Explain the reasons for the temperature uniformity deviation of the high and low temperature alternating heat and humidity test chamber. High and low temperature alternating heat and humidity test chamber. It is mainly used for high and low temperature test chambers and wet heat test chambers. Mainly for the aviation, aerospace, automotive, motorcycle, military, petroleum, chemical, shipbuilding, electronics, communications and other scientific research and production units to provide a constant temperature or constant temperature climate for users to the whole machine (or components), electrical appliances , instruments, materials, coatings, coatings, etc. for the corresponding high and low temperature gradient test, damp heat test. Cold resistance test, damp heat storage, etc., in order to analyze and evaluate the performance and adaptability of the sample under the proposed environmental conditions. It can more accurately reflect the adaptability of electric and electronic products to high and low temperature and hot and humid environment changes during actual use, and the rapid collapse of exposed products is essential for the whole process of new product development, prototype test and product qualification test. Test method.
1. Brief description The high and low temperature environmental test chamber is generally composed of the test chamber, the control system, the refrigeration system, the heating/humidification system, the dehumidification system, and the air duct system. The temperature in the test chamber is determined by the evaporator, heater, humidifier, dehumidifier, etc. in the duct and the operating state of the fan. Whether the test temperature of the working space is uniform is a very important problem, because if the temperature unevenness exceeds the allowable deviation, the test conditions of the test sample are not consistent, and some samples or parts of the sample have high test temperature. Some are low, resulting in problems of trial or under-test, making the test reproducible, the data obtained from the test is not reliable, and ultimately the failure of the test.
2 Standard Specification for Temperature Uniformity and Measurement Method JJF101-2003 "Environmental Test Equipment Temperature and Humidity Calibration Specification" defines the temperature uniformity of the test chamber under steady state, and the working space is at a certain instantaneous temperature between test points. Difference.
Calibration method To calibrate the temperature and humidity points, generally select the lower limit, upper limit and middle point of the equipment use range, or select the actual temperature and humidity points according to the user's needs. The position of the test point shall be placed on the three calibration surfaces in the equipment working chamber, referred to as the upper, middle and third floors, and the middle layer, which is the calibration working surface through the parallel lower bottom surface of the geometric center of the studio, the test point and the inner wall of the working chamber. The distance is not less than 1/10 of the length of each side. When the wind tunnel is encountered, the distance is increased, but it cannot be 500 mm. If the device has a sample holder or sample car, the lower test point can be placed 10 mm above the sample holder or sample car. The number of test points: (1) When the equipment volume is less than 2m3, the temperature side pilot is 9 and the humidity side pilot is 3, and the O point is located in the middle geometric center. The humidity 涮 pilot is 4, and E, O, and tw are located at the geometric centers of the upper, middle, and lower layers, respectively.
(3). (When the working volume is greater than 50m3, the side pilot can be increased).
The calculation of the temperature uniformity: the arithmetic mean value of the difference between the high temperature and the lowest temperature in each test in the environmental test equipment under steady state, within 3omin (tested every 2 minutes). ,
3 Reasons for uneven temperature (1) The structure of the high-low temperature alternating heat and humidity test chamber largely affects the temperature uniformity of the working space, and it is difficult to achieve complete symmetry due to the structure, thereby adversely affecting temperature uniformity. The commonly used structure is that the front door is in front and the air-conditioned room is at the back of the box. This structure has good bilateral symmetry, and it is easy to reach the left and right temperature uniformity, but the structure and the lower asymmetry are completely different before and after. The unevenness of the working space temperature is generated. .
(2) There is heat loss due to the heat conduction effect of the tank wall (heat box) or leaky (low temperature box). In order to compensate for the heat loss, there is bound to be a difference in the supply air temperature. The air supply temperature of the high temperature box is higher than that in the box. The working temperature, the air supply temperature of the low temperature box is lower than the working temperature inside the box. Therefore, the temperature difference of the supplied air also causes the temperature in the upper chamber to be uneven.
4 Improve the uniformity of the test chamber to reduce the measurement of the signal difference (1) reduce the supply air temperature difference and increase the air supply volume The temperature difference between the supply air and the amount of air supply depends on the cooling load (for the low temperature test chamber) or the heat load ( The size of the high temperature test chamber). Under a certain constant temperature state, the density of the thousand air and the specific pressure of the air are constant, and the air supply volume is inversely proportional to the temperature difference of the air supply. In order to improve the temperature uniformity in the tank and reduce the temperature fluctuation, it is necessary to reduce the air supply. The temperature difference increases the amount of air supplied. However, if the temperature difference of the supply air is too small, the amount of air supplied will be too large, thereby increasing the operating cost. Or the wind speed in the box is too high, when the test method requires wind speed, it does not meet the requirements of the test method standard. Generally, the temperature difference of the supply air may be set at t•1-3°C.
(2) Improve the rationality of the design of the test box structure to achieve structural symmetry, such as designing the left and right air ducts; if two fans are used, one left-handed one right-handed is used to make the air uniform. If necessary, install the air distribution plate at the air outlet to adjust the wind direction to make the temperature inside the box uniform; do the sealing of the box to prevent local air leakage; select the thermal insulation material with excellent performance to ensure sufficient insulation thickness to reduce heat Loss; the connection between the inner bladder and the outer casing is thermally insulated to reduce local latent heat.
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