length stem. Normally, bi-metalthermometers have a sensitive length of 2.5"; RTD's usually have a sensitive length of1" or so; thermocouples have sensitive lengths of 1/4" or so; grounded thermocouplesare tip sensitive, and have a faster response to temperature changes than ungroundedtypes (but ungrounded thermocouples help prevent current loops and induced voltagesthat often destroy the thermocouple millivolt signal). Industrial liquid-in-glassthermometers come standard with either a 2" stem (Submarine Thermometers) or morecommonly, 3 1/2" stems (Standard Industrial and Retort); sometimes 6", 8", 9" 10" andup to 48" types can be found. Careful measurement of the "U" dimension is necessaryfor a correct well fit.

C. Velocity Rating Factor

Tapered shank wells provide greater stiffness for the same sensitivity. The higherstrength-to-weight ratio gives these wells a higher natural frequency than the equivalent

length straight shank well, thus permitting operation at higher fluid velocity. Anotherconsideration might be materials of construction; some wells made of stainless steel, forexample, may take higher temperatures, pressures and velocities than a brass one.Fluid, flowing by the well, forms a turbulent wake (the "von Karmen" trail) with afrequency based upon the diameter of the well and the velocity of the fluid. If the wakefrequency equals the natural frequency of the well, the well will literally shake itself topieces and break of f from the piping. Velocity tables are available from us for mosttypes of standard wells, materials, pressures and temperatures. For simplicity sake,


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brass is rated at 3500F, steel and stainless @ 10000F, monel @ 9000F service. Slightlyhigher velocities might by possible at lower temperatures. Typical ratings for straightstepped thermowells in maximum fluid velocity feet per second:

1/4" OD Stem "U" Dimension | Material of Construction | FPS

2.5" Brass | 207C.S. | 290

304 & 316 | 300

4.5" Brass | 75

C.S. | 105

304 & 316 | 109

D. Materials of Construction

To prevent electrolysis, the well should ideally be constructed of the same material asthe piping. Another consideration is the corrosive conditions the well will face, as well asstrength necessary to face these conditions. Wells are often cut from bar stock in brass,monel, 304 and 316 SS, or other special grades of stainless steel, inconel, hastelloy B &C, Nickel, and Titanium. The least expensive are steel and brass constructions as wellas 304 and 316 SS. For high temperature thermocouple use, 302 SS sheaths, siliconcarbide or porcelain sheaths are common well solutions.


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THERMOCOUPLES

INTRODUCTION

The basic theory of thermocouples dates back to 1821 when T.J. Seebeck discoveredthat a current is induced into a closed circuit of two dissimilar metals by heating one ofthe two junctions. And, as long as the temperature differences exists between the two

junctions, current will continue flowing through the circuit.

While the theory is nearly 150 years old, incorrect application of thermocouples stillaffects today's sophisticated industrial processes. In any temperature control system,the heart of that system is the temperature sensing device -- in this case, thermocouple.Without proper application or understanding of basic thermocouple circuits, even themost complicated system cannot function.

In his discovery, Seebeck also concluded that any two metals can be used. However,the magnitude and direction of the generated current are functions of the magnitude ofthe temperature difference between the junctions and the thermal properties of themetals used in the circuit. Therefore, not every combination of metals is acceptable forthermocouple usage.

THERMOELECTRIC CHARACTERISTICS

A thermocouple should have thermoelectric characteristics such that the electromotiveforce (emf) produced per degree of temperature change is sufficient to be detected by

standard measuring instruments. The device must also be capable of withstandingtemperature extremes for prolonged periods, rapid temperature changes, and corrosiveatmospheres while exhibiting reproducibility and a high degree of accuracy.

The Instrument Society of America (ISA) has established a type of code and limits-of-error specifications for thermocouple wire, shown below:

Table 1: Thermocouple Wire Specifications


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ISA LIMITS OF ERROR

DESCRIPTION ISA TYPE TEMPERATURE RANGE STANDARD SPECIAL

Copper/Constantan T -300 deg. F to -75 deg. F -- +/- 1%

-150 deg. F to -75 deg. F +/- 2% +/- 1%

-75 deg. F to +200 deg. F +/- 1.5% +/- 3/4 deg F

Iron/Constantan J 0 deg. F to +530 deg. F +/-4 deg F +/- 2 deg F

+530 deg F to +1400 deg F +/- 0.75% +/- 0.375%

Chromel/Constantan E +32 deg. F to +600 deg. F +/- 3 deg F --

+600 deg. F to +1600 deg. F +/- 0.5% --

Chromel/Alumel K 0 deg. F to +530 deg. F +/- 4 deg F +/- 2 deg F

+530 deg. F to +2300 deg. F +/- 0.75% +/- 0.375%

Platinum/Platinum(+10% Rhodium)

S 0 deg. F to +1000 deg. F +/- 5 deg F --

Platinum/Platinum(+13% Rhodium)

R+1000 deg. F to +2700 deg.

F+/- 0.5% --

Six thermocouples are covered by this system:

Copper vs. Constantan (ISA Type T) (Nickel Alloy)

Superior for use at sub zero temperatures. Withstands corrosion well andis recommended for temperatures within the range of -300 deg F to +700deg F.

Iron vs. Constantan (ISA Type J)

Above 1000 deg F, the rate of oxidation of the iron wire increases rapidlyand the thermocouple should be enclosed in a protection tube of suitablematerial. Protected Iron vs. Constantan thermocouples are recommendedfor temperatures up to 1600 deg F.

Chromel vs. Alumel (ISA Type K) (Nickel Chromium Alloy) or (Nickel Manganese,Aluminum, Silicon)


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These are trade names of Hoskins Manufacturing Company. Chromel vs.Alumel thermocouples have excellent characteristics when supplied withprotection tubes up to 2200 deg F.

Chromel vs. Constantan (ISA Type E)

Although only in limited use in industrial applications, this typethermocouple has the highest emf output of any standardized metallictype. They may be used in oxidizing, inert or reducing atmospheres to1600 deg F and at sub-zero temperatures they are not subject tocorrosion. Indications are that the future will see more consideration givento this combination.

Platinum vs. Platinum-Rhodium

Platinum vs. 90% Platinum + 10% Rhodium (ISA Type S) and Platinum vs.

87% Platinum + 13% Rhodium (ISA Type R) are used for temperatures upto approximately 3100 deg F, depending upon the atmosphere. Both typesshould always be provided with a high temperature ceramic protectiontube.

Various other combinations of materials are used for thermocouples throughout industrybut with far less frequency than the six basic types. Combinations of platinum andrhodium with various percentages of each have been regularly available for some years.Iridium vs. Iridium with 40, 50 or 60% Rhodium thermocouples have found acceptancein some high temperature applications up to 3600 deg F.

Proper thermocouple selection is primarily determined by the temperature range inwhich its use is intended. Other factors, such as atmosphere, abrasion, vibration, andlocation will determine the type, size, and configuration of the complete assembly whichincludes protection tubes and mounting facilities.

LOCATION

Proper location of the thermocouple is probably the most important factor in obtainingaccurate temperature control. Thermocouples should be in a position to have a definitetemperature relationship to the heat source and workload. A good 'rule of thumb' in

locating thermocouples is to place them between the workload and heat source. Thethermocouple should be located 1/3 the distance from the heat source and 2/3 thedistance to the workload.


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If a thermocouple is located too close to the heaters, a long warm-up time will result.The thermocouple will sense the heat before it reaches the workload, and this meansrapid on/off action of the controller. In effect, the controller is controlling the heater andnot the workload. In rare cases, voltage will be induced into the thermocouple circuit athigh temperatures when located too near the heaters.

When a thermocouple is located too close to the workload, there is a substantial delayin sensing the proper control point and the result is overshooting the temperature. Inmost cases, it is better to be too close to the heaters than the workload as once atemperature point is passed, it becomes difficult to cool the workload unless a forcedcooling system is used. Two thermocouples connected in parallel could be used, one


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located near the heaters and the other near the workload. Both will balance these twofactors and provide closer control.

Another consideration in location is when locating a thermocouple in a thermocouple

well. If it is not bottomed correctly, located at the bottom of the well, the thermocouplewill be reading the air temperature around it and not the temperature of the workload.


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COMPENSATION

The compensation method used by all millivoltmetermanufacturers is to attach a bi-

metallic spiral to the top hairspring of the coil suspension system. This spiral is selectedaccording to the range of the instrument and will deflect the indicating pointercorrespondingly with changes in ambient temperature. Once ambient is setmechanically, using a zero adjust screw, it is not necessary to change the setting duringthe operation of the instrument. In solid state instruments, the compensation is achievedelectronically by placing a temperature sensor, such as a thermistor or RTD, at the cold

junction to monitor its temperature. The signal from this sensor is used to compensatefor variations in cold junction temperature.

The automatic compensation for ambient temperature is sufficient in most industrialapplications. However, in laboratory experiments or critical control situations, when

maximum accuracy is desired, one of two cold junction compensation methods areused. One method is to place the cold junction in an agitated ice bath, shown below.The instrument will then be set at 32 deg F (0 deg C), which is the temperature of theice bath.


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In the other method, the cold junction is situated in a precisely controlled temperature

above ambient, as shown below. In this case, ambient compensation is not necessary.The mechanical zero adjustment is set at the cold junction temperature beingmaintained. The normal temperature being maintained is 150 to 200 degrees F at thecold junction.

In normal applications, if the cold junction is located too close to the heat source,conduction and radiation heating will cause inaccurate readings. Errors will also occurwhen using copper wire or the wrong thermocouple lead wire. When copper wire isused, the cold junction in effect remains at the thermocouple connector block instead ofthe instrument. This will cause the instrument to read low in most cases unless the cold

junction and instrument are known to have the same ambient temperature.

There is one application where cold junction compensation is not a factor. When twothermocouples are connected in series opposing, as shown below, a millivoltage isproduced which is the difference in millivolts between the temperature at boththermocouples. As the difference in degrees between the two thermocouples is beingmeasured, cold junction compensation is not necessary.


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Each millivolt measuring instrument is calibrated for both the type of thermocouplebeing used and the length and gauge of the lead wire. The thermocouple lead wire is ineffect in series with the thermocouple wire and the meter movement. Using wrongthermocouple lead wire can be avoided by simply following the color-coding used by allmanufacturers (Table 2, below). A solid state controller can be used with up to 100

ohms of external resistance without having to be recalibrated.

Table 2: Calibration symbols and color codes for thermocouple andextension wire

TypeISA

SymbolPositive

(+)Polarity-Color

CodeConductor

Negative(-)

Overall

Thermocouple J -- Iron +White

(Magnetic)Constantan - Red Brown

Extension JX -- Iron + White Constantan - Red Black

Thermocouple

T --

Copper + Blue Constantan - Red Brown

ExtensionTX --

Copper+ Blue Constantan - Red Blue

ThermocoupleE --

Chromel+ Tan Constantan - Red Brown

ExtensionEX --

Chromel+ Tan Constantan - Red Brown

ThermocoupleK --

chromel+ Yellow

Alumel(Magnetic)

- Red Yellow

ExtensionKK --

Chromel + Yellow Alumel - Red Brown

ThermocoupleS -- PT10% RH

+ -- Platinum - -- --

ThermocoupleR -- PT13% RH

+ -- Platinum - -- --

Extension SX -- + Black Alloy 11 - Red Green


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Copper

When a millivoltmeteris calibrated, a series resistance (commonly called a calibratingspool) is used between the moving element coil of the instrument and the thermocoupletip. The resistance of the wire must be determined and used in the calibration of the

instrument. If the resistance of the thermocouple wire and extension wire is higher thanthe instrument is calibrated for, the temperature readings will be low and if theresistance is lower, the temperature readings will be high.

Where the thermocouple and extension wire are a significant portion of the circuit, thenwe must also consider the resistance change of the thermocouple wire at elevatedtemperatures. It may be necessary to calibrate instruments at the operatingtemperature. As an example: 5 feet of .020 dia. platinum vs. platinum 10% Rhodiumthermocouple wire, would have a resistance of 2.3 ohms. At 2500 deg F, the resistancewould be 2.3 x 3.5 ohms, or 8.5 ohms. A millivoltmeter with a sensitivity of 10 ohms pervolt would have an error of approximately 4% at 2500 deg F. The effects of temperature

on the thermocouple and thermocouple extension wire are shown in Table 3.

To illustrate the effects of incorrect lead length calibration on the millivoltmeters, wehave charted the errors that can result for various ranges and thermocouples bydeviating in resistance from the calibrated lead length. Table 4 is based upon 10 ohmsper millivolt sensitivity instrument. Instruments with less sensitivity would show greatererrors. A meter with a 5 ohm per millivolt sensitivity would have errors twice as great.

Table 3: Thermocouple resistance change with temperature

Multiplying factor for various temps; both wires same gauge

200 deg F 400 deg F 800 deg F 1600 deg F 2500 deg FIron-Constantan 1.02 1.05 1.11 1.22 ----

Chromel Alumel 1.05 1.14 1.30 1.62 2.01

Chromel-Constantan 1.13 1.33 1.7 2.5 ----

Plat. 10% RH - Platinum 1.13 1.34 1.83 2.67 3.50

Plat. 13% RH - Platinum 1.13 1.33 1.80 2.60 3.40

Table 4: Deviation in Ohms from calibrated lead length


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1 -- 0-2000 deg F C/A (4.02% at 20 Ohms)

2 -- 0-1200 deg F I/C, 0-1600 deg F C/A (4.97% at 20 Ohms)

3 -- 0-800 deg F I/C, 0-600 deg F C/A (7.65% at 20 Ohms)

4 -- 0-500 deg F CU/C, 0-2200 deg F PLT/PLT + 13% RH

0-2400 deg F PLT/PLT + 10% RH (14.32% at 20 Ohms)

5 -- 0-300 deg F I/C, o-350 deg F CU/C (22.15% at 20 Ohms)

THERMOCOUPLE CONNECTION

There are two common errors in connection thermocouple circuits. One is to connectthe extension lead wire completely reversed. In this case, you would receive a lowreading because the reversal causes the emf generated at the connection of thethermocouple and extension lead wire to be subtracted from the emf generated by thethermocouple. A more obvious error is to completely reverse the thermocouple. Theinstrument in this case will read downscale with an increase in temperature.


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Some control instruments feature 'thermocouple break protection' which means that inthe event of an open or broken thermocouple, a small voltage is applied to theinstrument which will cause it to read full scale and turn off the external circuit. Thus, inthe event the thermocouple breaks because of a mechanical shock or vibration or if it isover-exposed to extremely high temperature and deterioration sets in, an unattendedprocess will not overheat because of the loss of control.

Another consideration in the thermocouple use is that the leads wires should never runin the same conduit with electrical lines. This may induce currents in the thermocouplewire, resulting in instrument errors and poor control. However, if this cannot beavoided, or if the induced currents are being picked up at the thermocouple itself,then one side of the thermocouple lead wire should be grounded through a 1.0microfared paper capacitor at one of the thermocouple terminals in theinstrument. In emergencies, a direct ground will sometimes work as well.


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Occasionally, because of atmospheric conditions, corroding may occur on connectionswhich cause a loss of the millivolt signal. Or, a poor connection between the lead wireand thermocouple could cause loss of signal.

The gauge size of the wire used in thermocouples is again dependent upon the

application. Usually, when longer life is required, for the higher temperature ranges, thelarger size wires are chosen. When sensitivity is the prime concern, the smaller sizesshould be used.

GLOSSARY OF TERMS

CALIBRATE - General: to determine the indication or output of a measuring device withrespect to that of a standard.

CALIBRATE - Thermocouple: to determine the emf developed by a thermocouple with

respect to temperature established by a standard.

CALIBRATION POINT - General: a specific value, established by a standard, at whichthe indication or output of a measuring device is determined.

CALIBRATION POINT - Thermocouple: a temperature, established by a standard, atwhich the emf developed by a thermocouple is determined.

CELSIUS - The designation of the degree on the International Practical TemperatureScale. Also used for the name of the Scale, as "Celsius temperature scale." Formerly(prior to 1948) called "centigrade."

CENTIGRADE - The designation of the degree on the International Temperature Scaleprior to 1948. (See Celsius)

COAXIAL THERMOCOUPLE ELEMENT - A thermocouple element consisting of athermoelement in wire form, within a thermoelement in tube form with the twothermoelements insulated form each other and from the tube except at the measuring

junction.

CONNECTION HEAD - A housing enclosing a terminal block for an electricaltemperature-sensing device and usually provided with threaded openings for

attachment to a protecting tube and for attachment of conduit.

ELECTROMOTIVE FORCE - (emf) The electrical potential difference which produces ortends to produce an electric current.

EXTENSION WIRE - A pair of wires having such temperature-emf characteristicsrelative to the thermocouple with which the wires are intended to be used that, when


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properly connected to the thermocouple, the reference junction is transferred to theother end of the wires.

FAHRENHEIT - The designation of the degree and the temperature scale usedcommonly in public life and engineering circles in English-speaking countries. Related

to the International Practical Temperature Scale by means of the equation:

Degrees Fahrenheit = (Degrees Celsius x 1.8) + 32 Degrees Celsius = (Degrees Fahrenheit - 32) / 1.8

FREEZING POINT - The fixed point between the solid and liquid phases of amaterial when approached from the liquid phase under a pressure of onestandard atmosphere (101325 N/m squared). For a pure material, this is also themelting point.

ICE POINT - The fixed point between ice and air-saturated water under a

pressure of one standard atmosphere (101325 N/m squared). This temperatureis 0 deg C on the International Practical Temperature Scale.

KELVIN - The designation of the thermodynamic temperature scale and thedegree on this scale. This kelvin scale was defined by the Tenth GeneralConference on Weights and Measures in 1954 by assigning the temperature of273.16 degrees Kelvin to the triple point of water. Also the degree on theInternational Practical Kelvin Temperature Scale.

MELTING POINT - The fixed point between the solid and liquid phases of amaterial when approached from the solid phase under a pressure of one

standard atmosphere (101325 N/m squared). For a pure material, this is also thefreezing point.

PROTECTING TUBE - A tube designed to enclose a temperature-sensing deviceand protect it from the deleterious effects of the environment. It may provide forattachment to a connection head, but is not primarily designed for pressure-tightattachment to a vessel.

RANGE - The region between the limits within which a quantity is measured. It isexpressed by stating the lower and upper range-values.

REFERENCE JUNCTION - That junction of a thermocouple which is at a knowntemperature.

REFERENCE POINT - (liquid-in-glass thermometer) A temperature at which athermometer is checked for changes in bulb volume.


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REFRACTORY METAL THERMOCOUPLE - A thermocouple whosethermoelements have melting points above that of 60 percent platinum, 40percent rhodium, 1935 deg C (3515 deg F)

RESISTANCE, INSULATION - (sheathed thermocouple wire) The measured

resistance between wires or between wires and sheath multiplied by the length ofthe wire expressed in megohms (or ohms) per foot (or meter) of length. (NOTE:The resistance varies inversely with the length.)

SEEBECK COEFFICIENT - the rate of change of thermal emf with temperatureat a given temperature. Normally expressed as emf per unit of temperature.synonymous with thermoelectric power.

SEEBECK EMF - The net emf set up in a thermocouple under condition of zerocurrent. It represents the algebraic sum of the Peltier and Thompson emf.Synonymous with thermal emf.

SHEATHED THERMOCOUPLE - A thermocouple having its thermoelements,and sometimes its measuring junction, embedded in ceramic insulationcompacted within a metal protecting tube.

SHEATHED THERMOCOUPLE MATERIAL - One or more pairs ofthermoelements (without measuring junction(s)) embedded in ceramic insulationcompacted within a metal protecting tube.

THERMOCOUPLE - Two dissimilar thermoelements so joined as to produce athermal emf when the junctions are at different temperatures.

THERMOCOUPLE ASSEMBLY - An assembly consisting of a thermocoupleelement and one or more associated parts such as terminal block, connectionhead, and protecting tube.

THERMOCOUPLE ELEMENT - A pair of bare or insulated thermoelementsjoined at one end to form a measuring junction and intended for use as athermocouple or as part of a thermocouple assembly.

THERMOCOUPLE (TYPE E, B, J, K, R, S, OR T) - A thermocouple having anemf-temperature relationship corresponding to the appropriate letter-designated

table in ASTM Standard E 230, Temperature Electromotive Force (EMF) Tablesfor Thermocouples, within the limits of error specified in that Standard.

THERMOPILE - A number of thermocouples connected in series, arranged sothat alternate junctions are at the reference temperature and at the measuredtemperature, to increase the output for a given temperature difference betweenreference and measuring junctions.


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THERMOWELL - A closed end reentrant tube designed for the insertion of atemperature-sensing element, and provided with means for pressure-tightattachment to a vessel.

WORKING STANDARD THERMOCOUPLE - A thermocouple that has had its

temperature-emf relationship determined by reference to a secondary standardof temperature.


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Typical 4-20 mA Loop

Notes:

1. Loop impedance must be considered, to prevent overloading the power supply.2. If any one instrument is disconnected, the complete control loop will fail to operate3. 20 AWG shielded twisted pair cable is recommended. The twisted pair configuration

tends to distribute any electrical noise in a common mode; it is on both wires and tendsto cancel itself out.

4. Wiring needs to be run away form sources of EMI, such as 120V or larger power lines orequipment that generates EMI.

5. When using a shield, some simple rules apply:a. A shield should be grounded on one end only by using the green wire from the

power source and not the negative (-) lead of the sensor connector or thenegative (-) of any of the power supplies.

b. Think of the shield as a flexible conduit in the wiring scheme and remember that

metal conduits are always at earth ground. Attach it to ground at one end only toprevent it from becoming a conductor and creating a ground hazard.

c. The recommended point of grounding is at the instrument, since it is difficultfinding a good ground near a D/P or next to an Electronic Sensor Control Box.

Current Loops

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