Selection of temperature and humidity test chamber should be made from the following five aspects: volume, temperature range, humidity range, control mode and temperature change rate.
1. Volume selection
When placing the tested products (components, components, components or the whole machine) into the climatic environment box for test, in order to ensure that the ambient atmosphere around the tested products can meet the environmental test conditions specified in the test specification, the following provisions shall be followed between the working size of the climatic box and the overall size of the tested products:
A) the volume of the tested product (W× D ×H) shall not exceed (20 ~ 35) % of the effective working space of the test chamber (20% is recommended).It is recommended that no more than 10% of the product is heated during the test.
B) the ratio between the upwind section area of the test product and the total area of the test chamber and the test chamber on the section shall not be greater than (35 ~ 50) % (35% is recommended).
C) the distance between the outer surface of the tested product and the wall of the test chamber shall be kept at least 100-150mm (150mm is recommended).
The above three provisions are actually interdependent and unified.Take a cube box of 1 cubic meter as an example, the area ratio is 1 :(0.35 ~ 0.5) and the volume ratio is 1 :(0.207 ~ 0.354).The ratio of the equivalent volume 100 ~ 150mm from the wall is 1 :(0.343 ~ 0.512).
In summary, the working chamber volume of the climatic and environmental test chamber shall be at least 3 ~ 5 times of the overall volume of the tested product.The reasons for this provision are as follows:
(1) after the test piece is placed in the box, it occupies a smooth passage, and the narrow passage will lead to the increase of airflow velocity.The heat exchange between the accelerated airflow and the specimen under test.This is inconsistent with the reproduction of environmental conditions, because in the relevant standards, it is stipulated that the air velocity around the test sample in the test chamber shall not exceed 1.7m/s for the environmental tests involving temperature, in order to prevent the test sample and the surrounding atmosphere from producing an unpractical heat conduction.When the average wind speed in the test chamber is 0.6-0.8m /s, no more than 1m/s, and meets the space and area ratio specified by the requirements of a) and b), the wind speed in the flow field may increase (50-100) %, and the average wind speed is (1-1.7) m/s.Meet the standard requirements.If the volume of the test piece or the area of windward break is increased without restriction in the test, the actual wind speed of the airflow will increase to exceed the wind speed specified in the test standard, and the validity of the test results will be doubted.
(2) the climate box cavity environment parameters (such as temperature, humidity, salt fog sedimentation rate, etc) the precision of the indicators are in the condition of no-load test results, once the placement is after the test pieces, to test chamber cavity uniformity of environment parameters will influence, the test piece of space, the greater the this effect is more serious.Measured test data show that the flow field in the windward and leeward surface temperature can reach 3 ~ 8 ℃, serious when can be big to above 10 ℃.Therefore, the requirements of a) and b) must be satisfied as far as possible to ensure the uniformity of environmental parameters around the tested products.
(3) according to the principle of heat conduction, the walls near the temperature of the air flow and the flow field usually center 2 ~ 3 ℃, the temperature difference between the upper and lower limit of high and low temperature, also may reach 5 ℃.Box wall temperature and near wall flow field temperature and difference of 2 ~ 3 ℃ (depending on the chest wall structure and materials) test temperature atmospheric environment, the greater the difference with the outside world, the temperature difference is bigger, therefore, away from the walls (100 ~ 150 mm) distance in space is not use of space.
2. Selection of temperature range
At present, the range of temperature test chamber for substantially abroad – 73 ~ + 177 ℃, or – 70 ~ + 180 ℃.Most domestic manufacturers is commonly – 80 ~ + 130 ℃, 60 ~ + 130 ℃, and 40 ~ + 130 ℃, also has a high temperature to 150 ℃.These temperature ranges can usually meet the temperature test needs of most domestic military and civil products. Unless there is a special need, such as the installation location close to the engine and other heat sources, the temperature limit cannot be blindly increased.Because the higher the upper limit temperature, the greater the temperature difference between the inside and outside of the box, and the worse the uniformity of the flow field inside the box.The smaller the available studio space.On the other hand, the higher the upper limit temperature value, the higher the heat resistance requirement of the insulation material (such as glass wool, etc.) in the box wall sandwich.The higher the sealing requirement of the box, the higher the production cost of the box.
3. Selection of humidity range
The humidity index given by domestic and foreign environmental test boxes is mostly 20 ~ 98% RH or 30 ~ 98% RH. If there is no dehumidification system in the wet and hot test box, the humidity range is 60 ~ 98%. This kind of test box can only do high humidity test, but its price is much lower.It is important to note that the corresponding temperature range, or dew point temperature, should be indicated after the humidity indicator.Because is directly related to the temperature, relative humidity for the same moisture content, the higher the temperature, relative humidity, the less, such as the moisture content of 5 g/Kg (1 Kg dry contains 5 grams of water vapour in the air), when the temperature is 29 ℃, relative humidity 20% RH and temperature is 6 ℃, relative humidity is 90% RH, when the temperature dropped below 4 ℃, relative humidity above 100%, can appear in the casing dewing phenomenon.
To achieve high temperature and high humidity, only need to spray steam or atomized water droplets into the air of the box body 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, the air flowing in the box needs to be dehumidified, so that the air becomes dry.At present, most of the temperature and humidity boxes at home and abroad adopt the principle of refrigeration and dehumidification.When wet air passes through the cold tube, its relative humidity will reach 100% RH, because the air is saturated and dews on the light tube, making the air drier.This way of dehumidification in theory to the dew point temperature below freezing, but when the cold surface temperature arrived at 0 ℃, the light tube surface condensation water will freeze, thus affecting the light pipe on the surface of heat exchange, decreased dehumidifying capacity.Because it is impossible to seal the box, the humid air in the atmosphere will infiltrate into the box, making the dew point temperature rise., on the other hand, moist air flows between the light tube is just at the moment and light pipe (cold) contact saturated state separate out water vapor, therefore the dehumidification methods of the casing is not the dew point temperature is below 0 ℃.With the actual dew point temperature is 5 ~ 7 ℃.Dew point temperature of 5 ℃ or moisture content is 0.0055 g/Kg, corresponding to the relative humidity of 20% RH temperature 30 ℃.If required temperature 20 ℃ relative humidity to 20% RH, at this time of the dew point temperature is 3 ℃, the cooling dehumidification is difficult, must choose the air drying system to achieve.
4. Selection of control mode
The temperature and humidity test chamber has two conditions: constant test chamber and alternating test chamber.
The common high and low temperature test box generally refers to the constant high and low temperature test box, its control mode is: set a target temperature, the test box has the ability of automatic constant temperature to the target temperature point.The control mode of the constant temperature and humidity test chamber is similar. It sets a target temperature and humidity point, and the test chamber has the ability to automatically keep the temperature to the target temperature and humidity point.High and low temperature alternating test chamber is set high and low temperature change, loop of one or more programs, test chamber has the ability to complete test process according to the preset curve, and can be within the scope of the ability of heating, cooling rate, control temperature, cooling rate, which can be set according to the curve of the slope control temperature, cooling rate.Similarly, the high and low temperature alternating wet-heat test chamber also has the ability to preset temperature and humidity curves and control them according to the preset.Alternating test, of course, the function of the box has a constant test box, but alternating test chamber of the manufacturing cost is higher, because alternating test chamber configuration curve automatic recording equipment, process control instrument, still must solve test chamber in the studio open the refrigerator, under the condition of high temperature, therefore, alternating experiment box price is constant box generally higher than the price of more than 20%.Therefore, we should be practical and realistic in order to test the needs of the method as a starting point, the selection of constant test box or alternating test box.
5. Selection of temperature change rate
Ordinary high and low temperature test chamber has no index of cooling speed, and the time from the ambient temperature to the nominal temperature is generally 90 ~ 120min.High and low temperature alternating test box, high and low temperature alternating hot and humid test chamber temperature change speed requirements, the warming rate generally requires 1 ℃ / min, within the scope of the rate of speed is adjustable.And rapid temperature change chamber temperature rate faster, heating, cooling rate can reach 3 ℃ / min ~ 15 ℃ / min, in a certain period of heating temperature, cooling rate, and even can reach above 30 ℃ / min.
Various specifications, the speed of rapid temperature change chamber temperature range are generally the same, that is 60 ~ + 130 ℃, but the inspection cooling speed variable temperature range is different, according to the experiment of different requirements, rapid temperature change chamber temperature range, have a plenty of 55 ~ + 80 ℃, and others – 40 ~ + 80 ℃.