How is a steam pressure gauge protected against excessive temperature?

A steam pressure gauge is protected against excessive temperature primarily by using a siphon. A siphon, often referred to as a siphon tube or trap, is a U-shaped pipe that is filled with liquid, usually water. This liquid absorbs heat from the steam before it reaches the gauge, thereby reducing the temperature of the steam that comes into contact with the gauge itself.

This cooling effect helps to prevent the high temperatures of the steam from damaging the sensitive internal components of the pressure gauge, which could otherwise result in inaccurate readings or failure of the gauge due to thermal stress.

The other options do not provide the same protective effect. A temperature gauge would measure temperature but would not provide direct protective benefits against high temperatures affecting a pressure gauge. A pressure relief valve is designed to release pressure, not to mitigate temperature effects. Lastly, while an air pocket might have some insulating properties, it is not a standard or effective method to protect a pressure gauge from excessive heat.