1. Equipment selection basis
The types of natural environmental factors and induced environmental factors that exist on the surface of the earth and in the atmospheric space cannot be counted at present. Among them, there are more than a dozen factors that have a greater impact on the use and life of engineering products (equipment). Engineers engaged in the research of environmental conditions of engineering products summarize the environmental conditions induced by the existence of nature and human activities into a series of test standards and specifications to guide the environmental and reliability tests of engineering products. For example, to guide military products to conduct environmental tests GJB150-the National Military Standard of the People's Republic of China "Environmental Test Method for Military Equipment", GB2423 to guide electrical and electronic products to conduct environmental tests-National Standard of the People's Republic of China "Guide to Environmental Test Methods for Electrical and Electronic Products" . Therefore, Hanyang Technology, 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 agency and various industrial departments have also formulated a series of verification procedures for environmental test equipment and testing instruments. Such as the People's Republic of China National Standard GB5170 "Electrical and Electronic Products Environmental Test Equipment Basic Parameter Verification Method", and the JJG190-89 "Electric Vibration Test Bench System Trial Verification Regulations" promulgated and implemented by the State Technical Supervision Bureau. 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.
2. 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 beyond expectation to completely and accurately reproduce the environmental conditions existing in nature in the laboratory. However, within a certain tolerance range, people can completely and accurately simulate the external environmental conditions experienced by engineering products during use, storage, and transportation. This paragraph is summarized in the language of engineering, that is, "The environmental conditions (including the 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 box used for military product testing must 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 is in the environmental test specified by the same test specification The comparability of the test results obtained under the conditions inevitably requires the repeatability of the environmental conditions provided by the environmental test equipment. This means that 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 metrological verification department in accordance with 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 various technical indexes and accuracy indexes in the verification regulations, and the use time should not exceed the time limit specified in the verification cycle. If the commonly used electric vibration table meets the technical indicators such as excitation force, frequency range, load capacity, etc., it must also meet the requirements of precision indicators such as lateral vibration ratio, table acceleration uniformity, harmonic distortion, etc. as specified in the verification regulations. And the use period after each verification is two years, and more than two years must be re-verified before it can be put into use.
3. Measurability 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 to a certain tolerance range, but also to ensure the reproducibility and repeatability of the 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 bringing unnecessary losses. At present, various test specifications generally require that the accuracy of the parameter test should not be less than one-third of the error allowed by the test conditions.
4. Exclusiveness of environmental test conditions
Each time an environmental or reliability test is conducted, there are strict regulations on the types, values ​​and tolerances of environmental factors, and the environmental factors that are not required for the test are excluded from penetration, so as to judge and analyze the product failure during the test or after the test. When the fault mode is n, provide an accurate basis. Therefore, in addition to providing the specified environmental conditions, the environmental test equipment is not allowed to add other environmental stress interference to the tested product. The magnetic flux leakage, acceleration signal-to-noise ratio, and the ratio of the total root-mean-square value of acceleration in and out of band as defined in the electric vibration table verification regulations. 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 testing, especially reliability testing, has a long test period. The object of the test is sometimes a high-value military product. During the test, the test personnel often have to conduct patrols or test work around the site, so the environmental test equipment must be operational. Safety, easy operation, reliable use, long working life and other characteristics to ensure the normal conduct 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 product and the test equipment itself.
Third, the choice of temperature and humidity box
1. Selection of volume
When placing the tested product (components, components, parts or complete machine) in the climate environment box for testing, in order to ensure that the atmosphere around the tested product can meet the environmental test conditions specified in the test specifications, the working size of the climate box and the tested product The following points should be followed between the outer dimensions:
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 to select no more than 10% for products that generate heat during the test.
b) The ratio of the windward cross-sectional area of ​​the tested product to the total area of ​​the test chamber studio on the cross-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 box should be kept at least 100-150mm, (150mm is recommended).
The above three points are actually interdependent and unified. Taking a cubic meter box as an example, the area ratio of 1: (0.35 to 0.5) is equivalent to the volume ratio of 1: (0.207 to 0.354). 100 to 150 mm from the wall of the box corresponds to a volume ratio of 1: (0.343 to 0.512).
In summary, the above three points stipulate that the working 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, it squeezes the smooth channel, and the narrowing of the channel will cause the increase of 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-related environmental tests all stipulate that the air velocity around the test sample in the test chamber should not exceed 1.7m / s to prevent the test sample and the surrounding atmosphere from generating unrealistic results. Heat conduction. When the no-load average wind speed in the test chamber is 0.6 ～ 0.8m / s, no more than 1m / s, and the space and area ratios specified in a) and b) are met, the wind speed of the flow field may increase (50 ~ 100)%, the average maximum wind speed is (1 ~ 1.7) m / s. Meet the requirements of the standard. If the volume or upwind cross-sectional area of ​​the test piece is increased without restrictions during the test, the actual airflow wind speed will increase beyond the maximum wind speed specified in the test standard during the actual test, and the validity of the test results will be doubted.
2) Accuracy indicators of environmental parameters (such as temperature, humidity, salt fog settling rate, etc.) in the working chamber of the climate chamber are the results of testing under no-load conditions.Once the test piece is placed, the environment inside the working chamber of the test chamber The uniformity of the parameters will have an effect. The larger the space occupied by the test piece, the more serious this effect will be. The actual test data show that the temperature difference between the windward and leeward surfaces in the flow field can reach 3-8 ℃, and in severe cases it can be as high as 10 ℃. Therefore, the two requirements a) and b) must be met 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 air flow near the wall of the box usually differs from the center temperature of the flow field by 2 to 3 ℃, and it may reach 5 ℃ at the upper and lower limits of high and low temperatures. The difference between the temperature of the box wall and the temperature of the flow field near the box wall is 2 to 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 above temperature difference. (100 ～ 150mm) The space within the distance is unusable space.
2. Selection of temperature range
At present, the range of foreign temperature test chambers is generally -73 ～ + 177 ℃, or -70 ～ + 180 ℃. Most domestic manufacturers are generally -80 ~ +130 ℃, -60 ~ +130 ℃, -40 ~ +130 ℃, there are high temperatures to 150 ℃. These temperature ranges can usually meet the needs of most domestic military and civilian product temperature tests. Unless there are special needs, such as products installed near the engine and other heat sources, the upper temperature limit cannot be blindly increased. Because the higher the upper limit temperature, the greater the temperature difference between inside and outside the box, and the worse the uniformity of the flow field inside the box. The available studio size is smaller. On the other hand, the higher the upper limit temperature value, the higher the heat resistance requirement of the thermal insulation material (such as glass wool, etc.) in the wall sandwich. The higher the sealing requirements of the cabinet, the higher the manufacturing cost of the cabinet.
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 damp heat test chamber does not have a dehumidification system, the humidity range is 60-98%. 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 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 containing 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%, condensation will occur in the box phenomenon.
To achieve high temperature and high humidity, only water vapor or atomized water droplets need to be sprayed into the air of the cabinet 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 the air flowing inside the cabinet needs to be dehumidified to make the air dry. At present, most of the temperature and humidity boxes at home and abroad use the principle of refrigeration and dehumidification, which is to add a group of refrigeration light pipes in the air conditioning room of the cabinet. When the humid air passes through the cold pipe, its relative humidity will reach 100% RH, because the air is saturated and condenses on the light pipe, making the air drier. This dehumidification method can theoretically reach a dew point temperature below zero, but when the cold spot surface temperature reaches 0 ° C, the water droplets on the surface of the light pipe will freeze, which affects the heat exchange on the surface of the light pipe and reduces the dehumidification capacity. And because the box cannot be sealed completely, 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 pipes only reaches saturation at the moment of contact with the light pipe (cold spot) to precipitate water vapor, so it is difficult for this dehumidification method to make the dew point temperature in the cabinet below 0 ℃. The lowest dew point temperature actually reached is 5-7 ° C. A dew point temperature of 5 ° C is equivalent to an absolute moisture content of 0.0055g / Kg, and a 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 at this time is -3 ° C. It is very difficult to use the cooling method to dehumidify. An air drying system must be used to achieve this.
4. Selection of control method
There are two cases of temperature and humidity test chamber: constant test chamber and alternating test chamber.
The ordinary high and low temperature test box generally refers to the constant high and low temperature test box, and its control method is: setting a target temperature, the test box has the ability to automatically thermostat to the target temperature point. The control and control method of the constant temperature and humidity test box is also similar. Set a target temperature and humidity point. The test box has the ability to automatically thermostat to the target temperature and humidity point. The high and low temperature alternating test chamber has 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 it can be accurately controlled within the range of the maximum heating and cooling rate capability The heating and cooling 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 preset temperature and humidity curves, and the ability to control according to the preset. Of course, the alternating test chamber has 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 an automatic curve recording device, a program control instrument, and the test chamber must also be solved in the work room. When the temperature is higher, the problem of turning on the refrigerator and so on, therefore, the price of the alternating test chamber is generally more than 20% higher than the price of the constant test chamber. Therefore, we should be realistic and take the need of the test method as the starting point, choose a constant test box or an alternating test box.
5. Selection of variable temperature rate
Ordinary high and low temperature test chambers do not have an index of cooling rate, and the time from the ambient temperature 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 heat and humidity test chamber all have the requirements of temperature change rate, and the temperature change rate generally requires 1 ℃ / min, and the speed can be adjusted within the range of this rate. The temperature change rate of the rapid temperature change test chamber is faster, and the heating and cooling rate can reach 3 ℃ / min ～ 15 ℃ / min. In some temperature sections, the heating and cooling rate can even reach more than 30 ℃ / min.
The temperature range of the rapid temperature change test box of various specifications and speeds is generally the same, that is, -60 ~ +130 ℃, but the temperature change range for evaluating the cooling rate is not the same. According to different test requirements of the test, the rapid temperature change Some of the temperature range of the test chamber is -55 ～ + 80 ℃, and some is -40 ～ + 80 ℃.
Regarding the temperature change rate of the rapid temperature change test chamber, there are two ways to mention it. One is the average temperature rising and falling speed of Hanyang Technology, and the other is the linear temperature rising and falling speed (actually the average speed every 5 minutes). The average speed of the whole process refers to the ratio of the difference between the highest temperature and the lowest temperature to the time within the variable temperature range of the test chamber. At present, the technical parameters of the temperature change rate provided by various foreign environmental test equipment manufacturers refer to the overall average rate. The linear temperature rising and falling speed refers to the rate of temperature change that can be guaranteed in any period of every 5 minutes. In fact, for the rapid temperature change test chamber, it is the most difficult and most critical period to ensure the linear temperature rise and fall speed. In the last 5 minutes of the temperature fall period, the temperature fall rate that the test chamber can reach. From a certain perspective, the linear temperature rise and fall speed (average speed every 5min) is more scientific. Therefore, it is best for the test equipment to have two parameters: the whole average temperature rising and falling speed and the linear temperature rising and falling speed (average speed every 5 minutes). Generally speaking, the linear heating and cooling speed (average speed every 5min) is 1/2 of the average heating and cooling speed of the whole process.
6. Wind speed
The relevant standards stipulate that the wind speed in the temperature and humidity box should be less than 1.7m / s during the environmental test.For the test itself, the smaller the wind speed, the better.If the wind speed is too large, the heat exchange between the surface of the test piece and the flowing air inside the box will be accelerated. The authenticity of the test is unfavorable. But 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 box and the multi-factor comprehensive environmental test box such as temperature, humidity, vibration, etc., in order to pursue the temperature change rate, the velocity of the circulating air flow in the box must be accelerated, and the wind speed is usually 2 to 3 m / s. Therefore, for different purposes, the restriction of wind speed is different.
7. Temperature fluctuation
Temperature fluctuation is a relatively easy parameter to realize. The actual temperature fluctuation of most test chambers produced by all environmental test equipment manufacturers can be controlled within ± 0.3 ℃.
8. Uniformity of temperature field
In order to more accurately simulate the actual environmental conditions suffered by the product in nature, it must be ensured that the periphery of the tested product is at the same temperature and environmental conditions during the environmental test. For this reason, the temperature gradient and temperature in the test box must be Volatility is limited. The general provisions of the national military standard GJB150.1-86 military equipment environmental test methods clearly stipulate that "the temperature of the measurement system near the test sample should be within ± 2 ℃ of the test temperature, and its temperature should not exceed 1 ℃ / m or the maximum value. It is 2.2 ℃ (the test sample does not work) ".
9. Precision control of humidity
The humidity in the environmental test box is mostly measured by the wet and dry bulb method. The manufacturing standard of environmental test equipment GB10586 requires that the relative humidity deviation should be within ± 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 ℃. Otherwise, it is difficult to meet the requirements of humidity control accuracy.
10. Cooling method selection
If the test box is equipped with a refrigeration system, the refrigeration system needs to be cooled. The test box has two forms: air cooling and water cooling.
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