Level Control Switches

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685E Electric Liquid Level Indicator for Atmospheric Vessels

The Model 685E Liquid Level Transmitter is an electro-mechanical level control device, which accomplishes three (3) main functions. These are remote indication, local liquid level indication, and hi-low adjustable switch outputs. The unit continuously indicates the liquid level and transmits a proportional signal.


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 transmitter will still read 8’. The zero knob (green) moves the entire spring assembly and pointer, having the same effect as shifting the scale. The movement of the pointer rotates a shaft extending into the explosion proof housing where the electronic transmitter and switches are located. The shaft drives a potentiometer, thereby giving a continuous signal to the plug-in amplifier. The proportional signal is then transmitted to the remote indicating device in proportion to the liquid level which exists in the tank.

  • Complementary Adjustments: Zero and Span adjustments at transmitter.
  • 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.
  • Special Scales: Gallons, Pounds, Percent, etc. is available.
  • Long Life: All bearings are Teflon®, Delrin® or Moly Disulphide baked on Dry Film Lubricated Stainless Steel.
  • Weather Proof and Explosion Proof Housing.
  • Indicator Housing: Weatherproof
  • Proof Pressure: 100 psig
  • Temperature: -20˚F to +225˚F Standard
  • Pressure: 0 to 4 ounces pressure (Atmospheric)
  • Range (Vessel Height): 3’ thru 60’ (see 685 code sheet)