How to buy a satisfactory constant temperature and humidity box The choice of the volume of the inner box When placing the tested product (components, components, components or the complete machine) into the climate environment box for testing, the atmosphere around the tested product can be satisfied The environmental test conditions specified in the test specification, the dimensions of the climate chamber and the outer dimensions of the tested product shall comply with the following points:
a) The volume of the product under test (W × D × H) shall not exceed (20 ~ 35)% of the working space of the test chamber (20% is recommended). It is recommended not to use more than 10% of the products that produce heat during the test.
b) The ratio of the cross-wind area of ​​the product under test to the total area of ​​the chamber of the test chamber on the section shall not be greater than (35-50)% (35% is recommended).
c) The distance between the outer surface of the product being tested and the wall of the test chamber shall be at least 100-150mm (150mm recommended).
The above three provisions are actually interdependent and unified. Taking a cubic cubic box as an example, the area ratio of 1: (0.35 to 0.5) corresponds to a volume ratio of 1: (0.207 to 0.354). The ratio of 100 to 150 mm from the box wall corresponds to a volume ratio of 1: (0.343 to 0.512).
In summary, the above three requirements stipulate that the working chamber volume of the climate environment test box should be at least 3 to 5 times the volume of the outline of the tested product. The reasons for making this provision are the following:
1) After the test piece is placed in the box, it occupies a smooth channel. The narrowing of the channel will increase the airflow velocity. Accelerate the heat exchange between the air flow and the test piece. This is inconsistent with the reproducibility of environmental conditions, because in the relevant standards for temperature-related environmental tests, it is stipulated that the flow velocity of air around the test sample in the test box should not exceed 1.7m/s, so as to prevent the test sample and the ambient atmosphere from being produced inconsistent with the actual conditions. Heat conduction. When the average wind speed in the test chamber is 0.6-0.8m/s at no-load, not exceeding 1m/s, and the space and area ratios specified in the requirements of a) and b) are satisfied, the wind speed of the flow field may increase (50 ~ 100)%, with an average maximum wind speed of (1 to 1.7) m/s. Meet the requirements of the standard. If the volume of the test piece or the area facing the wind is increased without limit in the test, the actual wind speed of the test will increase beyond the maximum wind speed specified in the test standard, and the validity of the test result will be doubted.
2) The accuracy index of environmental parameters (such as temperature, humidity, and salt spray deposition rate) in the working chamber of the climate chamber is the result of testing under no-load conditions. Once the test object is placed, the environment of the chamber of the test chamber is The uniformity of the parameters will have an effect, and the greater the space occupied by the test pieces, the more serious this effect will be. The experimental test data shows that the temperature difference between the windward side and the leeward side in the flow field can reach 3~8°C, and when it is severe, it can be up to 10°C or more. Therefore, it is necessary to meet the requirements of a) and b) as much as possible so as 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 box wall is usually 2~3°C different from the center temperature of the flow field, and may reach 5°C when the upper and lower limits of high and low temperatures are reached. The temperature of the tank wall is different from the temperature of the flow field near the tank wall by 2 to 3°C (depending on the structure and material of the tank wall). The greater the difference between the test temperature and the external atmospheric environment, the greater the above temperature difference, and therefore, the distance from the tank wall. (100-150mm) The space within the distance is not available space.
The choice of two temperature ranges At present, the scope of foreign temperature test chambers is generally -73 ~ +177 °C, or -70 ~ +180 °C. Most domestic manufacturers are generally -80 ~ +130 °C, -60 ~ +130 °C, -40 ~ +130 °C, there are high temperatures to 150 °C. These temperature ranges can usually meet the needs of the vast majority of domestic military and civilian product temperature tests, unless there is a special need, such as the location of the installation close to the engine and other heat sources, can not blindly increase the temperature limit. Because the higher the upper limit temperature, the greater the temperature difference between the inside and outside of the box, the worse the uniformity of the flow field inside the box. The available studio volume is smaller. On the other hand, the higher the upper limit temperature value, the higher the heat resistance of the insulating material (such as glass wool, etc.) in the interlayer sandwiched in the box wall. The higher the box sealing requirements, the higher the manufacturing cost of the box.
The choice of the three humidity ranges The humidity indicators given by domestic and international environmental test chambers are mostly 20 to 98% RH or 30 to 98% RH. If the humidity test chamber has no dehumidification system, the humidity range is 60 to 98%. This type of test The box can only be tested for high humidity, 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 lower the relative humidity, such as the absolute moisture content of 5g/Kg (refers to 1g of dry air containing 5g of water vapor ) When the temperature is 29°C, the relative humidity is 20%RH, and the relative humidity is 90%RH when the temperature is 6°C. When the temperature drops below 4°C, 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 are sprayed into the air in the box to humidify it. Low temperature and low humidity are relatively difficult to control, because the absolute moisture content at this time is very low, and sometimes much lower than the absolute moisture content in the atmosphere, and the air flowing in the box needs to be dehumidified to make the air dry. At present, most of the domestic and foreign temperature and humidity cabinets use the principle of refrigeration and dehumidification. A set of cooling light tubes is added to the air conditioning room of the cabinet. When the humid air passes through the cold pipe, its relative humidity will reach 100% RH, and the air will become more dry as the air condenses on the light pipe. This kind of dehumidification method can theoretically reach the 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 will affect the heat exchange on the surface of the light pipe, and degrade the dehumidification capacity. And because the box cannot be absolutely sealed, the humid air in 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 pipes only reaches the saturated state at the moment of contact with the light pipe (cold spot) and water vapor is precipitated. Therefore, it is difficult for the dehumidification method to make the dew point temperature in the cabinet below 0° C. The lowest dew point temperature actually achieved is 5-7°C. The dew point temperature of 5°C corresponds to an absolute moisture content of 0.0055 g/Kg, and the 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, and it is very difficult to use the cooling mode to dehumidify. The air drying system must be used to achieve this.
The choice of four control methods The temperature and humidity test box has two cases: constant test box and alternating test box.
An ordinary high-low temperature test box generally refers to a constant high-low temperature test box. Its control method is: setting a target temperature, the test box has the ability to automatically thermostat to the target temperature point. The control method of the constant temperature and humidity test box is also similar. A target temperature and humidity point are set. The test box has the ability to automatically thermostat the target temperature and humidity point. The high and low temperature alternating test chamber has one or more programs for changing the temperature and the cycle. The test chamber has the ability to complete the test process according to the preset curve, and can accurately control the temperature within the range of the maximum temperature increase and cool rate. The rate of temperature increase and temperature drop can be controlled according to the slope of the set curve. In the same way, the high-low temperature alternating humidity tester also has preset temperature and humidity curves, and the ability to control according to presetting. Of course, the alternating test chamber has the function of a constant test chamber, but the manufacturing cost of the alternating test chamber is high, because the alternating test chamber needs to be equipped with an automatic curve recording device, a program control device, and the test chamber must be solved in the working chamber. When the temperature is high, the refrigerator and the like are turned on. Therefore, the price of the alternating test box is generally higher than the price of the constant test box by more than 20%. Therefore, we should seek truth from facts and take the test method as the starting point. We should use a constant test box or an alternating test box.
The choice of five variable temperature rates The common high and low temperature test boxes have no indication of the cooling rate. The time from ambient temperature cooling 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 humidity and heat test chamber have the requirements of temperature change rate. The temperature change rate is generally required to be 1°C/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 rate of temperature rise and fall can reach 3°C/min to 15°C/min. The temperature rise and temperature decrease rate can reach 30°C/min or more in some temperature zones.
The temperature range of rapid temperature change test chambers of various specifications and speeds are generally the same, ie, -60 to +130°C, but the temperature change range for checking the cooling rate is not the same. According to the test requirements for different tests, rapid temperature change test The temperature range of the box is -55 ~ +80 °C, and some -40 ~ +80 °C.
There are two ways to describe the temperature change rate of the rapid temperature change test chamber, one is the average temperature rise and fall speed in the whole course, and the other is the linear temperature rise and fall speed (actually, the average speed every 5 minutes). The overall average speed refers to the ratio of the difference between the maximum temperature and the minimum temperature to the time within the variable temperature range of the test box. At present, the technical parameters of the temperature change rate provided by foreign environmental test equipment manufacturers all refer to the average rate of the whole process. The linear temperature rise/fall rate refers to the temperature change rate that can be guaranteed in every arbitrarily 5 minute period. In fact, for a rapid temperature change test box, the most difficult and most critical step to ensure the linear temperature rise and fall is the cooling rate that the test box can achieve during the last 5 min of the cooling section. From a certain point of view, the linear temperature rise and lower speed (average speed every 5 minutes) is more scientific. Therefore, the test equipment is best to have the two parameters of the average lift temperature rate and the linear lift temperature rate (average speed every 5 minutes). In general, the linear temperature rise and fall speed (average speed every 5 minutes) is 1/2 of the average speed of the whole journey.
The relevant standards for six wind speeds stipulate that the wind speed in the temperature and humidity cabinet should be less than 1.7m/s during the environmental test. For the test itself, the smaller the wind speed, the better. The wind speed exceeds the speed of the accelerated heat exchange between the surface of the test piece and the flow in the tank. The authenticity of the test is unfavorable. However, in order to ensure the uniformity of the chamber of the test chamber, it is necessary to have circulating air in the chamber. However, rapid temperature change test chambers and multi-factor integrated environmental test chambers such as temperature, humidity, and vibration, in order to pursue a variable temperature rate, must accelerate the flow rate of circulating air in the box, and the wind speed is usually 2 to 3 m/s. Therefore, for different purposes of use, the speed limit is not the same.
VII Temperature fluctuation Temperature fluctuation is a relatively easy to implement parameter. The actual temperature fluctuation of most test chambers produced by all environmental testing equipment manufacturers can be controlled within ±0.3°C.
Octal field uniformity In order to more accurately simulate the actual environmental conditions that the product is subjected to in nature, it is necessary to ensure that the periphery of the test product is at the same temperature and environmental conditions in the environmental test. For this purpose, the temperature gradient inside the test box must be And temperature fluctuations are limited. In the general rule of the GJB150.1-86 military equipment environmental test method, it is stipulated that “the temperature of the measurement system near the test sample should be within ±2°C of the test temperature, and its temperature should not exceed 1°C/m or the total maximum value. 2.2 °C (test sample does not work)".
Nine Humidity Accuracy Control Environment The measurement humidity in the environmental test chamber is mostly wet and dry bulb method. The environmental test equipment manufacturing standard GB10586 requirements, relative humidity deviation should be ± 23% RH. In order to meet the requirements of humidity control accuracy, the temperature control accuracy of the humidity chamber is high, and the temperature fluctuation is generally less than ±0.2°C. Otherwise it is difficult to meet the requirements of humidity control accuracy.
Ten Cooling Mode Selection Test Chambers If you have a refrigeration system, the cooling system needs to be cooled. Test chambers are available in air-cooled and water-cooled versions. Air-cooled and water-cooled operating conditions The equipment is easy to install and only needs to be turned on. The ambient temperature should be lower than 28°C. If the ambient temperature is higher than 28°C, it will have a certain influence on the cooling effect (preferably with air conditioning). The circulating cooling water system must be configured.
Poor heat transfer effect (relative to water cooling method) Stable and good influence on the use environment will cause the air temperature around the test chamber to rise. Does not affect the temperature around the chamber. Larger noise (relative to the water cooling method) Smaller conclusions The choice of cooling method should be based on the actual situation in the laboratory. When conditions permit, priority should be given to choosing a water cooling method. However, if the cooling system of the selected test chamber is small, the water cooling method may be preferred. Air-cooled and water-cooled operating conditions The equipment is easy to install. It only needs to be turned on. The ambient temperature should be lower than 28°C. If the ambient temperature is higher than 28°C, it will have a certain influence on the cooling effect (preferably equipped with air conditioning). The circulating cooling water system must be configured.
Poor heat transfer effect (relative to water cooling method) Stable and good influence on the use environment will cause the air temperature around the test chamber to rise. Does not affect the temperature around the chamber.
Larger noise (relative to the water cooling method) Smaller conclusions The choice of cooling method should be based on the actual situation in the laboratory. When conditions permit, priority should be given to choosing a water cooling method. However, if the cooling system of the selected chamber is small, water cooling can also be a priority.
Analysis of Condition of Temperature and Humidity Test Equipment
1) The production of temperature and humidity environmental testing equipment is a machinery manufacturing industry that has lasted for decades. The basic technologies (such as cooling and heating) have matured. With the advancement and development of industrial technology, many new application technologies are continuously introduced into environmental testing equipment, such as computer technology, fuzzy control theory, and multi-chamber synchronous operation. Therefore, it is difficult for manufacturers to create new products. Poorly conclusive conclusions. The equipment produced by various manufacturers with decades of manufacturing experience can be used. The working principle, the applied technology, and even the structure and materials all have many similarities. However, due to the constraints of the production methods of “non-standard, multi-variety, and small-batch†environmental testing equipment, the reliability of the product has always been a problem that has plagued domestic and foreign manufacturers. The key lies in the fact that the manufacturer must be meticulous in management, complete in process, complete in testing means, and responsible for quality (eg through ISO9000 demonstration). At the same time, the environmental testing equipment is a kind of durable goods. Some problems and design flaws need to be used for many years before they can be exposed. Only by statistical analysis can the problems be discovered. A mature, highly reliable product often requires years of improvement and improvement. Therefore, environmental testing equipment manufacturers have a certain scale of production and production history, which is one of the necessary conditions for products with high reliability.
2) Quality control of the production process. The quality control of the domestic production process has greatly improved. The quality control of the production process is related to the management level and production habits of the manufacturer. The demonstration through the ISO9000 is a sign that the quality control of the production process has gone to standardization.
Domestic and foreign manufacturers of temperature and humidity environmental testing equipment have accumulated rich experience in the production of climate and environmental testing. All are available for use and are reliable in quality. As to which one should be selected, which one is more in line with the requirements, in addition to some of the principles and specific technical indicators mentioned in the previous section, there are some other factors that are worthy of consideration and consideration.
a) Performance/price ratio.
On the basis of fully validating the technical parameters and reliability of environmental testing equipment, cost-effectiveness is an important factor that should be considered. The import of environmental testing equipment, without exception, is sold through agents in the country. The price factor depends not only on the manufacturer’s costs and profits, but also includes many intermediate costs and sales commissions. Under the same manufacturing cost, reducing intermediate fees is an important factor in reducing the price of orders. Products with the same technical indicators in the same country and manufacturing costs are roughly the same. Products produced by different countries (western developed countries) will not be too different even if there are differences. .
b) The degree and ability to meet user needs.
The user orders a temperature and humidity environmental test box whose requirements vary according to the needs of product testing. For example, three comprehensive test boxes are often used for product components or components and environmental stress screening tests, at this time the equipment's forced dehumidification capacity and additional air The possibility of a drying system is a project that the user should focus on. If some users want to use three comprehensive test chambers for environmental testing (such as a damp heat test), the wind speed adjustability is a factor that must be considered.
c) After-sales service and equipment failure processing capabilities.
It is impossible for any one device to fail without failure. Once a problem occurs, the user can enjoy timely maintenance service, or can quickly determine the troubleshooting method according to the device's own fault diagnosis capability, so that the device can restore performance as soon as possible. And put it back into use. In this regard, domestic and foreign manufacturers have already or are making active efforts, such as the establishment of a rapid response maintenance team, such as adding diagnostic and fault warning system software in computer control systems, users can provide information as soon as possible according to the computer Deal with the fault.
