Correct selection of high and low temperature test chamber volume

When the tested product (components, components, components or complete machine) is placed in the high and low temperature test chamber for testing, in order to ensure that the atmosphere around the test product can meet the environmental test conditions specified in the test specification, the working dimensions of the high and low temperature test chamber are The following rules should be followed between the dimensions of the product being tested:
1. The volume of the product to be tested (W × D × H) shall not exceed 20 to 35% of the effective working space of the test chamber. It is recommended to use no more than 10% for products that generate heat during the test.
2. The ratio of the windward sectional area of ​​the tested product to the total area of ​​the test chamber on the section is not more than 35 to 50%.
3. The distance between the outer surface of the tested product and the wall of the test chamber shall be at least 100-150 mm.
In summary, the above three points stipulate that the working chamber volume of the high and low temperature test chamber should be at least 3 to 5 times the outer volume of the tested product. The reasons are as follows:
â—† The tested product is placed in the box and squeezes the smooth passage. The narrowing of the channel will result in an increase in the flow rate of the airflow and accelerate the heat exchange between the airflow and the product being tested. This is inconsistent with the reproduction of environmental conditions, because in the relevant standards, the temperature flow around the test specimens in the test chamber should not exceed 1.7 m/s, so as to prevent the test specimens and the surrounding atmosphere from being produced. The heat transfer in the test chamber at no load is 0.6-0.8 m/s, not exceeding 1 m/s. When the space and area ratio specified in the above requirements are met, the wind speed of the flow field may increase (50-100). %, the average maximum wind speed is (1 ~ 1.7) m / s, meeting the requirements of the standard. If the volume of the tested product or the windward cross-sectional area is increased without restriction in the test, the airflow velocity will increase to exceed the maximum wind speed specified in the test standard during the actual test, and the validity of the test results will be suspected.
◆The accuracy index of the environmental parameters in the working chamber of the high and low temperature test chamber is the result of the test under no-load condition. Once the product to be tested is placed, the uniformity of the environmental parameters in the working chamber of the test box will be affected, and the product to be tested is occupied. The greater the space, the more serious this effect will be. The measured test data shows that the temperature difference between the windward and leeward sides in the flow field can reach 3-8 °C, and in severe cases it can be as large as 10 °C. Therefore, it is necessary to meet the above requirements as much as possible to ensure the uniformity of the environmental parameters around the product being tested.
◆ According to the principle of heat conduction, the temperature of the airflow near the tank wall is usually 2 to 3 °C from the center temperature of the flow field, and may reach 5 °C at the upper and lower limits of the high and low temperature. The temperature of the tank wall is different from the temperature of the flow field near the tank wall by 2 to 3 ° C (depending on the structure and material of the tank wall). The greater the difference between the test temperature and the external atmospheric environment, the greater the above temperature difference. The space within the distance of the wall (100 to 150 mm) is an unusable space.

Correct selection of control methods for high and low temperature test chambers

The high and low temperature test chambers have two conditions: constant test chamber and alternating test chamber. Ordinary high and low temperature test chambers generally refer to constant high and low temperature test chambers, which are controlled by setting a target temperature and the ability of the test chamber to automatically constant temperature to the target temperature point. The control method of the constant temperature and humidity test chamber is similar. Set a target temperature and humidity point. The test chamber has the ability to automatically adjust the temperature to the target temperature and humidity point. The high and low temperature alternating test chamber has the program of setting one or more high and low temperature changes and circulation. The test chamber has the ability to complete the test process according to the preset curve, and can accurately control within the range of maximum heating and cooling rate capability. The rate of temperature rise and temperature drop, that is, the temperature rise and temperature drop rate can be controlled according to the slope of the set curve. Similarly, the high and low temperature alternating heat and humidity test chamber also has a preset temperature and humidity curve, and the ability to control according to presets. Of course, the alternating test chambers all have the function of a constant test chamber, but the manufacturing cost of the alternating test chamber is higher, because the alternating test chamber needs to be equipped with a curve automatic recording device, a program control device, and the test chamber must be solved in the working chamber. When the temperature is high, the refrigerator is turned on. Therefore, the price of the alternating test chamber is generally 20% higher than the price of the constant test chamber. Therefore, we should be realistic and take the test method as the starting point, choose a constant test box or an alternating test chamber.

Correct selection of high and low temperature test chamber temperature range

At present, the range of foreign temperature test chambers is generally -73 to +177 ° C, or -70 to +180 ° C. Most domestic manufacturers generally -70 ~ +150 ° C, -60 ~ +150 ° C, -40 ~ + 150 ° C. These temperature ranges usually meet the needs of most military and civilian products for temperature testing in the country. Unless there are special needs, such as products with heat sources close to the engine, do not blindly increase the upper temperature limit. Because the higher the upper limit temperature, the greater the temperature difference between the inside and outside of the tank, the worse the uniformity of the flow field inside the tank, and the smaller the available working chamber volume. On the other hand, the higher the upper limit temperature value, the higher the heat resistance requirement for the heat insulating material (such as glass wool, etc.) in the tank wall interlayer, the higher the sealing property of the box, and the higher the manufacturing cost of the box.

Correct selection of the temperature change rate of the high and low temperature test chamber

◆ Ordinary high and low temperature test chambers have no index of cooling rate. The time from ambient temperature to nominal minimum temperature is generally 90 to 120 minutes. The high and low temperature alternating test chamber and the high and low temperature alternating wet heat test chamber all have the requirements of variable temperature speed, and the temperature change rate generally requires 1 ° C / min, and the speed is adjustable within the range of this rate. The rapid temperature change test chamber has a faster temperature change rate, and the temperature rise and temperature drop rate can reach 3 ° C / min ~ 15 ° C / min, in some temperature ranges, the temperature rise and temperature drop rate can even reach 30 ° C / min or more.
◆The rapid temperature change of various specifications and speeds The temperature range of the high and low temperature test chambers is generally the same, namely -60~+130°C, but the variable temperature range of the evaluation cooling speed is not the same. According to the different test requirements of the test, it is fast. The temperature change range of the high and low temperature test chambers is -55 to +80 °C, and some are -40 to +80 °C.
â—† There are two kinds of formulas for the variable temperature rate of the high temperature and low temperature test chamber. One is the average temperature rise and fall speed, and the other is the linear temperature rise and fall speed (actually the average speed every 5 minutes). The average speed of the whole process refers to the ratio of the difference between the highest temperature and the lowest temperature and the time in the temperature range of the test box. At present, the technical parameters of the variable temperature rate provided by the foreign environmental test equipment manufacturers refer to the whole average rate. The linear temperature rise and fall speed refers to the variable temperature rate that can be guaranteed in any 5 min period. In fact, for the rapid temperature change high and low temperature test chamber, the most difficult and critical part of ensuring the linear temperature rise and fall is the cooling rate that can be achieved by the test chamber during the last 5 min period of the cooling section. Therefore, the test equipment preferably has two parameters: the average average temperature rise and fall speed and the linear temperature rise and fall speed (average speed per 5 minutes). Generally speaking, the linear temperature rise and fall speed (average speed per 5 minutes) is 1/1 of the average average temperature rise and fall speed. 2.

Wind speed of high and low temperature test chamber

According to the relevant standards, the wind speed in the temperature and humidity chamber should be less than 1.7m/s when conducting environmental tests. For the test itself, the wind speed is as small as possible. The wind speed is too high to accelerate the heat exchange between the surface of the test piece and the flowing airflow in the tank. The authenticity of the test is unfavorable. However, in order to ensure the uniformity of the test chamber working chamber, it is necessary to have circulating air in the test chamber. However, the rapid temperature change high and low temperature test chamber and the multi-factor comprehensive environmental test chamber such as temperature, humidity and vibration, in order to pursue the temperature change rate, the flow velocity of the circulating airflow in the tank must be accelerated, and the wind speed is usually 2 to 3 m/s. Therefore, the wind speed limit is different for different purposes.

Temperature uniformity of high and low temperature test chamber

In order to more accurately simulate the actual environmental conditions that the product suffers in nature, it is necessary to ensure that the periphery of the tested product is in the same temperature environment condition in the environmental test. For this reason, the temperature gradient and temperature fluctuation in the test chamber must be Limit it. In the national military standard GJB150.1-86 military equipment environmental test method general provisions clearly stipulate "the temperature of the measurement system near the test sample should be within ± 2 ° C of the test temperature, the temperature of the work does not exceed 1 ° C / m or the total maximum It is 2.2 ° C (test sample does not work)".

Temperature fluctuation of high and low temperature test chamber

Temperature fluctuations are a relatively easy to implement parameter. The actual temperature fluctuations of most test chambers produced by all environmental test equipment manufacturers can be controlled within ±0.3 °C.

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