There are many factors that need to be considered when purchasing a constant temperature and humidity chamber, among which the volume of the chamber must be determined first. In order to ensure that the surrounding environment of the tested product can meet the environmental test conditions specified in the test specification, it is concluded that the volume of the chamber is at least 3 to 5 times the outer volume of the tested product. The reasons for this requirement are as follows:
 
I. According to the principle of heat conduction, the temperature of the airflow near the wall of the chamber is usually 2℃～3℃ different from the temperature of the center of the flow field, and it may reach 5℃ at the upper and lower limits of high and low temperatures. The temperature of the chamber wall differs from the temperature of the flow field near the chamber wall by 2℃～3℃ (depending on the structure and material of the chamber wall). The greater the difference between the test temperature and the ambient temperature of the outside atmosphere, the greater the above-mentioned temperature difference, so the space within the distance from the chamber wall (100mm～150mm) is unusable space.
 
II. After the test piece is placed in the chamber, it occupies the air flow channel in the flow field, and the narrowing of the channel will cause the air flow velocity to increase, and  accelerate the heat exchange between the airflow and the test piece, which is inconsistent with the reproducibility of environmental conditions. Because in the relevant standards for temperature environmental tests, it is stipulated that the air velocity around the test sample in the test chamber should not exceed 1.7m/s to prevent the test sample and the surrounding atmosphere from generating unrealistic heat conduction. When the test chamber is empty, the internal average wind speed is 0.6m～0.8m, not exceeding 1m/s. When the space and area ratio specified in the requirements are met, the wind speed of the flow field may increase and meet the requirements of the standard. If the volume or cross-sectional area of the test piece is increased without restriction during the test, the airflow wind speed during the actual test will increase beyond the maximum wind speed specified by the test standard, and the validity of the test results will be questioned.
 
III. The accuracy indicators of the environmental parameters (such as temperature, humidity, etc.) in the working chamber of the constant temperature and humidity chamber are the test results under no-load conditions. Once the test piece is placed, it will affect 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 serious this impact will be. The actual test data shows that the temperature difference between the windward side and the leeward side of the flow field can reach 3-8℃, and it can be as large as 10℃ in severe cases. Therefore, this requirement must be met as much as possible to ensure the uniformity of environmental parameters around the tested product.