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Knowledge about environmental chamber test
I. Overview
1. Military products must inevitably be affected by environmental conditions in the process of production, storage, transportation and use. There are usually three cases of such environmental conditions:
1)Climatic environment conditions---high temperature, low temperature, humidity, mold, salt fog, sand dust, rain and sunshine, etc.
2) Flight environment conditions - low temperature, low pressure, high temperature, low pressure, temperature shock, acceleration, vibration, impact
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 have continuously introduced their best environmental testing equipment to meet market demand.
During the “Ninth Five-Year Plan†period, more than 60 units of environmental testing equipment such as Beijing, Shanghai, Chengdu, Xi’an, Shenyang, Wuhan, and Guangzhou’s defense systems were used in more than a few hundred units (cases). The statistical analysis of them used in the 1980s accounted for 61. The use of equipment in the 1970s accounted for 10 in the 29th and 60s, and these test equipments played a full role.
2. Military standards now in effect â— US military standard MIL-STO-810
â— GJB150~20-86 equipment environmental testing method â— GB/T5170-96 Basic parameters verification method for environmental testing equipment of electric and electronic products â— GB 2423 basic environmental testing for electric and electronic products â— GB10592-89 high and low temperature test box technical conditions â— GB10589-89 low temperature Technical Conditions of Test Chamber â— Technical Conditions of GB10586-89 Damp Test Chamber
Second, environmental testing equipment measurement problems
1. Temperature impact test and temperature change test Temperature impact test: This test is characterized by the initial and final test of the product performance before and after the test to determine whether the product can withstand the rapid changes in temperature without requiring the product to change in temperature. Work during the process and meet the performance requirements, high and low temperature box in the test process, high and low temperature conversion General provisions of this conversion time is less than 5 minutes (this is also the measurement required by the indicator).
Temperature change test: Assess the product's ability to withstand resistance and normal working capability during temperature changes. Use a test box to limit the rate of temperature change. For example: 10°C/min 15°C/min 20°C/min
2. Temperature tolerance In many environmental test method standards, test condition tolerance is an important technical indicator. It reflects the requirements of the environmental test on the control level of the equipment and reflects the normativeness of the environmental test. Therefore, it is of utmost importance to correctly understand the provisions of the head standards on the tolerances of the test conditions, so as to improve the reliability and scientificity of environmental test results.
The national standard GB2423.1 low temperature test method stipulates that the allowable temperature deviation range is ±3°C. Here, ±3°C does not mean that point or at that time, the requirements of the entire working space.
Another problem that should also be pointed out is that the ±2°C temperature tolerance is the concept of permissible deviation. When the temperature gradient of the GJB150 military standard is 2.2°C maximum, the temperature field uniformity requirement is the relationship between the temperature of each point in a certain instant temperature field. . This temperature tolerance of ±2°C must not be regarded as the setting range of the nominal value. Example 60±2°C
Here 60°C is the nominal value, ±2°C is the tolerance When setting the environmental condition parameter value, the temperature setting value can only be 60°C instead of 62°C, or 58°C, due to the control system drift adjustment In the error and test chamber temperature field inhomogeneity, the test temperature test results can not be 60 °C, the allowable deviation but the range is not more than 62 °C
Or no lower than 58°C
3. Measurement of Relative Humidity in Environmental Test Equipment Relative humidity is an important parameter in environmental tests. Measuring the relative humidity reasonably and accurately is of great significance for ensuring the environmental conditions.
The relative humidity is the ratio of the partial pressure of water vapour in the air and the partial pressure of saturated vapour 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 in the air and the partial pressure of saturated water vapor. It is obtained by measuring the dry bulb temperature and converting it. e=etw-APt-tw
The basis for measuring the relative humidity using the wet-dry bulb method is the air condition. If the relative humidity measured by the dry-wet-bulb method during the temperature change phase is inaccurate, it can only be used as a reference. When the air condition is stable, the relative humidity is only affected by the A value. Impact A=0.0000165+6.75/V
Equation A shows that the value of A depends on the air flow rate through the wet bulb, so the air flow rate through the wet bulb is an important factor affecting the relative humidity measurement. Experiments have shown that when the air flow rate is greater than 2.5 m/s, the relative temperature The influence is not obvious, but the test box is under confined conditions, requiring multiple RH measurement points to be placed in the working space at the time of the inspection. In this way, in the working space generally <1 m3, there will be a number of wind sources that can generate local air flow speeds greater than 2.5 m/s, which will destroy the original working state of the test box. Obviously, it is not appropriate to conclude:
â— Use the wet-bulk method to measure the relative humidity. The above factors are the influencing factors that bring about the accuracy of measuring relative humidity.
â— In the process of measuring the relative humidity using the wet-bulk method, the flow rate of the air through the wet bulb is an important factor affecting the results. It is maintained at 2.5 m/s or more, and the actual test chamber is about 0.6 m/s. If it does not meet the requirements, it can be used. The relative humidity table for the relative airflow speed.
Wet and dry balls for measuring relative humidity:
1) Select two thermometers with the same model and the same basic characteristics. The axis lines of the two thermometers should be parallel. The distance between the thermometers should not be less than three times the total diameter of the wet ball sensor (including wet ball yarns)
2) The cup is best covered with a lid and filled with distilled water. The distance from the water in the cup to the bottom of the wet bulb is 30mm.
3) Wet bulb water penetration is high (70-80) mm. (Required by ZBY157-83)
4) Read out the indication of the wet and dry bulb thermometer, calculate the difference between wet and dry bulb thermometers, and determine the relative humidity value at the visit temperature according to GB6999-86.
4. Nominal value of the environmental test equipment verification The nominal value of the environmental parameter of the working space is one of the important characteristics of the environmental test equipment. It is the basis for calculating the environmental parameter deviation and the basis for the environmental test equipment verification work.
1) Definition of nominal value: The value marked on the gage to indicate its characteristics or guide its use is called the nominal value.
2) Characteristics of the nominal value: It is a measurement result, unique to the measuring instrument, a known given value.
3) The role of the nominal value in the environmental test equipment verification work: 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 standards is an ideal value. The factors that affect 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 closeness 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 set 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.
III. Measuring items, methods and calculations of environmental chambers
1. Explanation of terms:
1) Volatility: The amount of change in the central point of the work space over time when the environmental test equipment is in a steady state.
2) Uniformity: The difference between the test points (temperature and humidity) at a certain moment in the working space under steady state conditions of the environmental test equipment.
3) Deviation: When the equipment is in a stable state, the actual measured maximum value of the measuring points in the working space and the deviation between the measured minimum value and the nominal value.
2, measurement characteristics: temperature and humidity deviation, temperature and humidity fluctuations, temperature and humidity uniformity
3, measurement methods:
1) Measurements are performed 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 equipment use range, and can also select actual commonly used temperature and humidity points according to user needs.
3) Position of the measuring point:
â— When the equipment volume is less than 2m3, there are 9 temperature points and 3 humidity points â— The equipment volume is greater than 2 m3, 15 temperature points and 4 temperature points
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