The Gammon Gauge Differential Pressure Indicator is an instrument designed to give a simple rating as to the condition of the filter elements in an aviation fuel filter vessel. It does this by comparing the inlet pressure with the outlet pressure, and displaying the difference.
The actual pressure is irrelevant to the condition of the filter elements, except when they are plugged.
To read this difference simply, the Gammon Gauge uses a piston in a very tight fitting glass tube. The actual clearance is a fraction of an inch. For this reason, the Gammon Gauge uses a small filter, GTP-2233, located in the top of the housing.
Recently, it has been discovered that some enterprising individuals are selling similar "AN" type filters as "equal" to the Gammon filter. An "AN" filter is an "Army/Navy" (AN) design meant for use in various hydraulic systems. There are a number of similar looking filters of this type. We have reviewed three of these filters, and all three failed to meet the requirements in at least one aspect of the construction. There are several different "AN" style filters, all about the same size, but differing in several ways.
All three we have had sent to us from the field had incorrect seals and less filter paper, and two didn't seal tightly. One had a seal incompatible even in jet fuel. The reason is simple, the Gammon Gauge filter is not an "AN" filter. Howard Gammon, our founder (and my Dad), wanted to design the best possible filter for the Gammon Gauge, and started with the basic size of an “AN” filter, but maximized better and specifically for the application. He never anticipated that people would substitute an "AN" filter.
But in at least one case we know of, a supplier even labeled the box the filter came in as "GTP-2233", which is beyond an incorrect substitution, it is a counterfeit.
The true Gammon Gauge filters have the part number GTP-2233 printed on the end cap. It has 30 pleats of filter paper and a Viton seal made to seal tightly into the gauge, so tightly that removing a Gammon Gauge filter can be slightly difficult, which is why we have a tool for this removal.
Incorrect filters cannot be counted on to keep dirt, rust and debris from getting to the piston and jamming it, or causing errors in readings. The GTP-2233 design allows for a long life and reliable readings. For these reasons, we had to inform the EI, JIG, A4A/ATA and IFQP organizations that these incorrect replacement elements have found their way into our industry, and this prevents us from being able to "stand behind" the Gammon Gauge if these incorrect filters are used in our gauge.
Simply said, if the filter doesn't have GTP-2233 printed on it and 30 pleats of paper, it isn't genuine and cannot be counted on to give accurate readings.