Common applications for liquid level sensors.For technicians repairing HVAC systems, one of the most common yet potentially unsafe tasks performed is refrigerant recovery. Part of making repairs and doing standard maintenance, the procedure – with liquid, vapor, and push-pull methods used – involves two possible DOT recovery tanks, both of which shouldn’t be filled beyond 80-percent capacity. Helping you reach this mark is a float switch inside the tank.

What a Float Switch Does

No matter which method you use, the unit continues to recover until the switch shuts down; then, the tank indicates that with a lamp. Once you go to purge the recovery unit, essential at the end of every service, the switch gets activated again and won’t turn off until the tank has been fully cleared.

While this sounds simple enough, the procedure itself has some inherent safety issues. The switch only turns off the recovery machine but doesn’t stop the flow of refrigerant, which can potentially result in an overfilled tank, which then becomes a hazard to the technician.

Additionally, as this occurs, if a tank is recovering a large amount of cold refrigerant from a system at a higher temperature, the refrigerant will continue to flow to the coldest point. When this occurs, you risk overfilling, even when the switch shuts the machine off.

Furthermore, overfilling may lead to too much pressure buildup inside the tank, which can then damage the float sensor. When this occurs, the shutoff switch fails to perform, and the technician, as a result, isn’t aware when it reaches the 80-percent mark.

Compatible Float Switches

A comparison of vertical and horizontal float switchesBecause of all issues delineated above, not every float switch is right for the job. For one, any switch installed needs to meet EPA standards for overfill tanks and additionally must be compatible with commercially available refrigerants.

Taking these factors into account, the standard switch for a DOT-approved tank tends to have a single-level design, possibly with multiple floats, actuation points, and termination styles. The model generally includes a brass stem with a Teflon or a stainless steel float.

As one potential solution, vertical float switches, mounted to the tank’s top or bottom, feature this two-part configuration, with a stationary stem and one or more hermetically sealed reed switches inside. A wire goes to the tank’s outside. As refrigerant fills up the tank, the float moves with the liquid’s level; when it lines up with the reed switch, it sends a signal through the wire to the tank’s outside, indicating the vessel must be shut off.

As another solution, horizontal float switches offer a similar style of operation, albeit they’re added through a hermetically sealed hole on the tank’s side. Here, when the liquid rises past the float’s level, the switch gets activated and sends a signal to the outside.

Through SMD Fluid Controls, both horizontal and vertical float switches come in several configurations, with stainless steel and plastic options available. As replacing your DOT tank’s float switch is key to its performance, be ready with a replacement. To discuss the best fit and custom solutions, contact our team today.


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