Calibration

PERSONAL COMPUTER

3.3.5 Calibration

Compatible results from the calorimeter were a pre-requisite. In order to achieve this it was necessary to a dual calibration method. In essence the calibration of the calorimeter is accomplished in three steps. In the first step the signal conditioners for each computer are calibrated to ensure equal response to temperature changes. The second step calibrates the temperature probes to ensure correct readings and accurate control of the calorimeters. When this has been achieved , the heater control system is adjusted to provide an adiabatic environment for the sample .

3.3.5.1 Signal Conditioner Adjustment

The three thermal probes for the calorimeter are connected to the personal computer via three individual signal conditioners and each unit has a facility for span adjustment.

This part of the procedure is accomplished by placing all 6 probes (3 from each calorimeter) in a freezing pOint ice - water mixture and then adjusting the software factors for the computer to read ODC . The probes are then placed in water at a temperature 70DC , which is normally above the required operating temperature of the calorimeters. The spans of the individual signal conditioners are then adjusted to match the temperature reading on a thermometer placed in the water.

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Chapter 3 -Laboratory Procedures

3.3.5.2 Thermal Probe Calibration

In this procedure, the probes for measuring the temperature of the sample, small tank and large tank are tied together with a calibrating thermometer by means of elastic bands and placed in the small tank. They are positioned so that the extremities of the probes and the bulb of the thermometer are at the same depth in the water and the tops of the probes are just below the water level. The computer is then set to maintain a steady temperature in the tank. Then the temperature readings of the probes on the computer screen are compared with the calibration thermometer. The differences are determined and entered into the computer in order to equalise the readings for the three probes on the screen and simultaneously to align them with the actual temperature of the water in the tank.

When the temperature probes have been satisfactorily correlated, the calorimeter is set in heating mode and the water in the small tank is heated to 70 DC to encompass its normal operating range.

During this phase, the temperature readings from the probes are recorded by the computer for analysis. Since adiabatic control of the calorimeter is effected by the difference between sample and small tank readings, this parameter may require further adjustment to compensate for any measurement discrepancies related to temperature in the operating range of the apparatus.

The data from the computer is extracted and the differences in temperature between the sample and the small tank probes are calculated. These results are then plotted against the sample temperature. Fig. 3.3 shows a typical result.

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10 20 30 40 50 60 70 80

SAMPLE TEMPERATURE

°c

Figure. 3.3: Thermal Probe Calibration.

Chapter 3 - Laboratory Procedures

In the example shown above it can be noted that the readings have a characteristic fluctuation of about 0.02 DC and there is a slight negative drift in the probe temperature difference with increasing temperature.

The software accommodates corrections for this drift when the slope and intercept from a simple straight line regression of the calibration data are entered into the computer.

When the slope and intercept corrections have been made the calorimeter is again calibrated as per the foregoing procedure in order to check on the validity of these corrections. Figure 3.4 shows the results obtained after making the corrections suggested by the regression of the data that was used for figure 3.3.

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20 25 30 35 40 45 50 55 60 65

Sample Probe Temperature

° c

Figure 3.4: Corrected Thermal Probe Calibration 3.3.5.1 Adiabatic Performance Calibration .

The preceding procedure ensures that the temperatures in the calorimeter are measured accurately. In order to complete the calibration it is necessary to ensure that the calorimeter behaves adiabatically. This is accomplished by adjusting the switching parameters of the heaters until the calorimeter can maintain an inert sample a fixed temperature over a sustained period.

This step is necessary because a number of external factors , which are difficult to quantify, can de-stabilize the temperatures of the system. Such factors include:

• the temperature difference between ambient and the small tank.

• heat induced to the tank water by the stirring mechanism.

• heat losses from the sample via the ther mal probe cable.

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Chapter 3 - Laboratory Procedures

An inert sample of silica sand with a thermal probe is installed in the sample chamber that is in turn placed in the small tank. The system is then set to heat to a pre-determined temperature.

When the sample reaches this temperature, the system is set in adiabatic mode. The temperatures of the tank water and the sample are then allowed to equalise .

At this point the heater parameters are adjusted until the sample maintains a constant temperature. The temperatures in the system are then monitored over a period of 18 hours to ensure that the calorimeter can maintain the sample at the constant temperature.

This process is repeated at every 10 degree interval in the operating range of the calorimeter.

Fig.3.S shows the results of a typical test where the sample has been maintained at a temperature of approximately 60 ac.

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Time in hours

Figure 3.5: Adiabatic Calibration.