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Models T20 & T21 top-mounting single-stage and dual-stage float switches

Single Stage Switch

T20 units are simple, reliable float switches, designed for top mounting into tanks or vessels. T20 units utilize a single switch mechanism and float. It can be used for virtually any type of open or closed process or storage vessel, with either threaded or flanged type mounting, and actuating depths of up to 48 inches (1219 mm).

Dual Stage Switch

T21 units combine the functions of two separate narrow differential level controls in a single, compact, easy-to-install instrument. Two independent switch mechanisms are employed to provide two actuating levels at least 8” apart, at actuating depths of up to 48 inches (1219 mm), for applications such as high and low level alarm operation. Model T21 tandem float switches are available with optional cages to help stabilize the floats under turbulent conditions.


Float level switches

All float operated liquid level controls operate on the basic buoyancy principle which states "the buoyancy force action on an object is equal to the mass of liquid displaced by the object." As a result, floats ride on the liquid surface partially submerged and move the same distance the liquid level moves. Because of this, they are normally used for narrow level differential applications such as high level alarm or low level alarm.

Float level switches


  • Float diameters of 3" × 5", 4" and 4½" available
  • Tank connections available in 1" NPT, cast iron, forged, or stainless steel flanges
  • Choice of switch mechanism: ◦ Dry contact ◦ Hermetically sealed ◦ Pneumatic
  • Choice of switch enclosure: ◦ NEMA 1 carbon steel for pneumatics ◦ TYPE 4X/7/9 Class I, Div. 1, Groups C & D, polymer coated aluminum ◦ TYPE 4X/7/9 Class I, Div. 1, Group B, polymer coated aluminum


  • NACE
  • ATEX approved housing
  • Housing heater
  • Float guide cage
  • Tropicalized switch mechanism
  • Special flange face finishes
  • Submersible housing
  • Elevated temperature

Operating principle

The buoyancy force of a process liquid on the float causes the float to move with the liquid surface. The movement of the float moves an attraction sleeve into the field of the switch magnet, pulling the magnet toward the sleeve and actuating the switch.

Float level switches
Float level switches


ATEX Ex d, Ex ia
EAC (GOST) Ex d, Ex ia
IEC Ex d, EX ia
Inmetro Ex d
Korea Ex d
SIL SIL 2 (1oo1)
Marine Lloyd's Register of Shipping (LRS)

Other approvals are available. Consult factory for more details.


Approvals & certificates

ATEX Ex d certificate Mechanicals
ATEX Ex i certificate Mechanicals
Canadian certificate Mechanical Switches - IS, NI
FM certificate Mechanical Switches
FMEDA Report Models T20/T21
IECEx d certificate Mechanicals
IECEx I certificate Mechanicals and Tuffy® II switch

Instruction manuals

Models T20/T21 I&O Manual
T20 and T21 units are simple, reliable float switches, designed for top mounting into tanks or vessels.

Product catalogs

Models T20/T21
T20 and T21 units are simple, reliable float switches, designed for top mounting into tanks or vessels.

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