Introduction: Environmental awareness is deeply rooted in people's minds. Monitoring of the environment can better protect the environment. Environmental monitoring requires the use of many sophisticated scientific equipment. The selection of environmental and reliability test equipment should follow the following basic principles.

1. Reproducibility of environmental conditions

Complete and accurate reproduction of the environmental conditions present in nature in the test chamber is elusive. However, within a certain range of tolerances, one can exactly and approximately simulate the external environmental conditions that engineering products undergo during use, storage and transportation. This paragraph is summarized in the language of the project, which is "the environmental conditions surrounding the tested product (including the platform environment) created by the test equipment should meet the requirements of the environmental conditions and tolerances specified in the product test specification." For example, the temperature box used for military product testing not only meets the requirements of the national military standard GJB150.3-86, GJB150.4-86 according to different uniformity and temperature control accuracy. Only in this way can we ensure the reproducibility of environmental conditions in environmental tests.

2. Reproducibility of environmental conditions

An environmental test equipment may be used for multiple tests of the same type of product, and a tested engineering product may also be tested in different environmental test equipment in order to ensure that the same product is tested in the same environment as the test specification. The comparability of the test results obtained under the conditions necessarily requires that the environmental conditions provided by the environmental test equipment be reproducible. This means that the stress levels (eg, thermal stress, vibrational stress, electrical stress, etc.) imposed on the tested product by the environmental testing equipment are consistent with the requirements of the same test specification.

The repeatability of the environmental conditions provided by the environmental test equipment is provided by the national metrological verification department after it is qualified according to the verification procedures established by the national technical supervision institution. For this purpose, the environmental testing equipment must be required to meet the requirements of the technical indicators and accuracy indicators in the verification procedures, and the time limit specified in the verification period shall not be exceeded in the use time. If the use of very common electric vibration table to meet the technical parameters such as excitation force, frequency range, load capacity, etc., must also meet the requirements of the accuracy specifications such as lateral vibration ratio, table acceleration uniformity, harmonic distortion degree specified in the verification procedures. , And the use period after each verification is two years, more than two years must be re-certified before they can be put into use.

3. Measurable control of environmental condition parameters

The environmental conditions provided by any environmental test equipment must be observable and controllable not only to limit the environmental parameters within a certain tolerance range, but also to guarantee the reproducibility and repeatability of test conditions. Moreover, it is also necessary to proceed from the safety of product testing in order to prevent the uncontrolled environmental conditions from causing damage to the tested product and bring about unnecessary losses. At present, the accuracy of the parameters required by various test specifications is not to be less than one third of the allowable error of the test conditions.

4. Exclusivity of environmental test conditions

Each time an environmental or reliability test is conducted, there are strict rules on the types, magnitudes, and tolerances of environmental factors, and environmental factors that are not required for the test are excluded to allow judgement and analysis of product failure during or after the test. In the case of a failure mode, it provides an exact basis. Therefore, it is required that the environmental testing equipment is not allowed to add other environmental stress disturbances to the tested product, in addition to providing the specified environmental conditions. For example, the magnetic flux leakage in the table, the signal-to-noise ratio of acceleration, and the ratio of the total rms acceleration in and out of the belt, as defined in the verification procedures for the electric vibration table. Random signal tests, harmonic distortion, and other accuracy indicators are all verification items that are designed to ensure the uniqueness of environmental test conditions.

5. Safety and reliability of test equipment

Environmental tests, especially reliability tests, have long test cycles. The objects of the tests are sometimes high-value military products. During the test, the testers often operate patrols or test work around the site. Therefore, the environmental test equipment must be operated. Safety, easy operation, reliable use, long working life, etc., to ensure the normal operation of the test itself. All kinds of protection and alarm measures and safety interlocking devices of the test equipment should be perfect and reliable to ensure the safety and reliability of the test personnel, the tested products and the test equipment itself.

The selection of temperature and humidity boxes follows the basic principles:

1, the choice of volume

When placing the tested product (components, components, components, or the whole machine) into the climate environment box for testing, in order to ensure that the atmosphere around the tested product meets the environmental testing conditions specified in the test specification, the working dimensions of the climate chamber and the tested product The following points should be observed between the outside dimensions:

a) The volume of the product under test (W × D × H) shall not exceed (20 ~ 35)% (20% recommended) of the effective working space of the test chamber. It is recommended not to use more than 10% of products that produce heat during the test.

b) The ratio of the cross-wind area of ​​the product under test to the total area of ​​the chamber of 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 product being tested and the wall of the test chamber shall be at least 100-150mm (150mm recommended).

The above three provisions are actually interdependent and unified. Taking a cubic cubic box as an example, the area ratio of 1: (0.35 to 0.5) corresponds to a volume ratio of 1: (0.207 to 0.354). The ratio of the volume of 100 to 150 mm from the box wall is 1: (0.343 to 0.512).

In summary, the above three requirements stipulate that the working chamber volume of the climate environment test box should be at least 3 to 5 times of the volume of the outline of the tested product. The reasons for making this provision are the following:

1) After the test piece is placed in the box, it occupies a smooth channel. The narrowing of the channel will increase the airflow velocity. Accelerate the heat exchange between the air flow and the test piece. This is inconsistent with the reproducibility of environmental conditions, because in the relevant standards for temperature-related environmental tests, it is stipulated that the flow velocity of air around the test sample in the test box should not exceed 1.7m/s, so as to prevent the test sample and surrounding atmosphere from producing non-realistic results. Heat conduction. When the average wind speed in the test chamber is 0.6-0.8m/s at no-load, not exceeding 1m/s, the wind speed of the flow field may increase when the space and area ratios specified in the requirements of a) and b) are satisfied. ~ 100)%, the average maximum wind speed is (1 ~ 1.7) m/s. Meet the requirements of the standard. If the volume of the test piece or the area facing the wind is increased without limitation during the test, the actual wind speed of the test will increase beyond the maximum wind speed specified in the test standard, and the validity of the test result will be questioned.

2) The accuracy index of environmental parameters (such as temperature, humidity, and salt spray deposition rate) in the working chamber of the climate chamber is the result of testing in the no-load state. Once the test object is placed, the environment of the test chamber is in the working chamber. The uniformity of the parameters will have an effect, and the greater the space occupied by the test pieces, the more serious this effect will be. The experimental test data shows that the temperature difference between the windward side and the leeward side in the flow field can reach 3~8°C, and when it is severe, it can be up to 10°C or more. Therefore, it is necessary to meet the requirements of a) and b) as much as possible so as to ensure the uniformity of the environmental parameters around the tested product.

3) According to the principle of heat conduction, the temperature of the air flow near the box wall is usually 2~3°C different from the center temperature of the flow field, and may reach 5°C when the upper and lower limits of high and low temperatures are reached. The temperature of the tank wall differs from the temperature of the flow field near the tank wall by 2 to 3°C (depending on the structure and material of the tank wall). The greater the difference between the test temperature and the outside atmospheric environment, the greater the above temperature difference, and therefore, the distance from the tank wall. (100-150mm) The space within the distance is not available space.

2, the choice of temperature range

At present, the scope of foreign temperature test chambers is generally -73 ~ +177 °C, or -70 ~ +180 °C. Most domestic manufacturers are generally -80 ~ +130 °C, -60 ~ +130 °C, -40 ~ +130 °C, there are high temperatures to 150 °C. These temperature ranges can usually meet the needs of the vast majority of domestic military and civilian product temperature tests, unless there is a special need, such as the location of the installation close to the engine and other heat sources, can not blindly increase the upper temperature limit. Because the higher the upper limit temperature, the greater the temperature difference between the inside and outside of the box, the worse the uniformity of the flow field inside the box. The available studio volume is smaller. On the other hand, the higher the upper limit temperature value, the higher the heat resistance of the insulating material (such as glass wool, etc.) in the interlayer sandwiched in the box wall. The higher the box sealing requirements, the higher the manufacturing cost of the box.

3, the choice of humidity range

The humidity indicators given by domestic and international environmental test chambers are mostly 20 to 98% RH or 30 to 98% RH. If the humid heat test chamber does not have a dehumidification system, the humidity range is 60 to 98%. This type of test chamber can only be made high. Wet test, but its price is much lower. It is worth noting that the corresponding temperature range should be indicated after the humidity index, or the lowest dew point temperature should be given. Because the relative humidity is directly related to the temperature, for the same absolute moisture content, the higher the temperature, the lower the relative humidity, such as the absolute moisture content of 5g/Kg (refers to 1g of dry air containing 5g of water vapor ) When the temperature is 29°C, the relative humidity is 20%RH, and the relative humidity is 90%RH when the temperature is 6°C. When the temperature drops below 4°C, the relative humidity exceeds 100%, condensation will occur in the box. phenomenon.

Source: Dongguan Yuanyao Electronic Technology Co., Ltd.