How to choose environmental test equipment and reliability test equipment

First, the basis for equipment selection: the types of natural environmental factors and induced environmental factors existing on the earth's surface and atmospheric space, at present, it is not possible to calculate an exact number, which has a greater impact on the use and life of engineering products (equipment) more than a dozen kinds of factors. Engineers who are engaged in the research of environmental conditions of engineering products summarize the environmental conditions that exist in nature and induced by human activities into a series of test standards and norms to guide the environmental and reliability tests of engineering products. For example, to guide military products to conduct environmental tests GJB150 - the People's Republic of China's national military standard "Military equipment Environmental test Method", to guide electrical and electronic products to conduct environmental tests GB2423 - the People's Republic of China's national standard "Electrical and electronic products Environmental Test Method Guide" and so on. Therefore, the main basis for our selection of environmental and reliability test equipment is the test specifications and test standards of engineering products. Secondly, in order to standardize the tolerance of environmental test conditions in test equipment and ensure the control accuracy of environmental parameters, the national technical supervision agency and various industrial departments have also formulated a series of verification regulations for environmental test equipment and testing instruments. For example, the national standard of the People's Republic of China GB5170 "Basic parameter Verification method of environmental testing equipment for electrical and electronic products", and the JJG190-89 "Trial Verification Regulations of Electric Vibration test Bench System" promulgated and implemented by the State Bureau of Technical Supervision. These verification regulations are also an important basis for selecting environmental and reliability test equipment, and test equipment that does not meet the requirements of these verification regulations is not allowed to be put into use.

Second, the basic principles of equipment selection: The selection of environmental and reliability test equipment should follow the following five basic principles: 1, the reproducibility of environmental conditions in the laboratory complete and accurate reproduction of the environmental conditions existing in nature is an elusive thing. However, within a certain tolerance range, people can accurately and approximately simulate the external environmental conditions experienced by engineering products in the process of use, storage, transportation, etc. This paragraph is summarized in engineering language, that is, "the environmental conditions created by the test equipment around the tested product (including the platform environment) should meet the environmental conditions and tolerance requirements specified in the product test specification." For example, the temperature box used for military product testing should not only meet 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 the reproducibility of environmental conditions in environmental tests be guaranteed. 2, repeatability 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 in the same test specification under the environmental test conditions of the test results are comparable, The environmental conditions provided by the environmental test equipment must be reproducible. That is to say, the stress level (such as thermal stress, vibration stress, electrical stress, etc.) applied to the product being tested by the environmental test equipment is consistent with the requirements of the same test specification. The repeatability of the environmental conditions provided by the environmental test equipment is guaranteed by the national metrological verification department after passing the verification procedures formulated by the national technical supervision agency. To this end, it must be required that the environmental test equipment can meet the requirements of the technical indicators and accuracy indicators in the verification regulations, and the use time does not exceed the time limit specified in the verification cycle. Such as the use of very common electric vibration table in addition to meet the exciting force, frequency range, load capacity and other technical indicators, but also must meet the requirements of the verification regulations stipulated in the lateral vibration ratio, table acceleration uniformity, harmonic distortion degree and other accuracy indicators, and the service cycle after each verification is two years, more than two years must be re-verified before it can be put into use. 3. The environmental conditions provided by any environmental test equipment must be observable and controllable, which is not only to limit the environmental parameters within a certain tolerance range, to ensure the reproducibility and repeatability of the test conditions, but also to start from the safety of the product test is also necessary. In order to prevent out-of-control environmental conditions leading to damage to the tested products, bringing unnecessary losses. At present, various test specifications generally require that the accuracy of parameter testing should not be less than one-third of the error allowed by the test conditions. 4. Exclusion of environmental test conditions Each time an environmental or reliability test is carried out, there are strict regulations on the category, quantity value and tolerance of environmental factors, and unless the environmental factors required by the test penetrate them, in order to provide an exact basis for judging and analyzing product failure and failure mode during the test or after the test. Therefore, in addition to providing the specified environmental conditions, the environmental test equipment is not allowed to attach other environmental stress interference to the tested product. For example, the mesa magnetic leakage, the acceleration signal-to-noise ratio, and the total RMS value ratio of the in-band plus speed are limited in the verification regulations of the electric vibration table. Random signal test, harmonic distortion degree and other accuracy indexes are designed to ensure the uniqueness of environmental test conditions. 5, the safety and reliability of the test equipment environmental test, especially the reliability test, the test cycle is long, the test object is sometimes high value military products, during the test process, the test personnel often have to operate around the site patrol or test work, so the environmental test equipment must have the characteristics of safe operation, easy operation, reliable use, long working life, etc. To ensure the normal conduct of the test itself. Various protection, alarm measures and safety interlocking devices of test equipment should be perfect and reliable to ensure the safety and reliability of test personnel, tested products and test equipment itself.

1, volume selection When the tested product (components, components, components or whole machine) is placed into the climate environment chamber for test, in order to ensure that the atmosphere around the tested product can meet the environmental test conditions specified in the test specification, the working size of the climate chamber and the outer size of the tested product should follow the following provisions: 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 not to use more than 10% of the products that heat up in the test. b) The ratio of the upwind fault area of the tested product to the total area of the Test chamber on this section is not 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 should be at least 100 ~ 150mm (150mm is recommended). The above three provisions are actually interdependent and unified. Taking 1 cubic meter cube box as an example, the area ratio of 1: (0.35 ~ 0.5) is equivalent to the volume ratio of 1: (0.207 ~ 0.354). 100 ~ 150mm from the wall of the box is equivalent to a volume ratio of 1: (0.343 ~ 0.512). In summary, the above three points stipulate that the working chamber volume of the climate environment test chamber should be at least 3 to 5 times the outer volume of the tested product. The reasons for this provision are as follows: (1) After the test piece is placed into the box, the smooth channel is squeezed, and the narrowing of the channel will lead to an increase in the airflow velocity. Accelerate the heat exchange between the air flow and the part under test. This is inconsistent with the reproduction of environmental conditions, because in the relevant standards for environmental tests involving temperature, it is stipulated that the air flow rate around the test sample in the test box should not exceed 1.7m/s, in order to prevent the test sample and the surrounding atmosphere from producing unrealistic heat conduction. When the average wind speed in the test box is 0.6 ~ 0.8m/s, not more than 1m/s, when the space and area ratio specified in the requirements of a) and b) is met, the wind speed of the flow field may increase by (50 ~ 100) %, and the average maximum wind speed is (1 ~ 1.7) m/s. Meet the requirements specified in the standard. If the volume of the test piece or the windward section area is increased without restriction in the test, the actual wind speed will increase to exceed the maximum wind speed specified in the test standard, and the validity of the test results will be doubted. (2) The precision indicators of environmental parameters in the working chamber of the climate chamber (such as temperature, humidity, salt spray settlement rate, etc.) are the results of detection in the no-load state. Once placed in the tested part, the uniformity of environmental parameters in the working chamber of the test chamber will be affected, and the larger the space occupied by the test part, the more serious the impact will be. The measured test data show that the temperature difference between the windward side and the leeward side in the flow field can reach 3 ~ 8℃, and it can reach more than 10℃ when it is severe. Therefore, it is necessary to meet the two requirements of a] and b) as far as possible to ensure the uniformity of 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 at the upper and lower limits of high and low temperatures. The temperature of the box wall and the temperature of the flow field near the box wall differ by 2 ~ 3℃ (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 above, therefore, the space within the distance from the box wall (100 ~ 150mm) is unusable space. 2, the choice of temperature range At present, the range 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, and also have 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 are special needs, such as the installation location near the engine and other heat sources of the product, can not blindly increase the temperature limit. Because the higher the upper limit 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 space. On the other hand, the higher the upper temperature value, the higher the heat resistance requirements of the insulation materials (such as glass wool, etc.) in the interlayer of the box wall. The sealing requirements of the box are also higher, so that the production cost of the box increases. 3, the selection of humidity range at home and abroad, the humidity index given by the environmental test box is mostly 20 ~ 98%RH or 30 ~ 98%RH, if the wet heat test box does not dehumidification system, the humidity range is 60 ~ 98%, this type of test box can only do high humidity test, but its price is much lower. It is worth noting that the corresponding temperature range should be indicated after the humidity indicator, or the lowest dew point temperature should be given. Because the relative humidity is directly related to temperature, for the same absolute moisture content, the higher the temperature, the smaller the relative humidity, such as the absolute moisture content is 5g/Kg (refers to 1 kg of dry air contains 5 grams of water vapor), when the temperature is 29℃, the relative humidity is 20%RH, when the temperature is 6℃, the relative humidity is 90%RH, when the temperature drops below 4℃, the relative humidity is 20%RH. If the relative humidity exceeds 100%, there will be condensation in the box. To achieve high temperature and high humidity, it is only necessary to spray steam or atomized water droplets into the box air for humidification. Low temperature and low humidity is relatively difficult to control, because the absolute moisture content at this time is very low, sometimes much lower than the absolute moisture content in the atmosphere, the need to dehumidify the air flowing in the box, so that the air becomes dry. At present, most of the temperature and humidity boxes at home and abroad use the principle of refrigeration dehumidification, which is to add a group of refrigeration light tubes in the air preconditioning room of the box. When the wet air passes through the cold tube, its relative humidity will reach 100%RH, because the air is saturated on the light tube condensation, so that the air becomes drier. This dehumidification method can theoretically reach the dew point temperature below zero, but when the cold point surface temperature reaches 0℃, the dew droplets on the surface of the optical tube will freeze, thus affecting the heat exchange on the surface of the optical tube, so that the dehumidification ability is reduced. And because the box can not be absolutely sealed, the wet air in the atmosphere will penetrate into the box, so that the dew point temperature rises. On the other hand, the wet air flowing between the optical tube only reaches saturation at the moment of contact with the optical tube (cold point) and precipitated water vapor, so this dehumidification method is difficult to make the dew point temperature in the box below 0 ° C. The actual lowest dew point temperature is 5 ~ 7℃. The dew point temperature of 5℃ is equivalent to the absolute moisture content of 0.0055g/Kg, corresponding to the relative humidity of 20%RH temperature of 30℃. If the required temperature is 20 ° C into the relative humidity of 20%RH, at this time the dew point temperature is -3 ° C, the use of refrigeration dehumidification is very difficult, must use the air drying system to achieve. 4, the choice of control mode temperature, humidity test box has a constant test box, alternating test box two cases. Ordinary high and low temperature test chamber generally refers to a constant high and low temperature test chamber, and its control mode is: set a target temperature, and the test chamber has the ability to automatically maintain a constant temperature to the target temperature point. The control mode of the constant Temperature And Humidity Test Chamber is similar, setting a target temperature and humidity point, the test chamber has the ability to automatically maintain a constant temperature to the target temperature and humidity point. The high and low temperature alternating test chamber has the ability to set one or more high and low temperature changes and cycles, the test chamber has the ability to complete the test process according to the preset curve, and can accurately control the rate of heating and cooling within the range of the maximum temperature rise and cooling rate, that is, the rate of heating and cooling can be controlled according to the slope of the set curve. Similarly, the high and low temperature alternating wet heat test chamber also has a preset temperature, humidity curve, and the ability to control according to the preset. Of course, the alternating test box has the function of a constant test box, but the manufacturing cost of the alternating test box is higher, because the alternating test box needs to be configured with a curve automatic recording device, program control instrument, and must solve the problem of opening the refrigerator when the temperature of the test box is higher in the chamber, so the price of the alternating test box is generally more than 20% higher than the price of the constant test box. Therefore, we should be realistic to the needs of the test method as a starting point, choose a constant test chamber or an alternating test chamber. 5, the choice of temperature change rate ordinary high and low temperature test chamber has no cooling speed index, the time from the ambient temperature to the nominal minimum temperature is generally 90 ~ 120min. The high and low temperature alternating test chamber and the high and low temperature alternating wet and heat test chamber have the requirements of changing temperature speed, and the changing temperature rate is generally required to be 1℃/min, and the speed can be adjusted within the range of this rate. The rapid temperature change test chamber has a faster temperature change rate, and the heating and cooling rates can reach 3 ° C /min ~ 15 ° C /min, and the heating and cooling rates can even reach more than 30 ° C /min in some temperature segments. The temperature range of the rapid temperature change test chamber of various specifications and speeds is generally the same, that is, -60 ~ +130℃, but the temperature change range of the examination cooling speed is not the same, according to the different test requirements of the test, the temperature change range of the rapid temperature change test chamber is -55 ~ +80℃, and some are -40 ~ +80℃. There are two formulations for the temperature change rate of the rapid temperature change test chamber, one is the average rise and fall rate of the whole process, and the other is the linear rise and fall rate (in fact, the average speed every 5 minutes). The average speed of the whole process is the ratio of the difference between the highest temperature and the lowest temperature in the temperature range of the test chamber to the time. At present, the technical parameters of variable temperature rate provided by foreign environmental test equipment manufacturers all refer to the average speed of the whole process. Linear rising and cooling rate refers to the temperature change rate that can be guaranteed in any 5min period. In fact, for the rapid temperature change test chamber, the most difficult and critical part of ensuring the linear rise and cooling speed is the cooling rate that the test chamber can achieve within a period of 5 min at the end of the cooling section. From a certain point of view, the linear rising and cooling speed (the average speed every 5 minutes) is more scientific.