Here is some info and some tips & requirements for using the OS-1.

  • Cable length: 10 feet / 3m
  • Hole size: 9/16" / 15mm 
  • An OS-1 indicates CLOSED when in water or as OPEN when out of water. An easy way to remember this is COVERED = CLOSED.


  • The OS-1 must be oriented properly. An OS-1 may be mounted horizontally, or may be mounted with the clear pyramid pointing UP. It must not be mounted with the clear pyramid pointing down; if improperly mounted pointing downward, a drop of water clinging to the tip could result in a false indication of water level.


  • The OS-1 may be mounted through a hole of the proper size in the side wall of your preferred ATO or saltwater reservoir. If you mount the OS-1 in the sidewall, the silicone gasket MUST be on the flange side of the optical sensor (inside the reservoir), not on the nut side of the sensor (outer side of the container). This is necessary to ensure a proper seal.


  • One common use for an OS-1 is as a low-level sensor in an ATO reservoir or saltwater reservoir. It is important to mount the sensor high enough above the bottom of the reservoir to alert you in the event that the water level gets too low, yet be high enough to positively prevent a pump from drawing air or running dry. For a PMUP, the recommended minimum height of the optical sensor is 2.5" / 65mm


This use of an optical sensor also requires suitable programming of the Apex. Let's say that you have named the FMM port for your reservoir's low-level optical sensor as "ResLow". In the output which controls the pump, add a statement (after your other programming) to shut the pump OFF if the water level drops below the ResLow sensor:

If ResLow OPEN Then OFF

In your email alarm output, add a statement to trigger an alert if the reservoir water level drops below the ResLow optical sensor:

If ResLow OPEN Then ON