Ⅰ. Choosing a Humidity Range
About Temperature and Humidity Test Chamber Selection，humidity specifications for environmental test chambers, both domestically and internationally, are mostly between 20% and 98% RH or 30% and 98% RH. If a humidity test chamber lacks a dehumidification system, its humidity range is limited to 60% to 98% RH. These chambers can only perform high-humidity tests, but are significantly more affordable. It's important to note that the corresponding temperature range or the minimum dew point temperature should be specified after the humidity specification. This is because relative humidity and temperature are directly correlated. For the same moisture content, the higher the temperature, the lower the relative humidity. For example, at a moisture content of 5g/kg (5g of water vapor per kilogram of dry air), at 29°C, the relative humidity is 20% RH; at 6°C, it reaches 90% RH; and when the temperature drops below 4°C, the relative humidity exceeds a certain limit, causing condensation to form inside the chamber.
To achieve a high-temperature, high-humidity environment, simply spray water vapor or atomized water droplets into the chamber to humidify the air. However, controlling low-temperature, low-humidity environments is relatively difficult because the humidity content in the air is very low, sometimes even significantly lower than that of the atmosphere. This necessitates dehumidification of the air flowing through the chamber to dry it out. Currently, most temperature and humidity chambers, both domestically and internationally, utilize refrigeration dehumidification. This involves adding a set of refrigerated light pipes to the chamber's air conditioning chamber. As the moist air passes through the refrigerated light pipes, its relative humidity (RH) reaches saturation. This saturated air forms condensation on the light pipes, further drying the air. Theoretically, this dehumidification method can achieve a sub-zero dew point temperature. However, when the surface temperature of the cold spot drops to 0°C, the condensation droplets on the light pipes freeze, impairing heat exchange on the light pipes and reducing dehumidification capacity. Furthermore, the chamber itself cannot be completely sealed, allowing humid air from the atmosphere to seep into the chamber, raising the dew point temperature. Furthermore, the moist air flowing between the light tubes only reaches saturation and releases water vapor upon contact with the tubes (cold spots). Therefore, this dehumidification method rarely reduces the dew point temperature inside the chamber below 0°C; the lowest dew point temperature achievable is 5-7°C. A dew point of 5°C corresponds to a moisture content of 0.0055g/kg, which corresponds to a relative humidity of 20% at a temperature of 30°C. If a relative humidity of 20% is required at 20°C, the dew point is -3°C, making dehumidification using refrigeration very difficult. Therefore, an air drying system should be used when conducting tests in low-temperature and low-humidity environments.
II. Environment within the Climate Chamber
The accuracy of parameters (such as temperature, humidity, and salt spray deposition rate) is typically measured under no-load conditions. However, placing the test object in the chamber will affect the uniformity of these environmental parameters within the chamber. The larger the space occupied by the test object, the more severe this effect. Actual test data shows that the temperature difference between the windward and leeward sides of the flow field can reach 3–8°C, and in severe cases, even exceed 10°C. Therefore, it is important to meet both requirements (a) and (b) to ensure uniformity of the environmental parameters surrounding the test product.
Based on the principles of heat conduction, the temperature of the airflow near the chamber walls typically differs by 2–3°C from the temperature at the center of the flow field. In extreme cases of high and low temperatures, the temperature difference can reach 5°C. Furthermore, there is an additional 2–3°C difference between the chamber wall temperature and the temperature of the flow field near the walls (the specific value depends on the chamber wall structure and material). Furthermore, the greater the temperature difference between the test temperature and the ambient air, the greater this temperature difference. Therefore, the space within 100-150mm of the chamber wall is unusable.
Regarding the temperature range, currently, the temperature range of temperature test chambers abroad is generally -73 to +177°C or -70 to +180°C. Most domestic manufacturers produce test chambers with temperature ranges of -80 to +130°C, -60 to +130°C, or -40 to +130°C, with some products capable of reaching temperatures as high as 150°C. These temperature ranges generally meet the temperature testing requirements of most domestic military and civilian products. Unless there are special requirements, such as when the product is installed near a heat source such as an engine, the upper temperature limit should not be blindly increased. This is because a higher upper temperature limit increases the temperature difference between the inside and outside of the chamber, impairing the uniformity of the internal flow field, and reducing the available working chamber volume. Furthermore, a higher upper temperature limit places greater demands on the heat resistance of the insulation material (such as glass wool) in the chamber wall interlayer and the sealing of the chamber, which in turn increases the cost of the chamber.
III. Volume Selection
When testing a product (component, assembly, subassembly, or complete device) in a climatic chamber, to ensure that the atmosphere surrounding the product meets the environmental test conditions specified in the test specification, the following requirements must be met regarding the working dimensions of the chamber and the overall dimensions of the product under test:
1. The volume of the product under test (W×D×H) must not exceed 20-35% of the effective working space of the chamber (20% is recommended). For products that generate heat during testing, a volume of no more than 10% is recommended.
2. The ratio of the windward cross-sectional area of the product under test to the total working space of the chamber on that cross-sectional area must not exceed 35-50% (35% is recommended).
3. The outer surface of the product under test must maintain a minimum distance of 100-150 mm from the chamber wall (150 mm is recommended).
The above three requirements are interdependent and unified. Taking a 1 cubic meter cubic 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 100-150 mm from the box wall is equivalent to a volume ratio of 1:(0.343-0.512). Summarizing the above three points, the working chamber volume of the climatic environmental test chamber should be at least 3-5 times the outer volume of the product under test.
The reason for this requirement is that when the test object is placed in the chamber, it squeezes the smooth passage. The narrowing of the passage increases the airflow velocity, accelerating the heat exchange between the airflow and the test object. This is inconsistent with the reproduction of environmental conditions. Relevant standards for temperature environmental testing stipulate that the air velocity around the test specimen in the test chamber should not exceed 1.7 m/s to prevent unrealistic heat transfer between the test specimen and the surrounding atmosphere. When the test chamber is unloaded, the average wind speed within the chamber is 0.6-0.8 m/s, not exceeding 1 m/s. When the space and area ratios specified in requirements a) and b) are met, the wind speed in the flow field may increase by (50-100)%, with an average high wind speed of (1-1.7) m/s. This meets the requirements specified in the standard. If the volume or windward cross-sectional area of the test piece is increased without restriction during the test, the actual airflow speed will increase to a level exceeding the high wind speed specified in the test standard, and the validity of the test results will be questioned.
Ⅳ. Control Method Selection:
(Constant Temperature and Humidity vs. Alternating Temperature and Humidity - i.e., Single-Point and Programmable)
Temperature and humidity test chambers come in two types: constant and alternating. Conventional high and low temperature test chambers are generally constant. Their control method involves setting a target temperature, and the chamber automatically maintains a constant temperature at that target. Constant temperature and humidity test chambers operate similarly: setting a target temperature and humidity, and the chamber automatically maintains a constant temperature at those targets. Alternating high and low temperature test chambers have one or more programmable high and low temperature cycles. They can complete the test according to a preset curve and control the heating and cooling rates within a wide range of heating and cooling rates, i.e., they can control the heating and cooling rates according to the slope of the set curve. Similarly, alternating high and low temperature humidity test chambers also have preset temperature and humidity curves and can be controlled accordingly.
Ⅴ. Selecting a Temperature Ramp Rate
Common high and low temperature test chambers generally do not specify a specific cooling rate; the time required to cool from ambient temperature to the nominal low temperature is typically 90 to 120 minutes. In contrast, high and low temperature alternating test chambers and high and low temperature alternating damp heat test chambers have specific temperature ramp rate requirements, generally requiring a ramp rate of 1°C/min, with adjustable rates within this range. Rapid temperature ramp test chambers, on the other hand, have even faster ramp rates, capable of heating and cooling at rates ranging from 3°C/min to 15°C/min. Within specific temperature ranges, these rates can even exceed 30°C/min.
Rapid temperature ramp test chambers of various specifications and ramp rates generally maintain a consistent temperature range of -60°C to +130°C. However, when evaluating the cooling rate, the temperature range varies. Depending on the testing requirements, rapid temperature ramp test chambers may have a range of -55°C to +80°C or -40°C to +80°C.
There are two ways to describe the temperature ramp rate of a rapid temperature change test chamber (QINSUN Instruments can repair all types of environmental testing equipment): the average ramp rate over the entire range, and the linear ramp rate (essentially the average rate per 5 minutes). The average ramp rate refers to the ratio of the difference between the highest and lowest temperatures within the test chamber's temperature range to the time required. Currently, the temperature ramp rate specifications provided by international environmental testing equipment manufacturers all refer to the average ramp rate over the entire range. The linear ramp rate, on the other hand, refers to the guaranteed ramp rate achieved by the test chamber within any continuous 5-minute period. In practice, the difficulty and key to ensuring a linear ramp rate for a rapid temperature change test chamber lies in the cooling rate achieved during the first 5 minutes after the cooling phase. In a sense, the linear ramp rate (average rate per 5 minutes) is a more scientific and reasonable metric.
Therefore, test equipment typically provides both the average ramp rate over the entire range and the linear ramp rate (average rate per 5 minutes). Generally speaking, the linear ramp rate (average rate per 5 minutes) is approximately half the average ramp rate for the entire process.
Ⅵ. Temperature Fluctuation
Temperature fluctuation is a relatively easy parameter to measure. Most test chambers produced by all environmental testing equipment manufacturers can actually control temperature fluctuations within a range of ±0.3°C.
Ⅶ. Wind Speed
Relevant standards stipulate that the wind speed within a temperature and humidity chamber during environmental testing should be less than 1.7 m/s. For the test itself, the lower the wind speed, the better. Excessive wind speeds can accelerate heat exchange between the test piece's surface and the airflow within the chamber, compromising test accuracy. However, circulating air is essential to ensure uniformity within the test chamber. However, for rapid temperature change test chambers and those that combine temperature, humidity, and vibration, wind speeds are typically limited to 2-3 m/s to achieve faster temperature changes and increase the speed of the circulating airflow within the chamber. Therefore, wind speed limits vary depending on the intended use.
Ⅷ. Temperature Field Uniformity
To more accurately simulate the actual environmental conditions encountered by a product in nature, the surrounding area of the test product is maintained under uniform temperature conditions during environmental testing. To this end, temperature gradients and temperature fluctuations within the test chamber are limited. GJB 150.1-86, General Rules for Environmental Test Methods for Military Equipment, clearly states that "the temperature of the measurement system near the test specimen shall be within ±2°C of the test temperature, with the temperature fluctuation not exceeding 1°C/m or a maximum total of 2.2°C (when the test specimen is not operating)."
Ⅸ. Cooling Method Selection
If the test chamber has a refrigeration system, the refrigeration system must be cooled. Test chambers are available in air-cooled or water-cooled configurations.
Ⅹ. Humidity Accuracy Control
Humidity measurement in environmental test chambers is often performed using the dry-bulb method. GB10586, the manufacturing standard for environmental test equipment, requires a relative humidity tolerance of ±2%RH. To meet these humidity control accuracy requirements, humidity test chambers require high temperature control accuracy, with temperature fluctuations generally less than ±0.2°C. Otherwise, achieving these humidity control accuracy requirements is difficult.
在线客服