Types of pressure gages
The two main types of pressure gages are
1. Bourdon tube type
2. Diaphragm type
In single-tube Bourdon gauge the pressure is applied to inside of a bent tube of oval cross section which is closed at one end and connected at the other to boiler pressure. The closed end is attached, by links and pins, to a toothed quadrant, which is turn meshes with a small pinion on the central spindle. As pressure builds up inside the over tube, it attempts to assume a circular cross section, thus tending to straighten out lengthwise. This action turns the spindle by the links and gearing, causing the needle to move and register the pressure on a graduated dial.