When measuring the heat of the sample, it is necessary to determine the heat capacity of the calorimeter first, and when calibrating, the heat capacity is generally tested for 5 times repeatedly. Calculate the average value and relative standard deviation of the results of 5 repeated tests, and the relative standard deviation should not exceed 0.20%; if it exceeds 0.20%, make up another test and take the average value of the results of 5 times that meet the requirements, and fix it to 1J/K as the heat capacity of the instrument. If the relative standard deviation of the results of any 5 times is checked over 0.20%, the test conditions and operation techniques should be carefully checked and the problems should be corrected, then the calibration should be carried out again and all the existing results should be discarded has the advantages of simple and convenient operation, less influence by the outside world and accurate results).
Before using the new calorimeter, it is necessary to determine the effective working range of its heat capacity. The method is to perform at least 8 heat capacity calibration tests with benzoic acid, with the mass of benzoic acid tablets generally ranging from 0.7g ------1.3g, or the mass of benzoic acid tablets determined according to the range of calorific values (temperature rise) likely to be involved in the sample being measured. At least 2 replicate determinations are made at each of the two endpoints. Then, plot the temperature rise versus heat capacity value using the temperature rise Δt (tn-t0) as the horizontal coordinate and the heat capacity E as the vertical coordinate. If there is no obvious systematic change in the heat capacity value observed from the graph throughout the range, the heat capacity of the calorimeter can be regarded as a constant; if there is an obvious correlation between the observed heat capacity value and the temperature rise, the relationship between E and △t is obtained by the method of one-dimensional linear regression as follows
and calculate the estimated variance and relative standard deviation of the linear regression equation, the relative standard deviation should not exceed 0.20%. All results should be included in the calculation, except for the test results of incomplete combustion* which must be discarded. If the precision meets the requirements, the heat value used can be determined by the above equation based on the actual temperature rise △t when determining the heat of the specimen.
If the precision does not meet the requirements, a new set of calibration should be performed after finding the cause and solving the problem.
The calibrated value of heat capacity is valid for 3 months and should be re-calibrated after this period*. However, the following conditions should be immediately re-tested:
a. Replacement of the calorimetric thermometer;
b. Replacement of large parts of calorimeter such as oxygen vertebrae, connecting rings
c. The difference in temperature between the calibrated heat capacity and the inner cylinder during the determination of heat generation is more than 5K;
d. After the calorimeter has undergone a large move.
If there is no significant change in the calorimetric heat system, the difference between the recalibrated heat capacity value and the previous heat capacity value should not be more than 0.25%, otherwise, the test procedure should be checked and the problem solved before recalibration.
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