First, the basis for equipment selection
The types of natural environmental factors and induced environmental factors that exist in the earth's surface and the atmosphere are not yet counted. There are no more than a dozen factors that have a greater impact on the use and life of engineering products (equipment). Engineers engaged in the study of environmental conditions of engineering products classify the environmental conditions induced by nature and human activities into a series of test standards and specifications to guide the environmental and reliability tests of engineering products. For example, instructing military products to conduct environmental tests GJB150 - National Military Standard of the People's Republic of China "Environmental Test Methods for Military Equipment", GB2423 for guiding environmental testing of electrical and electronic products - National Standard of the People's Republic of China "Guidelines for Environmental Testing Methods for Electrical and Electronic Products", etc. . Therefore, the main basis for our selection of environmental and reliability test equipment is the test specifications and test standards for 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 agencies and various industrial departments have also formulated a series of verification procedures for environmental test equipment and instruments. Such as the People's Republic of China national standard GB5170 "Electrical and electronic products environmental test equipment basic parameters verification method", and the National Technical Supervision Bureau issued JJG190-89 "Electric Vibration Test Bench System Trial Verification Regulations" and so on. 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.
Second, the basic principles of equipment selection
The selection of environmental and reliability test equipment should follow the following five basic principles:
1. Reproducibility of environmental conditions
It is an elusive thing to completely and accurately reproduce the environmental conditions in nature in the laboratory. However, within a certain tolerance range, people can correctly and correctly simulate the external environmental conditions experienced by engineering products during use, storage, transportation, and the like. This passage is summarized in the language of the project, that is, "the environmental conditions (including the platform environment) created around the test equipment created by the test equipment should meet the environmental conditions and tolerances specified in the product test specifications." For example, the temperature box used for military product testing should not only meet the requirements of different uniformity and temperature control accuracy in the national military standard GJB150.3-86 and GJB150.4-86. 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 one engineered product may also be tested in different environmental test equipment in order to ensure that the same product is in the same test test. The comparability of the test results obtained under the conditions necessarily requires the environmental conditions provided by the environmental test equipment to be reproducible. That is to say, the stress level (such as thermal stress, vibration stress, electrical stress, etc.) applied to the product under test 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 according to the verification procedures formulated by the national technical supervision agency. To this end, environmental testing equipment must be required to meet the requirements of various technical indicators and accuracy indicators in the verification procedures, and the time limit specified in the verification period should not exceed the time limit. If the use of the very common electric vibration table meets the technical indicators such as excitation force, frequency range, load capacity, etc., it must also meet the requirements of the accuracy index such as lateral vibration ratio, mesa acceleration uniformity and harmonic distortion specified in the verification regulations. And the use period after each verification is two years, more than two years must be re-qualified before they can be put into use.
3. Measureability and control of environmental condition parameters
The environmental conditions provided by any environmental test equipment must be observable and controllable, not only to limit environmental parameters to within certain tolerances, but also to ensure reproducibility and repeatability of test conditions. Moreover, it is necessary to start from the safety of the product test, in order to prevent the damage of the test product caused by the out of control of the environmental conditions, resulting in unnecessary loss. At present, the accuracy of the parameter test generally required in various test specifications 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 carried out, there are strict regulations on the types, magnitudes and tolerances of environmental factors, and the environmental factors required for non-tests are excluded from infiltration to judge and analyze product failures during or after the test. In the case of failure mode, the exact basis is provided. Therefore, the environmental test equipment is not allowed to add other environmental stress interference to the test product except for the specified environmental conditions. For example, the leakage magnetic field of the table, the acceleration signal-to-noise ratio, and the ratio of the total rms value of the in-band and out-of-band acceleration are defined in the electric vibration table verification procedure. Accuracy indicators such as random signal test and harmonic distortion are all verification items 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 test objects are sometimes high-value military products. During the test, testers often have to conduct inspections or test work around the site. Therefore, environmental test equipment must be operated. Safe, easy to operate, reliable to use, long working life, etc., to ensure the normal operation of the test itself. The various protection, warning 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.
Third, the choice of temperature and humidity box
1, the choice of volume
When the test product (components, components, components or complete machine) is placed in the climatic environment box for testing, in order to ensure that the atmosphere around the test product can meet the environmental test conditions specified in the test specification, the working size of the climate chamber and the product to be tested The following points should be observed between the dimensions:
a) The volume of the test product (W×D×H) shall not exceed 20% to 35% of the effective working space of the test chamber (20% recommended). It is recommended to use no more than 10% for products that generate heat during the test.
b) The ratio of the windward area of ​​the test product to the total area of ​​the test chamber on the section is not more than (35 to 50)% (35% is recommended).
c) The distance between the outer surface of the test product and the wall of the test chamber shall be at least 100~150mm (150mm is recommended).
The above three points are actually interdependent and unified. Taking a 1 cubic meter cube box as an example, the area ratio is 1: (0.35 to 0.5), which corresponds to a volume ratio of 1: (0.207 to 0.354). The ratio of the volume to the volume of 100 to 150 mm from the wall of the tank is 1: (0.343 to 0.512).
In summary, the above three points stipulate that the working chamber volume of the climatic 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 in the box, the smooth passage is occupied, and the narrowing of the passage will result in an increase in the flow rate of the air flow. Accelerate the heat exchange between the gas stream and the test piece. This is inconsistent with the reproduction of environmental conditions, because in the relevant standards, the temperature flow around the test specimens in the test chamber should not exceed 1.7 m/s, so as to prevent the test specimens and the surrounding atmosphere from being produced. Heat transfer. When the average wind speed in the test chamber is vacant at 0.6-0.8 m/s and does not exceed 1 m/s, the wind speed of the flow field may increase when the space and area ratio specified by the two requirements of a) and b) are met. ~100)%, the average maximum wind speed is (1 to 1.7) m/s. Meet the requirements of the standard. If the volume of the test piece or the wind-breaking area is increased without restriction in the test, the airflow velocity 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 suspected.
2) The accuracy index of the environmental parameters in the working chamber of the climate chamber (such as temperature, humidity, salt spray settling rate, etc.) is the result of the test under no-load condition. Once the test piece is placed, the working environment of the test chamber The uniformity of the parameters will have an effect, and the greater the space occupied by the test piece, the more serious this effect will be. The measured test data shows that the temperature difference between the windward and leeward sides in the flow field can reach 3-8 °C, and in severe cases it can be as large as 10 °C. Therefore, the two requirements a) and b) must be met as much as possible to ensure the uniformity of the environmental parameters around the test product.
3) According to the principle of heat conduction, the temperature of the airflow near the tank wall is usually 2 to 3 °C from the center temperature of the flow field, and may reach 5 °C at the upper and lower limits of the high and low temperature. The temperature of the tank wall is different from the temperature of the flow field near the tank wall by 2~3 °C (depending on the structure and material of the tank wall). The greater the difference between the test temperature and the external atmospheric environment, the greater the above temperature difference, therefore, the distance from the tank wall The space within the distance (100 to 150 mm) is an unusable space.
2, the choice of temperature range
At present, the range of foreign temperature test chambers is generally -73 to +177 °C, or -70 to +180 °C. Most domestic manufacturers generally have -80 to +130 ° C, -60 to +130 ° C, -40 to +130 ° C, and high temperatures to 150 ° C. These temperature ranges usually meet the needs of most domestic military and civilian products for temperature testing. Unless there are special needs, such as installing products close to the heat source such as the engine, the temperature upper limit should not be blindly increased. Because the higher the upper limit temperature, the greater the temperature difference between the inside and outside of the tank, the worse the uniformity of the flow field inside the tank. The smaller the available studio volume. On the other hand, the higher the upper limit temperature value, the higher the heat resistance requirement for the heat insulating material (such as glass wool, etc.) in the tank wall interlayer. The higher the sealing requirements of the cabinet, the higher the manufacturing cost of the cabinet.
3, the choice of humidity range
The humidity indicators given by the environmental test chambers at home and abroad are mostly 20 to 98% RH or 30 to 98% RH. If the damp heat test chamber does not have a dehumidification system, the humidity range is 60 to 98%. This type of test chamber can only be high. Wet 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 minimum 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 smaller the relative humidity, such as the absolute moisture content of 5g / Kg (refers to 1 kg of dry air containing 5 grams of water vapor) ), when the temperature is 29 ° C, the relative humidity is 20% RH, the temperature is 6 ° C, the relative humidity is 90% RH, when the temperature drops below 4 ° C, the relative humidity exceeds 100%, condensation will appear in the box phenomenon.
To achieve high temperature and high humidity, it is only necessary to spray water vapor or atomized water droplets into the air of the cabinet to humidify. Low temperature and low humidity are relatively difficult to control because the absolute moisture content at this time is very low, and sometimes it is much lower than the absolute moisture content in the atmosphere. It is necessary to dehumidify the air flowing in the tank to make the air dry. At present, most of the temperature and humidity chambers at home and abroad adopt the principle of cooling and dehumidification, and a set of cooling tubes are added in the air pre-conditioning room of the cabinet. When the humid air passes through the cold pipe, its relative humidity will reach 100% RH, and the air will become drier due to air saturation on the light pipe. The dehumidification method can theoretically reach a dew point temperature below zero, but when the surface temperature of the cold spot reaches 0 ° C, the water droplets on the surface of the light pipe will freeze, thereby affecting the heat exchange on the surface of the light pipe, and the dehumidification ability is lowered. And because the box can not be absolutely sealed, the humid air in the atmosphere will penetrate into the box, so that the dew point temperature will rise. On the other hand, the humid air flowing between the light pipes is saturated with water at the moment of contact with the light pipe (cold spot), so that the dehumidification method is difficult to make the dew point temperature in the case below 0 °C. The lowest dew point actually achieved is 5 to 7 °C. The dew point temperature of 5 ° C corresponds to an absolute moisture content of 0.0055 g/Kg, and the temperature corresponding to a relative humidity of 20% RH is 30 ° C. If the required temperature is 20 ° C and the relative humidity reaches 20% RH, the dew point temperature is -3 ° C at this time. It is very difficult to dehumidify by means of refrigeration. It must be done by using an air drying system.
4, the choice of control methods
The temperature and humidity test chambers have two conditions: constant test chamber and alternating test chamber.
Ordinary high and low temperature test chambers generally refer to constant high and low temperature test chambers, which are controlled by setting a target temperature and the ability of the test chamber to automatically constant temperature to the target temperature point. The control method of the constant temperature and humidity test chamber is similar. Set a target temperature and humidity point. The test chamber has the ability to automatically adjust the temperature to the target temperature and humidity point. The high and low temperature alternating test chamber has the program of setting one or more high and low temperature changes and circulation. The test chamber has the ability to complete the test process according to the preset curve, and can accurately control within the range of maximum heating and cooling rate capability. The rate of temperature rise and temperature drop, that is, the temperature rise and temperature drop rate can be controlled according to the slope of the set curve. Similarly, the high and low temperature alternating heat and humidity test chamber also has a preset temperature and humidity curve, and the ability to control according to presets. Of course, the alternating test chambers all have the function of a constant test chamber, but the manufacturing cost of the alternating test chamber is higher, because the alternating test chamber needs to be equipped with a curve automatic recording device, a program control device, and the test chamber must be solved in the working chamber. When the temperature is high, the refrigerator is turned on. Therefore, the price of the alternating test chamber is generally 20% higher than the price of the constant test chamber. Therefore, we should be realistic and take the test method as the starting point, choose a constant test box or an alternating test chamber.
5, the choice of temperature change rate
Ordinary high and low temperature test chambers have no index of cooling rate, and the time from ambient temperature cooling to the nominal minimum temperature is generally 90 to 120 minutes. The high and low temperature alternating test chamber and the high and low temperature alternating damp heat test chamber all have the requirements of variable temperature speed, and the variable temperature rate generally requires 1 ° C / min, and the speed is adjustable within the range of this rate. The rapid temperature change test chamber has a faster temperature change rate, and the temperature rise and temperature drop rate can reach 3 ° C / min ~ 15 ° C / min. In some temperature ranges, the temperature rise and temperature drop rate can even reach 30 ° C / min or more.
The temperature range of the rapid temperature change test chamber of various specifications and speeds is generally the same, that is, -60～+130°C, but the temperature change range of the evaluation cooling speed is not the same. According to the different test requirements of the test, the rapid temperature change The test chamber has a temperature range of -55 to +80 °C, and some are -40 to +80 °C.
There are two ways to change the temperature change rate of the rapid temperature change test chamber. One is the average temperature rise and fall speed, and the other is the linear temperature rise and fall speed (actually the average speed per 5 min). The average speed of the whole process is the ratio of the difference between the highest temperature and the lowest temperature to the time in the temperature range of the test chamber. At present, the technical parameters of the variable temperature rate provided by the foreign environmental test equipment manufacturers refer to the whole average rate. The linear temperature rise and fall speed refers to the variable temperature rate that can be guaranteed in any 5 min period. In fact, for the rapid temperature change test chamber, the most difficult and critical part of ensuring the linear temperature rise and fall is the cooling rate that can be achieved by the test chamber during the last 5 min period of the cooling section. From a certain perspective, the linear temperature rise and fall (average speed per 5 min) is more scientific. Therefore, the test equipment preferably has two parameters of the average average temperature rise and fall speed and the linear temperature rise and fall speed (average speed per 5 min). In general, the linear temperature rise and fall (average speed per 5 min) is 1/2 of the average average temperature rise and fall.
6, wind speed
According to the relevant standards, the wind speed in the temperature and humidity chamber should be less than 1.7m/s when conducting environmental tests. For the test itself, the wind speed is as small as possible. The wind speed is too high to accelerate the heat exchange between the surface of the test piece and the flowing airflow in the tank. The authenticity of the test is unfavorable. However, in order to ensure the uniformity of the test chamber, it is necessary to have circulating air in the test chamber. However, the rapid temperature change test chamber and the multi-factor comprehensive environmental test chamber such as temperature, humidity and vibration, in order to pursue the temperature change rate, the flow velocity of the circulating airflow in the tank must be accelerated, and the wind speed is usually 2 to 3 m/s. Therefore, the wind speed limit is different for different purposes.
7, temperature fluctuations
Temperature fluctuations are a relatively easy to implement parameter. The actual temperature fluctuations of most test chambers produced by all environmental test equipment manufacturers can be controlled within ±0.3 °C.
8, temperature field uniformity
In order to more accurately simulate the actual environmental conditions suffered by the product in nature, it is necessary to ensure that the periphery of the tested product is at the same temperature and environmental conditions in the environmental test. For this reason, the temperature gradient and temperature fluctuation in the test chamber must be Limit it. In the national military standard GJB150.1-86 military equipment environmental test method general provisions clearly stipulate "the temperature of the measurement system near the test sample should be within ± 2 ° C of the test temperature, the temperature of the work does not exceed 1 ° C / m or the total maximum It is 2.2 ° C (test sample does not work).
9, humidity precision control
Most of the humidity measured in the environmental test chamber is dry and wet ball method, and the manufacturing standard GB10586 is required. The relative humidity deviation should be ±23% RH. In order to meet the requirements of humidity control accuracy, the temperature control accuracy of the humidity test chamber is high, and the temperature fluctuation is generally less than ±0.2 °C. Otherwise it is difficult to meet the requirements of humidity control accuracy.
10, cooling method selection
If the test chamber has a refrigeration system, the refrigeration system needs to be cooled. The test chamber is available in both air-cooled and water-cooled versions.
Air-cooled Water-cooled Conditions of use The device is easy to install and can be used only when the power is turned on. The ambient temperature should be lower than 28 ° C. If the ambient temperature is higher than 28 ° C, the cooling effect will be affected (preferably equipped with air conditioning). Poor heat transfer (relative to water cooling) Stable, good impact on the environment can cause the temperature of the air around the chamber to rise. Does not affect the temperature around the test chamber. Larger noise (relative to water cooling)Conclusion The choice of cooling method should be based on the actual situation of the laboratory. When conditions permit, priority is given to the choice of water cooling. However, if the cooling system of the selected test chamber is small, it can also be given priority.
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