Thermocouple test_Lab demonstration

1

Objectives:

The experiment is based on the following objectives

To be introduced with the thermo-electric thermometers
To understand the temperature current/voltage relationship
To learn temperature measurement by using thermocouple

Background:

Measurement of temperature probably is the most common phenomena observed in the industrial and engineering applications. Wrong measurement of temperature can bring serious technical and financial consequences. It is therefore important to use reliable devices that accurately measure temperature. Depending on the requirement and interest, various temperature measurements can be possible. These include thermocouples, thermometers, resistance temperature detectors (RTDs), thermistors etc. In this experiment we will work basically with thermocouples.

A thermocouple consists of two wires of two dissimilar materials which are joined together. When the two junctions of thermocouple are kept at different temperatures a small electric current is induced- the principle which is known as Seeback effect. Due to the flow of current a voltage drop occurs. This voltage drop is the function of the temperature difference between the two junctions. In practice, this voltage drop is measured which later is correlated with the temperature difference.

It is important to note that a thermocouple does not measure the temperature, but rather the temperature difference between the two junctions. In order to use a thermocouple to measure temperature directly, one junction must be maintained at a known temperature. This junction is called the reference junction having the reference temperature. The other junction, which is normally placed in contact with the body of unknown temperature, is called the measurement junction.














Figure 1: Seeback effect (Thomas Seeback, 1821)


Experimental procedure:

The experiment to be conducted in the laboratory is consisted with two parts:

Fabrication of thermocouples by using soldering
Determination of e.m.f. for two thermocouple in series


























Figure 2: Essential tools for thermocouple experiment

Fabrication of thermocouple:

Thermocouples can be composed of many different pairs of metals and the junctions can be formed in many different ways. For a variety of reasons, different pairs of metals are used for different applications. For our experiment we will use the K type thermocouple.

The K-type thermocouple is formed by joining the two wires composed with the alloy chromel and alumel (nickel/ 5% aluminium and silicon). The chromel is connected as positive leg whereas alumel is connected as the negative leg. The temperature range that can be measured by K-type thermocouple is between -200 to 1250°C. Experimental procedure for fabricating K-type thermocouple is as follows:

Find thermocouple wires, pliers, strippers, voltmeter/ammeter, ice bath, soldering iron with materials and plates
Strip approximately ½ inch of leads from both ends of the wires
Form the wires at one end into an oval shape so that the two wires nearly touch at the single point
Form a pool of solder in the soldering plate and keep it liquid by using the soldering iron
Dip the thermocouple to the pool so that the soldering material forms a bridge between the two wires
Leave the joint on the atmosphere for few seconds so that soldering stiffens. The soldering thermocouple at this point should be ready to use and this will end the first part of the experiment

Determination of e.m.f. for two thermocouple in series:

For the second part of the experiment the generated emf will be checked for two thermocouples in series. Thermocouples if joined in series are called thermophiles which can sustain high measurement sensitivity. The step-by-step procedure for fabricating series thermocouples and subsequently to measure emf is:

First of all, use a millivolt meter to measure the emf of a thermocouple you have made when the soldered junction is in room air and in your hand.
Place the soldered junction in melting ice, measure the emf and determine the polarity of the thermocouple.
Check if your reading agrees with that given in the table attached.
The room temperature may be obtained with a mercury-in-glass thermometer.
Connect two thermocouples in series and keep two junctions in melting ice as shown in the figure below.
Measure the emf. Is the emf doubled? Comment on.



















Figure 3: Two thermocouples in series
Conclusions:
The use of thermocouples for temperature measurement is quite diverse. Although wide range of thermocouples are available, in this experiment the K-type is used which is one of the most extensively used thermocouples in the industrial applications. Joining thermocouples and thermophiles (several thermocouples in series) are not so difficult, however, the generated emf by these two types are different. For two thermocouples in series, the measured emf should be twice as big as the single thermocouple. This result is expected from the learning outcome of this thermocouple laboration.
ENE 226 Thermocouple lab