The high and low temperature test chamber is applicable to the high and low temperature reliability test of industrial products. Under the condition of high temperature and low temperature cycle changes, inspect the performance indexes of components and materials of electronic and electrical products, automobiles and motorcycles, aerospace, ship weapons, scientific research institutions and other related products.
When consulting about experimental projects, we believe that we will often be asked [How large is your test sample?]. In fact, the purpose of this question is to clearly understand whether the model of the test chamber can meet the placement requirements of the tested samples.
According to the relevant standards, the volume of the contents of the high and low temperature test chamber shall be at least 3~5 times of the overall volume of the tested product. There are three main reasons why the high and low temperature test chamber cannot be filled with samples when conducting experiments:
1. Influence of heat exchange between air flow and tested product
After the tested sample is put into the box, the channel will change volume, which will increase the air flow speed and accelerate the heat exchange between the air flow and the tested product. This is inconsistent with the reproduction of environmental conditions, because the relevant standards stipulate that the air flow velocity around the test sample in the test chamber shall not exceed 1.7m/s, to prevent the test sample and the surrounding atmosphere from not meeting the actual heat conduction. The average wind speed in the test chamber is 0.6-0.8m/s under no load, which does not exceed about 1m/s. If the space and area ratio specified in the above requirements are met, the wind speed in the flow field may increase by 50% - 100%, and the average limit wind speed shall not exceed 1.7m/s, meeting the requirements specified in the standard. If the volume or windward sectional area of the tested sample is increased without restriction during the test, the actual wind speed will increase to exceed the limit wind speed specified in the test standard, and the validity of the test results will be doubted.
2. Influence of uniformity of environmental parameters around the tested product
The accuracy index of environmental parameters in the working room is the result of detection under no-load condition. Once the tested product is put into the chamber, the uniformity of environmental parameters in the chamber will be affected. The larger the space occupied by the tested product, the more serious the impact will be. The experimental data show that the temperature difference between the windward side and the leeward side of the flow field can reach 3-8 ℃, and even more than 10 ℃ in severe cases. Therefore, it is necessary to meet the requirements specified in the standard as far as possible to ensure the uniformity of environmental parameters around the tested product.
3. Influence of temperature difference between box wall, near box wall and flow field center
According to the heat conduction principle, the temperature of the air flow near the box wall is generally 2-3 ℃ different from the temperature of the center of the flow field, and it can also reach 5 ℃ at the upper and lower limits of high and low temperatures. The difference between the temperature of the box wall and the temperature of the flow field near the box wall is 2-3 ℃, which depends on the structure and materials of the box wall. The greater the difference between the test temperature and the external atmospheric environment, the greater the above temperature difference. Therefore, the space between 100-150mm and the box wall is unusable.
Then, how can the samples be correctly placed in the high and low temperature test chamber?
First of all, the test products (parts, components, components or complete machine) should be placed in the middle of the high and low temperature test box. The air duct above should not be covered, nor should they be placed directly on the surface of the box. The placement distance between samples should be adjusted according to the size, shape, weight and quantity of samples.
To ensure that the atmosphere around the product meets the environmental test conditions in the test specification, the volume of the product (W × D × H) Do not exceed the effective working space in the test box (20%~35%), and it is recommended that the heated products do not exceed 10% during the test. The windward sectional area of the tested product shall not be greater than the total area of the test box on the section (35%~50%). At the same time, the tested product shall be at least 100~150mm away from the left and right sides of the box wall.
If external power supply is required, the wires shall be uniformly connected to the side of the test hole, and the wires shall not be entangled in the equipment, especially there shall be no damage or other dangerous factors.
In addition, products should not be placed too densely. If the product is liquid or thick liquid, it must be packed in a container before being put into the box; Rubber products need to be cushioned with plates; If the product is volatile, flammable, explosive or has a spontaneous ignition point, the temperature should not be set too high, and the liquid should not spill out. For some small parts and components, appropriate trays should be prepared in advance.
Therefore, in the process of testing, if the standard requirements are met, you can correctly place the test samples of the high and low temperature test chamber according to the above contents, and reserve the test space for the main samples, so as to obtain more accurate test data, which is conducive to the development and improvement of the product.
If the number of samples to be done is large and the test equipment is insufficient, the test can only be conducted in batches, so that more accurate test data can be obtained.
