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How We Helped The US Coast Guard With A Marine Tank Level Switch

United States Coast Guard Fast Response Cutter boats use the FL level switch from APGThe United States Coast Guard (USCG) was in need of a level switch solution to measure 4 different level points (30%, 60%, 75%, 90%) in raw sewage tanks installed on their Fast Response Cutter boats. Ultrasonic sensors were used before, but the USCG was not satisfied with the overall performance.

In reality, the turbulent movement of liquid in a boat at sea is not a good environment for ultrasonic sensors. They cannot easily lock on the liquid target and send a reliable reading.

The solution, instead, was an FL float level switch that was modified to meet the USCG’s needs.

The FL uses single pole single throw (SPST) reed switches and a magnetic float at each switch point. The magnets in the float cause the switches to either open or close when the tank fluid passes over the floats.

Normally when the FL float is placed on the stem, there is a very small distance between the stem and the inside diameter of the float. In this sewage application, the standard FL float would not work because of build-up in-between the float and stem. It is very likely that the floats would stick and not move across the switch points.

To resolve this dilemma, 3” floats with a large inside diameter were chosen. This creates a large gap between the stem and the float, which keeps the float from sticking and allows it to move freely.

The FL level switch can have up to 7 different switch points, and has a few stainless steel stem, mounting, and float material choices. The FL level switch is mounted with either with an NPT plug or flange. Due to the caustic nature of the application, the US Coast Guard chose 316L SS for the float, stem, and mount.

The standard FL level switch housing (not in contact with the liquid) is made from aluminum alloy, but to save cost and weight a lighter, less expensive nylon housing was chosen.

For this specific application, the USCG wanted the level switch to activate indicator lights as the tank level rises. To counteract turbulence, a time delay was programmed into the controller to keep the lights from firing as waves temporarily lift floats past the level switch points - potentially giving a false level.

By using an FL level switch built to unique needs, the USCG has a rugged sensor that will give years of service in an incredibly harsh environment.

We would like to talk to you about your specific level switch needs (or any other level sensor need, for that matter). Contact us if you have any questions about level switches and customization.

Image credit: U.S. Coast Guard

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