Selection Guide for Temperature and Humidity Environment Test Chamber

oneBasic principles for equipment selection

1Reproducibility of environmental conditions

It is impossible to fully reproduce the environmental conditions that exist in nature in the laboratory. However, within a certain tolerance range, people can accurately and approximately simulate the external environmental conditions that engineering products undergo during use, storage, transportation, and other processes. This statement can be summarized in engineering language as follows:“The environmental conditions around the tested product created by the experimental equipment（Including platform environment）Should meet the environmental conditions and tolerance requirements specified in the product testing specifications'The temperature chamber used for military product testing should not only meet the national military standardsGJB150.3-86TheGJB150.4-86The high and low temperature test values and test times specified for different types of products should also meet the requirements for temperature field uniformity and temperature control accuracy in the test specifications. Only in this way can the reproducibility of environmental conditions be ensured in environmental testing.

2Repeatability of environmental conditions

An environmental testing equipment may be used for multiple tests of the same type of product, and a tested engineering product may also be tested in different environmental testing equipment. In order to ensure the comparability of test results obtained for the same product under the same environmental testing conditions specified in the testing specifications, it is necessary to require the environmental conditions provided by the environmental testing equipment to be reproducible. That is to say, the stress level applied to the tested product by the environmental testing equipment（Such as thermal stress, vibration stress, electrical stress, etc）The requirements for the same test specification are consistent.

The repeatability of environmental conditions provided by environmental testing equipment is guaranteed by the national metrological verification department after passing the verification according to the verification regulations formulated by the national technical supervision agency. Therefore, it is necessary to require environmental testing equipment to meet the requirements of various technical indicators and accuracy indicators in the calibration regulations, and to not exceed the time limit specified in the calibration cycle in terms of usage time. If a very common electric vibration table is used, in addition to meeting technical indicators such as excitation force, frequency range, and load capacity, it must also meet the requirements of precision indicators such as lateral vibration ratio, table acceleration uniformity, harmonic distortion, etc. specified in the calibration regulations. Moreover, the service life after each calibration is two years, and after two years, it must be re calibrated and qualified before being put into use.

3Measurability of environmental condition parameters

The environmental conditions provided by any environmental testing equipment must be observable and controllable. This is not only to limit the environmental parameters within a certain tolerance range and ensure the reproducibility and repeatability requirements of the test conditions, but also necessary for the safety of product testing, in order to prevent damage to the tested product and unnecessary losses caused by uncontrolled environmental conditions. At present, various experimental standards generally require that the accuracy of parameter testing should not be less than one-third of the allowable error under experimental conditions.

4Exclusion of environmental testing conditions

Each environmental or reliability test conducted has strict regulations on the category, magnitude, and tolerance of environmental factors, and excludes the infiltration of non test required environmental factors, in order to provide accurate basis for judging and analyzing product failure and failure modes during or after the test. Therefore, it is required that environmental testing equipment not only provide the specified environmental conditions, but also not allow additional environmental stress interference to the tested product. As defined in the verification regulations for electric vibration tables, the table leakage magnetic flux, acceleration signal-to-noise ratio, and total root mean square value ratio of in band and out of band acceleration. The accuracy indicators such as random signal inspection and harmonic distortion are all established as verification items to ensure the reliability of environmental testing conditions.

5Safety and reliability of experimental equipment

Environmental testing, especially reliability testing, has a long testing cycle and sometimes targets high-value military products. During the testing process, testing personnel often need to operate or test around the site. Therefore, it is required that environmental testing equipment must have the characteristics of safe operation, convenient operation, reliable use, and long service life to ensure the normal progress of the testing itself. The various protection, alarm measures, and safety interlocking devices of the testing equipment should be complete and reliable to ensure the safety and reliability of the testing personnel, the tested products, and the testing equipment itself.

twoSelection of Temperature and Humidity Chamber

1Selection of Volume for Temperature and Humidity Chamber

Test the product to be tested（Components, assemblies, parts, or complete machines）When conducting tests in a climate chamber, in order to ensure that the atmosphere around the tested product meets the environmental conditions specified in the test specifications, the dimensions of the working space of the climate chamber and the overall dimensions of the tested product should follow the following regulations:

　　a）Volume of the tested product（W×D×H）Not exceeding the effective working space of the test chamber（20-35）%（Recommended selection20%）For products that generate heat during testing, it is recommended to choose products that do not exceed30%.

　　b）The ratio of the windward cross-sectional area of the tested product to the total area of the working chamber of the test box on that section is not significant

Yu（35-50）%（Recommended selection35%）.

　　c）The distance between the outer surface of the tested product and the wall of the test chamber should be maintained at least100-150mm，

（Recommended selection150mm）.

The above three provisions are actually interdependent and unified. with1Taking a cubic meter cube box as an example, the area ratio is1：（0.35~0.5）Equivalent to a volume ratio of1:（0.207~0.354）Distance from the box wall100-150mmEquivalent to a volume ratio of1:（0.343~0.512）.

Summarize the above three provisions,The working chamber volume of the climate environment test chamber is at least equal to the external volume of the tested product3~5times,The reasons for making such regulations are as follows:

　　1）After the test piece is placed in the box, it occupies the airflow channel in the flow field, and the narrowing of the channel will lead to an increase in airflow velocity. Accelerating the heat exchange between the airflow and the test piece is inconsistent with the reproducibility of environmental conditions, as relevant standards stipulate that the airflow velocity around the test sample in the test chamber should not exceed1.7m/sTo prevent unrealistic heat conduction between the test specimen and the surrounding atmosphere, the average wind speed inside the test chamber during no-load operation is0.6~0.8mnot exceed1m/sSatisfya）Theb）When the space and area ratio specified by the two requirements are met, the wind speed in the flow field may increase（50~100）%The average maximum wind speed is（1.17）m/sMeet the requirements specified in the standards. If the volume or windward cross-sectional area of the test piece is increased without restrictions during the experiment, the actual airflow speed during the test will exceed the maximum wind speed specified in the test standard, and the validity of the test results will be questioned.

　　2）Environmental parameters inside the working chamber of the climate chamber（Such as temperature, humidity, salt spray settling rate, etc）The accuracy indicators of the test are all the results of testing under no-load conditions. Once the test piece is placed, it will have an impact on the uniformity of the environmental parameters inside the working chamber of the test box. The larger the space occupied by the test piece, the more severe this impact will be. Experimental data shows that the temperature difference between the windward and leeward sides in the flow field can reach3~8℃In severe cases, it can be as big as10℃The above. Therefore, it is necessary to meet as much as possiblea）Theb）Two requirements to ensure the uniformity of environmental parameters around the tested product.

　　3）According to the principle of heat conduction, the temperature of the airflow near the box wall is usually different from the temperature at the center of the flow field2~3℃At high and low temperatures, it may also reach5℃The temperature of the box wall differs from the temperature of the flow field near the box wall2~3℃（Depending on the structure and material of the box wall）The greater the difference between the experimental temperature and the external atmospheric environment temperature, the greater the aforementioned temperature difference. Therefore, the distance from the box wall is（100~150mm）The space within the distance is unusable.

2Temperature range

At present, the temperature range of temperature test chambers abroad is generally（-73 ~ +177℃）, or（-70 ~ +180℃）Most domestic manufacturers are（-70 ~ +120℃）There are also high temperatures150℃This temperature range can usually meet the temperature testing needs of the vast majority of military and civilian products, unless there are special requirements, such as products installed near the engine or other heat sources, the smaller the working chamber volume that cannot be blindly used. On the other hand, the higher the upper limit temperature, the better the insulation material in the interlayer of the box wall（Like glass wool, etc）The higher the heat resistance requirements. The higher the requirement for the sealing of the box, the higher the production cost of the box.

3Humidity range

The temperature indicators provided by environmental test chambers both domestically and internationally are generally20~95%RHor20~98%RHIn fact, these two indicators are * consistent, because the humidity control accuracy is usually±（3~5）% RH,And the maximum humidity can only be100% RH,It is worth noting that the corresponding temperature range should be indicated after the humidity index,Or provide the lowest dew point temperature possible.Because relative humidity is directly related to temperature,For the same moisture content,The higher the temperature,The lower the relative humidity,Such as humidity contentdfor5g/kg （point1Kilogram of dry air contains5Grams of water vapor）,When the temperature is29℃time,relative humidity is20% RH,The temperature is6℃time,relative humidity is99% RH,When the temperature drops to4℃below,Relative humidity exceeds100%,There will be condensation inside the box.

To achieve high temperature and high humidity, simply spray steam or atomized water droplets into the air of the box for humidification. Low temperature and humidity are relatively difficult to control because 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 flowing air inside the box to make the air drier. At present, the vast majority of temperature and humidity chambers both domestically and internationally adopt the principle of refrigeration and dehumidification, which involves adding a set of refrigeration light pipes to the air conditioning room of the chamber. When humid air passes through a cold pipe, its relative humidity will reach100% RHDue to air saturation and condensation on the light pipe, the air becomes drier. This dehumidification method theoretically can reach dew point temperatures below zero degrees, but when the surface temperature of the cold spot reaches0℃following. The actual lowest dew point temperature achieved is5~7℃Dew point temperature5℃Equivalent to a moisture content of0.0055g/kgCorresponding to relative humidity20%RHThe temperature is30℃If temperature is required20℃When the relative humidity reaches20% RHAt this point, the dew point temperature is-3℃It is difficult to dehumidify using refrigeration, and an air drying system must be selected to achieve it.

4Wind speed

According to relevant standards, the wind speed inside the temperature and humidity chamber during environmental testing should be less than1.7m/sIf the wind speed is too high, it will accelerate the heat exchange between the surface of the test piece and the airflow inside the box, which is not conducive to the test. However, in comprehensive environmental tests involving multiple factors such as temperature, humidity, and vibration, in order to pursue a high temperature change rate, it is necessary to accelerate the flow rate of the circulating airflow inside the box. The wind speed is usually within the range of2.5~3.0m/sTherefore, the wind speed limit varies for different usage purposes.

5Temperature field

In order to simulate the actual environmental conditions that products experience in nature more accurately, it is necessary to ensure that the surrounding area of the tested product can be under the same temperature environment conditions during environmental testing. Therefore, it is necessary to limit the temperature gradient and temperature fluctuation inside the test chamber. In the national military standardGJB150.1-86The general principles of environmental testing methods for military equipment clearly stipulate“The temperature of the measurement system near the test sample should be within the range of the test temperature±2℃within,Its temperature gradient does not exceed1℃/mOr the total maximum value is2.2℃（The test sample does not work）'As mentioned above, the requirements for the temperature field include three aspects:

　　a）Accuracy of temperature measurement.

The test specification specifies the nominal value of the test temperature（Nominal temperature value）The experimental equipment measured the temperature field center web（Or air outlet, return air outlet）Actual value of experimental temperature（Temperature of workspace indicator point）There is an error between these two.

　　b）Temperature fluctuation.

The actual temperature data is not a constant, but a quantity that fluctuates up and down over time. The mathematical expected value can be obtained by statistically averaging over time（mean）And with one standard deviation.

According to the requirements of the experimental specifications, the above two items, including the error between the mathematical expected value and the nominal value, are the systematic error）Add standard deviation（I.e. random error）The total sum（Temperature control accuracy）Not exceeding the specified error range,GJB150.1-86The regulation is as follows:±2℃.

According to national standardsGB/T5170.1-1995The temperature deviation of the test chamber refers to the deviation between the maximum and minimum temperatures measured at each testing point in the workspace and the nominal temperature within a specified time when the test chamber is in a stable state. High temperature deviation not exceeding±2℃The low-temperature temperature deviation is not greater than±3℃.

　　c）Temperature field gradient.

This indicator is to control the uniformity of the temperature field inside the control box. That is to ensure consistency in the temperature around the tested product in the extension of space. National Army StandardGJB150The temperature gradient specified in the document is1℃/mThe requirements are usually used for3m3The above test chamber, with the maximum value being2.2℃Then use more3m3The following test chamber.

In theory, temperature gradient is applicable to all Lv production tests, but when the wind speed is greater than1.7m/sIn the test chamber, due to the enhanced vortex effect of the airflow in the flow field, the uniformity of the flow field deteriorates, making it difficult to meet the national military standardsGJB150requirement On the other hand, in pursuit of high temperature change rates（If greater than10℃/m）In the test chamber, it is necessary to increase the airflow speed（greater than2m/s）At this time, the temperature difference between the windward and leeward sides of the tested product increases with the increase of wind speed. Therefore, there is no strict requirement for temperature field uniformity in temperature and humidity chambers that pursue high temperature change rates in relevant test specifications and verification regulations. Of course, for environmental testing equipment manufacturers, efforts are being made to achieve products that combine high temperature change rates and good temperature field uniformity to meet user requirements. Like the American Environmental Testing Company（Envirotronics）Temperature and humidity chamber for production, temperature change rate15℃/mThe uniformity of the temperature field is±2.2℃Inside.

6Precision control of humidity.

Most humidity measurements in environmental test chambers are done using the inexpensive dry wet bulb method, especially in high humidity environments（80~100%RH）Under certain circumstances, the temperature difference between dry and wet bulb1℃Corresponding to the difference in relative humidity5%about. According to the principle that the accuracy of the testing device should not be lower than one-third of the allowable deviation under the test conditions（Refer to the national military standardGJB150.1-86）Temperature difference1℃The allowable deviation for temperature measurement of dry and wet balls is required to be±0.7℃The measurement accuracy of temperature is achieved by hydraulic pressure0.2℃Similarly, the relative humidity error is±3%RHThe required temperature measurement accuracy should reach±0.1℃Therefore, the error in measuring relative humidity using the dry wet bulb method is usually±（3~5）% RHWith such high temperature measurement accuracy, the precision of humidity control can be imagined.

In order to overcome the shortcomings of low measurement accuracy and excessive influence of human operation in the dry wet bulb method, solid humidity sensitive heads have been developed in the past decade（Like a capacitive humidity sensor）After solving the problem of low temperature（-55~70℃）After addressing the issue with its use, it has been applied to environmental testing equipment and its measurement accuracy can reach±1~3%RHThe temperature and humidity chambers produced by American companies are equipped with solid-state humidity sensors.

The accuracy of humidity control is not only limited by the humidity measurement method, but also related to the humidification method. There are many humidification methods in the environmental test chamber, and the following three are commonly used:

　　1）Water tray humidification. It is divided into natural evaporation and electric heating evaporation, and this humidification method has a slow humidification rate. Low control accuracy, mostly suitable for constant temperature and humidity chambers.

　　2）Spray steam humidification. The humidification rate is fast, but the fluctuation of humidification is large, making it easy to achieve rapid control and good control accuracy. However, due to the spray100℃Steam will cause a small amount of interference with the temperature state inside the box.

　　3）Spray water. The particle diameter of atomized water is generally around10~50mThis method has the advantage of steam humidification without the need for an external steam boiler, and can adjust the temperature of the sprayed particles to be the same as the test temperature inside the box, making it a good humidification method.

7Dew point temperature selection

Dew point temperature refers to the cooling of moist air to a saturated state under given conditions of constant moisture content（The relative temperature is100% RH）The temperature at that time. Therefore, the dew point temperature of the air state is given, which also gives the moisture content of the air, based on the enthalpy of the air——The humidity chart can display the relative temperature values at different temperatures corresponding to this moisture content. The lower the dew point temperature, the lower the humidity content of the air inside the box, and the corresponding relative humidity at the same temperature is also lower. The lowest dew point temperature that a temperature and humidity environment test chamber can achieve is related to the dehumidification method used. The commonly used refrigeration dehumidification method has a lowest dew point temperature of approximately5~7℃After a long period of stable dehumidification, the dew point temperature can also reach2~5℃If it is necessary to require the dew point temperature to reach0℃The following requires the use of additional devices such as solid or liquid moisture absorbers, or other drying and dehumidification devices. The temperature and humidity box produced by the American environmental testing company can be optionally equipped with a dew point temperature of-40℃The dry air blowing system and air drying system.

threeAnalysis of the condition of temperature and humidity testing equipment

　　1）The production of temperature and humidity environment testing equipment is a mechanical manufacturing industry that has been ongoing for decades, and its basic technology（Like cooling and heating）Already mature. However, with the advancement and development of industrial technology, many new application technologies are constantly being introduced into environmental testing equipment, such as computer technology, fuzzy control theory, multi box synchronous operation, etc. Therefore, it is difficult to give manufacturers an arbitrary conclusion on which is better or worse. The scale equipment produced by various manufacturers with decades of manufacturing experience can be used, and their working principles, applied technologies, and even structures, materials, etc. have many similarities. As long as the manufacturer manages meticulously and takes quality seriously and responsibly（if passedISO9000argumentation）Their products all have conditions for existence and value for use.

　　2）Although the working principle and application technology of domestic and foreign environmental testing equipment are basically the same, there are still gaps in the following three aspects, resulting in a high failure rate and shorter continuous guarantee period of performance indicators for domestically produced environmental testing equipment.

　　　a）The quality of components and parts is relatively poor, especially for refrigeration compressor units and expansion units

Valves, main path solenoid valves, and electronic components of control systems. Most of the failures of domestically produced environmental testing equipment are caused by the quality and service life of components and parts.

　　　b）The production process is relatively backward. The production process and equipment, including the technical level of construction personnel, have a significant impact on the service life of the product. The layout of pipelines in refrigeration systems involves resonance during operation, loss along the refrigerant flow, ease of maintenance, and smooth recovery of compressor oil throughout the entire process. For example, the welding process and quality of pipelines, pipeline cleaning and leak detection techniques, etc. Production equipment（Such as CNC variable board, CNC bending pipe, nitrogen protection welding machine, etc）It can be bought quickly with money, but the production process is a long-term accumulation of experience and cannot be achieved overnight.

　　　c）Quality control in the production process. The quality control of domestic production processes is far less strict than that of foreign companies, so there are more hidden dangers in the equipment. These hidden dangers will gradually be exposed and cause malfunctions during use. The quality control of the production process is related to the management level and production habits of the manufacturerISO9000The argument is a sign of the standardization of quality control in the production process.

　　　3）There are manufacturers in the United States that produce temperature and humidity environmental testing equipment abroadEnvirotronics（Environmental testing）TheThermotron（Thermal measurement）Germany'sWeissTheVotsch（Fuqi）Company, ItalyACSCompany, JapanTaBalCompanies, etc. The above-mentioned companies have accumulated rich experience in producing climate and environmental testing equipment. The equipment provided is for use and reliable in quality. As for which company should be selected and which model of product is more in line with the requirements, in addition to the principles and specific technical specifications mentioned earlier in this article, there are also some factors that deserve attention and consideration.

　　　a）performance/Price ratio.

The import of environmental testing equipment is invariably sold domestically through agents, and the price factor not only depends on the manufacturer's costs and profits, but also includes many intermediate costs and sales commissions. Reducing intermediate costs is an important factor in lowering order prices at the same manufacturing cost. Products with the same technical specifications in the same country have roughly similar manufacturing costs, but in different countries（Refers to developed Western countries）Even if there are differences in the products produced, they will not be too significant.

　　　b）The degree and ability to meet user needs.

The user orders a temperature and humidity environment test chamber, and its requirements vary according to the needs of product testing. For example, some three comprehensive test chambers are often used for environmental stress screening tests of product components or assemblies. At this time, the forced dehumidification ability of the equipment and the possibility of additional air drying systems are the key issues that the user should be concerned about. If some users want to use the three comprehensive test chamber for environmental testing（Like wet heat test）The adjustability of wind speed is a factor that must be considered.

　　　c）Ability to provide after-sales service and handle equipment malfunctions.

It is impossible for any device to fail during use. Once a problem occurs, users can enjoy timely maintenance services or quickly determine the fault handling method based on the device's own fault diagnosis ability, so that the device can recover its performance as soon as possible and be put back into use. In this regard, domestic and foreign manufacturers have made or are making active efforts, such as establishing rapid response maintenance teams and adding fault diagnosis and warning system software to computer control systems. Users can quickly handle faults based on the information provided by the computer.