We only know four basic principles for choosing an environmental Test Chamber, so where should we start? Here I will specifically explain how to choose an environmental Test Chamber from several aspects such as volume, temperature, humidity and control methods.
volume selection
Put the product to be tested (parts, assemblies, parts or complete machine) into the climate environment Test Chamber for testing, and then ensure that the environment around the product under test meets the environmental test conditions specified in the test specification, as follows: the size of the climate chamber and the Measured product dimensions should follow between points.
The volume of the product under test shall not exceed (20-35)% (recommended 20%) of the effective working space of the laboratory.
The proportion of the windward area of the tested product to the total area of the laboratory working room is not more than (35-50)% (recommended 35%).
The distance between the outer contour surface of the tested product and the inner wall of the Test Chamber is at least 100~150mm (recommended 150mm).
Based on the above three regulations, the volume of the working chamber of the climate Test Chamber should be at least 3 to 5 times the volume of the product to be tested. The main reason for this regulation is that the test piece is placed in a crowded channel in the box. The narrowing of the channel will increase the air flow, which will affect the uniformity of the environmental parameters in the test box and occupy more space. The test piece, the more serious this effect.
According to the principle of heat conduction, the temperature of the airflow near the inner wall of the cavity usually differs from the center of the flow field by 2°C to 3°C, and it can even reach 5°C at high or low temperatures. The greater the difference between the test temperature and the external atmospheric environment, the greater the above-mentioned temperature difference . Therefore, the space within the distance (100-150mm) from the inner wall of the cavity is an unusable space. The actual test data shows that the temperature difference between the windward side and the leeward side of the flow field can reach 3°C~8°C, and in severe cases it can reach more than 10°C. Therefore, it is necessary to meet the two requirements of 1.1 and 1.2 as much as possible, so as to ensure the consistency of the surrounding environment parameters of the tested product.
For example, in the relevant standards of environmental tests involving temperature, there is such a regulation: the air velocity around the test piece in the Test Chamber should not exceed 1.7m/s, so as to prevent the unreality of the test piece and the surrounding environment. Spread the heat. If the volume or windward cross-sectional area of the test piece is not limited, the actual test airflow velocity will increase to exceed the maximum wind velocity specified in the test standard, and the validity of the test results will be questioned.
Choice of temperature range
At present, the range of Temperature Test Chambers is generally -40°C~+130°C, and some instruments can reach -70°C at low temperature and +150°C or even +180°C at high temperature. For the temperature range of -40°C~+130°C, it can already meet the temperature test requirements of most military and civilian products. Unless there are special needs, such as the installation location is close to the heat source outside the product such as the engine, do not blindly increase the upper limit of the temperature.
The higher the upper temperature limit, the greater the temperature difference between the inside and outside of the chamber, the worse the uniformity of the flow field in the chamber, and the smaller the volume of the working chamber. On the other hand, the higher the upper limit temperature, the higher the heat resistance requirements for the insulating material (such as glass wool, etc.) in the interlayer of the chamber wall, and the higher the requirements for the chamber sealing, which increases the cost of producing the box.
Selection of Humidity Range
The humidity index given by the environmental Test Chamber is mostly 20%~98% or 30%~98%. If the temperature and humidity Test Chamber does not have a dehumidification system, and the humidity range is 60% to 98%, this kind of Test Chamber can only do high humidity tests.
To achieve high temperature and high humidity, you only need to spray water vapor or atomized water droplets into the chamber air for humidification. Low temperature and low humidity are relatively difficult to control, because the absolute moisture content at this time is very low, sometimes far lower than the absolute moisture content in the atmosphere, and it is necessary to dehumidify the air flowing in the room to make the air dry.
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 of 5g/Kg (refers to 5g of water vapor in 1kg of dry air). When the temperature is 29°C, the relative humidity is 20%, when the temperature is 6°C, the relative humidity is 90%, and when the temperature is lower than 4°C, the relative humidity is above 100%, condensation will occur in the inner cavity.
Choice of control method
There are two kinds of temperature and humidity Test Chambers: constant Test Chamber and alternating Test Chamber. Ordinary high-low Temperature Test Chambers generally refer to constant temperature high-low Temperature Test Chambers. Its control method: set a target temperature, and the Test Chamber has the ability to automatically maintain the temperature to the target temperature point. The control method of the constant temperature and humidity Chamber is similar. Set a target temperature and humidity point, and the test box has the ability to automatically constant temperature to the target temperature and humidity point.
The high-low temperature alternating test box can be set with one or more high-low temperature change and cycle programs. The test box has the ability to complete the test process according to the preset program, and can accurately control the heating and cooling speed within the maximum temperature rise range and cooling speed. That is, the heating and cooling speed can be controlled according to the slope of the set curve. Of course, the alternating test box has the function of a constant test box, but the manufacturing cost of the alternating test box is relatively high, because the alternating test box needs to be equipped with an automatic curve recording device, a program controller, and it is necessary to solve the problem of opening the refrigerator under high temperature conditions in the test studio, etc. question.
Fluctuations in temperature and humidity
In order to more accurately simulate the actual environmental conditions that the product is exposed to in nature, it is necessary to ensure that the surroundings of the tested product are under the same temperature environmental conditions in the environmental test. For this reason, the temperature gradient and temperature fluctuation chamber in the test need to be limited. Temperature fluctuation is a relatively easy parameter to realize. The actual temperature fluctuation in most Test Chambers produced by all environmental test equipment manufacturers can be controlled within the range of ±0.3°C.
The relative humidity deviation in the environmental Test Chamber should be -3%~+2%. In order to meet the requirements of humidity control accuracy, the temperature control accuracy requirements in the environmental Test Chamber are relatively high, and the temperature fluctuation is generally less than ±1°C, otherwise it is difficult to meet the humidity control accuracy requirements.
