1.        Objectives:

In this experiment, it is required to measure the temperature of water bath and furnace by various instruments. The temperature of the mercury in glass thermometer will be taken as a reference and the rest of instruments will be calibrated with respect to it.

2.        Introduction:

Temperature measuring bench (Cussons Unit) is designed to demonstrate several commonly used methods of temperature measurements, and provides the means for calibration and accuracy comparisons of different methods. Features are providing so that many faults commonly occurring in thermocouple systems can be demonstrated. In addition, the unit may be used to be to provide a temperature measuring facility for use with other laboratory experiments.

3.        Theory and Analysis:

3.1         Thermocouple:

It is one of the most electrical effective measuring device. When two dissimilar wires of metals are joined together as in Fig. 1 an emf will exist between the two points A and B, which is primarily a function of junction temperature (T). All thermocouple circuits must have at least two junctions. If the temperature of one junction is known, then the temperature of the other junction may be easily calculated using the thermoelectric properties of the materials. The known temperature is called the reference temperature. It is common to use the temperature of ice as a reference temperature (ice bath).

Fig. 1 Thermocouple circuit.

The relation when one junction is at 0 ˚C can be expressed mathematically as:

Log E=A log T + B

Where:

·         E = emf in microvolts.

·         T = temperature in ˚C

·         A, B constants depending on the wire forming the junction.

4.        Description of Experimental Setup and Instrumentation:

4.1         The main components of the experimental setup are:

4.1.1        Thermocouple

The thermocouple principle is that if two dissimilar wires are fused at each end (but otherwise separated) and one end junction is heated to a higher temperature than the other, an emf is produced causing a current to flow around the loop. The actual value of emf will be dependent on the nature of the materials and the temperature difference between the two junctions.

When temperature is measured by thermocouple, that part of circuit which connects the thermocouple to indicating instrument is as important a part of the measuring unit as the thermocouple itself. Ideally, one end of the thermocouple (the cold junction) is kept at fixed temperature (preferably 0 ˚C) so that the emf generated by the thermocouple at a given temperature of the 'hot' junction is always the same. In practice, however, a cold junction is not always kept at an un varying temperature. Compensating leads are therefore employed to transmit to the measuring instrument. With the minimum of error, a thermocouple signal from a cold junction which is at an unknown temperature. For highly an accurate temperature measurement under laboratory conditions, the thermocouple wires should be led into an ice flask-or similar constant temperature source –from which connections are made to the instrument by pure copper wires. If the cold junction is not at 0 ˚C, it should be remembered that the emf output must be connected to follow the different temperature. The second thermocouple may be connected in series with hot junction thermocouples, so that the cold junction effects at the terminals are cancelled out. The second thermocouple then becomes the cold junction, and may be conveniently placed in the ice path for 0 ˚C reference, when using mv digital indicator. The direct reading thermocouple instruments modern commercial design suitable for practical applications with a minimum of error and therefore employs automatic cold junction compensation.

4.1.2        Liquid expansion thermometers (liquid-in-glass thermometer).

The indication of a simple liquid-filled thermometer depends simply on the difference in the coefficient of volumetric expansion of the filling liquid relative to the envelope containing it. The thermometers supplied are mercury in glass type. The ranges and accuracies are given in the specification for optimum accuracy the immersion depths specified should be adhered to. Although relatively low cost and simple mercury in glass thermometers are capable of high orders of accuracy over the designed temperature range. The thermometers supplied are general laboratory grade but are of sufficient accuracy to provide a reference for calibration of the other temperature sensing devices provided.   

 4.1.3        Electrical resistance temperature thermometers. 

Electrical resistance thermometers work on principle that when metal wire is heated, its electrical resistance increases progressively with increasing temperature. The relationship between variations in the resistance of wire and the temperature can be determined for any given wire. By measuring the resistance using a suitable bridge the temperature can be determined. In the cussons unit a platinum resistance probe is provided together precision power supply and resistance network giving a direct digital indication of the probe resistance in ohms. The temperature /resistance characteristics of the probe supplied are as follows:

 Resistance at 0˚   100+ 0.1Ω

 Temperature coefficient 0.385 Ω/˚C

4.1.4        Bi-metallic temperature indicator

The indicator incorporates a bimetallic strip consisting of a metal with a high coefficient of expansion (e.g. brass), and the other allow coefficient of expansion (e.g. invar). An increase in temperature will produce bending in the direction of the metal with the lower coefficient of expansion, and decrease in temperature bending in the direction of the metal with the higher coefficient of expansion. The resulting moment is mechanically coupled to the indicating pointer to give a direct temperature readout. 

Fig. 2 Bi-metallic strip.

 4.1.5        Hot water bath

The hot water bath comprises a dewar flask whose lid is fitted with a combined heater unit. The heater unit incorporates an adjustable thermostatic temperature control and over temperature cutout. When used with distilled water the thermostat may be set to boil the water to provide a 100 ºC reference point, (at 760 mm Hg barometric pressure) the water in bath is constantly circulated barometric pressure). The water in bath is constantly circulated by the stirrer so that when the thermostat is set below 100 ºC, the temperature throughout the bath is constant.

4.1.6        Ice point bath

The ice point bath comprises a dewar flask which is filled with crushed melting ice. The flask is provided with a lid so that thermal insulation is very high. When the crushed ice is made using pure water (distilled is preferred), the temperature in the bath will be close to the theoretical triple point of water (0.01ºC). The lid incorporates three holes so that three temperature measuring devices may be inserted simultaneously. 

4.1.7        Furnace

The furnace comprises a brass block with holes drilled to accept a mercury in glass thermometer (standard thermometer), bi-metallic temperature indicator and thermocouple (type k). An electrical heating element is fitted into the block to be equidistant from the three temperature measuring holes, ensuring that the temperature is equal at each of the three points. The block is mounted in lidded metal enclosure which providing an air space around the block, so that heat loss is reduced and temperature stability improved. The electrical supply to heater is provided from a variable transformer with a front panel control so that the rate of heating and final temperature can be controlled as desired. An over temperature thermostat is mounted on the rear of the block and is set to limit the maximum temperature to 250 ºC. This thermostat should not normally require adjustment, however, if required, an adjusting screw is provided access to which is provided through a hole in the rear of the furnace enclosure.

4.1.8        Use with the resistance thermometer/thermocouple

Set the Ohm/mv selector to the mv position. For correct operation thermocouples, should be connected in pairs of the same as shown in figure below. In each case the red wire should be connected to the black terminal, and the yellow, blue or white wire connected to the red terminal. The red wire should be connected between the black terminals of both hot and cold junctions as shown. In this configuration, there are no spurious junction errors and the cold junction is referenced at 0 ˚C. The mv display may be converted to an accurate temperature in ˚C by reference to the appropriate tables in appendices. The hot source thermocouple may be placed in the ice bath for 0 ˚C check and the boiling hot water bath for 100 ˚C check. Alternatively, the thermocouples may be checked against the thermometers.

Fig. 3 Thermocouple principal (type k).

4.2         Instrumentation used in this experiment:

·         Thermocouple.

·         Bi-metallic strip.

·         Mercury-in-glass thermometer.

·         Resistance thermometer.

5.        Test Conditions and Experimental Procedure:

5.1         Precautions:

Before running the engine, the following should be checked:

1.      Make sure that the ice bath is at the triple point of water.

2.      Make you eye normal to the thermometer when you take the reading.

5.2         Procedure:

1.      Prepare the water bath and put the heater in it.

2.      Turn on the furnace then, increase the temperature gradually in the hot water bath and take the reading after 5 ºC (in the thermometer) for both the resistance (Ω) and thermocouple (mv),

3.      Read the temperature of the furnace after a bout 5 ºC (thermometer) for the thermocouple (ºC) and bimetallic strip (ºC).

4.      After recording these readings, make calibration for thermocouple, resistance thermometer, bi-metallic strip and use the mercury-in-glass thermometer as a reference.  

6.        Results:

Record calculated results on the attached summary results sheets following the recorded measured data tables.

plot the following:

·         The calibration curves for thermocouple, resistance thermometer and bi-metallic strip, use the mercury-in-glass thermometer as a reference. 

7.        Discussions:

 Discuss the results you obtained.

8.        References:

·         List all references you might have used.

Calibration in Industry 

Calibration Curve for thermocouple against mercury thermometer