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Thermatel® TD1/TD2 thermal dispersion switches

The Thermatel® Model TD1/TD2 switch is utilized in flow, level and interface applications. The TD1/TD2 is popular for pump protection, specifically low flow detection with the unique spherical tip design, to prevent pumps from running dry. Continuous diagnostics with fault indication, temperature compensation and fast response time make the TD1/TD2 the latest in thermal dispersion technology.

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The product

Thermal dispersion switch technology

The thermal switches are based on convective heat transfer. One sensor is at the process temperature and the other is being heated by a constant power. As the flow rate increases, the temperature difference between the sensors decreases. A set point is established so when that specific temperature difference is reached the relay changes state. This can be on either increasing or decreasing flow or flow/no flow. When used in a level or interface application it is primarily the thermal conductivity of the fluid that will provide the difference in heat transfer.


  • Temperature compensated to provide repeatable alarm under varying process temperatures
  • Continuous diagnostics detect sensor fault
  • Non-linear mA output signal can be used for trending, diagnostics and repeatable flow/level indication
  • Detects minimum flow or presence / absence of flow
  • Excellent low flow sensitivity
  • Simple calibration / installation


  • Hot tap retractable probe
  • Hygienic design
  • NACE construction
  • Window to view alarm status
  • Integral or remote electronics
  • Flow body for very low flow detection

Operating principle

The THERMATEL sensor consists of two RTD elements. One is the reference and the second is heated to a temperature above the process temperature. The electronics detect the temperature difference between the two elements. The temperature difference is greatest in air, then decreases when cooling occurs due to a change in media. An increase in the flow rate further decreases the temperature difference.

The set point is adjusted for the switch to alarm at the desired temperature difference. Once the set point is reached, the relay will change state.


ATEX Ex d, Ex d+ib
IEC Ex d, Ex d+ib
Inmetro Ex d, Ex d+ib
Korea Ex d
SIL SIL 1 (1oo1)

Other approvals are available. Consult factory for more details.

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