What protects the bourdon tube in a steam pressure gauge?

The bourdon tube in a steam pressure gauge is protected by a siphon. The siphon is a device that helps to prevent steam from directly entering the bourdon tube. This protection is necessary because steam can be extremely hot and can damage the sensitive components of the gauge. By using a siphon, a water-filled loop is created which not only provides a barrier against high temperatures and potential damage but also helps to cool the steam before it reaches the bourdon tube. This ensures accurate pressure readings and extends the life of the gauge, particularly in high-temperature applications.

The other options, while related to pressure and temperature management, do not serve the specific function of protecting the bourdon tube in this context. A pressure relief valve is designed to release excess pressure to prevent system failures, a thermal expansion joint accommodates movements from thermal expansion in piping, and a pressure regulator controls the pressure of the fluid reaching a system but does not provide any direct protection to the bourdon tube itself.