Precautions for purchasing constant temperature and humidity test chambers

Introduce from 4 major aspectsHumidity ChamberSelection techniques and precautions, including equipment selection criteria, analysis of temperature and humidity testing equipment conditions, basic principles for equipment selection, and selection of temperature and humidity chambers.

One Equipment selection criteria

The exact number of natural environmental factors and induced environmental factors that exist on the surface of the Earth and in atmospheric space cannot be determined at present. There are dozens of factors that have a significant impact on the use and lifespan of engineering products (equipment) in muffle furnaces. Engineers engaged in the study of environmental conditions for engineering products have compiled and summarized the environmental conditions that exist in nature and are induced by human activities into a series of testing standards and specifications to guide the environmental and reliability testing of engineering products. For example, GJB150- the National Military Standard of the People's Republic of China for Environmental Testing of Military Equipment, which guides environmental testing of military products, and GB2423- the National Standard of the People's Republic of China for Environmental Testing of Electrical and Electronic Products, which guides environmental testing of electrical and electronic products. Therefore, the main basis for selecting environmental and reliability testing equipment is the testing specifications and standards of engineering products.

Secondly, in order to standardize the tolerance of environmental testing conditions in experimental equipment and ensure the control accuracy of environmental parameters, national technical supervision agencies and various industrial departments have also formulated a series of calibration regulations for environmental testing equipment and detection instruments. Such as the national standard GB5170 of the People's Republic of China "Basic Parameter Calibration Method for Environmental Testing Equipment of Electrical and Electronic Products", and JJG190-89 "Trial Calibration Regulations for Electric Vibration Test Stand System" issued and implemented by the State Administration of Technical Supervision. These verification regulations are also an important basis for selecting environmental and reliability testing equipment. Testing equipment that does not meet the requirements of these verification regulations is not allowed to be put into use.

II Analysis of the condition of temperature and humidity testing equipment

1. Foreign manufacturers of temperature and humidity environmental testing equipment include Envirotronics and Thermotron from the United States, Weiss and Votsch from Germany, ACS from Italy, and TaBal from Japan. 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 to choose and which model of product better meets the requirements, in addition to the principles and specific technical specifications mentioned earlier in this article, there are also some factors worth paying attention to and considering for the muffle furnace.

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 equivalent manufacturing costs, and products produced in different countries (referring to developed Western countries) may not differ significantly even if there are differences.

b. 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, adding fault diagnosis and warning system software to computer control systems, and allowing incubator users to quickly handle faults based on the information provided by the computer.

c. The degree and ability to meet user needs

1. When a user orders a temperature and humidity environment test chamber, the requirements vary depending on the needs of the 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 key concerns for users. If some users want to use the three comprehensive test chamber for environmental testing (such as wet heat testing), the adjustability of wind speed is a factor that must be considered.

The production of temperature and humidity environment testing equipment is a mechanical manufacturing industry that has been ongoing for decades, and its basic technologies (such as refrigeration and heating) have matured. 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 has fine management and takes quality seriously and responsibly (such as through ISO9000 certification), their products have the conditions for existence and value for use.

Although the working principle and application technology of environmental testing equipment are basically the same both domestically and internationally, there are still gaps in the following three aspects, resulting in a higher failure rate and shorter continuous guarantee period of performance indicators for domestically produced environmental testing equipment compared to foreign ones.

a. The quality of components and parts is relatively poor, especially the electronic components of refrigeration compressor units, expansion valves, main path solenoid valves, and 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. The production equipment (such as CNC variable plates, CNC bending pipes, nitrogen protection welding machines, etc.) can be quickly purchased with money, but the production process is a long-term experience accumulation process that 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 manufacturer. The demonstration of ISO9000 is a sign of the standardization of quality control in the production process.

III Basic principles for equipment selection

The selection of environmental and reliability testing equipment should follow the following five basic principles:

1. Safety 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 incubator 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.

2. Reproducibility 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 the language of engineering, which means that "the environmental conditions (including platform environment) created by the testing equipment around the tested product should meet the requirements of the environmental conditions and their tolerances specified in the product testing specifications. The temperature chamber used for military product testing should not only meet the high and low temperature test values and test times specified for different types of products in the national military standards GJB150.3-86 and GJB150.4-86, but 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.

3. Repeatability 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. This means that the stress levels (such as thermal stress, vibration stress, electrical stress, etc.) applied by environmental testing equipment to the tested product are consistent with the requirements of the same testing specification.

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.

4. Measurability 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.

5. Exclusion 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.

4 Selection of Temperature and Humidity Chamber

1. Precision control of humidity.

Most humidity measurements in environmental test chambers are done using the inexpensive dry wet bulb method. Under high humidity conditions (80-100% RH), the temperature difference between dry and wet bulbs is 1 ℃, corresponding to a relative humidity difference of about 5%. According to the principle that the accuracy of the testing device should not be lower than one-third of the allowable deviation of the test conditions (see the national military standard GJB150.1-86), a temperature difference of 1 ℃ requires a tolerance of ± 0.7 ℃ for the temperature measurement of the dry and wet balls themselves. Therefore, the temperature measurement accuracy of hydraulic pressure should reach 0.2 ℃. Similarly, the relative humidity error is ± 3% RH. The temperature measurement accuracy should reach ± 0.1 ℃. Therefore, the error in measuring relative humidity using the dry wet bulb method is usually ± (3-5)% RH. With such temperature measurement accuracy, the precision of humidity control can be imagined.

In order to overcome the shortcomings of low measurement accuracy and excessive human operation of the wet dry bulb method, solid humidity sensitive heads (such as capacitive humidity sensors) have been applied to environmental testing equipment in the past decade after solving the problem of low temperature (-55~70 ℃) use, and their measurement accuracy can reach ± 1-3% RH. The temperature and humidity chambers produced by American companies are all equipped with solid 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:

a. 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.

b. 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 injection of 100 ℃ steam, there will be a small amount of interference with the temperature state inside the box.

c. Spray water. The particle diameter of atomized water is generally between 10~50 μ. This 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. It is a good humidification method.

2. Dew point temperature selection

Dew point temperature refers to the temperature at which humid air is cooled to a saturated state (relative temperature of 100% RH) under given conditions of constant moisture content. Therefore, the dew point temperature of the air state is given, which also gives the moisture content of the air. From the enthalpy humidity graph of the air, the relative temperature values at different temperatures corresponding to this moisture content can be found. 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 can be achieved by a temperature and humidity environment test chamber is related to the dehumidification method used. The commonly used refrigeration dehumidification method has a lowest dew point temperature of about 5-7 ℃. After a long period of stable dehumidification, the dew point temperature can also reach 2-5 ℃. If it is necessary to require the dew point temperature to be below 0 ℃, additional devices such as solid or liquid desiccants or other drying and dehumidification devices need to be used.

3. Selection of temperature and humidity chamber capacity

When placing the test product (components, assemblies, parts or whole machine) into a climate chamber for testing, in order to ensure that the atmosphere around the test product can meet the environmental conditions specified in the test specifications, the working space size of the climate chamber and the overall size of the test product should follow the following regulations:

a. The volume of the tested product (W × D × H) shall not exceed (20-35)% of the effective working space of the test chamber (20% is recommended). For products that generate heat during testing, it is recommended to use no more than 30%.

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 shall not exceed (35-50)% (35% is recommended).

c. The distance between the outer surface of the tested product and the wall of the test chamber should be maintained 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). 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 external volume of the tested product. The reasons for making this regulation 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 specimen is inconsistent with the reproducibility of environmental conditions, as relevant standards stipulate that the air flow velocity around the test specimen in the test chamber should not exceed 1.7m/s for temperature and environmental tests to prevent unrealistic heat conduction between the test specimen and the surrounding atmosphere. When the average wind speed in the test chamber is 0.6~0.8m under no-load conditions, it should not exceed 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)%, and the average maximum wind speed is (1.17) m/s, which meets the requirements of the standard. 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) The accuracy indicators of environmental parameters (such as temperature, humidity, salt spray settling rate, etc.) in the working chamber of the climate chamber are all detected under no-load conditions. Once the test piece is placed, it will have an impact on 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 severe this impact will be. Experimental data shows that the temperature difference between the windward and leeward sides in the flow field can reach 3-8 ℃, and in severe cases, it can exceed 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.

3) 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, and may even reach 5 ℃ at high and low temperatures. 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 greater the difference between the experimental temperature and the external atmospheric temperature, the greater the aforementioned temperature difference. Therefore, the space within a distance of 100-150mm from the box wall is unusable.

4. Temperature range

At present, the temperature range of temperature test chambers abroad is generally (-73~+177 ℃) or (-70~+180 ℃). Most domestic manufacturers use (-70~+120 ℃), but there are also high temperatures up to 150 ℃. 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 higher the heat resistance requirements for insulation materials (such as glass wool) in the interlayer of the box wall. The higher the requirement for the sealing of the box, the higher the production cost of the box.

5. Humidity range

The temperature indicators provided by environmental testing chambers both domestically and internationally are generally 20~95% RH or 20~98% RH. In fact, these two indicators are * consistent, because the humidity control accuracy is usually ± (3~5)% RH, and the maximum humidity can only be 100% RH. It is worth noting that the corresponding temperature range should be specified after the humidity indicator, or the minimum dew point temperature should be given Because relative humidity is directly related to temperature, for the same moisture content, the higher the temperature, the lower the relative humidity. For example, if the moisture content d is 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 99% RH. When the temperature drops below 4 ℃ and the relative humidity exceeds 100%, condensation will occur 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 reaches 100% RH, as the air saturates and condenses on the light pipe, making the air drier. This dehumidification method theoretically can reach dew point temperatures below zero degrees, but when the surface temperature of the cold spot reaches below 0 ℃. The actual lowest dew point temperature achieved is 5-7 ℃. A 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 a relative humidity of 20% RH is required at a temperature of 20 ℃, with a dew point temperature of -3 ℃, it is difficult to use refrigeration for dehumidification, and an air drying system must be selected to achieve it.

6. Wind speed

According to relevant standards, the wind speed inside the temperature and humidity chamber during environmental testing should be less than 1.7m/s. If the wind speed is too high, it will accelerate the heat exchange between the surface of the test piece and the flowing airflow inside the chamber, 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 velocity of the circulating airflow inside the chamber. The wind speed is usually between 2.5 and 3.0 m/s. Therefore, the wind speed limit varies for different usage purposes.

7. Temperature 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 General Principles of Environmental Test Methods for Military Equipment (GJB150.1-86) of the National Military Standard, it is clearly stipulated that "the temperature of the measurement system near the test sample should be within ± 2 ℃ of the test temperature, and the temperature gradient should not exceed 1 ℃/m or the total maximum value should be 2.2 ℃ (when the test sample is not working). As mentioned above, the requirements for temperature field include three aspects:

a. Accuracy of temperature measurement.

The test specification specifies the nominal value of the test temperature (nominal temperature value), while the test equipment measures the actual value of the test temperature at the center of the temperature field (or at the outlet or return outlet) (indicated temperature in the workspace). There is an error between the two.

b. Temperature fluctuation.

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

According to the requirements of the experimental specifications, the sum of the above two items, including the error between the mathematical expected value and the nominal value (i.e. systematic error) plus the standard deviation (i.e. random error) (temperature control accuracy), shall not exceed the specified error range, which is specified as ± 2 ℃ in GJB150.1-86.

According to the national standard GB/T5170.1-1995, the temperature deviation of the test chamber refers to the deviation between the maximum and minimum temperatures measured at each testing point in the working space and the nominal temperature within the specified time when the test chamber is in a stable state. The deviation of high temperature shall not exceed ± 2 ℃, and the deviation of low temperature shall not exceed ± 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. The requirement of a temperature gradient of 1 ℃/m specified in the national military standard GJB150 is usually used for test chambers with a capacity of 3m3 or more, while the maximum value of 2.2 ℃ is more commonly used for test chambers with a capacity of 3m3 or less.

In theory, temperature gradient is applicable to all production tests, but in test chambers with wind speeds greater than 1.7m/s, due to the enhanced vortex effect of airflow in the flow field, the uniformity of the flow field deteriorates, making it difficult to meet the requirements of the national military standard GJB150. On the other hand, in test chambers that pursue high temperature change rates (such as greater than 10 ℃/m), it is necessary to increase the airflow speed (greater than 2m/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 calibration 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.

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