I. Overview
1. Military products inevitably invade with environmental conditions during production, storage, transportation and use. There are usually three situations in which such environmental conditions invade:
1) Climate and environmental conditions: high temperature, low temperature, humidity, mold, salt fog, sand dust, rain and sunshine, etc.
2) Flight environment conditions Low temperature and low pressure, high temperature and low pressure, temperature shock, acceleration, vibration, shock
3) Transportation environmental conditions Collision, vibration, impact, etc. In order to adapt to and withstand the invasion of these environmental conditions, domestic and foreign manufacturers continue to introduce their own best environmental testing equipment to meet market demand. During the "Ninth Five-Year Plan" period, more than 60 units of national defense systems in seven regions (Beijing, Shanghai, Chengdu, Xi'an, Shenyang, Wuhan, and Guangzhou) put in hundreds of environmental testing equipment (boxes). Statistical analysis included the 61%, put into use in the 1970s accounted for 29%, put into use in the 1960s accounted for 10%, these test equipment have played a full role.
2. The current military standards â— US military standard MIL-STO-810
â— GJB150 ~ 20-86 military equipment environmental test method â— GB / T5170-96 electrical and electronic products environmental test equipment basic parameter verification method â— GB 2423 electrical and electronic products basic environmental test â— GB10592-89 high and low temperature test chamber technical conditions â— GB10589-89 Low temperature test chamber technical conditions â— GB10586-89 wet and hot test chamber technical conditions
2. Several measurement problems of environmental test equipment
1. Temperature shock test and temperature change test Temperature shock test: This test is characterized by initial and final product performance tests only before and after the test to determine whether the product can withstand sharp changes in temperature, and does not require the product to change in temperature Work in the process and meet the performance requirements. During the test, the high and low temperature conversion of the high and low temperature box generally stipulates that the conversion time is less than 5 minutes (this is also the index required for measurement).
Temperature change test: To test the resistance and normal working ability of the product during temperature change, use a test box to limit the rate of temperature change. For example: 10 ℃ / min 15 ℃ / min 20 ℃ / min
2. Temperature tolerance In many environmental test method standards, the test condition tolerance is an important technical indicator, which reflects the requirements of environmental testing on the control level of the equipment and reflects the standardization of environmental testing. Therefore, it is of utmost importance to correctly understand the provisions of the heading standards on the tolerance of test conditions, thereby improving the reliability and scientificity of environmental test results.
The national standard GB2423.1 low temperature test method stipulates the allowable temperature deviation range of ± 3 ℃, where ± 3 ℃ does not refer to that point or at that time, the emu's requirements for each point of the entire working space.
Another problem that should be pointed out is that the temperature tolerance of ± 2 ℃ is the concept of allowable deviation. When the temperature gradient of the GJB150 military standard is 2.2 ℃ at the maximum, the uniformity of the temperature field is the relationship between the temperature of each point in the temperature field . The temperature tolerance of ± 2 ° C should never be regarded as the setting range of the nominal value. Example 60 ± 2 ℃
Here 60 ℃ is the nominal value, ± 2 ℃ is the tolerance. When setting the environmental condition parameter value, the temperature setting value can only be 60 ℃, not 62 ℃, or 58 ℃, due to the drift adjustment of the control system In the error and the non-uniformity of the temperature field of the test box, the actual measurement result of the test temperature cannot be 60 ℃, the allowable deviation is not more than 62 ℃
Or not lower than 58 ℃
3. Measurement of relative humidity in environmental testing equipment Relative humidity is an important parameter in environmental testing. It is important to ensure that the relative humidity is measured reasonably and accurately.
The relative humidity is the ratio of the partial pressure of water vapor in the air to the partial pressure of saturated water vapor at the same temperature u = e / es * 100%, RS
The relative humidity in the test chamber is usually not obtained by measuring the partial pressure of water vapor and the saturated partial pressure of water vapor. It is obtained by measuring the dry bulb temperature and converting it. E = etw-AP (t-tw)
The basis of measuring the relative humidity using the wet and dry bulb method is the air condition. If the relative humidity is measured by the wet and dry bulb method during the temperature change stage, it is not accurate and can only be used as a reference. In the case of stable air, the relative humidity is only affected by the A value. Influence A = 0.00001 (65 + 6.75 / V)
According to the formula, the value of A depends on the flow velocity of the air flowing through the wet bulb, so the flow velocity of the air flowing through the wet bulb is an important factor that affects the measurement of relative humidity. The experiment proves that when the air flow velocity is greater than 2.5m / s, the relative temperature The effect is not obvious, but the test box is under closed conditions, and it is required to arrange multiple relative humidity measurement points in the work space during the inspection. In this way, in the working space generally <1 m3, there will be multiple wind sources that can produce local airflow velocity greater than 2.5 m / s, which will destroy the original working state of the test chamber, which is obviously inappropriate:
â— Use the wet and dry bulb method to measure relative humidity. The above factors are the factors that influence the accuracy of measuring relative humidity.
â— The process of measuring relative humidity with the wet and dry bulb method, the airflow velocity through the wet bulb is an important factor that affects the result, keep above 2.5 m / s, and the actual test chamber is about 0.6m / s, if it can not meet the requirements, it can be used Relative humidity lookup table for the corresponding air velocity.
Method for measuring relative humidity with wet and dry bulbs:
1) Select two thermometers with the same model and basically the same characteristics. The axes of the two thermometer sensors should be parallel, and the distance between the thermometers should not be less than 3 times the total diameter of the wet ball sensor (including wet ball yarn sleeve)
2) The water cup should be covered and covered with distilled water. The distance between the water surface and the bottom of the wet bulb in the water cup is 30mm.
3) The penetration height of wet bulb water is (70 ~ 80) mm. (Required by ZBY157-83)
4) Read the indication values ​​of the wet and dry bulb thermometers respectively, calculate the difference between the wet and dry bulb thermometers, and find out the relative humidity value under the visiting temperature according to GB6999-86.
4. Nominal value in the verification of environmental test equipment The nominal value of the working space environmental parameter is one of the important characteristics of the environmental test equipment, the basis for calculating the deviation of the environmental parameters, and the basis of the verification work of the environmental test equipment.
1) Definition of nominal value: The value marked on the measuring tool to indicate its characteristics or guide its use is called the nominal value.
2) The characteristic of the nominal value: it is a measurement result, unique to the measuring instrument, and a known value.
3) The role of the nominal value in the verification work of environmental test equipment: the environmental conditions required in the environmental test are provided by the test equipment, and the nominal value of the environmental conditions specified in the test method standard is an ideal value. This factor, which affects the actual value of the environmental conditions provided by the test equipment, can only be close to the nominal value and cannot reach the nominal value. The degree of proximity depends on the accuracy of the test equipment control system. Its role is reflected in two aspects:
â— The nominal value is the basis for the setting value of the test equipment control system.
â— The nominal value is the basis for calculating the deviation of environmental conditions at various points in the working space of the test equipment.
3. Measurement items, methods and calculations of the environmental test chamber
1. Explanation of terms:
1) Fluctuation: the amount of change of the parameter of the center point of the working space with time when the environmental test equipment is in a stable state.
2) Uniformity: the difference between the test points (temperature and humidity) at a certain moment in the working space of the environmental test equipment in a stable state.
3) Deviation: In the stable state, the measured maximum and minimum measured values ​​of the measurement points in the working space deviate from the nominal value.
2. Measurement characteristics: temperature and humidity deviation, temperature and humidity fluctuation, temperature and humidity uniformity
3. Measurement method:
1) The measurement is carried out under no-load conditions. If there is a load, the load should be stated in the certificate.
2) The measurement of turbidity and humidity points should generally select the upper limit, lower limit and center point of the use range of the equipment, and the actual commonly used temperature and humidity points can also be selected according to user needs
3) Location of measuring point:
â— When the equipment volume is less than 2m3, there are 9 temperature points and 3 humidity points
â— Equipment volume is greater than 2 m3, 15 temperature points, 4 temperature points
Calculation (take temperature as an example)
Fluctuation: △ tf = ± (tomax-tomin) / 2
Uniformity: △ tu = Σ (ttmax-ttmin) / n
I = 1
Deviation: â–³ Tmax = Tmax-T standard â–³ Tmin = Tmin-T standard
1. Military products inevitably invade with environmental conditions during production, storage, transportation and use. There are usually three situations in which such environmental conditions invade:
1) Climate and environmental conditions: high temperature, low temperature, humidity, mold, salt fog, sand dust, rain and sunshine, etc.
2) Flight environment conditions Low temperature and low pressure, high temperature and low pressure, temperature shock, acceleration, vibration, shock
3) Transportation environmental conditions Collision, vibration, impact, etc. In order to adapt to and withstand the invasion of these environmental conditions, domestic and foreign manufacturers continue to introduce their own best environmental testing equipment to meet market demand. During the "Ninth Five-Year Plan" period, more than 60 units of national defense systems in seven regions (Beijing, Shanghai, Chengdu, Xi'an, Shenyang, Wuhan, and Guangzhou) put in hundreds of environmental testing equipment (boxes). Statistical analysis included the 61%, put into use in the 1970s accounted for 29%, put into use in the 1960s accounted for 10%, these test equipment have played a full role.
2. The current military standards â— US military standard MIL-STO-810
â— GJB150 ~ 20-86 military equipment environmental test method â— GB / T5170-96 electrical and electronic products environmental test equipment basic parameter verification method â— GB 2423 electrical and electronic products basic environmental test â— GB10592-89 high and low temperature test chamber technical conditions â— GB10589-89 Low temperature test chamber technical conditions â— GB10586-89 wet and hot test chamber technical conditions
2. Several measurement problems of environmental test equipment
1. Temperature shock test and temperature change test Temperature shock test: This test is characterized by initial and final product performance tests only before and after the test to determine whether the product can withstand sharp changes in temperature, and does not require the product to change in temperature Work in the process and meet the performance requirements. During the test, the high and low temperature conversion of the high and low temperature box generally stipulates that the conversion time is less than 5 minutes (this is also the index required for measurement).
Temperature change test: To test the resistance and normal working ability of the product during temperature change, use a test box to limit the rate of temperature change. For example: 10 ℃ / min 15 ℃ / min 20 ℃ / min
2. Temperature tolerance In many environmental test method standards, the test condition tolerance is an important technical indicator, which reflects the requirements of environmental testing on the control level of the equipment and reflects the standardization of environmental testing. Therefore, it is of utmost importance to correctly understand the provisions of the heading standards on the tolerance of test conditions, thereby improving the reliability and scientificity of environmental test results.
The national standard GB2423.1 low temperature test method stipulates the allowable temperature deviation range of ± 3 ℃, where ± 3 ℃ does not refer to that point or at that time, the emu's requirements for each point of the entire working space.
Another problem that should be pointed out is that the temperature tolerance of ± 2 ℃ is the concept of allowable deviation. When the temperature gradient of the GJB150 military standard is 2.2 ℃ at the maximum, the uniformity of the temperature field is the relationship between the temperature of each point in the temperature field . The temperature tolerance of ± 2 ° C should never be regarded as the setting range of the nominal value. Example 60 ± 2 ℃
Here 60 ℃ is the nominal value, ± 2 ℃ is the tolerance. When setting the environmental condition parameter value, the temperature setting value can only be 60 ℃, not 62 ℃, or 58 ℃, due to the drift adjustment of the control system In the error and the non-uniformity of the temperature field of the test box, the actual measurement result of the test temperature cannot be 60 ℃, the allowable deviation is not more than 62 ℃
Or not lower than 58 ℃
3. Measurement of relative humidity in environmental testing equipment Relative humidity is an important parameter in environmental testing. It is important to ensure that the relative humidity is measured reasonably and accurately.
The relative humidity is the ratio of the partial pressure of water vapor in the air to the partial pressure of saturated water vapor at the same temperature u = e / es * 100%, RS
The relative humidity in the test chamber is usually not obtained by measuring the partial pressure of water vapor and the saturated partial pressure of water vapor. It is obtained by measuring the dry bulb temperature and converting it. E = etw-AP (t-tw)
The basis of measuring the relative humidity using the wet and dry bulb method is the air condition. If the relative humidity is measured by the wet and dry bulb method during the temperature change stage, it is not accurate and can only be used as a reference. In the case of stable air, the relative humidity is only affected by the A value. Influence A = 0.00001 (65 + 6.75 / V)
According to the formula, the value of A depends on the flow velocity of the air flowing through the wet bulb, so the flow velocity of the air flowing through the wet bulb is an important factor that affects the measurement of relative humidity. The experiment proves that when the air flow velocity is greater than 2.5m / s, the relative temperature The effect is not obvious, but the test box is under closed conditions, and it is required to arrange multiple relative humidity measurement points in the work space during the inspection. In this way, in the working space generally <1 m3, there will be multiple wind sources that can produce local airflow velocity greater than 2.5 m / s, which will destroy the original working state of the test chamber, which is obviously inappropriate:
â— Use the wet and dry bulb method to measure relative humidity. The above factors are the factors that influence the accuracy of measuring relative humidity.
â— The process of measuring relative humidity with the wet and dry bulb method, the airflow velocity through the wet bulb is an important factor that affects the result, keep above 2.5 m / s, and the actual test chamber is about 0.6m / s, if it can not meet the requirements, it can be used Relative humidity lookup table for the corresponding air velocity.
Method for measuring relative humidity with wet and dry bulbs:
1) Select two thermometers with the same model and basically the same characteristics. The axes of the two thermometer sensors should be parallel, and the distance between the thermometers should not be less than 3 times the total diameter of the wet ball sensor (including wet ball yarn sleeve)
2) The water cup should be covered and covered with distilled water. The distance between the water surface and the bottom of the wet bulb in the water cup is 30mm.
3) The penetration height of wet bulb water is (70 ~ 80) mm. (Required by ZBY157-83)
4) Read the indication values ​​of the wet and dry bulb thermometers respectively, calculate the difference between the wet and dry bulb thermometers, and find out the relative humidity value under the visiting temperature according to GB6999-86.
4. Nominal value in the verification of environmental test equipment The nominal value of the working space environmental parameter is one of the important characteristics of the environmental test equipment, the basis for calculating the deviation of the environmental parameters, and the basis of the verification work of the environmental test equipment.
1) Definition of nominal value: The value marked on the measuring tool to indicate its characteristics or guide its use is called the nominal value.
2) The characteristic of the nominal value: it is a measurement result, unique to the measuring instrument, and a known value.
3) The role of the nominal value in the verification work of environmental test equipment: the environmental conditions required in the environmental test are provided by the test equipment, and the nominal value of the environmental conditions specified in the test method standard is an ideal value. This factor, which affects the actual value of the environmental conditions provided by the test equipment, can only be close to the nominal value and cannot reach the nominal value. The degree of proximity depends on the accuracy of the test equipment control system. Its role is reflected in two aspects:
â— The nominal value is the basis for the setting value of the test equipment control system.
â— The nominal value is the basis for calculating the deviation of environmental conditions at various points in the working space of the test equipment.
3. Measurement items, methods and calculations of the environmental test chamber
1. Explanation of terms:
1) Fluctuation: the amount of change of the parameter of the center point of the working space with time when the environmental test equipment is in a stable state.
2) Uniformity: the difference between the test points (temperature and humidity) at a certain moment in the working space of the environmental test equipment in a stable state.
3) Deviation: In the stable state, the measured maximum and minimum measured values ​​of the measurement points in the working space deviate from the nominal value.
2. Measurement characteristics: temperature and humidity deviation, temperature and humidity fluctuation, temperature and humidity uniformity
3. Measurement method:
1) The measurement is carried out under no-load conditions. If there is a load, the load should be stated in the certificate.
2) The measurement of turbidity and humidity points should generally select the upper limit, lower limit and center point of the use range of the equipment, and the actual commonly used temperature and humidity points can also be selected according to user needs
3) Location of measuring point:
â— When the equipment volume is less than 2m3, there are 9 temperature points and 3 humidity points
â— Equipment volume is greater than 2 m3, 15 temperature points, 4 temperature points
Calculation (take temperature as an example)
Fluctuation: △ tf = ± (tomax-tomin) / 2
Uniformity: △ tu = Σ (ttmax-ttmin) / n
I = 1
Deviation: â–³ Tmax = Tmax-T standard â–³ Tmin = Tmin-T standard
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