The selection of equipment should follow the following five basic principles:

1. Reproducibility of environmental conditions

It is unattainable 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 experienced by engineering products during use, storage, transportation, and other processes. This statement can be summarized in engineering language as follows: "The environmental conditions (including platform environment) created by the testing equipment around the tested product should meet the environmental conditions and tolerance requirements specified in the product testing specifications.". The temperature box used for military product testing should not only meet the high and low temperature test values and test time specified in the national military standards GJB150.3-86 and GJB150.4-86 for different types of products, 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 tests.

2. 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 the test results of the same product under the environmental testing conditions specified in the same testing specification, it is necessary to require the environmental conditions provided by the environmental testing equipment to be repeatable. This means that the stress levels (such as thermal stress, vibration stress, electrical stress, etc.) applied to the tested product by environmental testing equipment are consistent with the requirements of the same test specification.

The repeatability of the 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 and accuracy indicators in the calibration regulations, and to not exceed the time limit specified in the calibration cycle in terms of usage time. The widely used electric vibration table must not only meet the technical indicators such as excitation force, frequency range, and load capacity, but also meet the accuracy requirements specified in the calibration regulations such as lateral vibration ratio, table acceleration uniformity, harmonic distortion, etc. Moreover, the usage cycle after each calibration is two years. If it exceeds two years, it must be re calibrated and qualified before it can be put into use.

3. 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, ensure the reproducibility and repeatability requirements of the test conditions, but also from the perspective of product testing safety, in order to prevent damage to the tested product caused by uncontrolled environmental conditions and unnecessary losses. The general requirement in various test specifications is that the accuracy of parameter testing should not be less than one-third of the allowable error under the test conditions.

4. Exclusivity of environmental test 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 provides the specified environmental conditions, but also does not allow additional environmental stress interference on the tested product. As specified in the verification regulations for electric vibration tables, the table leakage magnetic field, acceleration signal-to-noise ratio, and the ratio of the total root mean square values of the acceleration within and outside the band are limited. The inspection of random signals and accuracy indicators such as harmonic distortion are all calibration items developed to ensure the consistency of environmental test conditions.

5. Safety and reliability of testing equipment

Environmental testing, especially reliability testing, has a long testing cycle and sometimes targets high-value military products. During the testing process, testers often need to operate or test around the site. Therefore, it is required that environmental testing equipment must have characteristics such as safe operation, convenient operation, reliable use, and long working life to ensure the normal progress of the test itself. The various protection, alarm measures, and safety interlocking devices of the testing equipment should be complete and reliable to ensure the safety and reliability of the testing personnel, the tested product, and the testing equipment itself.