Currently, for all types of calorimeters available, assuming the instrument is qualified and the operation is correct, high-accuracy test results can be obtained as long as the following points are noted before the formal test.
Firstly, regarding the equipment itself, the quality of each component directly affects the obtained data. The more precise the instrument's performance and the higher the level of automation, the less error caused by human operation can be minimized. Specifically: the oxygen bomb should not be affected by the high temperatures and corrosive products generated during combustion, thus avoiding thermal effects; the propeller-type stirrer should remain stable. The stirring efficiency should ensure that the time from ignition to the endpoint in the heat capacity calibration does not exceed 10 minutes, while avoiding excessive stirring heat; accurate thermometer measurement is of particular importance; the combustion vessel should ensure complete combustion of the sample without being corroded or generating thermal effects; the outer barrel temperature is affected by the number of tests, thus increasing the uncertainty of heat capacity compared to instruments where the outer barrel temperature is not affected by the number of tests.
Secondly, attention should be paid to every detail in actual operation to avoid systematic and random errors caused by human mistakes.
1. Ambient Temperature Equilibrium. Calorimeters calculate the calorific value of a substance by reading the temperature rise, with an accuracy of one ten-thousandth of a degree Celsius. Therefore, equilibration between the instrument and the environment before measurement is crucial. First, the laboratory should be located in a separate room, and other experiments should not be conducted simultaneously in the same room. Strong heat sources and fans should be avoided. Ideally, the laboratory should face north to avoid direct sunlight. Second, maintain a stable ambient temperature. There should be no strong air convection indoors, and doors and windows should be avoided during the experiment. If possible, air conditioning should be turned on before testing to equilibrate the temperature between the equipment and the environment. The room temperature should be kept as constant as possible, generally within the range of 15℃ to 35℃. When the ambient temperature stabilizes at a certain point, and the temperature of the outer casing is fully consistent with the ambient temperature, the heat capacity of the calorimeter's measuring system is determined, yielding test results with good reproducibility and accuracy.
2. When preparing samples, ensure that the analytical base sample is dry and fine. If the sample is too wet or the coal sample is too coarse, it may lead to incomplete combustion of the sample, resulting in lower test results and invalidation.
3. For samples with poor coal quality, low volatile matter, easy splashing during combustion, or high moisture content, a suitable proportion of standard coal should be added, or the weighed sample should be wrapped in lens paper of known mass and calorific value and pressed firmly by hand before being placed in the combustion dish. For samples with incomplete combustion, an asbestos pad or asbestos wool can be placed at the bottom of the combustion dish as a liner. Quartz combustion dishes do not require any liner. If combustion is still incomplete even with a liner, the oxygen pressure can be increased to 3.0 MPa–3.2 MPa.
4. During oxygen bomb charging, ensure sufficient oxygen pressure for complete combustion of the sample. First, check that the oxygen pressure gauge is intact and sensitive, and that the indicated pressure is correct. The pressure must be increased slowly during charging, and maintained for 0.5 to 1 minute after reaching the specified pressure. If the oxygen pressure in the cylinder drops below 5.0 MPa, the charging time should be extended accordingly.
The oxygenation pressure must not be too low or too high.
5. The oxygen bomb must be kept completely airtight during the experiment. This is to ensure both safety and data accuracy. If air bubbles are observed in the water, it indicates a leak. New oxygen bombs and those with newly replaced parts should undergo a 15.0 MPa hydrostatic test before use. Furthermore, structures related to the oxygen bomb's strength should be frequently inspected, such as the threads of the cup body and connecting rings, the oxygen valve, and the connection between the electrodes and the bomb cover. If significant wear or loosening is found, repairs should be made, and the bomb should be retested before use. Additionally, the oxygen bomb should be periodically inspected.
Perform a hydrostatic test, and after each hydrostatic test, the oxygen bomb must not be used for more than one year.
6. Coal samples with low volatility and high moisture content often experience incomplete combustion without treatment, leading to significant deviations in results. Therefore, after each test, it is necessary to check whether there is any unburned coal in the combustion pan and oxygen bomb. If so, the samples should be treated and measured again.
7. Instruments should be sent to metrology authorities for verification periodically, and the correction values in the verification certificate should be calibrated as necessary.
