Key points when choosing high and low temperature boxes
1. Volume selector
When introducing the tested product (components, components, parts or complete machine) into the environmental chamber for testing, to ensure that the surrounding atmosphere of the tested product can meet the environmental test conditions specified in the test specification, and that the following rules are observed between the working size of the climate chamber and the external dimensions of the tested product:
a)The tested product The volume (W×D×H) must not exceed (20-35)% of the effective working space of the test chamber. It is recommended not to use more than 10% for products that generate heat during the test.
b) The ratio of the cross-sectional area facing the wind of the tested product to the total area of the working chamber of the test chamber on this cross-section is not greater than (35-50)%.
c) The distance between the surface of the product to be tested and the wall of the test chamber, keep a distance of at least 100~150mm.
The above three rules are in fact interdependent and unified. Taking a cubic box of 1 cubic meter as an example, the area ratio is 1: (0.35-0.5), which corresponds to a volume ratio of 1:(0.207-0.354) 100~150mm from the box wall corresponds to a volume ratio of 1:(0.343~0.512)
2. Selecting the temperature range
Currently testing the range of external temperature chambers is generally -73 ～+177℃ or -70～+180℃ Most domestic manufacturers generally have temperatures ranging from -80 to +130℃, from -60 to +130℃, from -40 to +130℃, and sometimes untilto 150 °C. These temperature ranges can normally meet the temperature test requirements of most military and civilian products in China. If there are no special requirements, such as products installed near heat sources such as engines, the upper temperature limit should not be blindly increased. This is because the higher the upper limit temperature, the greater the temperature difference between the inside and the outside of the box and the worse the uniformity of the flow field inside the box. The available studio volume is also smaller. On the other hand, the higher the upper limit temperature value, the higher the heat resistance requirements for the insulating material (such as glass wool, etc.) in the intermediate layer of the box wall. The requirement for the airtightness of the box body is also higher, which increases the manufacturing costs of the box body.
3. Choice of control methods
There are two types of Temperature and humidity test chambers: constant test chamber and alternating test chamber.
normal height The low temperature test chamber generally refers to a constant high and low temperature test chamber, and its control method is: set a target temperature, and the test chamber has the ability to automatically adjust the temperature to the target temperature point The constant temperature and humidity test chamber is also similar, providing a target temperature and-humidity point on, the test box has the ability to automatically maintain the temperature at the target temperature and humidity point. The high and low temperature alternate test box has one or more high and low temperature alternation and cycle programs, and the test box has the Ability, the curve completes the testing process, and can accurately control the heating and cooling rate within the range of maximum heating and cooling rate capabilities, that is, the heating and cooling rate can be controlled according to the slope of the set curve. Likewise the high and low temperature alternating humidity test. The chamber also has a preset temperature and humidity curve and the ability to control according to the preset. Of course, the interchangeable test chamber has the function of a constant test chamber, but the manufacturing cost of the interchangeable test chamber is relatively high, since the interchangeable test chamber is equipped with an automatic curve recordingThe problem of switching on the refrigerator when the temperature in the working room is high must be equipped with a program control. Therefore, the price of the alternating test box is generally 20% higher than that of the constant test box. Above. Therefore, we should consider the needs of the test method as a starting point for seeking truth from facts, and choose a constant test chamber or an alternating test chamber.
4. Humidity range selection
Most moisture indicators declared by domestic and foreign environmental testing chambers are 20 -98% RH or 30-98% RH if test box for humidhte heat does not have a dehumidification system, the humidity range is 60-98%. This type of test box can only perform high humidity tests, but its price is high. It should be noted that after the humidity index, the corresponding temperature range should be indicated, or the minimum dew point temperature should be specified Since the relative humidity is directly related to the temperature, the higher the temperature at the same absolute humidity, the lower the relative humidity. The humidity is 5 g/kg (based on 5 g of water vapor in 1 kg of dry air), at a temperature of 29 °C the relative humidity is 20% RH, at a temperature of 6 °C C the relative humidity is 90% RH , when the temperature When the temperature falls below 4°C and the relative humidity exceeds 100%, condensation will occur in the cabinet.
To achieve high temperature and high humidity, you only need to spray water vapor or atomized water droplets into the air of the box, humidifying low temperature and low humidity are relatively difficult to control, because the absolute humidity at this time is very low, sometimes much lower than the absolute humidity in the atmosphere, and it is necessary to dehumidify the air flowing in the box to make the air dry.Current Most temperature and humidity cabinets apply the principle of cooling and dehumidification, which is to add a series of cooling light tubes in the air pre-conditioning space of the cabinet.When the humid air passes through the cold tubes, its relative humidity reaches 100%RH since the air-saturated dew condensation occurs on the light guideand the air is getting drier. This dehumidification method can theoretically reach sub-zero dew point temperature, but when the cold spot surface temperature reaches 0℃, the water droplets condensed on the surface of the light guide will freeze, which will affect the heat exchange on the surface of the light guide and reduce the dehumidifying performance, and because the box cannot be absolutely tight, moist air from the atmosphere penetrates into the box, causing the dew point temperature to rise, on the other hand, the air flowing between the light guides only reaches saturation at the moment of contact with the light guide (cold point) and excretes water vapor, therefore, with this dehumidification method, it is difficult to keep the dew point temperature in the box below 0 °C. The actual minimum dew point temperature is 5 ~ 7 °C. The dew point temperature of 5 °C corresponds to an absolute humidity of 0.0055 g/kg, and the temperaturer corresponds to a relative humidity of 20%RH of 30°C, when the temperature reaches 20°C and the relative humidity is 20%RH, the dew point temperature at that time is -3°C, dehumidification by cooling is very difficult, and air drying is required drying system can be realized.
5. Temperature change rate selection
Traditional high and low temperature test chambers do not have a cooldown rate indicator and the time to cool down from ambient temperature to the minimum nominal temperature is im Generally 90 ~ 120 min. The high and low temperature alternating testThe heating chamber and the high and low temperature wet heat alternating test chamber all have the requirement of temperature change speed, and the temperature change speed generally requires 1 ° C / min, and the speed can be adjusted within the range of this speed. The temperature change rate of the rapid temperature change test chamber is relatively fast, and the heating and cooling rate can reach 3 °C / min to 15 °C / min, and the heating and cooling rate can even reach more than 30 °C / min in some temperature ranges . The temperature range of the rapid temperature change test chamber with different specifications and speeds is generally the same, that is -60 to +130 ° C, but the temperature range of the evaluation cooling rate is not the same. According to different test requirements, the rapid temperature change test some temperature ranges of the box is as low as -55 up to +80°C, others at -40up to +80°C.
There are two indicators of the rate of temperature change of the test chamber for rapid temperature changes, one is the average rate of temperature rise and fall throughout the process and the other is the linear rate of temperature rise and fall (actually the average rate 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 within the variable temperature range of the test chamber. At present, the technical parameters of the temperature change rate specified by various foreign manufacturers of environmental testing equipment refer to the average rate of the whole process. The linear heating and KCooling rate refers to the rate of temperature change that can be guaranteed in any 5 minute period. In fact, for the rapid temperature change test chamber, the last section in the cooling section is the most difficult and critical section to ensure the linear heating and cooling speed. The cooling speed that the test chamber can achieve within 5 minutes. Some kind of angle science. Therefore, it is best if the test equipment has two parameters: the average heating and cooling rate throughout the process and the linear heating and cooling rate (average rate every 5 minutes). In general, the linear temperature rise and fall rate (average rate every 5 minutes) is half the average temperature rise and fall rate of the entire process.
6. Temperature fluctuations
Temperature variation is on relatively easy parameter to realize.The actual temperature variation of most test chambers of all manufacturers of environmental test equipment can be controlled within the range of ±0.3°C.
7. Wind Speed
Appropriate standards prescribe that the wind speed in the temperature - and humidity chamber in the implementation of von environmental tests should be less than 1.7 m/s, the smaller the wind speed, the better. Too high a wind speed accelerates the heat exchange between the surface of the test piece and the flowing air in the box, which is unfavorable for the authenticity of the test. However, in order to ensure uniformity in the working area of the test chamber, circulating air is required in the test chamber. However, for test chambers with rapid temperature change and test chambers with extensive environmental conditions such as temperature, humidity, vibration, etc., the flow speed of the circulating airflow in the chamber needs to be accelerated to track the temperature change rate, and the wind speed is usually 2~3m/s. Therefore, for different purposes, the wind speed limit is different.
8. Humidity Accuracy Check
Most of the humidity in the environmental testing chamber is measured by the wet and dry bulb method. The manufacturing standard for environmental testing equipment GB10586 requires the relative humidity deviation should be within ±23% RH.In order to meet the humidity control accuracy requirements, the temperature control accuracy of the humidity test chamber is relatively high, and the temperature fluctuation is generally less than ±0.2℃.Otherwise, it is difficult to to meet humidity control accuracy requirements.
9. Uniformity of the temperature field
In order to better simulate the actual environmental conditions to which the product will be exposed in nature, it is necessary to ensure that the environments of the tested product are on during the environmental test same temperature Under environmental conditions, temperature gradients and temperature fluctuations in the chamber must be limited.The general rules of the national military standard GJB150.1-86 for environmental test methods for military equipment clearly state that the temperature of the measuring system in the vicinity of the test sample must be within ± 2°C of the test temperature and the temperature should not exceed 1°C/m or the total maximum value. 2.2°C (the test sample does not work).
10. Cooling method selection
If the test chamber has a cooling system, will the cooling system needs cooling.There are two types of air cooling and water cooling in the test chamber.