Definition
A pressure gauge is a mechanical instrument. It is designed to measure the internal pressure, vacuum of a vessel or system. There is lots of variety of pressure gauges in the market according to size, styles.
Principles of Operation
Bourdon tubes are the main pressure sensing element used in most of the pressure gauges
When the pressure is applied to Bourdon tube (sensing element), it flexes and the resulting motion is transmitted as a measurement through a mechanical movement that we can see at the dial face.
Selection of a pressure gauge
Case
Cases are available in a wide variety of materials and configurations. The combination of material and configuration is generally determined by the demands of the application, as well as the interest of the gauge purchaser.
If the pressure gauge is purchased for a severe environment rugged the case construction will be.
Wetted Parts and Pressure Medium
Under normal operating conditions, only the tube and socket assembly (Wetted Parts) of a pressure gauge will come into contact with the fluid being measured(Pressure Medium). The selection of the assembly will be determined by the composition of the medium. Air, gas, steam, water and other non corrosive media are usually satisfied by a bronze or brass bourdon tube and brass socket assembly. Stainless steel or Monel wetted parts are used when the medium contains corrosive elements or when high operating pressures or temperatures will be encountered
Connection and Mounting
The socket connection provides an entrance port for the process medium as well as a means to mount the gauge to a pipeline or vessel. Male connections can be provided with NPT, BSPT, or other thread styles, in sizes from 1/ 8″ through1 / 2″.
Window and Ring
The window provides a means for viewing the measurement, as well as protection of the dial face and pointer of the instrument, and is normally held in place by a ring, which is screwed or snapped to the case of the gauge. Plastic, clear glass and laminated glass are common window materials. Ring styles include threaded, bayonet, friction, crimped and hinged, depending upon the case type.
Pointer
There are essentially three types of pointers available on Pressure Gauges:
- Micro-adjustable pointer (providing high quality and accuracy)
- Friction-type pointer (providing adjustability and durability)
- Plain, non-adjustable pointer (providing economic reliability).
Measurement Range and Dial
The maximum operating pressure of the application should not exceed 75% of the measurement range selected. Therefore, the specified range should be twice the normal operating point of the gauge to avoid damage to the gauge internals. A wide variety of measurement ranges is available, from 30″ Hg vacuum through 20,000psi pressure. Ranges are indelibly presented in black figures and markings upon a white dial face.
Environmental Conditions
Pressure gauge selection will be influenced by the environmental conditions under which the gauge is expected to perform. Condensation or waterproofing concerns can be addressed through the use of hermetically sealed or weatherproofed gauges. Case construction should be considered when selecting a gauge to be used in conditions of extreme or sustained heat or cold, or where the atmospheric environment may contain corrosive elements.
Liquid Filling
Liquid filling (glycerin, silicone or other fills) can prolong the life of a pressure gauge by minimizing wear on the gauge internals resulting from vibration or oscillation. Liquid filling also acts as a permanent lubricant to the moving parts of the instrument.
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