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this switch is "normally closed" and should show near-zero resistance
It is frequently used for oil return lines and pressure sensor ports
Compare your reading against the Lochinvar sensor resistance chart for the current ambient temperature
compressed elastomer is mandatory to prevent weeping or catastrophic leaks under system pressure
McDonnell & Miller 310488 Switch Assembly R-410A this switch is "normally closed"The McDonnell & Miller 310488 is a field replaceable SWA 42S Snap Switch Assembly engineered specifically for Series 42S low water cut off and pump controllers. This single pole, single throw (SPST) mechanism provides precise automatic reset functionality, ensuring consistent boiler feed pump control and high limit safety across residential, commercial, and industrial steam applications. Built with a robust housing and high quality internal contacts,