Each year during the cold weather months we (and probably all other manufacturers of Stored Pressure fire extinguishers) experience a rash of complaints about “leakers.” Usually it turns out that the extinguishers have been subjected to abnormally cold temperatures in transit.
Most if not all extinguishers are filled and pressurized at 68-70 degrees F. Pressure gauges are calibrated to show the normal CHARGED pressure at this temperature. The GREEN area of any pressure gauge is intended to show a CHARGED condition for the extinguisher through the temperature range for which the extinguisher is rated (ie. –40 degrees through +120 degrees F). An extinguisher which has been subjected to temperature ranges between –40 degrees and +70 degrees F. will register between the left edge of the GREEN pie and the upright CHARGED position. Conversely, extinguishers exposed to abnormally high temperatures will indicate a reading higher than (to the right of) the upright position.
If manufacturers were to purposely overpressurize the extinguishers to account for anticipated low temperatures during transit into colder climates, warm weather would bring complaints of overcharged extinguishers.
The pressure/temperature relationship is noticeably greater in halon (vaporizing liquid) extinguishers than in dry chemical units because halon itself expands and contracts in relationship to temperature. It is for this reason that the GREEN pie area on halon gauges is larger than on dry chemical or water gauges.
WHEN IN DOUBT: CONDITION THE EXTINGUISHER TO 70 DEGREES F. (21 DEGREES C) FOR SEVERAL HOURS TO OBTAIN AN ACCURATE INDICATION.
A FEW ADDITIONAL PRESSURE GAUGE ITEMS SHOULD BE NOTED:
Always use the proper replacement gauge. All gauges used by Amerex on new extinguishers or sold as replacement parts are marked as follows: DRY CHEMICAL (also used for Class D Dry Powder), WATER (also used for Loaded Stream/Anti-Freeze, FFFP, and AFFF foam) or HALON.
Avoid using a pressure gauge with a stem of dissimilar metal to the extinguisher valve body. Use “epoxy coated” brass stem gauges on aluminum bodies and either “coated” or “uncoated” brass stem gauges on brass, chrome plated valves.
Always use a stainless steel or oil filled bourdon tube type gauge on WATER, LS/AF and FOAM extinguishers. This will prevent bourdon tube corrosion, which could cause a leak or possibly make the extinguisher inoperable.
To avoid potential damage to the gauge bourdon tube, never set your pressure regulator to more than 25 psi above the extinguisher operating pressure.