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a) The volume of the test product (W×D×H) shall not exceed 20% of the effective working space of the test box (20% is recommended). It is recommended to use no more than 10% for products that generate heat during the test.
b) The ratio of the windward area of ​​the test product to the total area of ​​the test chamber on the section is not more than (35 to 50)% (35% is recommended).
c) The distance between the outer surface of the test product and the wall of the test chamber shall be at least 100~150mm (150mm is recommended).
The above three points are actually interdependent and unified. Taking a 1 cubic meter cube box as an example, the area ratio is 1: (0.35 to 0.5), which corresponds to a volume ratio of 1: (0.207 to 0.354). The ratio of the volume to the volume of 100 to 150 mm from the wall of the tank is 1: (0.343 to 0.512).
In summary, the above three points stipulate that the working chamber volume of the climatic environment test chamber should be at least 3 to 5 times the outer volume of the tested product. The reasons for this provision are as follows:
1) After the test piece is placed in the box, the smooth passage is occupied, and the narrowing of the passage will result in an increase in the flow rate of the air flow. Accelerate the heat exchange between the gas stream and the test piece. 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. Heat transfer. When the average wind speed in the test chamber is vacant at 0.6-0.8 m/s and does not exceed 1 m/s, the wind speed of the flow field may increase when the space and area ratio specified by the two requirements of a) and b) are met. ～100)%, the average zui high wind speed is (1 to 1.7) m/s. Meet the requirements of the standard. If the volume of the test piece or the wind-breaking area is increased without restriction in the test, the airflow velocity will increase to exceed the high wind speed specified in the test standard during the actual test, and the validity of the test results will be suspected.
2) The environmental parameters of the climate chamber working chamber (such as temperature, humidity, salt spray sedimentation rate, etc.) are the results of the test under no-load conditions. Once the test piece is placed, the working environment of the test chamber is The uniformity of the parameters will have an effect, and the greater the space occupied by the test piece, 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, the two requirements a) and b) must be met as much as possible to ensure the uniformity of the environmental parameters around the product under test.
3) 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 2~3 °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, therefore, the distance from the tank wall The space within the distance (100 to 150 mm) is an unusable space.
2. Selection of 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 have -80 to +130 ° C, -60 to +130 ° C, -40 to +130 ° C, and high temperatures to 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. The available studio volume is also smaller. 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 requirements of the cabinet, the higher the manufacturing cost of the cabinet.
3, the choice of humidity range The humidity indicators given by the environmental test chambers at home and abroad are mostly 20 ~ 98% RH or 30 ~ 98% RH, if the damp heat test box does not have a dehumidification system, the humidity range is 60 ~ 98%, this category The test chamber can only be tested for high humidity, but it is much cheaper. It is worth noting that the corresponding temperature range should be indicated after the humidity indicator, or the low dew point temperature of zui should be given. Because the relative humidity is directly related to the temperature, for the same absolute moisture content, the higher the temperature, the smaller the relative humidity, such as the absolute moisture content of 5g / Kg (refers to 5 grams of water vapor in 1 kg of dry air) ), when the temperature is 29 ° C, the relative humidity is 20% RH, the temperature is 6 ° C, the relative humidity is 90% RH, when the temperature drops below 4 ° C, the relative humidity exceeds 100%, condensation will appear in the box phenomenon.
To achieve high temperature and high humidity, it is only necessary to spray water vapor or atomized water droplets into the air of the cabinet to humidify. Low temperature and low humidity are relatively difficult to control because the absolute moisture content at this time is very low, and sometimes it is much lower than the absolute moisture content in the atmosphere. It is necessary to dehumidify the air flowing in the tank to make the air dry. At present, most of the temperature and humidity chambers at home and abroad use the principle of refrigeration and dehumidification. A set of cooling tubes is added to the air pre-conditioning room of the cabinet. When the humid air passes through the cold pipe, its relative humidity will reach 100% RH, and the air will become drier due to air saturation on the light pipe. This dehumidification method can theoretically reach a dew point temperature below zero, but when the surface temperature of the cold spot reaches 0 ° C, the water droplets on the surface of the light pipe will freeze, thereby affecting the heat exchange on the surface of the light pipe, and the dehumidification ability is lowered. And because the box can not be absolutely sealed, the humid air in the atmosphere will penetrate into the box, so that the dew point temperature will rise. On the other hand, the humid air flowing between the light pipes is saturated with water at the moment of contact with the light pipe (cold spot), so that the dehumidification method is difficult to make the dew point temperature in the case below 0 °C. The actual zui low dew point temperature is 5 to 7 °C. The dew point temperature of 5 ° C corresponds to an absolute moisture content of 0.0055 g/Kg, and the temperature corresponding to a relative humidity of 20% RH is 30 °C. If the required temperature is 20 ° C and the relative humidity reaches 20% RH, the dew point temperature is -3 ° C at this time. It is very difficult to dehumidify by means of cooling, and it must be achieved by using an air drying system.
4. Selection of control method The temperature and humidity 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 circulations. The test chamber has the ability to complete the test process according to the preset curve, and can be accurate within the range of Zui's large temperature rising and cooling rate capabilities. By controlling the rate of temperature rise and temperature drop, the temperature rise and temperature drop rates 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.
5. Selection of variable temperature rate The ordinary high and low temperature test chamber has no index of cooling rate. The time from ambient temperature drop to nominal zui low temperature is generally 90-120 min. 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 temperature range of the rapid temperature change test chamber of various specifications and speeds is generally the same, that is, -60～+130°C, but the temperature change range of the evaluation cooling speed is not the same. According to the different test requirements of the test, the rapid temperature change The test chamber has a temperature range of -55 to +80 ° C, and some are -40 to +80 ° C.
There are two ways to change the temperature change rate of the rapid temperature change 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 high temperature of zui and the low temperature of zui in the temperature range of the test chamber. 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 test chamber, the difficulty of ensuring the linear temperature rise and fall speed is the key to the cooling rate of the test chamber in a 5 minute period after the cooling stage Zui. From a certain perspective, the linear temperature rise and fall (average speed every 5 minutes) is more scientific. Therefore, the test equipment has the two parameters of the average average temperature rise and fall speed and the linear temperature rise and fall speed (average speed per 5 minutes). In general, the linear temperature rise and fall (average speed per 5 minutes) is 1/2 of the average temperature rise and fall of the whole process.
6. According to the relevant standards of wind speed, 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 of the flow of the test piece and the flow inside the box. Exchange, 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 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.
7. Temperature fluctuations 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.
8. Temperature field uniformity In order to more accurately simulate the actual environmental conditions suffered by the product in nature, it is necessary to ensure that the periphery of the tested product is at the same temperature and environmental conditions in the environmental test. For this reason, the temperature in the test chamber must be Gradient and temperature fluctuations are limited. 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 zui The value is 2.2 ° C (test sample does not work).
9. Accuracy of humidity control Most of the humidity measured in the environmental test chamber is dry and wet ball method, the manufacturing standard GB10586 of environmental test equipment, the relative humidity deviation should be ±23% RH. In order to meet the requirements of humidity control accuracy, the temperature control accuracy of the humidity test chamber is high, and the temperature fluctuation is generally less than ±0.2 °C. Otherwise it is difficult to meet the requirements of humidity control accuracy.
10. Cooling method Select the test box. If there is a refrigeration system, the cooling system needs to be cooled. The test chamber is available in both air-cooled and water-cooled versions.
How to identify a good constant temperature and humidity box
1. Selection of volume When the test product (components, components, components or complete machine) is placed in the climatic environment box 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 climate chamber works. The following points should be observed between the dimensions and the dimensions of the product to be tested: