The selection of instrument types must meet the requirements of process production. For example, whether remote transmission, automatic recording or alarm is required; whether the properties of the medium being measured (such as the temperature, viscosity, corrosiveness, degree of contamination, flammability and explosion of the measured medium, etc.) Environmental conditions (such as humidity, temperature, magnetic field strength, vibration, etc.) require instrument types, etc. Therefore, the correct selection of the instrument type according to the process requirements is an important prerequisite for ensuring the normal operation of the instrument and safe production.
For example, the spring tube of ordinary pressure gauges is mostly made of copper alloy (alloy steel is used for high pressure), while the material of the spring tube of ammonia pressure gauges is made of carbon steel (or stainless steel), and copper alloys are not allowed. Because ammonia reacts chemically with copper, it will explode, so ordinary pressure gauges cannot be used for ammonia pressure measurement.
The oxygen pressure gauge can be exactly the same as the ordinary pressure gauge in terms of structure and material, but the oxygen pressure gauge must be oil-free. Because oil enters the oxygen system, it is easy to cause an explosion. When the used oxygen pressure gauge is calibrated, oil cannot be used as the working medium like ordinary pressure gauges, and the oxygen pressure gauge should be strictly avoided to contact with oil during storage. If the existing pressure gauge with oil must be used to measure the oxygen pressure, it must be repeatedly cleaned with carbon tetrachloride before use, and carefully checked until there is no oil pollution.