thermocouple

4/7/2014 Experiment 1

http://www.eng.fsu.edu/~alvi/EML4304L/webpage/experiment4.html 1/9

Experiment 4Dynamic response of temperature measuring device

(Transient heat transfer)

Download Experiment Description

Objectives Theoretical Background Apparatus

Experimental Procedure Question to be Answered Download Data Sheet

Objectives

This experiment has two main goals. First, to introduce the basic operating principles of several common methods oftemperature measurement such as, liquid-in-glass thermometers, thermocouples and thermistors and how to calibrate these

devices. Second, to introduce the concept of dynamic response of thermal systems, ways of measuring this response and

factors, which influence this behavior.

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Theoretical Background

Temperature Measuring Devices

Thermocouples

When a pair of electrical conductors (metals) are joined together, a thermal emf is generated when the junctions are at differenttemperatures. This phenomenon is known as the Seebeck effect. Such a device is called a thermocouple. The resultant emfdeveloped by the thermocouple is in the millivolt range when the temperature difference between the junctions is ~ 100 0C. To

determine the emf of a thermocouple as a function of the temperature, one junction is maintained at some constant referencetemperature, such as ice-water mixture at a temperature of 0 0C. The thermal emf, which can be measured by a digital

voltmeter as shown in Figure 1, is proportional to the temperature difference between the two junctions. To calibrate suchthermocouple the temperature of the second junction can be varied using a constant temperature bath and the emf recorded asa function of the temperature difference between the two nodes.

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Figure 1 Measuring the EMF of a Thermocouple

The output voltage, E, of a simple thermocouple circuit is usually written in the form,

(1)

where T is the temperature in 0C, and E is based on a reference junction temperature of 0 0C. The constants A, B and C aredependent on the thermocouple material.

Providing a fixed reference temperature for the reference junction using an ice bath can make the use of a thermocouplecumbersome. Hence, commercially available thermocouples usually consist of two leads terminating in a single junction. Theleads are connected to a thermocouple signal conditioning ‘box’ containing an electrical circuit which provides a referencevoltage equal to that produce by a reference junction placed at 0 0C, a process called ‘ice point compensation’. Thesethermocouple signal conditioners or ‘power supplies’ usually display the temperature directly and or provide a voltage output

that is proportional to the thermocouple temperature. A similar thermocouple signal conditioner with a digital temperaturedisplay and an analog voltage output is used in the present experiment.

Thermistors

The thermistor, a thermally sensitive resistor, is a solid semi conducting material. Unlike metals, thermistors respond inversely

to temperature, i.e., their resistance decreases as the temperature increases. The thermistors are usually composed of oxides ofmanganese, nickel, cobalt, copper and several other nonmetals. The resistance is generally an exponential function of thetemperature, as shown in Equation 2:

(2)

where R0 is the resistance at a reference temperature, T0, while b is a constant, characteristic of the material. T0, the reference

temperature, is generally taken as 298 K (25 0C). Since all measurements made with thermistors can be reduced to detectingthe resistance changes, the thermistor must be placed in a circuit and the resistance changes recorded in terms of thecorresponding voltage or current changes. The formula relating the voltage (or current) changes to the resistance changes for a



given circuit has to be determined theoretically or empirically, or by a combination of both.

In the design of thermistor circuits, one must take the precaution that within the range of the operating conditions, the circuitremains stable at all times. Thermistor resistance varies inversely with temperature. The voltage applied directly across athermistor causes its temperature to rise, and its resistance to decrease. Sufficiently high voltage may cause thermal "runaway"(curve A in Figure 2), in which condition, higher currents and temperatures are induced until the thermistor fails, or the power isreduced. A series resistor, introduced to limit current, ensures stability (curve B). Thermal "runaway" will, in all probability,

permanently damage the thermistor, or change its characteristic properties.

Figure 2 Thermistor Behavior and Thermal Runaway

To increase the precision of the measurement, one should add a voltage divider to the circuit shown in Figure 4(a). This will

convert it to a Wheatstone bridge circuit, as shown in Figure 4(b). The out-of-balance voltage, DV, can then be measured andrelated to the resistance of the thermistor. A correct choice of resistors R2 and R3 will remove the mean DC value of DV.

Note that although the bridge circuit can increase the precision of the readings, the sensitivity is still the same as for the simplevoltage divider circuit shown in Figure 4(a). The simple DC bridge circuit of Figure 4(b) is generally satisfactory for mostapplications.



Figure 3 Thermistor Circuit

Considering this circuit, we shall now derive the relation between T and DV. In general,

(3)

Assume R1 = R3. Then,

(4)

Rearranging for RT,

(5)

The relation between T and RT is given by,



(6)

or,

(7)

Substituting for RT from Equation 5, we have

(8)

If we further assume R1 = R2 = R3 = Rb, we have,

(9)

T is not a linear function of V, and so any linear analog recorder will be in error when linear interpolation is used betweencalibration points (for small ranges in temperatures, the error may be negligible). If we measure E along with our scans of theDVs, then the only unknowns in Equation 9 are R0 and b. These unknowns are determined by calibration experiments. You

will perform a 3 or 4 point static calibration of both the thermocouple and the thermistor. Back to the top

Dynamic Response of Thermal Systems

When temperature measurements of a transient process are made, it is important to verify that the dynamic response of themeasuring device is fast enough to accurately track the time varying temperature. In the second part of this experiment, we willstudy the influence of different parameters on the transient response characteristics of a thermal system. In this experiment, wewill measure the response of thermocouple (or a modified thermocouple). The thermocouple is modeled as a spherical ball, asshown in Figure 4. The thermocouple temperature is, T, mass m and specific

Figure 4 – Thermocouple bead modeled as a simple thermal system

heat capacity c. If the sphere is suddenly exposed to an environment at temperature Tµ, then, after making the appropriateassumptions, the energy balance for this transient process is given by:

The solution for the above first order equation is the well known exponential decay given as:



where the time constant t = mc/hA. In this experiment, we will examine the influence of properties suh as mass, surface areaand specific heat capacity of the bead on its dynamic response.

Apparatus

The following apparatus is used in conducting the experiments:

1. Constant temperature bath: The constant temperature bath is capable of providing liquids at constant temperatures betweenapproximately 10 to 90 0C. Several different temperatures will be used in the calibration procedure. Note how the settings aremade and set the bath for a low temperature.

2. Thermocouples: The thermocouple used in this experiment is connected to a power supply, which has a digital temperaturedisplay and an analog output. The analog output is connected to the ADC card. The thermocouple will be calibrated byplacing it in the constant temperature bath and recording the digital display and the voltage output using the computer and theADC card.

3. Thermistor: Examine the thermistor provided; it will already be connected to a Wheatstone bridge circuit. You will calibrateit by placing it in the constant temperature bath along with the thermocouple and recording the output voltage. Thermocoupleswith different beads.

4. Wheatstone bridge circuit: For the thermistor.

5. Personal Computer and Analog-to-Digital (ADC) converter: This will be used to digitize and record the voltage signal formthe thermocouple and thermistor as a function of time.

6. Resistance Temperature Detector (RTD): The RTD will be immersed in the constant temperature bath for the duration ofthe experiment. The temperature indicated by the RTD will serve as the reference temperature (i.e., actual temperature of the

bath).

Click here to see apparatus

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Experimental Procedure

I. Thermocouple and Thermistor Static Calibration The static calibration for the thermocouple and the thermistor will be done at the same time.

1. Connect the outputs of the thermocouples and the thermistor to the appropriate channels on the ADC card. Start theLabView program that is used for data acquisition. Ask the TA’s for assistance.

2. Place the thermocouple and the thermistor in the constant temperature bath. Place the in the constant temperature bath. Starting with the bath at the lowest setting, between 20 –0 0C.

3. Record the temperature of the RTD.

4. Record the thermocouple temperature and the voltage reading.

5. Record the thermistor out-of-balance voltage, DV, and

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6. Repeat for three other different temperatures.

II. Time Response Measurement

1. Set the constant temperature bath between 20 and 40 0C.

2. Using the LabView program provided, monitor and record the outputs of the thermocouple as you rapidly move thethermocouple from the constant temperature bath and place them in the ice bath.

3. Now change the constant temperature bath temperature to somewhere between 60 and 80 0C and repeat step 2.

4. Without changing the constant bath temperature, repeat step 2 with the larger size “thermocouples”. Note the diameter andthe material of all the thermocouple beads.

Question to be Answered

1. Discuss the principles of operation of thermocouples and thermistors. Compare the advantages and drawbacks of using thetwo devices.

2. Plot the static calibration data for the thermocouple and the thermistor, i.e. plot temperature vs. voltage. Does the voltageoutput confirm the expected trends? How and why?

3. Is the temperature in the constant temperature bath truly constant? Did you notice temperature fluctuations, and if so, whatwas their magnitude? What is the effect of these fluctuations on the accuracy of your static calibration?

4. Is there a discrepancy in the RTD temperature and the temperature displayed on the thermocouple digital display? Is thediscrepancy constant over the entire temperature range? Discuss reasons for this discrepancy.

5. Determine the time constants for the smallest thermocouple at the two different temperature settings. Would you expect the

time responses to be the same or different and why?

6. Determine and compare the time constants for the larger diameter thermocouples. Keeping in mind the physical parameterswhich govern the time response, is this trend expected? Back to the top

Experiment 4 Data Sheet

Note: Please note the units of the quantities which are being measured, when recording data. For example, when measuring voltage, if the voltmeter reads 16 mV, then write down 16 mV instead of just 16.

I. Inspect the constant temperature bath.



II. Thermocouple analysis:

1. Record output voltage of thermocouple:

Both terminals in air:_________

One terminal held between fingers and the other terminal in an ice bath:________

III & IV. Thermocouple and Thermistor calibration

1. Record the following:

a) Supplied voltage, E: __________

b) Wheatstone bridge resistor values:

R1 = _______________, R2 = ______________, and R3 = ______________

2. Place one junction of the thermocouple in the ice bath and the other in the constant temperature

bath. Also place the thermistor in the constant temperature bath. Record the temperature of the constant temperature bath (via RTD), the thermocouple voltage (V) and the out-of-balance voltage of the thermistor (DV) in the following table. Repeat for ten different bath temperatures.

No. RTD temperature (oC) Thermocouple (V) Thermistor (DV)

0

1

2

3

4

5

6

7

8

9

10

V: Time response:

DO NOT DISCONNECT THE WHEATSTONE BRIDGE CIRCUIT. With the circuit in place, connect thermistor



(DV) output and thermocouple output to the ADC card in the computer. The data will be recorded using a LabView program, your lab TA will assist you with this part of the experiment.

Note on Breadboard use:

The supplied breadboard should be used to develop the wheatstone bridge circuit as discussed in the lab handbook. The breadboard is simply a convenient tool for building up circuits. The breadboard consists of rows of small, electrically connected connection points in a grid pattern. Interconnected connection points form a bus. The board has three sections that are divided to allow for easy circuit connection. Each section has two long buses running from top to bottom labeled A and B. These buses are normally used for power connections from the terminal posts located on the top of the board or from the two long buses running left to right on the top of the board (also labeled A and B). Small buses consisting of five connection points are situated on the left and right of the power bus. These buses are only connected from left to right and are not connected to the power bus. Each section of the board is independent of the others. The breadboard configuration allows for very complex circuits to be constructed and allows for quick construction of the wheatstone bridge circuit needed for this experiment. The supplied wire set and resistors are pushed into the connection points in the required configuration as illustrated in the lab handbook. Consult the lab TA for further assistance.

YOU NEED TO GET THE LAB INSTRUCTOR'S SIGNATURE BEFORE LEAVING.

The student has performed the experiment satisfactorily and has cleaned the work area.

___________________________ _______________

(Lab assistant's signature) Date

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