How to choose power supply testing environmental test equipment

1. Basis for equipment selection
It is currently impossible to count the exact number of natural environmental factors and induced environmental factors existing on the earth's surface and in the atmosphere. Among them, there are no less than a dozen factors that have a great impact on the use and life of engineering products (equipment). Engineers engaged in the study of environmental conditions of engineering products have sorted out and summarized the environmental conditions induced by natural existence and human activities into a series of test standards and specifications to guide the environmental and reliability tests of engineering products. For example, GJB150, the national military standard of the People's Republic of China, "Environmental Test Methods for Military Equipment", guides the environmental testing of military products, and GB2423, the national standard of the People's Republic of China, "Guide to Environmental Test Methods for Electrical and Electronic Products", guides the environmental testing of electrical and electronic products. 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 procedures for environmental test equipment and testing instruments. For example, the National Standard of the People's Republic of China GB5170 "Basic Parameter Verification Methods for Environmental Test Equipment of Electrical and Electronic Products", and the JJG190-89 "Trial Verification Procedures for Electric Vibration Test Bench System" promulgated and implemented by the State Administration of Technical Supervision. These verification procedures are also an important basis for selecting environmental and reliability test equipment. Test equipment that does not meet the requirements of these verification procedures is not allowed to be put into use.

[page]2. Basic principles for equipment selection
The selection of environmental and reliability test equipment should follow the following five basic principles:
1. Reproducibility of environmental conditions
It is impossible to fully and accurately reproduce the environmental conditions existing in nature in the test room. However, within a certain tolerance range, people can correctly and approximately simulate the external environmental conditions that engineering products are subjected to during use, storage, transportation, etc. This passage can be summarized in engineering language, that is, "the environmental conditions (including platform environment) created by the test equipment around the tested product should meet the environmental conditions and tolerance requirements specified in the product test specifications." For example, the temperature chamber used for military product testing must not only meet the requirements of uniformity and temperature control accuracy in the national military standards GJB150.3-86 and GJB150.4-86. Only in this way can the reproducibility of environmental conditions in environmental testing 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 the comparability of the test results of the same product under the environmental test conditions specified in the same test specification, the environmental conditions provided by the environmental test equipment must be repeatable. That is to say, the stress level (such as thermal stress, vibration stress, electrical stress, etc.) applied by the environmental test equipment to the tested product 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 metrology and verification department after passing the verification according to the verification procedures formulated by the national technical supervision agency. To this end, it is necessary to require that the environmental test equipment can meet the requirements of various technical indicators and accuracy indicators in the verification procedures, and the use time does not exceed the time limit specified in the verification cycle. For example, the commonly used electric vibration table must not only meet the technical indicators such as exciting force, frequency range, load capacity, etc., but also meet the requirements of precision indicators such as lateral vibration ratio, table acceleration uniformity, harmonic distortion, etc. specified in the verification procedures. Moreover, the use cycle after each verification is two years. If it exceeds two years, it must be re-verified before it can be put into use.
3. Measurability and controllability of environmental condition parameters
The environmental conditions provided by any environmental test equipment must be observable and controllable. This is not only to limit the environmental parameters within a certain tolerance range and ensure the reproducibility and repeatability of the test conditions, but also necessary from the perspective of product test safety, so as to prevent the environmental conditions from getting out of control and causing damage to the tested product, resulting in 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. Exclusivity of environmental test conditions
Every time an environmental or reliability test is conducted, there are strict regulations on the types, values ​​and tolerances of environmental factors, and environmental factors that are not required for the test are excluded from infiltration, so as to provide an accurate basis for judging and analyzing product failure and failure modes during or after the test. Therefore, it is required that the environmental test equipment, in addition to providing the specified environmental conditions, does not allow other environmental stress interference to the tested product. For example, the table leakage magnetic field, acceleration signal-to-noise ratio, and total root mean square value ratio of acceleration inside and outside the band specified in the verification regulations of the electric vibration table. Accuracy indicators such as random signal inspection and harmonic distortion are all verification items formulated to ensure the uniqueness of environmental test conditions.
5. Safety and reliability of test equipment
Environmental tests, especially reliability tests, have long test cycles, and the test objects are sometimes high-value military products. During the test, test personnel often have to operate and patrol or test around the site. Therefore, environmental test equipment must have the characteristics of safe operation, convenient operation, reliable use, and long working life to ensure the normal progress of the test itself. Various protections, alarm measures and safety interlocking devices of the test equipment should be complete and reliable to ensure the safety and reliability of the test personnel, the tested products and the test equipment itself.

[page] III. Selection of temperature and humidity chamber
1. Selection of volume
When the test product (components, assemblies, parts or complete machines) is placed in a climate chamber for testing, in order to ensure that the atmosphere around the test product can meet the environmental test conditions specified in the test specifications, the working dimensions of the climate chamber and the outer dimensions of the test product should comply with the following provisions:
a) The volume (W×D×H) of the test product shall not exceed (20-35)% of the effective working space of the test chamber (20% is recommended). For products that generate heat during the test, it is recommended to use no more than 10%.
b) The ratio of the windward cross-sectional area of ​​the test product to the total area of ​​the test chamber working room on the cross-sectional area shall not exceed (35-50)% (35% is recommended).
c) The distance between the outer surface of the test 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 a 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). 100-150 mm from the box wall is equivalent to the volume ratio of 1: (0.343-0.512).
To summarize the above three provisions, the working chamber volume of the climate environment test chamber should be at least 3-5 times the outer volume of the tested product. The reasons for making such a provision are as follows:
(1) After the test piece is placed in the box, it occupies the smooth channel, and the narrowing of the channel will lead to an increase in the air flow rate. Accelerate the heat exchange between the airflow and the test piece. This is inconsistent with the reproduction of environmental conditions, because the relevant standards for temperature environment tests stipulate that the air flow rate around the test sample in the test chamber should not exceed 1.7 m/s to prevent the test sample and the surrounding atmosphere from generating unrealistic heat conduction. When the test chamber is unloaded, the average wind speed in the test chamber is 0.6-0.8 m/s, not exceeding 1 m/s. When the space and area ratios specified in the requirements of a) and b) are 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. The requirements specified in the standard are met. If the volume or windward cross-sectional area of ​​the test piece is increased without restriction during the test, the air flow speed will increase to exceed the maximum wind speed specified in the test standard during the actual test, and the validity of the test results will be questioned.
(2) The accuracy indicators of the environmental parameters in the working chamber of the climate chamber (such as temperature, humidity, salt spray deposition rate, etc.) are all the results of testing under no-load conditions. Once the test piece is placed, it will affect the uniformity of the environmental parameters in the working chamber of the test chamber. The larger the space occupied by the test piece, the more serious this impact will be. The actual test data show that the temperature difference between the windward side and the leeward side in the flow field can reach 3-8°C, and in severe cases it can be as large as 10°C or more. Therefore, it is necessary to meet the requirements of a〕 and b〕 as much as possible to ensure the uniformity of the environmental parameters around the tested product.
(3) According to the principle of heat conduction, the temperature of the airflow near the box wall is usually 2 to 3℃ different from the temperature at the center of the flow field, and may reach 5℃ at the upper and lower limits of high and low temperatures. The temperature of the box wall is 2 to 3℃ different from the temperature of the flow field near the box wall (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 above temperature difference. Therefore, the space within a distance of 100 to 150mm from the box wall is unusable space.
2. Selection of temperature range
At present, the range of foreign temperature test chambers is generally -73 to +177℃, or -70 to +180℃. Most domestic manufacturers generally use -80 to +130℃, -60 to +130℃, -40 to +130℃, and there are also high temperatures up to 150℃. These temperature ranges can usually meet the needs of temperature tests for most domestic military and civilian products. Unless there are special needs, such as products installed near heat sources such as engines, the upper temperature limit should not be increased blindly. Because the higher the upper temperature limit, 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 volume. On the other hand, the higher the upper temperature limit, the higher the heat resistance requirements for the insulation materials (such as glass wool, etc.) in the interlayer of the box wall. The higher the requirements for the sealing of the box, the higher the production cost of the box.
[page]3. Selection of humidity range
The humidity indicators given by domestic and foreign environmental test chambers are mostly 20-98%RH or 30-98%RH. If the wet heat test chamber does not have a dehumidification system, the humidity range is 60-98%. This type of test chamber can only perform high humidity tests, but its price is much lower. It is worth noting that the corresponding temperature range should be indicated after the humidity indicator, or the minimum dew point temperature should be given. Because relative humidity is directly related to temperature, for the same absolute moisture content, the higher the temperature, the lower the relative humidity. For example, when the absolute moisture content is 5g/Kg (meaning that 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 exceeds 100%, and condensation will occur in the box.
To achieve high temperature and high humidity, you only need to spray water vapor or atomized water droplets into the air in the box for 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 in the atmosphere, and it is necessary to dehumidify the air flowing in the box to make the air dry. At present, most temperature and humidity chambers at home and abroad adopt the principle of refrigeration and dehumidification, which is to add a group of refrigeration light tubes in the air pre-conditioning room of the box. When the wet air passes through the cold tube, its relative humidity will reach 100%RH, and condensation will form on the light tube due to air saturation, making the air drier. This dehumidification method can theoretically reach a dew point temperature below zero, but when the surface temperature of the cold spot reaches 0℃, the water droplets condensed on the surface of the light tube will freeze, thereby affecting the heat exchange on the surface of the light tube and reducing the dehumidification capacity. Because the box cannot be absolutely sealed, the humid air in the atmosphere will penetrate into the box, causing the dew point temperature to rise. On the other hand, the humid air flowing between the light tubes only reaches a saturated state and precipitates water vapor at the moment of contact with the light tube (cold spot), so this dehumidification method is difficult to make the dew point temperature in the box below 0℃. The actual minimum dew point temperature reached is 5~7℃. The dew point temperature of 5℃ is equivalent to an absolute moisture content of 0.0055g/Kg, and the corresponding temperature of 20%RH is 30℃. If the temperature is required to reach 20%RH at 20℃, the dew point temperature at this time is -3℃, and it is very difficult to dehumidify by refrigeration, and an air drying system must be used to achieve it.

Air-cooled and water-cooled
Conditions of use The equipment is easy to install and can be used by simply connecting the power supply. The ambient temperature should be lower than 28°C. If the ambient temperature is higher than 28°C, it will have a certain impact on the cooling effect (it is best to be equipped with air conditioning). A circulating cooling water system is required.
The heat exchange effect is poor compared to the water cooling method. Stable and good The
impact on the use environment will cause the air temperature around the test chamber to rise. It does not affect the temperature around the test chamber. The
noise is large (relative to the water cooling method) Small
Conclusion The choice of cooling method should be based on the actual situation of the laboratory. If conditions permit, water cooling is preferred. However, if the refrigeration system of the selected test chamber is small, water cooling can also be given priority.

[page]IV. Analysis of the status of temperature and humidity test equipment1
〕The production of temperature and humidity environmental test equipment is a mechanical manufacturing industry that has lasted for decades, and its basic technology (such as refrigeration and heating) has matured. With the advancement and development of industrial technology, many new application technologies have been continuously introduced into environmental test equipment, such as computer technology, fuzzy control theory, multi-box synchronous operation, etc. Therefore, it is difficult to give a manufacturer an arbitrary conclusion on which is better. The equipment produced by each manufacturer with decades of manufacturing experience can be used, and their working principles, applied technologies, and even structures and materials have many similarities. However, due to the constraints of the "multi-non-standard, multi-variety, small batch" production method of environmental test equipment, product reliability has always been a problem that plagues domestic and foreign manufacturers. The key is that the manufacturer must have fine management, complete technology, complete testing methods, and be responsible for quality (such as passing ISO9000 certification). At the same time, environmental test equipment is a durable product, and some problems and design defects need to be exposed after years of use, and the problem can only be found through statistical analysis. A mature and highly reliable product often requires many years of improvement and perfection. Therefore, it is one of the necessary conditions for the environmental test equipment manufacturer to have a certain production scale and production history, which is one of the necessary conditions for the product to have high reliability.
2) Quality control of the production process. The quality control of the domestic production process has been greatly improved. The quality control of the production process is related to the management level and production habits of the manufacturer. Passing the ISO9000 demonstration is a sign that the quality control of the production process is standardized.
Domestic and foreign manufacturers of temperature and humidity environmental test equipment have accumulated rich experience in the production of climate and environmental tests. All the products provided are available for use and are reliable in quality. As for which one should be selected and which model of product meets the requirements better, in addition to some principles and specific technical indicators described in the previous article, there are some factors worthy of attention and consideration.
a) Performance/price ratio.
On the basis of fully verifying the technical parameters and reliability of environmental test equipment, cost performance is an important factor that should be considered. The imported environmental test equipment is sold domestically through agents without exception. The price factor depends not only on the cost and profit of the manufacturer, but also on many intermediate link costs and sales commissions. Under the same manufacturing cost, reducing the intermediate cost is an important factor in reducing the order price. The manufacturing cost of products with relevant technical indicators in the same country is roughly the same. Even if there are differences, the products produced in different countries (referring to developed Western countries) will not be too big.
b) The degree and ability to meet user needs. When
users order a temperature and humidity environment test chamber, their requirements vary according to the needs of product testing. For example, some three-in-one test chambers are often used for product parts or components and environmental stress screening tests. At this time, the forced dehumidification capacity of the equipment and the possibility of adding an air drying system are the items that users should focus on. If some users want to use the three-in-one test chamber for environmental testing (such as wet heat testing), the wind speed adjustability is a factor that must be considered.
c) After-sales service and equipment failure handling capabilities.
It is impossible for any equipment to not fail during use. Once a problem occurs, can the user enjoy timely maintenance services, or can the user quickly determine the fault handling method based on the fault diagnosis capability of the equipment itself, so that the equipment can recover its performance as soon as possible and be put back into use. In this regard, domestic and foreign manufacturers have made or are making positive efforts, such as establishing rapid response maintenance teams, and adding diagnostic and fault warning system software to computer control systems, so that users can handle faults as quickly as possible based on the information provided by the computer.