May I ask if silicone oil can be used in pressure gauges for Class A explosion-proof areas?

This post was last edited by Shutong on 2022-4-6 at 11:25. First, let’s talk about Class A explosion-proof areas: 1. Liquids with a flash point below 28°C ； 2. Liquids with an explosion limit of less than 10% ； 3. Substances that can decompose on their own at room temperature or oxidize in air, thereby leading to rapid spontaneous ignition or explosion ； 4. Substances that, at room temperature and in contact with water or water vapor in the air, can produce flammable gases that lead to combustion or explosion ； 5. A strong oxidizing agent that is highly prone to combustion or explosion when exposed to acids, heat, impact, friction, catalysis, or flammable inorganic substances such as organic compounds or sulfur ； 6. Substances that can cause combustion or explosion when subjected to impact, friction, or contact with oxidizers or organic substances ； 7. Production carried out in enclosed equipment at a temperature equal to or greater than the substance’s own autoignition point. Furthermore, silicone oil (1) has the best viscosity-temperature properties among liquid lubricants, with little change in viscosity over a wide temperature range. Its freezing point is generally below –50 °C, with some reaching as low as –70 °C. When stored at low temperatures for extended periods, the appearance and viscosity of this oil remain unchanged. It is a base oil suitable for use over a wide temperature range, including both high and low temperatures. (2) Excellent thermal oxidation stability, such as a thermal decomposition temperature >300°C, low evaporation loss (2% only after 30 days at 150°C), and minimal changes in viscosity and acid value during oxidation tests (at 200°C for 72 hours). So I think silicone oil can be used to fill pressure gauges, but pressure gauges for oxygen and oxidizing media must not be filled with any oil!

I agree with the views of Shutong and Haiyou. Filling pressure gauges with silicone oil is a common practice, taking advantage of its excellent chemical stability and wide operating temperature range. Since the silicone oil is placed inside the sealed diaphragm of the pressure sensing element, it does not have any adverse effect on explosive environments from a usage perspective. As suggested by bookboy Haiyou, when silicone oil is used in oxygen-based media, high-pressure air media, or strong oxidizing agent media (such as chlorine, nitrogen oxides, etc.), it cannot be used in such situations because silicone oil may react with these oxidizing agents; instead, liquid chlorinated paraffins or perfluorohydrocarbons should be used.

Silicone oil is added for shock absorption, and the amount used is very small, so it has no impact on explosion protection

This is not for shock absorption, but rather for pressure transmission; by utilizing the incompressibility of liquids, pressure is transmitted accurately and reliably to the pressure sensing element, based on the principle of Pascal’s law. If shock absorption is the goal, then a little air should be added to the chamber filled with silicone oil, so as to achieve better shock reduction; however, this will result in delayed and inaccurate pressure transmission.

Where does the definition of this “Class A explosion-proof area” come from? There’s no such definition in the standards, right? Factory mark?

Are you thinking of it as a transmitter? This is a pressure gauge; it has oil filled in its casing