Several problems to be paid attention to when measuring the moisture of rice by drying method
Drying method to determine rice moisture is the most classic and most accurate method, often used as the basis for rice milling machine proofreading. GB5497 [1] specifies four kinds of measuring methods, among which the equipment and operation required for the fixed temperature drying method are relatively simple, which is the preferred method for most users. The author realized in the actual operation that some small operation details, such as inadvertent attention, often affect the accuracy of the measurement results. I will present some experiences for discussion and reference.
1 Problems to be aware of during sample preparation
1.1 Sample pulverization When smashing rice samples, according to the requirements of GB5497, there should be 90% through the 1.5mm round sieve of the rice milling machine , but since the husk is more difficult to break than the rice grains, the majority of the residue on the sieve is the chaff, and Due to the light weight of the chaff, when the residue that has not passed through the 1.5mm round sieve is ground into the pulverized sample by grinding with a mortar, uneven agitation tends to occur, and when the rice husks and rice grains have water content. When the difference is large (in most cases, the fresh rice husks have lower water content than the rice grains), the urban measurement results will be poorly reproducible.
1.2 Sample weight When using the fixed temperature and drying method to determine the water content of rice, the weight of the sample directly affects the accuracy of the measurement. For this reason, GB5497 specifies the sample weight as “aluminum bottom area × 0.126â€, but in actual operation It is impossible, and it is not necessary to control the weight of each sample very accurately in a very small range. According to experience, the sample weight can obtain accurate results within the range of ±15% of the above formula, for example: 4.5cm in diameter. Aluminum box, the sample weight can be 2 ± 0.3g (that is, 1.7-2.3g).
2 Determination of problems to be noted during operation
2.1 The number of samples processed at one time The test proved that the same sample was placed in different positions in the same oven, and the measurement results were very different. Therefore, GB5497 requires “put the sample on the baking net around the thermometer in the ovenâ€. After the test, it is necessary to achieve the measurement accuracy requirement of the double test result error within 0.2%. When the oven with the air blowing device is 44cm×34cm, and the aluminum box is 4.5cm high and 2.0cm high, each time Up to 9 samples can be baked. The samples should be arranged in a 3×3 rectangle and placed around the thermometer. The number of samples in the oven without the air blower can be more appropriate, but the area occupied by the sample should not exceed the area of ​​the drying net. 50%.
2.2 Weighing time after drying The sample GB5497 is required to be cooled in the desiccator to room temperature and then weighed. The test results show that, under normal circumstances, the time after the sample is dried and the dryer is shorter than 20 min or longer than 6 h affects the accuracy. The optimum time for weighing after drying is within 30 min to 4 h.
2.3 Climate impact test results show that when the rice moisture is detected by the drying method, the relative humidity of the air has an influence on the measurement results, especially in summer, so if the difference of water content of different samples is compared, it is best to at the same time. The drying measurement is carried out. When the measurement results at different times are compared with each other, the influence of climatic factors should be considered. More information: Rice Mill http://