I would like to ask about the explosion-proof rating of the instrument

Yes. The explosion hazard zone classification diagram is designed by electrical specialists; by referring to this diagram, we can determine the explosion-proof rating required for instruments. It is sufficient that the explosion-proof rating of the instruments meets or exceeds the level specified in the explosion hazard zone classification diagram. Of course, on the explosion hazard zone classification diagram, the ratings for different zones may vary; this is related to the type of flammable gas present in each zone. For example, if a zone contains hydrogen, while another zone contains only ordinary flammable gases, we generally design the system according to the highest requirements, that is, based on the C gas group classification. This prevents confusion among the instruments used on site. Therefore, in installations where hydrogen or acetylene is present, the explosion-proof rating of the instruments is typically E*aIICT4 or ExdIICT4. Of course, in devices that do not use hydrogen or acetylene, the explosion protection rating of the instruments is generally specified as E*aIIBT4 or ExdIIBT4. T4 refers to the requirements regarding the surface temperature of instruments, and it is divided into six levels; these levels are related to the ignition temperature of gases. T6 represents the highest requirement, indicating the maximum surface temperature that an instrument (an electrical device) is allowed to have due to heat generated by its own electrical consumption. Most process units are defined as Zone 2, so explosion-proof types such as dIICT4 can be used. But when the definition is Zone 0, only intrinsically safe types are allowed to be used. The area in Zone 0 that we can imagine at present is the part above where the floating roof of a floating roof tank meets the inner wall of the tank, as flammable gases are constantly leaking there.

The value of T depends on the medium, but in typical designs it is standardized to one or two temperatures to facilitate maintenance

I’m wondering, for a gauge that operates on 24V electricity, will its surface temperature exceed 85 degrees?

The intrinsically safe gauge of T6 will not exceed 85°C. With explosion-proof certification specifying such parameters, if you take it seriously, then you have to ensure that those specifications are met. What is the operating temperature for instruments in relation to this 85°C threshold? ... Once you start paying close attention to these details, things can get complicated; it’s possible to exceed the limits, and then, damn it, there might not be any instruments available anymore........ :lol

The explosion protection rating of instruments and that of electrical equipment are the same thing; in China, the standard applicable in both cases is GB 3836 \"Explosive Environments\"； Some instruments are inherently intrinsically safe, such as pneumatic instruments. In principle, anything that is connected to other electrical devices is not a natural intrinsically safe instrument (since intrinsically safety is a system). Devices such as thermal resistors, thermocouples, and weighing elements that have not been certified are not intrinsically safe instruments. The temperature range is selected as needed; of course, the instrument can easily achieve T6, while some electrical devices can only be selected according to specific requirements.