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    Wuxi Nanya Technology Co., Ltd., founded in 1996, is a high-tech enterprise integrating R&D, manufacturing and sales. It is located on the beautiful shore of the Taihu Lake Lake.

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    Selection of Temperature and Humidity Reliability Environmental Test Box

    Release time:2021-07-14

    Publisher:Environmental Testing chamber

    Number of views: 113

    1. Selection of volume

    When placing the tested product (component, component, component, or complete machine) into a climate chamber for testing, in order to ensure that the surrounding atmosphere of the tested product can meet the environmental test conditions specified in the test specifications, the following regulations should be followed between the working dimensions of the climate chamber and the overall dimensions of the tested product: a) The volume of the tested product (W × D × H) It should not exceed (20-35)% of the effective working space of the test chamber (20% is recommended). For products that generate heat during the experiment, it is recommended to choose no more than 10%. b) The ratio of the windward cross-sectional area of the tested product to the total area of the chamber chamber on this section should not exceed (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 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, an area ratio of 1: (0.35-0.5) is equivalent to a volume ratio of 1: (0.207-0.354). A distance of 100-150mm from the box wall is equivalent to a volume ratio of 1: (0.343-0.512). In summary, the working chamber volume of the climate environment test chamber should be at least 3-5 times the outer volume of the tested product, as stipulated in the above three points. The reasons for making this regulation 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 lead to an increase in the airflow velocity. Accelerate the heat exchange between the airflow and the tested object. This is inconsistent with the reproduction of environmental conditions, as in relevant standards, it is stipulated that the air flow rate around the test sample in the test chamber should not exceed 1.7m/s for temperature environment tests, in order to prevent the test sample and the surrounding atmosphere from generating unrealistic heat conduction. When the average wind speed in the test chamber is 0.6-0.8m/s under no load, not exceeding 1m/s. When the space and area ratio specified in points a) and b) are met, the wind speed in the flow field may increase by (50-100)%, with an average wind speed of (1-1.7) m/s. Meet the requirements specified in the standard. If the volume or windward cross-sectional area of the test piece is increased without restrictions during the test, the actual airflow speed during the test will increase to exceed the wind speed specified in the test standard, and the validity of the test results will be questioned.


    2. Climate box working chamber environment

    The accuracy indicators of parameters such as temperature, humidity, salt spray settling rate, etc. are all measured under no-load conditions. Once placed in the test piece, it will have an impact on the uniformity of environmental parameters in the working chamber of the test box. The larger the space occupied by the test piece, the more severe this impact will be. The measured experimental data shows that the temperature difference between the windward and leeward surfaces in the flow field can reach 3-8 ℃, and in severe cases, it can reach over 10 ℃. Therefore, it is necessary to meet the requirements of a and b as much as possible to ensure the uniformity of environmental parameters around the tested product. According to the principle of heat conduction, the temperature of the airflow near the box wall is usually 2-3 ℃ different from the temperature at the center of the flow field. At the upper and lower limits of high and low temperatures, it may also reach 5 ℃. The temperature of the box wall differs from the temperature of the flow field near the box wall by 2-3 ℃ (depending on the structure and material of the box wall). The larger the difference between the test temperature and the external atmospheric environment, the greater the above temperature difference. Therefore, the space within a distance of 100-150mm from the box wall is unusable. 2. At present, the range of temperature test chambers in foreign countries is generally between -70 to+180 ℃. Most domestic manufacturers generally operate at -70~+150 ℃. These temperature ranges can usually meet the needs of temperature testing for the vast majority of military and civilian products in China. Unless there are special needs, such as products installed near heat sources such as engines, the upper temperature limit cannot be blindly raised. Because the higher the upper limit temperature, the greater the temperature difference between the inside and outside of the box, and the poorer the uniformity of the flow field inside the box. The smaller the available studio volume. On the other hand, the higher the upper limit temperature value, the higher the heat resistance requirements for insulation materials (such as glass wool) in the interlayer of the box wall. The higher the requirement for sealing of the box, the higher the production cost of the box.


    3. Selection of humidity range


    The humidity indicators provided by domestic and foreign environmental test chambers are mostly 20-98% RH. If there is no dehumidification system in the damp heat test chamber, 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 or dew point temperature should be indicated after the humidity indicator. Because relative humidity is directly related to temperature, for the same moisture content, the higher the temperature, the lower the relative humidity. For example, with a moisture content of 5g/Kg (referring to 5 grams of water vapor in 1 kilogram of dry air), when the temperature is 29 ℃, the relative humidity is 20% RH, and when the temperature is 6 ℃, the relative humidity is 90% RH. When the temperature drops below 4 ℃ and the relative humidity exceeds 100%, condensation will occur inside the box. To achieve high temperature and humidity, only spray steam or atomized water droplets into the box air for humidification. Low temperature and humidity are relatively difficult to control, as the moisture content at this time is very low, sometimes much lower than the moisture content in the atmosphere. It is necessary to dehumidify the air flowing inside the box to make the air dry. At present, the vast majority of temperature and humidity boxes both domestically and internationally use the principle of refrigeration dehumidification, which involves adding a set of refrigeration light tubes in the air conditioning room of the box. When wet air passes through the cold tube, its relative humidity will reach 100% RH, as the air saturates and condenses on the light tube, making the air drier. This dehumidification method can theoretically reach a dew point temperature below zero degrees Celsius, but when the surface temperature of the cold point reaches 0 ℃, the water droplets on the surface of the light tube will freeze, affecting the heat exchange on the surface of the light tube and reducing the dehumidification ability. And because the box cannot be sealed, moist air from the atmosphere will seep into the box, causing the dew point temperature to rise. On the other hand, the moist air flowing between the light tubes only reaches saturation state at the moment of contact with the light tube (cold point) and separates water vapor, so this dehumidification method is difficult to keep the dew point temperature inside the box below 0 ℃. The actual dew point temperature reached is 5-7 ℃. The dew point temperature of 5 ℃ is equivalent to a moisture content of 0.0055g/Kg, corresponding to a relative humidity of 20% RH at a temperature of 30 ℃. If the temperature is required to reach 20 ℃ and the relative humidity reaches 20% RH, the dew point temperature at this time is -3 ℃. It is difficult to use refrigeration for dehumidification, and an air drying system must be selected to achieve this.


    4. Selection of control methods

    There are two types of temperature and humidity test chambers: constant test chamber and alternating test chamber. A regular high and low temperature test chamber generally refers to a constant high and low temperature test chamber, which is controlled by setting a target temperature, and the test chamber has the ability to automatically maintain a constant temperature to the target temperature point. The control method of the constant temperature and humidity test box is also similar, setting a target temperature and humidity point, and the test box has the ability to automatically stabilize to the target temperature and humidity point. The high and low temperature alternating test chamber has a program for setting one or more high and low temperature changes and cycles. The test chamber has the ability to complete the test process according to preset curves, and can accurately control the rate of heating and cooling within the range of heating and cooling rate capabilities. The heating and cooling rate can be controlled according to the slope of the set curve. Similarly, the high and low temperature alternating humidity and heat test chamber also has the ability to preset temperature and humidity curves, and control them according to the preset settings. Of course, high and low temperature alternating current test boxes all have the function of a constant test box, but the manufacturing cost of high and low temperature alternating current test boxes is higher because the alternating current test box needs to be equipped with automatic curve recording devices, program controllers, and also needs to solve problems such as starting the refrigeration system when the temperature in the laboratory is high.


    5. Selection of temperature change rate

    Ordinary high and low temperature test chambers do not have a cooling rate indicator, and the time from ambient temperature to nominal temperature is generally 90 to 120 minutes. Both the high and low temperature alternating test chamber and the high and low temperature alternating damp heat test chamber have requirements for temperature change speed. The temperature change rate generally requires 1 ℃/min, and the speed can be adjusted within this speed range. The rapid temperature change test chamber has a faster temperature change rate, with heating and cooling rates ranging from 3 ℃/min to 20 ℃/min. In some temperature ranges, the heating and cooling rates can even reach over 30 ℃/min. The temperature range of the rapid temperature change test box for various specifications and speeds is generally the same, namely -70~+150 ℃, but the temperature range for assessing the cooling rate is not the same. According to different test requirements, the temperature range of the rapid temperature change test box is -55~+85 ℃, and some are -40~+125 ℃. There are two ways to describe the temperature change rate of a rapid temperature change test chamber, one is the average temperature rise and fall speed throughout the entire process, and the other is the linear temperature rise and fall speed (actually the average speed every 5 minutes). The average speed throughout the entire process refers to the ratio of the difference between temperature and time within the temperature range of the test chamber. At present, the technical parameters of temperature change rate provided by various environmental testing equipment manufacturers abroad refer to the average rate throughout the entire process. The linear temperature rise and fall speed refers to the guaranteed temperature change rate within any 5-minute time period. In fact, for a rapid temperature change test chamber, the difficulty and key part of ensuring the linear temperature rise and fall speed is the cooling rate that the test chamber can achieve during a 5-minute cooling period. From a certain perspective, the linear temperature rise and fall speed (average speed every 5 minutes) is more scientific. Therefore, the experimental equipment has two parameters: the average temperature rise and fall speed throughout the entire process and the linear temperature rise and fall speed (average speed every 5 minutes). Generally speaking, the linear temperature rise and fall speed (average speed every 5 minutes) is half of the average temperature rise and fall speed throughout the entire process.


    6. Wind speed

    According to relevant standards, the wind speed in the temperature and humidity chamber during environmental testing should be less than 1.7m/s. For the test itself, the smaller the wind speed, the better. Excessive wind speed will accelerate the heat exchange between the surface of the test piece and the airflow inside the chamber, which is detrimental to the authenticity of the test. However, in order to ensure uniformity within the chamber, it is necessary to have circulating air inside the chamber. However, in order to pursue the rate of temperature change, it is necessary to accelerate the flow rate of the circulating airflow inside the chamber, which is usually between 2 and 3 meters per second, in the case of a rapid temperature change test chamber and a comprehensive environmental test chamber with multiple factors such as temperature, humidity, and vibration. Therefore, the restrictions on wind speed vary for different purposes of use.


    7. Temperature fluctuation

    Temperature fluctuation is a relatively easy parameter to achieve, and the actual temperature fluctuation of most test chambers produced by all environmental testing equipment manufacturers can be controlled within the range of ± 0.3 ℃.


    8. Uniformity of temperature field

    In order to more accurately simulate the actual environmental conditions experienced by the product in nature, it is necessary to ensure that the surroundings of the tested product are under the same temperature environment conditions in environmental testing. Therefore, it is necessary to limit the temperature gradient and temperature fluctuation inside the test chamber. In the General Principles of Environmental Testing Methods for Military Equipment in the National Military Standard GJB150.1-86, it is clearly stipulated 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 total value should be 2.2 ℃ (the test sample does not work).


    9. Precision control of humidity

    Most humidity measurements in environmental testing chambers use the wet and dry bulb method. The manufacturing standard for environmental testing equipment GB10586 requires a relative humidity deviation of ± 3% RH. To meet the requirements of humidity control accuracy, the temperature control accuracy of the humidity test chamber is relatively high, and the temperature fluctuation is generally less than ± 0.2 ℃. Otherwise, it is difficult to meet the requirements for humidity control accuracy.


    10. Cooling method selection

    If the test chamber is equipped with a refrigeration system, the refrigeration system needs to be cooled. The test chamber comes in two forms: air-cooled and water-cooled.


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