Temperature and humidity test chambers include constant temperature test chambers and alternating test chambers. Ordinary high and low temperature test chambers generally refer to constant high and low temperature test chambers.
Its control method is: to set the target temperature, and the inspection room has the ability to automatically stabilize the temperature to the target temperature point. The control method of the constant temperature and humidity test chamber is also similar. Set target temperature and humidity points. Temperature and humidity are automatically stabilized in the test chamber. The high and low temperature alternating test chamber has a program for setting one or more high and low temperature changes and cycles. The test chamber has the ability to complete the test process according to the preset curve, and can accurately control the temperature rise and deceleration within the maximum temperature rise and deceleration range, that is, the temperature rise and deceleration can be controlled according to the slope of the set curve.
Similarly, the high and low temperature alternating damp heat test chamber also has the ability to preset the temperature and humidity curve and control it according to the preset. Of course, the alternating test chamber has the function of the constant test chamber, but the manufacturing cost of the alternating test chamber is relatively high. Because the West Alternating Test Chamber needs to be equipped with an automatic curve recording device and a program controller, it is necessary to solve the problem of starting the refrigerator when the temperature in the chamber is high. Therefore, the price of the alternating test chamber is generally more than 20% higher than that of the constant test chamber. Therefore, we should seek truth from facts and choose a constant current test chamber or an alternating test chamber based on the needs of the test method.
2. Temperature change rate
Ordinary high and low temperature test chambers do not have a cooling rate index, and the time from ambient temperature to the nominal minimum temperature is generally 90 to 120 minutes. High and low temperature alternating test chambers and high and low temperature alternating damp heat test chambers have temperature change speed requirements. 1°C/min, the speed can be adjusted within this speed range. The temperature of the rapid temperature change test chamber changes quickly. The temperature rise and fall rate can reach 3°C/min~15°C/min, and the temperature rise and fall rate of some temperature sections can even reach more than 30°C/min.
The temperature range of rapid temperature change test chambers of various specifications and speeds is generally the same, namely- 60~+130℃, but the temperature range for checking the cooling rate is different. According to different test requirements, the temperature range of the rapid temperature change test chamber is -55~+80℃, and some are 40~+80℃. There are two ways to describe the temperature change rate of the rapid temperature change test chamber, one is the average temperature rise and fall speed of the whole process, and the other is the linear temperature rise and fall speed (actually the average speed every 5 minutes).
The total average speed refers to the ratio of the difference between the highest temperature and the lowest temperature and the time within the temperature range of the test chamber. At present, the technical parameters of temperature change rate provided by foreign environmental test equipment manufacturers all refer to the average change rate of the whole process. The linear heating and cooling rate refers to the temperature change rate that can be guaranteed every 5 minutes.
In fact, for the rapid temperature change test chamber, the most difficult and critical part to ensure the linear heating and falling speed is the cooling rate that the test chamber can achieve in the last 5 minutes of the cooling section. From a certain point of view, the linear heating and cooling rate (average rate every 5 minutes) is more scientific. Therefore, it is best for the test equipment to have two parameters: the average heating and cooling speed in the whole process and the linear heating and cooling speed (average speed every 5 minutes). Generally speaking, the linear heating and cooling speed (average speed every 5 minutes) is 1/2 of the overall average heating and cooling speed.
3. Wind speed
During an environmental test, the wind speed should not exceed 1.7m/s in the temperature and humidity test chamber. For the test itself, the lower the wind speed, the better. Excessive wind speed will accelerate the heat exchange between the surface of the test piece and the airflow in the box, which is not conducive to the authenticity of the test. However, in order to ensure the uniformity of the test chamber, there must be circulating air in the test chamber. However, for rapid temperature change test chambers and multi-factor comprehensive environmental test chambers such as temperature, humidity, and vibration, in order to pursue the temperature change rate, the flow rate of circulating air in the test chamber must be accelerated, and the wind speed is usually 2~3 m/s.
4. Temperature fluctuation
It is relatively easy to achieve temperature fluctuation. In fact, most of the test chambers produced by various environmental test equipment manufacturers can control the temperature fluctuation within ±0.3°C. Uniformity of temperature field In order to more accurately simulate the actual environmental conditions of the product in nature, it is necessary to ensure that the surroundings are in the same temperature environment. Therefore, it is very important to set up test temperature gradients and temperature fluctuations that meet the standards.
5. Humidity control
Generally, wet and dry bulb methods are used to measure humidity in environmental test chambers. Environmental test equipment manufacturing standards require a humidity deviation of ±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 humidity control accuracy requirements.
6. Cool down
If the test chamber is equipped with a refrigeration system, the refrigeration system needs to be cooled. There are two types of test chambers: air-cooled and water-cooled.
7. Volume selection
When the tested product (components, components, parts, or complete machine) is placed in the climate chamber for testing. In order to ensure that the ambient atmosphere of the tested product can meet the test conditions. The working dimensions of the climate chamber and the external dimensions of the product to be tested shall meet the following requirements:
a) The volume of the product under test (width x depth x height) shall not exceed (20~35)% (recommended) 20% of the effective working space of the test chamber. It is recommended to choose no more than 10% heat-generating products in the test.
b) The ratio of the windward cross-sectional area of the product under test to the total area of the test chamber on the cross-section should not be greater than (35-50)% (recommended 35%).
c) The distance between the outer surface of the tested product and the wall of the test chamber is at least 100-150mm (150mm is recommended).
The above three points are interdependent and unified. Taking a cubic meter cubic room as an example, an area ratio of 1: (0.35-0.5) is equivalent to a volume ratio of 1: (0.207-0354). From 100-150mm away from the tank wall, the volume ratio is 1: (0.343-0.512 On the basis of the above three points, the working chamber of the climate environment test chamber should be at least 3-5 times the external volume of the product being tested. The reasons are as follows:
1)As soon as the test piece is placed in the test chamber, the smooth channel is occupied, causing the air velocity to increase. Accelerates the heat exchange between the airflow and the quilt specimen. In contrast to the reproduced environmental conditions, the relevant standards stipulate that the air velocity around the test sample in the test chamber must not exceed 1.7m/s to prevent unrealistic heat transfer between the test sample and the surrounding atmosphere during the temperature environment test. When no load, the average wind speed in the test chamber is 0.6-0.8m/s, not more than 1m/s. When the space area ratio specified in a) and b) is met, the wind speed of the flow field may increase by (50-100)%, and the average maximum wind speed is (1-1.7) m/s. meet the requirements of the standard. If the volume of the specimen or the increase of the windward cross-sectional area is not restricted during the test,
2) The accuracy indicators of the environmental parameters (such as temperature, humidity, salt mist sedimentation rate, etc.) in the working room of the climate chamber are all in the no-load state
Once the specimen is placed in the test chamber, the test results will have an impact on the uniformity of the environmental parameters in the working chamber of the test chamber. The larger the space occupied by the specimen, the more serious the impact. The actual test data show that the temperature difference between the windward side and the leeward side of the flow field can reach 3-8°C, and in severe cases exceed 10°C. Therefore, the requirements of a) and b) must be met as much as possible to ensure the consistency of the ambient parameters of the tested product.
3) According to the principle of heat conduction, the temperature of the airflow near the tank wall is usually 2-3°C different from the temperature of the center of the flow field
The upper and lower limits can reach 5°C. The difference between the tank wall temperature and the flow field temperature near the tank wall is 2~3°C (depending on the tank wall structure and material). The temperature difference between the test temperature and the external atmospheric environment increases with the difference between the test temperature and the external atmospheric environment. Therefore, the space within the range of 100-150mm from the tank wall cannot be used.
8. Selection of temperature range
At present, the range of temperature test chambers in foreign countries is generally -73～+177℃ or -70～+180℃. Most domestic manufacturers generally -80 ~ +130 ℃, -60 ~ +130 ℃, -40 ~ +130 ℃. There are also high temperatures as high as 150°C. These temperature ranges can basically meet the temperature testing needs of most domestic military and civilian products. Unless there is a special need, such as the product being installed near a heat source such as an engine, the upper limit of the temperature cannot be raised blindly. Because the higher the upper limit temperature, the greater the temperature difference between the inside and outside of the cavity, and the worse the uniformity of the flow field in the cavity. Less studio space available. On the other hand, the higher the upper limit temperature, the higher the heat resistance requirements for the insulation material (such as glass wool) of the cavity wall interlayer.
9. Selection of humidity range
Domestic and foreign environmental test chambers usually give humidity indicators between 20-98%RH and 30-98%RH. In humid heat test chambers without dehumidification systems, the humidity range is 60-98%. This kind of test box can only do high humidity tests, but the price is much lower. It should be noted that the corresponding temperature range or minimum dew point temperature should be indicated after the humidity index. Because relative humidity is directly related to temperature, for the same absolute water content, the higher the temperature, the lower the relative humidity. For example, the absolute moisture content is 5 g/kg (referring to 5 g of water vapor in 1 kg of dry air).
When the temperature is 29°C, the relative humidity is 20%RH; when the temperature is 6°C, the relative humidity is 90%RH. When the temperature drops below 4°C and the relative humidity exceeds 100%, condensation will appear in the cabinet. To achieve high temperature and high humidity, steam or atomized water droplets can only be sprayed into the air of the cabinet for humidification. Low temperature and low humidity are more difficult to control, because the absolute moisture content at this time is very low, sometimes even much lower than the absolute moisture content in the atmosphere. To make the air in the chamber dry, it must be dehumidified.
Currently, most temperature and humidity test chambers at home and abroad use the principle of refrigeration and dehumidification, which involves adding a set of refrigeration light pipes to the air-conditioning room. As humid air passes through the cold pipe, its relative humidity will reach 100%. The extended air becomes drier as the air saturates and condenses on the light pipe. In theory, this method of dehumidification can bring the dew point temperature below zero. However, when the surface temperature of the cold spot reaches 0°C, the water droplets on the surface of the light pipe will freeze, which will affect the heat exchange on the surface of the light pipe and reduce the dehumidification capacity.
In addition, since the cavity is not completely sealed, humid air from the atmosphere will seep into the cavity, raising the dew point temperature. On the other hand, the moist air flowing between the light pipes is only saturated at the moment of contact with the light pipes (cold spot), and the water vapor is separated. Therefore, this dehumidification method is difficult to keep the fog point temperature in the box below 0°C. The actual minimum cloud point temperature is 5-7°C. A dew point temperature of 5°C is equivalent to an absolute moisture content of 0.0055g/kg, and a relative humidity of 20%RH corresponds to a temperature of 30°C. If the required temperature is 20°C and the relative humidity reaches 20%RH, the dew point temperature at this time is -3°C. Refrigeration and dehumidification are very difficult, and an air-drying system must be used.