The main points of the high and low temperature box are that after the test piece is placed in the box, it squeezes out 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 Heat conduction. When no-load, the average wind speed in the test chamber is 0.6-0.8m / s, not more than 1m / s. When the space and area ratios specified by the two points are met, the wind speed of the flow field may increase by (50-100)%. The average maximum wind speed is (1 ～ 1.7) m / s. Meet the requirements of the standard. If the volume of the test piece or the windward cross-sectional area is increased without restrictions during the test, the airflow wind speed during the actual test will increase to exceed the maximum wind speed specified by the test standard, and the validity of the test results will be doubted. Selection of volume When the test product (components, components, parts or complete machine) is placed in the climate environment box for testing, in order to ensure that the surrounding atmosphere of the test product can meet the environmental test conditions specified in the test specifications, the working size of the climate box and the test product The following points should be followed between the external dimensions. The volume of the test product (W × D × H) must not exceed (20 to 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. Sealing Machine,Pallet Strapping Machine,Carton Tape Sealing Machine,Adhesive Tape Sealing Machine Jining Myway Machinery Co., Ltd. , https://www.mwpackall.com
The high and low temperature box only needs to spray water vapor or atomized water droplets into the box air to achieve high temperature and high humidity. 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 cabinets 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, the air becomes drier. This 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, which affects the heat exchange on the surface of the light pipe and reduces the dehumidification capacity. And because the cabinet cannot be sealed completely, the humid air in the atmosphere will penetrate into the cabinet, 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 ° C. The lowest dew point temperature actually reached is 5-7 ° C. A dew point temperature of 5 ° C corresponds 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. The ordinary high and low temperature test chamber generally refers to the constant high and low temperature test chamber, and its control method is: setting a target temperature, and the test chamber 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 change and cycle programs. The test chamber has the ability to complete the test process according to the preset curve, and 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 presets. Alternating test chambers of high and low temperature chambers have the function of constant test chambers, but the manufacturing cost of alternating test chambers is relatively high, because the alternating test chambers need to be equipped with automatic curve recording devices and program control instruments, and the test chambers must be resolved The problem of turning on the refrigerator when the indoor temperature is high, 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 needs of the test method as the starting point, choose a constant test box or an alternating test box. Humidity indicators given by environmental test chambers at home and abroad are mostly 20-98% RH or 30-98% RH. If the humidity 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 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 is 5g / Kg (refers to 5 grams of water vapor in 1 kg of dry air ), 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.
The working chamber volume of the high and low temperature box should be at least 3 to 5 times the outer volume of the test product. The reasons for this regulation are as follows: The accuracy indicators of the environmental parameters (such as temperature, humidity, salt mist deposition rate, etc.) in the working chamber of the climate chamber are the results of testing under no-load conditions, once placed in the test piece It will affect the uniformity of the environmental parameters in the working chamber of the test chamber. The larger the space occupied by the test piece, the more serious this effect will be. 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 ° C, and may reach 5 ° C at the upper and lower limits of high and low temperatures. The temperature of the box wall is different from the temperature of the flow field near the box wall by 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. Therefore, the distance from the box wall (100 ～ 150mm) The space within the distance is unusable space. The actual test 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 be as high as 10 ℃. Therefore, two requirements must be met as far as possible to ensure the uniformity of the environmental parameters around the test product. 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 of the high and low temperature box is smaller. On the other hand, the higher the upper limit temperature value, the higher the heat resistance requirements of the thermal insulation material (such as glass wool, etc.) in the wall sandwich. The higher the sealing requirements of the box, the higher the manufacturing cost of the box.