In view of the wide application and importance of the oxygen bomb calorimeter, it is necessary to evaluate the uncertainty of the test results in view of the rigor of the test report. The size of the uncertainty is directly related to the quality of the measurement results. The smaller the uncertainty, the greater the reliability, the higher the quality of the measurement results, and the greater the use value of the measurement results.
Measurement uncertainty is, for a measurement result, characterizing the degree of dispersion of the result. According to the Measurement Uncertainty Guide, this dispersion is composed of many components. For these components, the uncertainty of the result according to a statistical distribution (Type A) or a probability distribution (Type B) of experience and other information; the Type A and Type B uncertainties are summed up by sum-of-squares , is the combined standard uncertainty. Purpose of measurement: To determine the value of the measurand or to obtain a usable measurement result. Measurement uncertainty is used to characterize the quality of the measurement result. This paper analyzes and calculates the uncertainty of EPS calorific value test based on the calorific value test principle of EPS.
2 Test principle
In this paper, IKA C2000 calorimeter is used to make EPS combustion calorific value test samples according to GB/T 14402-2007. In the peripheral isothermal mode, the sample burns in the oxygen bomb to release heat, which increases the temperature of the entire calorimetric system, and the two are proportional. Calculate the calorific value of polyurethane according to the temperature rise before and after the test.
The calibration of the heat capacity adopts the national first-class standard benzoic acid tablet.
3. Uncertainty evaluation of EPS combustion calorific value measurement
3.1 Mathematical model for measuring the heat of combustion of polyurethane
According to the temperature rise of water before and after combustion, calculate the combustion calorific value of polyurethane 2, see formula (1):
In the formula: Q——the combustion calorific value of EPS, J/g;
M——mass of EPS, g;
△T1——The temperature rise of the calorimetric system during calorimetry, °C;
E——The heat capacity of the calorimetric system, J/℃;
q1——The combustion calorific value of the ignition wire, J.
According to the definition of heat capacity, the heat capacity of the system can be obtained by calibration with standard benzoic acid tablets:
In the formula: Qs——the calorific value of the standard material, J/g;
ms——the mass of the standard substance, g;
△T——The temperature rise of the calorimetric system during calibration, °C.
According to the calorific value test principle and JJF 1059-1999 3 polyurethane combustion calorific value Q as the measured value, it is determined by the heat capacity E, the temperature rise ΔT1 of the system, the ignition heat q1, and the mass m of the polyurethane sample. According to the test principle and test procedure, m and E are independent of each other, △T1 is positively correlated with m, and E is negatively correlated with △T1 4. So there is a correlation coefficient:
Therefore, the synthetic standard uncertainty of polyurethane calorific value measurement is:
3.2 Evaluation of system heat capacity uncertainty
3.2.1 Identification of Uncertainty Sources
From the principle of heat capacity calibration and the test process, it can be known that the repeatability of the calibration heat capacity, the quality of the water in the inner cylinder, the standard calorific value of benzoic acid, the heat of ignition and the uncertainty of temperature measurement determine the uncertainty of the heat capacity of the system.
(1) Summarizing the above analysis, when carrying out the uncertainty analysis of the calorific value of polyurethane combustion, first determine the mathematical model used in the detection process, and based on the evaluation model, analyze the uncertainty components, and determine the composition of each uncertainty component. The standard uncertainty of type A and the standard uncertainty of type B are calculated separately for each component value, and finally the measurement uncertainty is reported.
(2) It can be seen from the calculation that although the uncertainty of the calorific value of polyurethane is large, the relative uncertainty is relatively small. Therefore, the calorific value of polyurethane measured under known test conditions has high accuracy.
(3) It can be seen from the calculation process that the measurement repeatability and the heat capacity of the calorimeter system cause a large uncertainty component. Therefore, in the calorific value test of the test sample, the accuracy of the instrument and the measurement method should be improved, such as in the preparation of the sample and the measurement method. During the debugging process, avoid the increase of repeatability uncertainty due to uneven sampling or unbalanced debugging; in addition, the proficiency of personnel operation is also an important factor leading to repeatability uncertainty, so in the quality inspection work, inspectors must A thorough understanding of standards and equipment, and on-the-job training are effective means to ensure the quality of calorific value testing data.
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