- CASE: Stainless Steel
- SOCKET: Brass (with powder paint coating)
- COMPATIBILITY CODE: M
- TEXT: Gauge/Valve Material Compatibility
Gauges have compatibility codes printed on their faces. These are usually the letter ‘A’ or ‘M’ with lines above, below or both above and below the letter. This is to signify the type of valve it can be used on:
- A letter with a line ABOVE AND BELOW it, or NO LINES at all, means the gauge can be used with either brass or aluminum valves
- A letter with a line BELOW it means the gauge can be used on brass valves only
- A letter with a line ABOVE it means the gauge can be used on aluminum only
Do not use a gauge specified for use on a brass valve on an aluminum valve. The contact of the two dissimilar metals will result in galvanic corrosion unless the gauge is coated with epoxy or a similar compound to minimize this risk.
PRINCIPLES OF OPERATION
- BACKGROUND: Older gauges operate using a ‘c-shaped’ Bourdon tube which straightens under pressure and returns to its original form when the pressure drops. The end of the tube was geared to turn the dial. New gauges (after 1964) use a spiral Bourdon tube without a gear.
- Incorrect Pressure Readings: Gauges are calibrated against atmospheric pressure, which is indicated when the gauge reads zero. Thus a cylinder pressure of 100 psi would move the pointer to 100. If there is a slow leak inside the case, however, and the pressure inside the gauge were 100 psi, the gauge would read zero. New gauges have pressure relief devices which relieve case pressure at 50 psi or less. There are many ways of creating leaks and leaks are not the only things that cause gauges to give false readings, so care should be used when installing them