Experiment 10: Measurement of temperature using Instrumentation Trainer.

Instrumentation trainer:

The instrumentation trainer provided by Dynalog India contains different sensors, electronics circuits and output display units. By using these sensors and electronics circuits one can design an instrumentation system. There is a temperature sensing unit which contains four temperature sensors such as ‘K’ type thermocouple, LM35 IC temperature sensor, a Platinum RTD and NTC thermistor. These transducers are mounted inside a clear plastic enclosure which contains a heater. The heater used here is basically a 33Ω 5W resistor. It generates heat when current is allowed to flow through it. Which then raises the temperature inside the enclosure and that temperature is used by the sensors for the experiments. In the case of NTC thermistors and thermocouples, an additional separate unit is mounted outside the heated enclosure. The externally mounted sensors are made available for comparison between ambient temperature and the temperature within the enclosure.

IC Temperature Sensor (LM35):

LM35 is an IC which is basically used as atmospheric temperature sensor. It looks like a BC547 transistor. It has three terminals named Vcc, GND and output. It gives an output of 10mV/^oC. Measurement of the output voltage therefore indicate the temperature directly in degree Celsius (^oC ). For example, at a temperature of 25^oC the output voltage will be 250mV.

 Procedure:

Connect the volt meter (in millivolt range) between INT o/p and 0V as shown in fig. Switch ON the power supply and note down the voltage. Multiply the output voltage in volt(V) with 100 or divide the output voltage in milivolt (mV) by 10 to get the temperature in ⁰C. This is the room temperature. Then connect the 12V to the heating element to raise the temperature of the enclosed chamber. Note the voltage reading at every minute and calculate its corresponding temperature value in ⁰C and ⁰K.

Observation:

Time (minutes)		0	1	2	3	4	5	6	7	8	9	10
Voltage (mV)
Temperature	0C
	0K

Note:

• Connect the 12V to the heater for some time (1 minute), then remove the patch cord, the temperature will go rising for some time. It helps to get stable temperature.
• Don’t let the temperature to go beyond 60⁰C before that stop heating the heater.

The Platinum RTD:

The RTD used here is Pt100, it has 100Ω resistance at 0⁰C. it has positive temperature coefficient (PTC) means the resistance increases as the temperature increases. The increase in resistance is linear, the relationship between resistance change and temperature rise being 0.385Ω/⁰C.

                                                                                      R_t = R_o + 0.385 t

R_t = resistance at temperature t⁰C.

R₀ = resistance at 0⁰C (100Ω)

Normally the unit would be connected to a DC supply via a series resistor and the voltage developed across the transducer is measured and the resistance is calculated.

  

Procedure:

Set the slider of the 10KΩ carbon resistor to midway and volt meter in 200mV range. Then connect the circuit as shown in fig. Switch ON the power supply. Calculate the room temperature(Y) from the LM35 o/p. Then calculate the R_t taking t as the room temperature (let it be XΩ). Then adjust the slider control of the 10KΩ resistor so that the voltage drop across the Platinum RTD would be X mV as indicated by the digital volt meter. This calibrates the platinum RTD for an ambient temperature of Y⁰C, since the resistance of the RTD at Y⁰C will be XΩ. Note that the voltage reading across the RTD in mV is the same as the RTD resistance in Ω. Connect the 12V supply to the heater element input and note the values of the voltage across the RTD and LM35 IC temperature sensor. Convert the two voltage readings to RTD temperature and RTD resistance and record the values. Plot the graph of RTD resistance ( Ω ) against temperature ( ⁰C).

Observation:

Time (minutes)		0	1	2	3	4	5	6	7	8	9	10
RTD Resistance
RTD Temperature	0C
	0K

 The NTC thermistor:

It is basically a resistance which has Negative Temperature Coefficient of resistance (NTC). Means the resistance falls as the temperature rises. And the resistance temperature characteristic being nonlinear. The resistance of the thermistors provided is of the order of 5KΩ at an ambient temperature of 20⁰C. Two similar units are provided, one being mounted inside the heated enclosure. This is connected to +5V supply and designated A. the other is mounted outside the heated enclosure, it is connected to 0V (ground) line and is designated as B.

To measure the resistance of the thermistor, a calibrated resistor is to be connected with the thermistor to +5V supply. For each reading the variable resistor is adjusted until the voltage at the junction of the thermistor and resistor is half of the supply voltage. For this setting there will be same voltage drop across the thermistor and resistor and, since the same current flows in each, there resistance must equal. Hence the value of resistance read from the calibrated resistor scale is the same as the resistance of the thermistor.

Procedure:

Connect the circuit as shown in fig., set the switch on Wheatstone bridge circuit to OUT to disconnect the 12K and R_x resistors from the circuit and set the calibrated variable resistor dial reading to approximately 500. Switch the power supply ON and adjust the resistor control until the voltage indicated by the volt meter is 2.5V and note the dial reading and corresponding temperature value from the IC temperature sensor. Connect the 12V supply to the heater element input socket and at one minute intervals note the values of dial reading to produce 2.5V across the resistance and also the temperature (from the IC temperature sensor). Plot the graph between the thermistor resistance and temperature.

Observation:

Time (minutes)		0	1	2	3	4	5	6	7	8	9	10
Temperature (from IC Transducer)	0C
	0K
Dial reading for 2.5V
Thermistor resistance (10 X Dial reading + 1KΩ)

Type ‘K’ thermocouple:

The materials used to construct the type ‘K’ thermocouple are alumel and chromel. The ends that are joined together are referred to as the ‘hot’ junction and the other end is  ‘cold’ junction. When the hot junction is heated an output voltage is obtained between the cold ends. And the magnitude of the emf depends on the temperature difference between the hot and cold junctions and on the materials used. For the ‘K’ type thermocouple the output voltage is fairly linear over the temperature range 0-100⁰C with the sensitivity 40.28 µV/⁰C. there are two thermocouples are provided with this unit, one being mounted within the heated enclosure, this being the active unit which will have its hot and cold junctions at different temperatures in operation. The other one is mounted outside of the heated enclosure and is incorporated in a heat sink with an LM 335 IC temperature sensor so that the temperature of the cold junction of the active thermocouple can be measured.

The second thermocouple is connected in series with the first with the wires of the same material connected together. The second thermocouple does not contribute to the output voltage because its hot and cold junctions are maintained at the same temperature. Due to low output voltage of the thermocouple amplification is required.

Procedure:

Connect the circuit as shown in fig., set the voltmeter to 200mV DC range and set amplifier #1 gain coarse to 10 and fine to 0.2. Switch the power supply ON and then set the offset control of amplifier #1 as follows:

Short circuit the input connections to the instrumentation amplifier and adjust the offset control for zero indication on the volt meter. Reconnect the thermocouple outputs to the instrumentation amplifier as shown in fig. the output voltage should still be zero with the hot and cold junctions at the same temperature. Find the temperatures of inside and outside of the enclosure by using digital mutimeter. Inside temperature can be measured from LM35 INT socket, and outside temperature can be measured from the REF output socket of the LM 335 provided on the k type thermocouple unit. To measure the outside temperature, keep the multimeter in millivolt range, measure the voltage at REF socket. The millivolt shown is multiplied by 100 to obtain the temperature directly in ⁰K. Connect the 12V supply to the heater and at one minute intervals, note the values of thermocouple voltage (mV), and the voltages representing the temperatures of the hot and cold junctions of the thermocouple. Plot the graph of thermocouple output voltage against temperature difference between hot and cold junctions.

Observation:

Time (minutes)		0	1	2	3	4	5	6	7	8	9	10
Temperature (from IC Transducer 0K)	Hot junction (INT)
	Cold junction(REF)
	Difference
Thermocouple output