1. Selection of temperature and humidity chamber volume
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When the product under test (component, assembly, component or complete machine) is placed in a climate environment chamber for testing, in order to ensure that the atmosphere around the product under test can meet the environmental conditions specified in the test specifications, the working space size of the climate chamber must be consistent with the product under test. The following regulations should be followed for the outer dimensions:
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a) The volume of the product under test (W×D×H) shall not exceed (20-35)% of the effective working space of the test chamber (20% is recommended). For products that generate heat during the test, it is recommended to use no more than 30%.
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b) The ratio of the windward cross-sectional area of the product being tested to the total area of the working chamber of the test chamber on that cross-section is not large
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At (35-50)% (35% is recommended).
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c) The distance between the outer surface of the product under test and the wall of the test chamber should be at least 100-150mm.
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(150mm is recommended).
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The above three provisions are actually interdependent and unified. Taking a 1m3 chamber as an example, the area ratio is 1: (0.35~0.5), which is equivalent to a volume ratio of 1: (0.207~0.354). A distance of 100-150mm from the chamber wall is equivalent to a volume ratio of 1: (0.343~0.512).
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To summarize the above three regulations, the working chamber volume of the climate environment test chamber is at least 3 to 5 times the outer volume of the product under test. The reasons for this provision are as follows:
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1) After the test piece is placed in the chamber, it occupies the airflow channel in the flow field. The narrowing of the channel will lead to a decrease in the airflow velocity. Accelerating the heat exchange between the airflow and the test piece, which is inconsistent with the reproducibility of environmental conditions, because the relevant standards for temperature environmental tests stipulate that the air flow rate around the test sample in the test chamber should not exceed 1.7m/s. In order to prevent unrealistic heat conduction between the test sample and the surrounding atmosphere, the average wind speed in the test chamber is 0.6~0.8m and does not exceed 1m/s when no-load, and meets the space and area specified in the two requirements a) and b). When compared, the wind speed in the flow field may increase by (50~100)%, and the average maximum wind speed is (1.17) m/s, which meets the requirements of the standard. If the volume or windward cross-sectional area of the test piece is increased without restriction during the test, the airflow speed during the actual test will increase beyond the highest wind speed stipulated in the test standard, and the validity of the test results will be questioned.
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2) The accuracy indicators of the environmental parameters (such as temperature, humidity, salt spray sedimentation rate, etc.) in the working chamber of the climate chamber are the results of detection under no-load condition. Once the test piece is placed, the environment in the working chamber of the test chamber will be affected. The uniformity of parameters will have an impact, and the larger the space occupied by the test piece, the more serious this impact will be. Experimental test data shows 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 can be as large as more than 10°C. Therefore, the two requirements a) and b) must be met as much as possible to ensure the uniformity of environmental parameters around the product being tested.
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3) According to the principle of heat conduction, the temperature of the air flow near the chamber wall is usually 2~3°C different from the temperature in the center of the flow field. It may reach 5°C at the upper and lower temperatures*. The temperature of the chamber wall and the temperature of the flow field near the chamber wall differ by 2~3°C (depending on the structure and material of the chamber wall). The greater the difference between the test temperature and the outside atmospheric temperature, the greater the above-mentioned temperature difference. Therefore, the space within the distance from the chamber wall (100~150mm) is unusable space.
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 2. temperature range
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At present, the temperature range of foreign temperature test chambers is generally (-73~+177℃), or (-70~+180℃). Most domestic manufacturers use (-70~+120℃), and there are also high temperatures up to 150℃. This temperature range can usually meet the temperature testing needs of most military and civilian products. Unless there are special needs, such as products installed close to heat sources such as engines, the volume of the working chamber that cannot be blindly utilized will be smaller. On the other hand, the higher the upper limit temperature, the higher the heat resistance requirements for the insulation materials (such as glass wool, etc.) in the chamber wall interlayer. The requirements for chamber sealing are also higher, which increases the production cost of the chamber.
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3． Humidity range
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The temperature indicators given by domestic and foreign environmental test chambers are generally 20~95%RH or 20~98%RH. In fact, these two indicators are the most consistent because the humidity control accuracy is usually ±(3~5)%RH. , and the maximum humidity can only be RH. It is worth noting that the corresponding temperature range should be indicated after the humidity indicator, or the minimum dew point temperature should be given. Because relative humidity is directly related to temperature, for the same moisture content , the higher the temperature, the smaller the relative humidity. For example, the humidity content d is 5g/kg (meaning that 1 kilogram of dry air contains 5 grams of water vapor). When the temperature is 29°C, the relative humidity is 20%RH, and the temperature When the temperature is 6℃, the relative humidity is 99%RH. When the temperature drops below 4℃ and the relative humidity exceeds, condensation will appear in the chamber.
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To achieve high temperature and high humidity, you only need to spray water vapor or atomized water droplets into the air of the chamber for humidification. Low temperature and low humidity are relatively difficult to control, because the moisture content at this time is very low, sometimes much lower than the moisture content in the atmosphere. It is necessary to dehumidify the air flowing in the chamber to make the air drier. At present, most temperature and humidity chamberes at home and abroad adopt the principle of refrigeration and dehumidification, which is to add a set of refrigeration light tubes in the air preconditioning chamber of the chamber. When the moist air passes through the cold tube, its relative humidity will reach RH, and condensation will form on the light tube due to air saturation, making the air drier. This dehumidification method can theoretically reach sub-zero dew point temperatures, but when the cold spot surface temperature reaches below 0°C. The actual lowest dew point temperature achieved is 5~7℃. A dew point temperature of 5°C is equivalent to a moisture content of 0.0055g/kg, and a temperature corresponding to a relative humidity of 20%RH is 30°C. If the relative humidity is required to reach 20% RH at a temperature of 20°C, the dew point temperature at this time is -3°C. It is very difficult to use refrigeration to dehumidify, and an air drying system must be used to achieve this.
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4． wind speed
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Relevant standards stipulate that the wind speed in the temperature and humidity chamber should be less than 1.7m/s when conducting environmental tests. Excessive wind speed will accelerate the heat exchange between the surface of the test piece and the airflow flowing in the chamber, which is detrimental to the test. However, there are many factors such as temperature, humidity, vibration, etc. In the comprehensive environmental test of factors, in order to pursue a high temperature change rate, the flow rate of the circulating air flow in the chamber must be accelerated. The wind speed is usually 2.5~3.0m/s. Therefore, the wind speed restrictions are different for different purposes.
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5. Temperature Field
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In order to more accurately simulate the actual environmental conditions that products encounter in nature, it is necessary to ensure that the surroundings of the product under test can be under the same temperature environment during environmental testing. To this end, the temperature gradient and temperature fluctuations in the test chamber must be controlled. be restricted. The national military standard GJB150.1-86 General Principles of Environmental Test Methods for Military Equipment clearly stipulates that “the temperature of the measurement system near the test sample should be within ±2°C of the test temperature, and its temperature gradient shall not exceed 1°C/m or the total Z maximum The value is 2.2℃ (the test sample does not work)". As mentioned above, the requirements for the temperature field include three aspects:
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a) Accuracy of temperature value.
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The test specification gives the nominal value of the test temperature (nominal temperature value), and the test equipment measures the actual test temperature value (temperature of the working space indication point) in the center of the temperature field (or air outlet, return air outlet). There is an error between.
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b) Temperature fluctuation.
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The data of the actual temperature value is not a constant value, but a quantity that fluctuates up and down over time. Using time statistical averages, a mathematical expectation value (mean) and a standard deviation can be obtained.
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According to the requirements of the test specification, the sum of the above two items (that is, the error including the mathematical expected value and the nominal value (system error)) plus the standard deviation (ie random error) (temperature control accuracy) is not greater than the specified error range, GJB150.1 -86 specifies ±2℃.
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According to the national standard GB/T5170.1-1995, the temperature deviation of the test chamber refers to the upper and lower deviation of the highest and lowest temperatures measured at each test point in the working space from the nominal temperature in the stable state of the test chamber within the specified time. The high temperature temperature deviation is not more than ±2℃, and the low temperature temperature deviation is not more than ±3℃.
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c) Temperature field gradient.
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This indicator is to control the uniformity of the temperature field in the chamber. That is to ensure that the temperature around the product under test is consistent in the extension of space. The national military standard GJB150 stipulates that the temperature gradient is 1°C/m, which is usually used in test chambers above 3m3. If the maximum value is 2.2°C, test chambers below 3m3 are mostly used.
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Theoretically, the temperature gradient is applicable to all production tests, but in a test chamber with a wind speed greater than 1.7m/s, due to the enhanced vortex effect of the airflow in the flow field, the uniformity of the flow field deteriorates, making it difficult to achieve National military standard GJB150 requirements. On the other hand, in a test chamber that pursues a high temperature change rate (such as greater than 10°C/m), the wind speed of the air flow must be increased (greater than 2m/s). At this time, the temperature difference between the windward and leeward sides of the product being tested increases with the wind speed. Therefore, in the relevant test specifications and calibration procedures, there are no strict requirements for temperature field uniformity for temperature and humidity chambers that pursue high temperature change rates. Of course, environmental testing equipment manufacturers are striving to achieve products with both high temperature change rates and good temperature field uniformity to meet user requirements. For example, in the temperature and humidity chamber produced by the American environmental testing company (Envirotronics), when the temperature change rate is 15°C/m, the uniformity of the temperature field is within ±2.2°C.
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6. Precision control of humidity.
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Humidity measurement in environmental test chambers mostly uses the inexpensive wet and dry bulb method. Under high humidity conditions (80~RH), the temperature difference between dry and wet bulbs is 1°C, which corresponds to a relative humidity difference of about 5%. According to the principle that the accuracy of the test device should not be less than one-third of the allowable deviation of the test conditions (see the national military standard GJB150.1-86), a temperature difference of 1°C requires that the allowable difference in temperature measurement of the dry bulb and the wet bulb itself is ±0.7°C. The temperature measurement accuracy of hydraulic pressure reaches 0.2℃. By analogy, the relative humidity error is ±3%RH. It is required that the temperature measurement accuracy should reach ±0.1℃. Therefore, the error of relative humidity measurement using the wet and dry bulb method is usually ±(3~5)%RH. With such a temperature measurement accuracy, the humidity control accuracy can be imagined.
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In order to overcome the shortcomings of the wet and dry bulb method's low measurement accuracy and excessive influence from human operation, solid humidity sensitive heads (such as capacitive humidity sensors) have been used in low temperature (-55~70℃) in recent decades. After the problem, it has been applied to environmental testing equipment, and its measurement accuracy can reach ±1~3%RH. The temperature and humidity chambers produced by the American company are all equipped with solid humidity sensors.
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In addition to being restricted by the humidity measurement method, the humidity control accuracy is also related to the humidification method. There are many methods of humidification in the environmental test chamber, and the following three are generally used:
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1) Water plate humidification. It is divided into natural evaporation and electric heating evaporation. This humidification method has a slow humidification rate. The control accuracy is low and is mostly suitable for constant temperature and humidity chambers.
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2) Spray steam for humidification. The humidification rate is fast, but the fluctuation of humidification is large. It is easy to achieve rapid control and the control accuracy is good. However, due to the injection of 100°C steam, there will be a small amount of interference on the temperature state in the chamber.
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3) Sprayed water. The particle diameter of atomized water is roughly 10~50μ. This method has the advantages of spraying steam for humidification, does not require an external steam boiler, and can adjust the temperature of the sprayed water particles to be the same as the test temperature in the chamber. It is a better method. Humidification method.
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7. Dew point temperature selection
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The dew point temperature refers to the temperature at which moist air is cooled to a saturated state (relative temperature is RH) under the conditions of a given moisture content. Therefore, the dew point temperature of the air state is given, and the moisture content of the air is also given. From the enthalpy-humidity diagram of the air, the relative temperature values at different temperatures corresponding to this moisture content can be found. The lower the dew point temperature, the smaller the moisture content of the air inside the chamber, and the smaller the corresponding relative humidity at the same temperature. The lowest dew point temperature that a temperature and humidity environment test chamber can achieve is related to the dehumidification method used. The lowest dew point temperature that is usually used for refrigeration and dehumidification is about 5~7℃. After a long period of stable dehumidification, the dew point temperature will also increase. It can reach 2~5℃. If the dew point temperature must be below 0℃, you need to use additional devices such as solid moisture absorbent or liquid moisture absorbent, or other drying and dehumidification devices. For example, the temperature and humidity chamber produced by the American Environmental Testing Company can be equipped with a dry air purging system and an air drying system with a dew point temperature of -40°C.