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685S Single Switch Liquid Level Indicator for Atmospheric Vessels

The Model 685S is a liquid head pressure type level control. It continuously indicates the liquid level in an atmospheric vessel in feet and inches. It has an adjustable switch which completes an electrical circuit to a solenoid, alarm, or other device when the level rises to the top predetermined point, and breaks the circuit when the level falls to the bottom predetermined point (or the reverse). In addition, the scale on the Model 685S is available calibrated for horizontal cylindrical vessels in pounds, gallons, barrels, percent, etc.

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685S Single Switch Liquid Level Indicator for Atmospheric Vessels


Liquid head pressure against a flexible elastomer diaphragm is transmitted directly to a leaf spring, which has a fixed pivot point at the upper end and an adjustable fulcrum point in the mid region of the leaf spring. For example, because 8’ of heavy liquid exerts more pressure on the diaphragm than 8’ of lighter liquid, the spring can be “stiffened” by moving the fulcrum down by the gravity adjustment knob (red), so the indicator will still read 8’. The zero knob (green) moves the entire spring assembly and pointer, having the same effect as shifting the scale. The switch trigger assembly is normally independent of the indicator mechanism and is free to tilt around the fixed pivot, except that it is weighted to assure a vertical position. At high or low levels, the trigger assembly tilts; thus actuating the cam and engaging the switch.


Since the unit measures liquid head pressure, the point at which it is installed on the vessel is the point that liquid measurement begins. For example, if the unit is mounted 1 foot above the bottom of an 8’ vessel, the liquid level from the bottom to the 1’ mounting will not be measured (or seen); therefore, the indicator should be “zeroed” at 1’ and the specific gravity knob should be calibrated accordingly ie. 1 to 8 feet or total of 7 foot span. For calibration and maintenance purposes, always install an isolation valve between the model 685S and the vessel. Also, install a drain petcock in the liquid inlet bottom, where a plug is generally located.


  • Simple Adjustments: One high adjustment dial for top level set point and one low dial for bottom level set point. One Zero Adjustment Knob and one Specific Gravity (span) Knob. Unit can be factory calibrated for a nominal cost or field adjusted in less than 15 minutes, without having to change the liquid level in the vessel.
  • Easy to Read: Scale reads the same as a hand tank gauge, making it unnecessary to convert psig of head pressure into feet and inches of liquid to get the correct reading.
  • Long Life: All bearings are Teflon®, Delrin® or Moly Disulphide baked on Dry Film Lubricated Stainless Steel.
  • Weather Proof and Explosion Proof Housing.


  • Explosion Proof: CSA Approved, Class I, Group C & D, Class II, Groups E, F and G hazardous locations.
  • Temperature: -20˚F to +225˚F Standard
  • Pressure: 0 to 4 ounces pressure (Atmospheric)
  • Range (Vessel Height): 3’ thru 60’ (see 685 code sheet)


  • SPDT Micro Switch
  • 5 Amp @ 125, 250 or 480 VAC
  • 1 /2 Amp @ 125 VDC, 1 /4 Amp @ 250 VDC
  • Electrical Connections: 1/2″ Female Pipe Thread, 3 /4″ Female Pipe Thread is also available as an option.

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