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How to choose temperature and humidity chamber
Jan 08, 2018

First, the choice of humidity range

Most of the humidity indicators given by environmental chamber at home and abroad are 20-98% RH or 30-98% RH. If there is no dehumidification system in the chamber, the humidity range is 60-98%. This kind of chamber can only do high Wet test, but its much lower price. It is noteworthy that after the humidity indicator should indicate the corresponding temperature range, or give the lowest dew point temperature. 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 (1 kg of dry air contains 5 grams of water vapor ), When the temperature is 29 ℃, the relative humidity is 20% RH, the temperature is 6 ℃, the relative humidity is 90% RH. When the temperature drops below 4 ℃ and the relative humidity exceeds 100%, condensation will appear in the box phenomenon. To achieve high temperature, high humidity only need to spray water vapor or atomized water into the air box, humidification. Low temperature and low humidity are relatively difficult to control because the absolute moisture content at this time is very low, sometimes much lower than the absolute moisture content of the atmosphere, requiring dehumidification of the air flowing inside the tank to make the air dry. At present, most of the temperature and humidity boxes at home and abroad are based on the principle of refrigeration and dehumidification, plus a set of refrigeration tubes in the air pre-conditioning chamber of the box. When the wet air passes through the cold pipe, the relative humidity will reach 100% RH, and the air becomes more dry due to air saturation on the light pipe. This dehumidification method can theoretically reach the zero point below the dew point temperature, but when the cold spot surface temperature reaches 0 ℃, the condensation of water droplets on the surface of the tube will freeze, Shenzhen Chong Jie instrument professional maintenance temperature and humidity chamber, thus affecting The heat exchange on the surface of the light pipe reduces the dehumidification capacity. And because the box can not be absolutely sealed, the air in the wet air will penetrate into the box, so that the dew point temperature rise. On the other hand, the moist air flowing between the light pipes only reaches the saturated state at the moment of contact with the light pipe (cold spot) and precipitates water vapor. Therefore, it is difficult for the dehumidification method to make the dew point temperature in the box below 0 ° C. The actual minimum dew point temperature reached 5 ~ 7 ℃. The dew point temperature of 5 ° C corresponds to an absolute moisture content of 0.0055 g / Kg and corresponds to a temperature of 30 ° C at a relative humidity of 20% RH. If the required temperature of 20 ℃ into the relative humidity of 20% RH, the dew point temperature at this time -3 ℃, the use of refrigeration dehumidification is very difficult, so do the low temperature and humidity environment test air drying system.

Second, the work chamber climate chamber environment

Parameters such as temperature, humidity, salt spray sedimentation rate and other precision indicators are in no-load test results, once the test piece into the test chamber on the uniformity of the environmental parameters will have an impact, The greater the space occupied by the test piece, the more serious this effect is. The experimental data show that the temperature difference between the windward and leeward surfaces in the flow field can reach 3 ~ 8 ℃, and in severe cases it can be as large as 10 ℃. Therefore, we must try to meet a〕, b〕 two requirements to ensure the uniformity of the environmental parameters of the product under test. According to the principle of heat conduction, the temperature of the airflow in the vicinity of the box wall usually varies from 2 to 3 ° C at the center of the flow field, and may reach 5 ° C at the upper and lower limits of the high and low temperatures. The temperature difference between the box wall and the flow field near the box wall 2 ~ 3 ° C (depending on the structure and material of the box wall) The greater the difference between the test temperature and the external atmospheric environment, the greater the temperature difference, therefore, (100 ~ 150mm) space within the distance is not available space. Temperature range of choice At present, the scope of foreign temperature chamber is generally -73 ~ +177 ℃, or -70 ~ +180 ℃. Most domestic manufacturers are generally -80 ~ +130 ℃, -60 ~ +130 ℃, -40 ~ +130 ℃, but also to high temperatures to 150 ℃. These temperature ranges can usually meet the needs of the vast majority of domestic military and civilian products temperature test, unless there is a special need, such as the installation location close to the engine and other heat sources, the product can not blindly raise the temperature. Because the higher the upper temperature, the greater the temperature difference between the inside and outside the box, the worse the uniformity of the flow field inside the box. The smaller the available studio size is. On the other hand, the higher the upper temperature limit, the higher the heat resistance requirements of the insulation materials (such as glass wool, etc.) in the sandwich wall. The higher the requirements of tightness of the box, the higher the manufacturing cost of the box is.

Third, the choice of volume

In order to ensure that the atmosphere around the product under test meets the environmental test conditions stipulated in the test specification when the product (component, component, component or complete machine) to be tested is put into a climate environment box, the working dimensions of the climate chamber and the product under test Outline dimensions should follow the following points:

1. The volume of the tested product (W × D × H) shall not exceed 20% ~ 35% of the effective working space of the chamber (20% is recommended). For the test in the heat of the product recommended selection of not more than 10%.

2. The tested product windward cross-sectional area and the cross-section of the total studio chamber ratio of not more than (35 ~ 50)% (35% recommended).

3. The surface of the product to be tested should be at least 100 ~ 150mm away from the wall of the test chamber (150mm recommended).

The above three points are actually interdependent and unified. Taking a cube of 1 cubic meter as an example, the area ratio is 1: (0.35-0.5), which corresponds to the volume ratio of 1: (0.207-0.354). From the box wall 100 ~ 150mm is equivalent to the volume ratio of 1: (0.343 ~ 0.512). Summarizing the above three points, the working chamber volume of the climate chamber should be at least 3 to 5 times the volume of the product to be tested.

The reason for making such a requirement is that the test piece is squeezed into a smooth passage when it is placed in the cabinet and the narrowing of the passage leads to an increase in the flow rate of the air. Accelerate the heat exchange between the flow and the test piece. This is inconsistent with the reproduction of environmental conditions, because the relevant standards for temperature and environmental tests require the test chamber around the test sample air flow rate should not exceed 1.7m / s, in order to prevent the test sample and the ambient atmosphere is not consistent with the actual The heat conduction. The average wind speed in the test chamber at no-load is 0.6-0.8 m / s and not more than 1 m / s, and the wind speed of the flow field may increase when the space and area ratios specified in the two requirements are met ~ 100)%, the average maximum wind speed is (1 ~ 1.7) m / s. Meet the standard requirements. If the test in the test to increase the volume or windward cross-sectional area, the actual test airflow speed will increase beyond the maximum wind speed specified in the test standards, the validity of the test results will be questioned.

Fourth, the control mode of choice: (constant temperature and humidity and temperature and humidity change - that is, single-point and programmable)

Temperature and humidity chamber has a constant test chamber, alternating chamber two cases. Ordinary high and low temperature chamber generally refers to the constant high and low temperature chamber, the control method is: set a target temperature, the chamber has the ability to automatically thermostat to the target temperature point. Constant temperature and humidity test chamber control and control methods are similar to set a target temperature and humidity points, the chamber has a constant temperature to the target temperature and humidity point capacity. High and low temperature alternating test chamber has set one or more high and low temperature changes, the cycle of the program, the test chamber has the ability to complete the test curve according to the preset process, and can be in the maximum temperature, the ability to cool down the rate of precise control Temperature, cooling rate, which can be set according to the slope of the curve control heating, cooling rate. Similarly, high and low temperature alternating hot and humid chamber also has a preset temperature and humidity curves, and the ability to control according to the preset.

Fifth, the choice of temperature rate

Ordinary high and low temperature test chamber cooling rate is not an indicator of cooling from ambient temperature to the nominal minimum temperature of the time is generally 90 ~ 120min. High and low temperature alternating chamber, high and low temperature alternating hot and humid chamber have the requirements of variable temperature speed, the rate of temperature generally require 1 ℃ / min, within the scope of this rate adjustable speed. The rapid temperature changing chamber has a fast temperature changing rate, and the heating and cooling rate can reach 3 ° C / min to 15 ° C / min. In some temperature ranges, the cooling rate can even reach 30 ° C / min or more. Rapid temperature changes of various specifications, speed The temperature range of the test chamber is generally the same, that is -60 ~ + 130 ℃, but the temperature range of assessment of cooling rate is not the same, according to the test of different test requirements, rapid temperature changes Test chamber temperature range is -55 ~ +80 ℃, while others are -40 ~ +80 ℃. About rapid temperature change chamber temperature rate there are two references, Shenzhen Chong Jie professional maintenance of various instrumentation environment test equipment, one is the entire average temperature rise and cooling speed, one is the linear temperature rise and cooling speed (in fact, every 5min Average speed). The average speed throughout the test chamber is the temperature range, the highest temperature and the lowest temperature difference between the time ratio. At present, the technical parameters of the variable temperature rate provided by various environmental test equipment manufacturers all refer to the entire average speed. Linear heating and cooling speed refers to any 5min time period, to ensure that the rate of change. In fact, for the rapid temperature change test chamber, it is very difficult to ensure the linear temperature rising and falling speed. The most crucial one is the cooling rate that the chamber can achieve during the last 5 min of the temperature drop period. In some ways, the linear temperature rise and cooling speed (every 5min average speed) is more scientific. Therefore, it is desirable that the test equipment has two parameters, the average temperature rise and fall speed and the linear rise and fall speed (average speed every 5 minutes). In general, the linear temperature rise and cooling rate (every 5 min average speed) is the entire average temperature rise and cooling speed of 1/2.

Sixth, the temperature fluctuation

Temperature fluctuation is a relatively easy to implement parameters, and most of the test chambers produced by all environmental testing equipment manufacturers can control the actual temperature fluctuation within the range of ± 0.3 ℃.

Seven, the wind speed

The relevant standard stipulates that the wind speed in the temperature and humidity box during the environmental test should be less than 1.7m / s. For the test itself, the smaller the wind speed, the better the wind speed will accelerate the heat exchange between the surface of the test object and the flow of air in the box. The authenticity of the test is not good. However, in order to ensure the uniformity of the chamber, it is necessary to have a circulating air in the chamber. However, rapid temperature change test chamber and temperature, humidity, vibration and other multi-factor comprehensive environmental chamber, in order to pursue the variable temperature rate, must speed up the circulating flow in the box body flow velocity usually 2 ~ 3m / s. Therefore, for different purposes, the wind speed limit is not the same.

Eight, temperature uniformity

In order to more accurately simulate the actual environmental conditions suffered by the product in nature, it is necessary to ensure that the surroundings of the product to be tested are under the same temperature and environmental conditions in the environmental test. To this end, the temperature gradient and the temperature fluctuation in the test chamber Limit. In the military standard GJB150.1-86 military equipment environmental testing methods in the general rule clearly stipulates that "the temperature of the measurement system near the test sample should be within ± 2 ℃ of the test temperature, the temperature does not exceed 1 ℃ / m or the total maximum 2.2 ℃ (test sample does not work) ".

Nine, cooling options

The test chamber, if equipped with a refrigeration system, requires cooling of the refrigeration system. Test chamber has air-cooled, water-cooled two forms.

Ten, humidity control accuracy

Most of the environmental chamber humidity measurement is the use of wet and dry bulb method, environmental testing equipment manufacturing standards GB10586 requirements, the relative humidity deviation should be ± 2% RH. In order to meet the requirements of humidity control accuracy, humidity chamber temperature control accuracy, temperature fluctuations generally less than ± 0.2 ℃. Otherwise it is difficult to achieve the accuracy of humidity control requirements.