These UL-listed Bourdon gauges have a stainless steel case, an all-brass threaded male inlet (no powder paint coating), and markings in both psi and kPa. 100 psi is the green charged zone and the dial range goes from 0 to 200 psi (0 to 1380 kPa). The male thread is 1/8" NPT (outside diameter ~3/8") and the gauge diameter is 1-5/8" (41 mm). These gauges work with water, AFFF foam and loaded stream handheld extinguishers.
The Extinguisher Gauge Wrench (item # 09838) makes installation and removal of gauges a snap, even in tight places.
The compatibility code on these gauges is M. 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. These gauges are for use on water, AFFF or loaded stream fire extinguishers only.
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) such as this one use a spiral Bourdon tube without a gear.
Note: 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* was 100 psi, the gauge would read zero. New gauges such as these 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.