Experiment No.1

TITLE: To plot & study the characteristics of RTD

APPARATUS: RTD,connecting wires , multimeter , thermometer

Theory

RTDsResistance temperature detectors (RTDs) operate on the principle of changes in electrical resistance of pure metals and are characterized by a linear positive change in resistance with temperature. Typical elements used for RTDs include nickel (Ni) and copper (Cu), but platinum (Pt) is by far the most common because of its wide temperature range, accuracy, and stability.

RTDs are constructed by one of two different manufacturing configurations. Wire-wound RTDs are constructed by winding a thin wire into a coil. A more common configuration is the thin-film element, which consists of a very thin layer of metal laid out on a plastic or ceramic substrate. Thin-film elements are cheaper and more widely available because they can achieve higher nominal resistances with less platinum. To protect the RTD, a metal sheath encloses the RTD element and the lead wires connected to it.

RTDs are popular because of their excellent stability, and exhibit the most linear signal with respect to temperature of any electronic temperature sensor. They are generally more expensive than alternatives, however, because of the careful construction and use of platinum. RTDs are also characterized by a slow response time and low sensitivity; and because they require current excitation, they can be prone to self-heating.

RTDs are commonly categorized by their nominal resistance at 0 °C. Typical nominal resistance values for platinum thin-film RTDs include 100 Ω and 1000 Ω. The relationship between resistance and temperature is very nearly linear and follows the equation

For <0 °C RT = R0 [ 1 + aT + bT2 +cT3 (T - 100) ]  (Equation 1)For >0 °C RT = R0 [ 1 + aT + bT2 ]

Where RT = resistance at temperature TR0 = nominal resistancea, b, and c are constants used to scale the RTD

The resistance/temperature curve for a 100 Ω platinum RTD, commonly referred to as Pt100, is shown below:

Figure 1. Resistance-Temperature Curve for a 100 Ω Platinum RTD, a = 0.00385

The most common RTD is the platinum thin-film with an a of 0.385%/°C and is specified per DIN EN 60751. The a value depends on the grade of platinum used, and also commonly include 0.3911%/°C and 0.3926%/°C. The a value defines the sensitivity of the metallic element, but is normally used to distinguish between resistance/temperature curves of various RTDs.

Table 1. Callendar-Van Dusen Coefficients Corresponding to Common RTDs

Standard Temperature Coefficient (a)

A B C

DIN 43760 0.003850

American 0.003911

ITS-90 0.003926

* For temperatures below 0 °C only; C = 0.0 for temperatures above 0 °C.

Procedure:1 Take water in the container

2 Insert RTD into water 3 Connect RTD to multimeter 4 Measure resistance of RTD for every two degree rise. 5 Plot the characteristics of R Vs T

Experiment No.2

TITLE: To plot & study the characteristics of Thermisttor

APPARATUS: Thermisttor,connecting wires , multimeter , thermometer

ThermistorsThermistors (thermally sensitive resistors) are similar to RTDs in that they are electrical resistors whose resistance changes with temperature. Thermistors are manufactured from metal oxide semiconductor material which is encapsulated in a glass or epoxy bead.

Thermistors have a very high sensitivity, making them extremely responsive to changes in temperature. For example, a 2252 Ω thermistor has a sensitivity of -100 Ω/°C at room temperature. In comparison, a 100 Ω RTD has a sensitivity of 0.4 Ω/°C. Thermistors also have a low thermal mass that results in fast response times, but are limited by a small temperature range.

Thermistors have either a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). The first has a resistance which decreases with increasing temperature and the latter exhibits increased resistance with increasing temperature. Figure 2 shows a typical thermistor temperature curve compared to a typical 100 Ω RTD temperature curve:

Figure 2. Resistance versus Temperature for a Typical Thermistor and RTD

RTD and Thermistor Measurement and Signal Conditioning

Because RTDs and thermistors are resistive devices, you must supply them with an excitation current and then read the voltage across their terminals. If extra heat cannot be dissipated, I2R heating caused by the excitation current can raise the temperature of the sensing element above that of the ambient temperature. Self-heating will actually change the resistance of the RTD or thermistor, causing error in the measurement. The effects of self-heating can be minimized by supplying lower excitation current.

The easiest way to connect an RTD or thermistor to a measurement device is with a 2-wire connection.

Figure 3. Making a 2-Wire RTD/Thermistor Measurement

With this method, the two wires that provide the RTD or thermistor with its excitation current are also used to measure the voltage across the sensor. Because of the low nominal resistance of RTDs, measurement accuracy can be drastically affected by lead wire resistance. For example, lead wires with a resistance of 1 Ω connected to a 100 Ω platinum RTD cause a 1% measurement error.

 

A 3-wire or 4-wire connection method can eliminate the effects of lead wire resistance. The connection places leads on a high impedance path through the measurement device, effectively eliminating error caused by lead wire resistance. It is not necessary to use a 3 or 4-wire connection method for thermistors because they typically have much higher nominal resistance values than RTDs. A diagram of a 4-wire connection is shown below.

Figure 4. Making a 4-Wire RTD Measurement

RTD and thermistor output signals are typically in the millivolt range, making them susceptible to noise. Lowpass filters are commonly used in RTD and thermistor data acquisition systems to effectively eliminate high frequency noise in RTD and thermistor measurements. For instance, lowpass filters are useful for removing the 60 Hz power line noise that is prevalent in most laboratory and plant settings.

One of the most frequently used temperature sensors is the thermocouple. Thermocouples are very rugged and inexpensive and can operate over a wide temperature range. A thermocouple is created whenever two dissimilar metals touch and the contact point produces a small open-circuit voltage as a function of temperature. This thermoelectric voltage is known as the Seebeck voltage, named after Thomas Seebeck, who discovered it in 1821. The voltage is nonlinear with respect to temperature. However, for small changes in temperature, the voltage is approximately linear, or

DV=S * DT

where DV is the change in voltage, S is the Seebeck coefficient, and DT is the change in temperature.

S varies with changes in temperature, however, causing the output voltages of thermocouples to be nonlinear over their operating ranges. Several types of thermocouples are available; these thermocouples are designated by capital letters that indicate their composition according to American National Standards Institute (ANSI) conventions. For example, a J-type thermocouple has one iron conductor and one constantan (a copper-nickel alloy) conductor.

For more information on thermocouples visit the thermocouple tutorial, or return to the top of the page .

Procedure:1 Take water in the container

2 Insert Thermisttor into water (air) 3 Connect Thermisttor to multimeter 4 Measure resistance of Thermisttor for every two degree rise. 5 Plot the characteristics of R Vs T

Experiment No.3

TITLE: To plot & study the characteristics of Thermocouple

APPARATUS: Thermocouple, connecting wires, multimeter, thermometer

Temperature Measurement with a Thermocouple

To measure a thermocouple Seebeck voltage, you cannot simply connect the thermocouple to a voltmeter or other measurement system, because connecting the thermocouple wires to the measurement system creates additional thermoelectric circuits.

Figure 1. J-Type Thermocouple

Consider the circuit illustrated in Figure 1, in which a J-type thermocouple is in a candle flame the temperature of which you want to measure. The two thermocouple wires are connected to the copper leads of a DAQ board. Notice that the circuit contains three dissimilar metal junctions – J1, J2, and J3. J1, the thermocouple junction, generates a Seebeck voltage proportional to the temperature of the candle flame. J2 and J3 each have their own Seebeck coefficient and generate their own thermoelectric voltage proportional to the temperature at the DAQ terminals. To determine the voltage contribution from J1, you need to know the temperatures of junctions J2 and J3 as well as the voltage-to-temperature relationships for these junctions. You can then subtract the contributions of the parasitic junctions at J2 and J3 from the measured voltage at junction J1.

Thermocouples require some form of temperature reference to compensate for these unwanted parasitic “cold” junctions. The most common method of cold-junction compensation is to measure the temperature at the reference junction with a direct-

reading temperature sensor and subtract the parasitic junction voltage contributions. This process is called cold-junction compensation. You can simplify computing cold-junction compensation by taking advantage of some thermocouple characteristics

Procedure:

1 Take water in the container 2 Insert Thermocouple into water (air) 3 Connect Thermocouple to multimeter 4 Measure Voltage (emf) of Thermocouple for every two degree rise. 5 Plot the characteristics of E Vs T

Experiment No.4

TITLE: To plot & study the characteristics of LVDT

APPARATUS:

LVDT, connecting wires, multimeter, thermometer

Theory of Operation

An LVDT is much like any other transformer in that it consists of a primary coil, secondary coils, and a magnetic core. An alternating current, known as the carrier signal, is produced in the primary coil. The changing current in the primary coil produces a varying magnetic field around the core. This magnetic field induces an alternating (AC) voltage in the secondary coils that are in proximity to the core. As with any transformer, the voltage of the induced signal in the secondary coil is linearly related to the number of coils. The basic transformer relation is:

(1)

where: Vout is the voltage at the output, Vin is the voltage at the input, Nout is the number of windings of the output coil, and Nin is the number of windings of the input coil.

As the core is displaced, the number of coils in the secondary coil exposed to the coil changes linearly. Therefore the amplitude of the induced signal varies linearly with displacement.

The LVDT indicates direction of displacement by having the two secondary coils whose outputs are balanced against one another. The secondary coils in an LVDT are connected in the opposite sense (one clockwise, the other counter clockwise). Thus when the same varying magnetic field is applied to both secondary coils, their output voltages have the same amplitude but differ in sign. The outputs from the two secondary coils are summed together, usually by simply connecting the secondary coils together at a common center point. At an equilibrium position (generally zero displacement) a zero output signal is produced.

The induced AC signal is then demodulated so that a DC voltage that is sensitive to the amplitude and phase of the AC signal is produced.

Procedure: 1. Make the connection as shown in the figure 2. Connect the o/p terminal to the multimeter. 3. Move the core from R to L & measure the voltage (emf). 4. When the core is at the center note down residual voltage. 5 . Plot the graph of emf Vs displacement of core

Experiment No.5

TITLE: To plot & study the characteristics of Capacitive Transducer

APPARATUS:

Capacitive Transducer kit, connecting wires, multimeter,

Theory

Capacitance is given by C= Є A / d

where Є = permitivity of medium A = area of the plate d = distances between the plate

Procedure: 1. Make the connection as shown in the figure

2. Connect the o/p terminal to the multimeter.

3. Move the Knob clockwise measure the capacitance.

4 . Plot the graph of capacitance Vs displacement

Experiment No.6

TITLE: To study the characteristics of digital Transducer

Digital Encoders

A digital optical encoder is a device that converts motion into a sequence of digital pulses. By counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute position measurements. Encoders have both linear and rotary configurations, but the most common type is rotary. Rotary encoders are manufactured in two basic forms: the absolute encoder where a unique digital word corresponds to each rotational position of the shaft, and the incremental encoder, which produces digital pulses as the shaft rotates, allowing measurement of relative position of shaft. Most rotary encoders are composed of a glass or plastic code disk with a photographically deposited radial pattern organized in tracks. As radial lines in each track interrupt the beam between a photoemitter-detector pair, digital pulses are produced.

Absolute encoder

The optical disk of the absolute encoder is designed to produce a digital word that distinguishes N distinct positions of the shaft. For example, if there are 8 tracks, the encoder is capable of producing 256 distinct positions or an angular resolution of 1.406 (360/256) degrees. The most common types of numerical encoding used in the absolute encoder are gray and binary codes. To illustrate the acion of an absolute encoder, the gray code and natural binary code dsisk track patterns for a simple 4-track (4-bit) encoder are illustrated in Fig 2 and 3. The linear patterns and associated timing diagrams are what the photodetectors sense as the code disk circular tracks rotate with the shaft. The output bit codes for both coding schemes are listed in Table 1.

Decimal code

Rotation range (deg.)

Binary code

Gray code

0 0-22.5 0000 0000

1 22.5-45 0001 0001

2 45-67.5 0010 0011

3 67.5-90 0011 0010

4 90-112.5 0100 0110

5 112.5-135 0101 0111

6 135-157.5 0110 0101

7 15.75-180 0111 0100

8 180-202.5 1000 1100

9 202.5-225 1001 1101

10 225-247.5 1010 1111

11 247.5-270 1011 1110

12 270-292.5 1100 1010

13 292.5-315 1101 1011

14 315-337.5 1110 1001

15 337.5-360 1111 1000

Table 1. 4-Bit gray and natural binary codes

The gray code is designed so that only one track (one bit) will change state for each count transition, unlike the binary code where multiple tracks (bits) change at certain count transitions. This effect can be seen clearly in Table 1. For the gray code, the uncertainty during a transition is only one count, unlike with the binary code, where the uncertainty could be multiple counts.

Since the gray code provides data with the least uncertainty but the natural binary code is the preferred choice for direct interface to computers and other digital devices, a circuit to convert from gray to binary code is desirable. Figure 4 shows a simple circuit that utilizes exclusive OR gates (XOR) to perform this function.For a gray code to binary code conversion of any number of bits N, the most signficant bits (MSB) of the binary and gray code are always identical, and for each other bit, the binary bit is the exlcusive OR (XOR) combination of adjacent gray code bits.

 

 

Fig 4. Gray code to binary code conversion

Incremental encoder

The incremental encoder, sometimes called a relative encoder, is simpler in design than the absolute encoder. It consists of two tracks and two sensors whose outputs are called channels A and B. As the shaft rotates, pulse trains occur on these channels at a frequency proportional to the shaft speed, and the phase relationship between the signals yields the direction of rotation. The code disk pattern and output signals A and B are illustrated in Figure 5. By counting the number of pulses and knowing the resolution of the disk, the angular motion can be measured. The A and B channels are used to determine the direction of rotation by assessing which channels "leads" the other. The signals from the two channels are a 1/4 cycle out of phase with each other and are known as quadrature signals. Often a third output channel, called INDEX, yields one pulse per revolution, which is useful in counting full revolutions. It is also useful as a reference to define a home base or zero position.

Figure 5 illustrates two separate tracks for the A and B channels, but a more common configuration uses a single track with the A and B sensors offset a 1/4 cycle on the track to yield the same signal pattern. A single-track code disk is simpler and cheaper to manufacture.

The quadrature signals A and B can be decoded to yield the direction of rotation as hown in Figure 6. Decoding transitions of A and B by using sequential logic circuits in different ways can provide three different resolutions of the output pulses: 1X, 2X, 4X. 1X resolution only provides a single pulse for each cycle in one of the signals A or B, 4X resolution provides a pulse at every edge transition in the two signals A and B providing four times the 1X resolution. The direction of rotation(clockwise or counter-clockwise) is determined by the level of one signal during an edge transition of the second signal. For

example, in the 1X mode, A= with B =1 implies a clockwise pulse, and B= with A=1 implies a counter-clockwise pulse. If we only had a single output channel A or B, it would be impossible to determine the direction of rotation. Furthermore, shaft jitter around an edge transition in the single signal woudl result in erroneous pulses..

Experiment No.8

TITLE: To plot & study the characteristics of Flapper nozzle system

APPARATUS:

Compressor ,Pneumatic tubing’s , I / P converter.

Theory

A regulated supply of pressure usually over 20 psig provides a source of air through restriction . Nozzle is open at end where gap exist between flapper & nozzle & air escapes in this region . If flapper moves down and closes off nozzle opening so that no air leaks then o/p pressure equals to maximum signal pressure.

If Flapper moves away from the nozzle then air start leaking & signal pressure starts decreasing. When the flapper is far away signal will stabilize at particular pressure.

Procedure:1. Build suitable supply pressure in the compressor2. Connect compressor to I/P converter through suitable tubings3. Vary the current in the steps of 2 mA.4. Note down corresponding o/p pressure.

Experiment No.7

TITLE: To study the characteristics of air purge method

APPARATUS:

Compressor ,Pneumatic tubing’s ,pressure gauge (u tube manometer), dip tube

Theory

Bubblers

Granulars

SlurriesInterfaces

TheoryAdvantages

Simplicity of design and low initial purchase cost are frequently given as advantages of bubblers, but

this is somewhat misleading. The system consists of a pipe, an air

Disadvantages

Calibration is directly affected by changes in product density. It is frequently also necessary to periodically clean this device. The tip of the pipe can collect material from the process, solidify, and plug the hole. Bubblers are not suitable for use in non-vented vessels.

Typical Bubbler Configuration

Practical Notes

Instrument air lines should be trace heated if there is a frost risk. Calibration of a bubbler system should be at maximum temperature to avoid overfills. Accuracy depends on a stable air supply and is limited by the regulator, which may be + 10% of full scale. In applications where the purge air is exposed to a hazardous substance, additional steps must be taken to contain any possible contamination.

Procedure1. Build suitable supply pressure in the compressor2 .Connect compressor to dip tube through suitable tubing’s

3. Increased the pressure till bubble starts to appear 4. Note down the pressure 5 .Calculate the level through suitable formulae

Experiment No.1

TITLE: Strain Gauges and the Wheatstone Bridge

APPARATUS: Strain gauge kit, connecting wires, Standard weights.

Object

Use strain gauges to measure the strain in a loaded steel cantilever.

Verify Hooke's Law.

Verify the calculation for the strain in a loaded cantilever.

Introduction

The electrical resistance (R) of a metal wire is given by (where is the resistivity), so the resistance is proportional to the length (L) and inversely proportional to the area (A). As the wire stretches it becomes longer and thinner (because the volume of the wire stays approximately the same). Hence the resistance of the wire is increased by stretching. When a wire is compressed it becomes shorter and fatter, this reduces the resistance. The material structure changes slightly during stretching and compression, this produce small resistivity changes.

Strain Gauges are thin wires that can be glued to a metal structure. When the structure flexes under a load the resistance of the strain gauges changes and this can be used to measure the strain in the structure. In this way, the strain in a structure (e.g. an oil rig or an aircraft wing) can be measured to verify the design calculations.

Theory

The change in resistance R in a strain gauge of resistance R is very nearly proportional

to the applied strain. Hence:

K is a constant known as the gauge factor and is the relative strain

. The gauges used in this experiment have K = 2.10 ± 0.02. Figure 1: Loaded Cantilever Beam.

The gauges are a distance D from the load (see figure 1), a load of mass m and weight mg is suspended from the cantilever beam (g is the acceleration due to gravity). The beam has thickness t and width w and is made from stainless steel with a Young's Modulus E

. The calculated strain due to the suspended mass is:

Therefore the relative change in the resistance of the strain gauge is given by:

The resistance changes in the strain gauges are very small, therefore the gauges are connected in a Wheatstone Bridge Circuit (see figure 2). The gauge on top of the beam is in tension, the gauge underneath the beam is in compression, hence strain causes equal and opposite resistance changes in the gauges. By using two gauges the effects of temperature variations on the gauge resistances are cancelled out.

The left hand end of the bridge circuit is at zero volts (see figure 1), the circuit is powered by the bridge excitation voltage VEX applied to the right hand end of the bridge (see figure 1).

If the strain increases the resistance of Gauge One from R to R + R then the resistance of Gauge Two is decreased from R to R - R. Hence the voltage VG (see figure 2) is given by:

To balance the Wheatstone Bridge the Zero Adjust resistor is adjusted to produce a

voltage of . Therefore the output voltage Vo of the Wheatstone bridge is given by:

Substituting then:

Figure 2: Strain Gauge Wheatstone Bridge Circuit.

Apparatus

Cantilever Beam apparatus, Bridge Power Supply, high sensitivity voltmeter, Wheatstone Bridge, digital voltmeter, 1 kg set of 100g masses (individually marked with accurate masses).

Preliminary Experiment

1. With the cantilever beam unloaded, measure and note the resistance of one of the gauges using a Digital Multimeter (in resistance mode). What is the change in gauge resistance when a 1 kg load is suspended from the beam?

Setting up the Wheatstone Bridge Circuit

1. Assemble the Wheatstone Bridge Circuit as shown in figure 2. 2. Connect the high sensitivity voltmeter to measure output voltage. 3. The Bridge Power Supply and the digital multimeter (in voltage mode) used to

measure the Bridge Excitation Voltage are connected using separate wires. This eliminates errors due to the resistance in the wires and connections.

4. Adjust the Zero Adjust resistor to obtain a zero output voltage when the cantilever beam is unloaded.

Experiment

1. Measure the bridge output voltage with beam loadings from 0.00 to 1.00 kg in 0.10 kg steps.

2. Plot a graph of output voltage (in Volts) on the vertical axis versus beam loading (in kg) on the horizontal axis.

3. Hooke's Law states that for elastic behaviour that the strain is proportional to the load applied, does your graph verify Hooke's Law?

4. If the design of the apparatus is correct, the output voltage of the strain gauge

circuit should be . Use this formula to calculate the output voltage you expect for one of the loadings you have measured.

5. Calculate the maximum experimental error in the expected value of Vo using the

formula . 6. Does the output voltage you expect agree with output voltage you have measured

within their respective experimental errors? What does this tell you?

Symbol Description Value Units

K Gauge Factor 2.10 ± 0.02 dimensionless

m suspended mass ± 0.01 per mass grams (g)

g acceleration due to gravity 9.816 ± 0.001metres per second squared (ms-

2)

D load to gauge distance 0.504 ± 0.003 metres (m)

VEX bridge excitation voltage estimated error volts (V)

E Young's modulus 195 ± 5 Giga Pascals (GPa)

wwidth (horizontal) of cantilever

2.53 ± 0.01 centimetres (cm)

tthickness (vertical) of cantilever

3.55 ± 0.05 millimetres (mm)

©  Mark Davison, 1997,  give feedback or ask questions   about this experiment.

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Experiment No.2

TITLE: Measurement of flow using head type flow meter

APPARATUS: U tube manometer, Flow experiment set-up

Theory

Introduction

A fluid passing though an orifice constriction will experience a drop in pressure across the orifice. This change can be used to measure the flowrate of the fluid.

To calculate the flowrate of a fluid passing through an orifice plate, enter the parameters below. (The default calculation involves air passing through a medium-sized orifice in a 4" pipe, with answers rounded to 3 significant figures.)

 Inputs

  Pipe (inlet) diameter upstream of orifice, Di:    

  Orifice diameter (less than the inlet diameter), Do:    

  Pressure difference across the orifice, p:    

  Fluid density, :    

  Flow Coefficient, Cf:    

Answers

  Velocity at the inlet, Vi:  1.76  m/s

  Volumetric Flowrate, Q:  13.9  l/s

  Mass Flowrate:  0.0179  kg/s

Select desired output units for next calculation.

Equations used in this Calculator

Electronic Instrumentat. As long as the fluid speed is sufficiently subsonic (V < mach 0.3), the incompressible

Bernoulli's equation describes the flow reasonably well. Applying this equation to a streamline traveling down the axis of the horizontal tube gives,

where location 1 is upstream of the orifice, and location 2 is slightly behind the orifice. It is recommended that location 1 be positioned one pipe diameter upstream of the orifice, and location 2 be positioned one-half pipe diameter downstream of the orifice. Since the pressure at 1 will be higher than the pressure at 2 (for flow moving from 1 to 2), the pressure difference as defined will be a positive quantity.

From continuity, the velocities can be replaced by cross-sectional areas of the flow and the volumetric flowrate Q,

Solving for the volumetric flowrate Q gives,

The above equation applies only to perfectly laminar, inviscid flows. For real flows (such as water or air), viscosity and turbulence are present and act to convert kinetic flow energy into heat. To account for this effect, a discharge coefficient Cd is introduced into the above equation to marginally reduce the flowrate Q,

Since the actual flow profile at location 2 downstream of the orifice is quite complex, thereby making the effective value of A2 uncertain, the following substitution introducing a flow coefficient Cf is made,

where Ao is the area of the orifice. As a result, the volumetric flowrate Q for real flows is given by the equation,

The flow coefficient Cf is found from experiments and is tabulated in reference books; it ranges from 0.6 to 0.9 for most orifices. Since it depends on the orifice and pipe diameters (as well as the Reynolds Number), one will often find Cf tabulated versus the ratio of orifice diameter to inlet diameter, sometimes defined as ,

The mass flowrate can be found by multiplying Q with the fluid density,

2.

Venturi Flowmeter Calculator

A fluid passing through smoothly varying constrictions experience changes in velocity and pressure. These changes can be used to measure the flowrate of the fluid.

To calculate the flowrate of a fluid passing through a venturi, enter the parameters below. (The default calculation involves air passing through a medium-sized venturi, with answers rounded to 3 significant figures.)

 Inputs

  Pipe diameter upstream of venturi, Da:    

  Diameter of venturi neck, Db:    

  Pressure difference measured by venturi, p:    

  Fluid density, :    

  Discharge Coefficient, C:    

Answers

  Velocity at A, V:  2.21  m/s

  Volume Flowrate, Q:  17.0  l/s

  Mass Flowrate:  0.0220  kg/s

Select desired output units for next calculation.

Equations used in the Calculation

As long as the fluid speed is sufficiently subsonic (V < mach 0.3), the incompressible Bernoulli's equation describes the flow. Applying this equation to a streamline traveling down the axis of the horizontal tube gives,

From continuity, the throat velocity Vb can be substituted out of the above equation to give,

Solving for the upstream velocity Va and multiplying by the cross-sectional area Aa gives the volumetric flowrate Q,

Ideal, inviscid fluids would obey the above equation. The small amounts of energy converted into heat within viscous boundary layers tend to lower the actual velocity of real fluids somewhat. A discharge coefficient C is typically introduced to account for the viscosity of fluids,

C is found to depend on the Reynolds Number of the flow, and usually lies between 0.90 and 0.98 for smoothly tapering venturis.

The mass flowrate can be found by multiplying Q with the fluid density,

Experiment No.3

TITLE: Measurement of flow using Rotameter

APPARATUS: U tube manometer, Flow experiment set-up

Controlling the flow in piping systems is a significant issue in the chemical process industries. Obviously, in order to control the flow in a pipe, the flow must be measured. This experiment will introduce you to three devices that are used to measure flow. One, the rotameter, is a simple mechanical device that is designed to be read by an operator. It is rugged, relatively inexpensive, and easily installed. The second, the orifice plate, can be set up to be read locally or remotely using pressure transducers. Both are designed for flows that do not contain significant amounts of solid material. The third, the magnetic flow meter, is a more sophisticated device than either the rotameter or the orifice plate. It requires that the flowing material be electrically conductive, but can measure flows with suspended material. Brief descriptions of the three devices are on the attached pages, along with the simplified directions and questions.

Below are some basic experimental guidelines that are common to all three sub-experiments.

1. Trace the piping in the flow loop so you understand the basic process of getting flow from the feed tank to the receiving tank .

2. Ensure that the water level in the receiving tank is above the pump that recycles the water in the receiving tank to the feed tank. Use the hose and tap to fill as necessary

3. Ensure that the feed tank is about 60% full of water. Use the recycle pump to pump from the receiving tank to the feed tank. Before activating the recycle pump, have the lab instructor verify that you have the valves in the correct position.

4. Refill the receiving tank to about 50% full.

5. Ensure that the water will only flow from the feed pump through the desired flow measuring device. Follow the piping and open and close the necessary valves to allow the water to flow from the feed pump, through the measuring device, and back to the receiving tank.

6. Close the final (discharge) valve just prior to the discharge into the receiving tank.

7. Have the lab instructor verify that your valving is correct for the measuring device you want to use.

8. Turn on the feed pump

9. Slowly open the discharge valve until you obtain a desired reading. Slowly change the discharge valve opening to obtain different flow rates.

10. When the receiving tank is about 50 - 60% full, activate the recycle pump.

11. Monitor the level in the receiving tank and turn off the recycle pump if the receiving tank level approaches the level of the top of the recycle pump.

12. When you are ready to leave the lab, have the lab instructor verify that you have left the flow loop in the proper condition.

Rotameter Experiment

The purpose of this portion of the experiment is to develop a calibration curve for a rotameter so that it may be used to measure flow in a pipe.

The rotameter is a very commonly used flow measuring device, especially in low pressure systems. It is used for both liquid and gas flows. As shown in Figure 1 below, this simple device is just a tapered tube with a metal bob on a central shaft. (The bob can actually be any material that does not react with the fluid being measured and the central shaft is not really necessary, but serves to keep the bob aligned in the tube.) As the fluid flows in the annular space between the bob and the tube, a drag force is generated that tends to pull the bob along. The force of gravity tends to resist this drag force. The drag force is proportional to the square of the velocity, whereas the gravitational force is proportional to the mass of the bob. If the drag force is greater than the gravitational force, the bob rises. As the bob rises, the annular space gets bigger and the velocity, and hence the drag force, drops. At some point, the drag force and gravitational force balance and the bob remains fairly steady. (If there is no balance point, the bob either sits at the bottom of the tube or slams against the top of the tube --- neither condition is very useful for measuring flow!)

As you will show in CHEG 3110, the flow rate through a rotameter is given by:

(1)

(2)

(3)

whereu = flow velocityVb = volume of the bobb = density of the bobf = density of the fluidAb = frontal area of the bobA = area of the annulus between the bob and tube wally = distance from the bottom of the meterf(y) = the function that defines the tube taperg = gravitational accelerationC = a drag coefficientQ = volumetric flow rate

The annular cross section (A) at any distance above the base is a function of the design of the meter. The proper choice of the bob diameter and the tube taper can make the functional term nearly linear such that the formula reduces to:

(4)

where Cm now contains the constants and the drag coefficient from equations (1) - (3).

While Cm could be evaluated for each meter, it is much more convenient to develop a calibration curve for a particular application. That will be the exercise in this portion of the lab.

Vary the flow rate through the rotameter by slowly opening the valve on the discharge line. Measure the flow with an appropriate container and a stop watch. Build a calibration curve for the flow meter. In equation (4), note that the flow is dependent on the density of the fluid and the bob, both of which change as the temperature changes. Therefore, you will need to record the fluid temperature for each of your observations. Take between five and ten different flows and rotameter positions for the calibration curve.

Some questions:

What calculations and assumptions would you have to make to build a calibration curve that would apply over a fairly large fluid temperature range?

Estimate the meter coefficient (Cm in equation 4) from your data.

Rotameter SchematicFigure 1

Flow

Bob

Orifice Plate Experiment

The purpose of this portion of the experiment is to calibrate an orifice plate so it may be used to measure flow.

Sufficiently far (about 5 pipe diameters) from an any obstruction, turbulent flow in a pipe is reasonably stable in that the velocity at any point in the flow cross section is the same as all other points, on average. Thus, at line 1 in Figure 2 below, the velocity is almost constant across the pipe cross section. In order to pass through the restricted opening at line 2 (the orifice), the flow must converge and accelerate in order to pass through the restriction. The flow forms a jet as it exits the orifice and the cross section of the jet continues to decrease for some small distance downstream1. Eventually, the flow diverges and within a few pipe diameters of the orifice the flow is again constant across the pipe cross section. The pressure exerted by the fluid is related to its velocity and it can be shown (and you will do so in CHEG 3110), that the flow rate through the orifice is given by:

(5)

Where Q is the volumetric flow rateA1 is the cross sectional area of the pipeA2 is the cross sectional area of the orificep is the pressure drop (P1 - P2)C is coefficient of discharge is the fluid density

The coefficient of discharge is dependent on the ratio A2/A1 and somewhat dependent upon the Reynolds Number. The Reynolds Number is used to characterize the flow of fluids and its definition should be remembered. The definition is:

whereD = pipe diameteru = fluid velocity = fluid density = fluid viscosity

Fortunately, this dependence on Re is relatively weak and for our purposes, we can treat C as a constant.

For a particular installation, A1 and A2 are constants, so many of the terms in equation (5) can be collected and (5) rewritten as:

(6)

In this experiment, vary the flow rate through the orifice plate by slowly closing the valve on the discharge side of the flow system setup. Measure the flow at any given valve position by using an appropriate container and stop watch. Record the pressure difference (p) at the same time as the flow is being measured.

A plot of Q vs will result in a calibration curve for the orifice plate. Is this a straight line? Equation (5) is dimensionally consistent and "C" is dimensionless. Convert everything to consistent units and determine the value and units of "K".

In this experiment, the pressure difference is displayed by two different methods and you are to develop calibrations curves for both methods. Method one uses the manometer, while method two uses the electronic display from the differential pressure transducer. (Note – this signal could be taken to a computer for display via a LabView vi.)

For the manometer, the pressure drop is directly proportional to the difference in height between the liquid heights in the two legs of the manometer. The differential pressure transducer displays a value that is proportional to the pressure difference. Is this displayed value a voltage or a current? Is the resulting calibration curve linear?

Since the fluid is water, the following properties are approximately true:

Density: 62.4 lbm/ft3

Viscosity: 0.01 centipoiseA1 = 1.049 inchesA2 = 0.777 inches

1 The point of the minimum jet diameter is called the vena contracta and ideally it is at this point that the down stream pressure measurement is made. In practice, however, the downstream pressure measurement is made in the flange near the orifice. Since the diameter of the vena contracta is a function of the orifice diameter, we can bundle all the variation into one coefficient (C in equation 5) and use the orifice diameter. Thus the value of C in equation 5 is a function of Re and A2/A1.

Orifice Plate Schematic

Figure 2

Magnetic Flow Meter Experiment

The magnetic flow meter is a relatively sophisticated device (compared to the rotameter or orifice plate). The flowing, conductive particles (ions or true particles) induce an electric potential which is measured by the flow meter electronics. Flow meters are sized for a maximum flow and the induced potential is essentially compared to the maximum permitted potential. In this way, the flow through the meter is related to the maximum measurable flow. The flow meter display can display the flow directly (in appropriate units) or as a percentage of the maximum measurable flow. The flow meter in this experiment displays the flow as a percent of the maximum.

Your goal in this experiment is to determine:

a) what is the maximum measurable flow rate?b) build a LabView vi to take the signal from the flow meter and display the

volumetric flow rate on a meter of some form. Be sure to include the flow units on the meter display.

P1 P2

1 2

Experiment No.4

TITLE: Measurement of Pressure

APPARATUS: U tube manometer, Pressure measurement set-up

Theory:

3Manometer Pressure

Manometers measure a pressure difference by balancing the weight of a fluid column between the two pressures of interest. Large pressure differences are measured with heavy fluids, such as mercury (e.g. 760 mm Hg = 1 atmosphere). Small pressure differences, such as those experienced in experimental wind tunnels or venturi flowmeters, are measured by lighter fluids such as water (27.7 inch H2O = 1 psi; 1 cm H2O = 98.1 Pa).

To calculate the pressure indicated by the manometer, enter the data below. (The default calculation is for a water manometer with a 10 cm fluid column, with the answer rounded to 3 significant figures.):

 Inputs

  Height of fluid column, h:    

  Fluid density, :    

Answers

  Manometer Pressure, p:  981  Pa

Equations used in the Calculation

The pressure difference between the bottom and top of an incompressible fluid column is given by the incompressible fluid statics equation,

where g is the acceleration of gravity (9.81 m/s2).

Diaphragm

A pile of pressure capsules with corrugated diaphragms in an aneroid barograph.

A second type of aneroid gauge uses the deflection of a flexible membrane that separates regions of different pressure. The amount of deflection is repeatable for known pressures so the pressure can be determined by using calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used.

Useful range: above 10-2 Torr [5] (roughly 1 Pa)

For absolute measurements, welded pressure capsules with diaphragms on either side are often used.

Shape:

Flat corrugated flattened tube capsule

Bellows

In gauges intended to sense small pressures or pressure differences, or require that an absolute pressure be measured, the gear train and needle may be driven by an enclosed and sealed bellows chamber, called an aneroid, which means "without liquid". (Early barometers used a column of liquid such as water or the liquid metal mercury suspended by a vacuum.) This bellows configuration is used in aneroid barometers (barometers with an indicating needle and dial card), altimeters, altitude recording barographs, and the altitude telemetry instruments used in weather balloon radiosondes. These devices use the sealed chamber as a reference pressure and are driven by the external pressure. Other sensitive aircraft instruments such as air speed indicators and rate of climb indicators (variometers) have connections both to the internal part of the aneroid chamber and to an external enclosing chamber

Bourdon Tube

The Bourdon Tube is a nonliquid pressure measurement device. It is widely used in applications where inexpensive static pressure measurements are needed.

A typical Bourdon tube contains a curved tube that is open to external pressure input on one end and is coupled mechanically to an indicating needle on the other end, as shown schematically below.

Typical Bourdon Tube Pressure GagesInternal linkages have been simplified.

The external pressure is guided into the tube and causes it to flex, resulting in a change in curvature of the tube. These curvature changes are linked to the dial indicator for a number readout. Alternatively, a strain gage circuit can be attached on the tube to convert the pressure-induced deflections into electric voltage signals.

These signals can then be output electronically, rather than mechanically with the dial indicator.

Pressure Units

  pascal(Pa)

bar(bar)

technical atmosphere

(at)atmosphere

(atm)torr

(Torr)

pound-force per

square inch(psi)

1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 145.04×10−6

1 bar 100,000 ≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.50377441 at 98,066.5 0.980665 ≡ 1 kgf/cm2 0.96784 735.56 14.223

1 atm 101,325 1.01325 1.0332 ≡ 1 atm 760 14.696

1 torr 133.322 1.3332×10−3 1.3595×10−3 1.3158×10−3 ≡ 1 Torr; ≈ 1 mmHg

19.337×10−3

1 psi 6.894×103 68.948×10−3 70.307×10−3 68.046×10−3 51.715 ≡ 1 lbf/in2

Example reading:  1 Pa = 1 N/m2  = 10−5 bar  = 10.197×10−6 at  = 9.8692×10−6 atm, etc

Experiment No. 8

TITLE:

To determine viscosity of given oil using Oswald’s Viscometer.

APPARATUS: Oswald’s Viscometer, Beaker (100 ml), Measuring cylinder. Stop watch, Retort stand. Samples of oil, Distilled water, Acetone, Alcohol.

STEPWISE PROCEDURE:

1. Wash, clean and dry the Oswald’s viscometer with water and acetone. 2. Fix up the Oswald’s viscometer on stand vertically. 3. Fill up the viscometer with distilled water of known volume to fill the viscometer bulb. 4. Suck up the distilled water from capillary end slightly above the upper mark A and hold it. 5. Set the stopwatch to zero. 6. Allow the water to flow down the capillary. 7. Start the stopwatch when the water level crosses mark A. 8. Stop the stopwatch when the water level touches the mark B, this will give time required for flow of liquid between A and B through the capillary.. 9. Repeat the same procedure three times. 10. Remove water from viscometer, clean dry it using alcohol and acetone. 11.Repeat the same procedure with different oil samples to know the time required to flow from mark A to mark B. 12. Note the specific gravity or density of oils.

OBSERVATION: Table for values of specific gravity and time for different oils.

Sr.No Solution Density

Time 1 Time 2 Time 3 Mean Time

1234

Experiment No.3

TITLE: To measure pressure using strain gauge.

APPARATUS: Strain Gauge, Instrumentation amplifier, display unit, bridge circuit, connecting wires

Theory:

Strain gauge is resistive transducer, whose resistance changes when it is subjected to stress, due to changes in length, area and resistivity. The sensitivity or gauge factor of strain gauge is defined as the ratio of unit change in resistance to unit change in length . 1. To discriminate between dummy gauge and active strain gauge. 2. To interpret the graph and comment. 3. Ability to measure the weight. 4. Ability to plot the graph between actual weight and Digital panel meter (DPM) readings.

. PROCEDURE:

1. Connect the Strain gauge to the terminals as shown in the figure. 2. Adjust the course and fine controls untill the bridge is balanced. 3. Apply 1 kg weight on the cantilever and adjust the gain of potentiometer so that DPM shows reading 1.0. 4. Remove the weight. 5. Repeat three times steps 3 and 4 till DPM display 1.00 for kg. Weight. 6. Note down the readings by increasing the weight in steps of 0.5 kg. 7. Draw the graph between the actual weight and DPM display reading. 8.0 OBSERVATIONS: Table for applied weight and Displayed weight

RESULT: The pressure measured by strain gauge is……………..………

CONCLUSION: Comment on the graph.).

QUESTIONS: Write answers to Q…., Q…., Q…., Q…., Q..(Question numbers are to be allotted by Subject Teacher)

1. What is piezo-resistive effect? 2. List the different types of strain gauges. 3. Which type of bridge is suitable for strain gauge measurement? 4. Draw the diagram of bonded strain gauge. 5. Draw the diagram of unbonded strain gauge. 6. Which type of strain gauge is used to obtain high accuracy? 7. State the type of strain gauge, which is used to measure large weight. 8. Draw the diagram of semiconductor type strain gauge. 9. What do you mean by gauge factor? Give its importance and typical value. 10. List other types of strain measurement methods. 11. What is the need of pressure measurement? Give an example. 12. List three advantages of unbonded strain gauges over bonded strain gauges. 13. Give metals with important characteristics of it to form strain gauges wires. 14. Explain in brief unbonded strain gauge measuring displacement or force. 15. For measurement of pressure ,the force applied to the surface must be-a) At right angleb) Slantc) Parallel.(Space for answers)

Experiment No.4

TITLE: Test & calibration of pressure gauges using Dead weight tester.

APPARATUS: pressure gauges, Dead weight tester. Standard weights.

Dead Weight Testers.

1 - Handpump2 - Testing Pump3 - Pressure Gauge to be calibrated4 - Calibration Weight5 - Weight Support6 - Piston7 - Cylinder8 - Filling Connection

Dead weight testers are a piston-cylinder type measuring device. As primary standards, they are the most accurate instruments for the calibration of electronic or mechanical pressure measuring instruments.

They work in accordance with the basic principle that P= F/A, where the pressure (P) acts on a known area of a sealed piston (A), generating a force (F). The force of this piston is then compared with the force applied by calibrated weights. The use of high

quality materials result in small uncertainties of measurement and excellent long term stability.

Dead weight testers can measure pressures of up to 10,000 bar, attaining accuracies of between 0.005% and 0.1% although most applications lie within 1 - 2500 bar. The pistons are partly made of tungsten carbide (used for its small temperature coefficient), and the cylinders must fit together with a clearance of no more than a couple of micrometers in order to create a minimum friction thus limiting the measuring error. The piston is then rotated during measurements to further minimise friction.

The testing pump (2) is connected to the instrument to be tested (3), to the actual measuring component and to the filling socket. A special hydraulic oil or gas such as compressed air or nitrogen is used as the pressure transfer medium. The measuring piston is then loaded with calibrated weights (4). The pressure is applied via an integrated pump (1) or, if an external pressure supply is available, via control valves in order to generate a pressure until the loaded measuring piston (6) rises and 'floats' on the fluid. This is the point where there is a balance between pressure and the mass load. The piston is rotated to reduce friction as far as possible. Since the piston is spinning, it exerts a pressure that can be calculated by application of a derivative of the formula P = F/A.

The accuracy of a pressure balance is characterised by the deviation span, which is the sum of the systematic error and the uncertainties of measurement.

Today's dead weight testers are highly accurate and complex and can make sophisticated physical compensations. They can also come accompanied by an intelligent calibrator unit which can register all critical ambient parameters and automatically correct them in real time making readings even more accurate.

Procedure.- 1. Pour clean mineral oil to approx 2/3 of reservoir capacity (castor 30 motor oil) is suitable & restrict slippage past free piston to reasonable level higher viscosity oil will cause free piston to move sluggishly .particularly at low Pressure but slippage will reduce. Opposite effect will be noticed with lower Viscosity oil. 2.a) Open a release valve b) Turn screw pump handle clockwise fully this will accept some air from the system which bubbled out in the oil cup c) Turn the handle anticlockwise fully to drawn the oil into the instrument . d) Repeat clockwise / anticlockwise turning of handle number of times until no bubble appears in the oil cup3. Install the gauge to be tested. a) Slowly turn screw pump clockwise .This will build up pressure & is indicated on the gauge

5. Rotate the weights with platform by hand to reduce to the effect of friction in the free piston. Continue to increase the pressure & also rotate the weights until the piston & weights rises up & polished portion is seen

6. Increase the weights in steps & observe the gauge readings7. Calculate the errors if any in the gauge & give comment upon the

observation