With the development of industry and information technology, environmental test has increasingly become an important means for various scientific research institutes and factories to test product quality. As one of the most important supporting equipment for environmental test and research, temperature and humidity test chamber is used more and more. In the previous literature and standard specifications, the main technical parameters of temperature and humidity deviation, fluctuation, temperature rise and fall rate and wind speed etc. humidity test chamber are discussed much, but there are not many studies on uniformity. Based on the above situation, this paper improves the uniformity by increasing the relative humidity in the temperature and humidity test chamber, and verifies the influence of the increase of relative humidity on heat transfer, which provides a certain reference for the calibration of temperature and humidity chamber.
1 Influence Factors
- 1.1 Chamber Structure
Because the temperature and humidity test chamber has different inner wall structures, the temperature and humidity of each part of the inner wall of the test chamber are also different, which affects the thermal convection in the studio, resulting in different uniformity and deviation of internal temperature and humidity.
- 1.2 Thermal Load
Some luminous and heating loads placed inside the test chamber or test samples that affect the overall internal heat convection will affect the uniformity of internal temperature and humidity. According to relevant standards, by default, the temperature and humidity uniformity of the temperature and humidity test chamber should be measured under no-load.
- 1.3 Heat Transfer
Because the heat transfer coefficients of the front, rear, left, upper and lower six surfaces of the inner wall of the test chamber are different, and there are detection holes, threading holes and test holes on the side of some chamber walls, there will be heat dissipation and heat transfer in some parts of the test chamber, resulting in uneven temperature in the chamber, resulting in uneven convective heat transfer radiated by the chamber wall, which greatly affects the temperature uniformity.
- 1.4 Thermal Radiation
The design of the space and internal structure of the test chamber, such as the placement position of the heating pipe, the design, power and position of the air duct, is difficult to reach the standard of uniformity and symmetry, which will inevitably lead to the uneven temperature and humidity inside the chamber.
1.5 Position and Volume of Test Samples
The unreasonable position of the test sample or the excessive volume of the sample will hinder the air heat convection in the chamber, thus affecting the uniformity of temperature and humidity. For example, placing the sample next to the inner wall or air duct on one side will seriously affect the thermal cycle in the chamber, resulting in uneven temperature and humidity. According to the requirements of the test standard, for the test of heat dissipation samples: the volume of heat dissipation samples is at most 1 / 5 of the volume of the test chamber.
- 1.6 Tightness
The tightness of the chamber and the door is not strict. For example, when the sealing strip is damaged and the door leaks, the uniformity of temperature and humidity in the temperature and humidity test chamber will be affected.
- 1.7 Wind Speed
Generally speaking, during the environmental test, the wind speed in the temperature and humidity chamber should not exceed 1.7 m/s. If the wind speed is too large, it will accelerate the heat exchange between the surface of the test piece and the flowing air flow in the chamber, and also accelerate the evaporation of moisture in the wet ball gauze, which is unfavorable to the test and inaccurate humidity measurement. Therefore, during the test, the smaller the wind speed is, the better. However, in order to ensure the uniformity of temperature and humidity in the chamber, it is necessary to circulate air in the test chamber. During the no-load test, the average wind speed in the chamber is 0.6 ~ 0.8 m/s.
- 1.8 Control Accuracy
After the test chamber reaches the set temperature and humidity point, there will be a certain fluctuation in a short time because the heat divergence is not completely uniform. By improving the control accuracy of the test chamber and reducing the temperature fluctuation, the temperature deviation can be reduced. For the high temperature test chamber, the temperature fluctuation can be reduced by PID continuous adjustment of heating power; For the low-temperature test chamber, the temperature fluctuation is generally controlled through heat balance, that is, after the test chamber reaches the set temperature, the refrigerator is still normally open, and the controlled heating power is used to balance the excess refrigeration capacity.
2 Metering Characteristics
The measurement characteristics of temperature and humidity test chamber shall mainly include temperature and humidity deviation, temperature and humidity fluctuation and temperature and humidity uniformity.
- 2.1 Deviation
The deviation of temperature and humidity test chamber refers to the difference between the average value of temperature and humidity displayed when the environmental test equipment reaches a stable state and the average value of actual temperature and humidity measured at the center of the workspace.
- 2.2 Fluctuation
The fluctuation of temperature and humidity test chamber refers to 1 / 2 of the difference between the maximum value and the minimum value of the temperature (humidity) at the central point of its workspace within 30 minutes after the set temperature (humidity) value reaches stability.
- 2.3 Uniformity
The uniformity of the temperature and humidity test chamber is defined as the arithmetic mean of the difference between the highest and lowest temperature (humidity) values in each test within 30 minutes after the set temperature (humidity) value reaches stability.
3. Correlation test
According to the experimental data, when the temperature in the temperature and humidity test chamber is constant and the relative humidity increases, the corresponding temperature and humidity uniformity and fluctuation will be greatly improved. The main reason is that air is a poor thermal conductor. Generally speaking, when the moisture content is low and the temperature is low, the thermal conductivity of the object is small, the thermal conductivity of the solid is greater than that of the liquid, and the thermal conductivity of the liquid is greater than that of the gas. This difference is largely due to the different molecular spacing in different states. For air, the greater the relative humidity, that is, the greater the ratio of the mass of water vapor in wet air to the mass of water vapor in saturated air at the same temperature and pressure, the greater the moisture in air and the greater the thermal conductivity. When the wind speed is constant, the better the thermal circulation and convection in the chamber, which greatly improves the uniformity and fluctuation of temperature and humidity in the chamber.
In the daily calibration work, it is found that the temperature and humidity fluctuation and uniformity of the same temperature and humidity test chamber under high humidity are better than those under low humidity. It is suggested that the greater the relative humidity, the greater the moisture in the air, the greater the thermal conductivity, so as to improve the thermal cycle in the chamber, Therefore, relevant verification experiments are designed and carried out.
In this paper, the uniformity, fluctuation and bias of temperature and humidity test chamber are theoretically analyzed and studied. By increasing the relative humidity in the chamber, the heat transfer in the chamber is improved, the uniformity in the chamber is greatly improved, and the temperature and humidity in the chamber are more stable. This provides a necessary understanding and reference for the calibration and use of temperature and humidity test chamber in the future.