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What is a Limiter?

 

A Limiter, or Limit Device, is an electronic protective device used to read an input value from a measuring device and, with a programmed setpoint limit value, stop a potentially unsafe condition from continuing. See the diagram example below.


 

The controlled variable is measured by a suitable sensor and converted to a signal acceptable to the limiter. The limiter receives that signal and converts it to a value representing the appropriate unit of measure. A desired limit value (setpoint) is programmed into the device to represent the line separating the unsafe condition from the “safe zone”. This limit setpoint can define unsafe as high/over condition or low/under condition. A fail-safe latching relay in the limiter is used to open the electrical circuit to stop the process, and it cannot be reset by the operator until the process is back in safe condition. Limiters are FM approved for temperature applications in particular but can be used to stop many potentially unsafe process conditions not requiring agency approvals.

Example:

 

Gems RFA Series continuous flow rate sensor measures coolant volume from 8 – 60 GPM on a machine tool cutting application. It sends a 0-10 VDC scaling linear signal to a CAL MaxVU Rail Limiter. The limiter converts that signal to read anywhere from 8 to 60 gallons per minute on the display. The limiter is programmed to open the relay connected to the electrical circuit powering the cutting machine when the GPM drops down to 15 GPM or less. This protects the machine tooling from overheating causing possible damage to expensive parts. The circuit remains open preventing operation to continue until flow is once again above the 15 GPM setpoint and the operator manually resets the device. The pump’s optimal flow rate is 30 GPM, so a second relay is programmed to set off a visual alarm light when the flow deviates 5 GPM above or below the 30 GPM rate. This is called a “band alarm”.

 

Indicators can have auxiliary functions:

  • Deviation and band Alarms
  • Linear signal output for retransmission
  • Digital input remote reset function
  • Communications

 

Gems Sensors & Controls and West Control Solutions are happy to offer a wide range of limiters to help with your process protection needs. See more HERE.