Level Measurement

Pneumatic Level Transmitter

The pneumatic level transmitter has the advantage of no leakage will matter to use in the process industry and the pneumatic signal can be feed backed to control the pneumatic valve.

Construction of Pneumatic level transmitter

Pneumatic level transmitter

The valve and float arrangement so the float arm compresses a spring and the spring force is opposed by a bellows that contains our output pressure.

Flow through the valve doesn’t change based on position of the float, it responds to differences between the position of the float and the balance of forces of the spring and the bellows.

Working of Pneumatic Level transmitter

  • As the level increases the nozzle is moved away from the baffle so more air bleeds out at the nozzle. The pressure in the output bellows decreases so the spring pushes down on the float and upon the baffle, following the nozzle.
  • When the level falls the nozzle is pressed against the baffle so the pressure increases and the bellows compresses the spring to push the baffle down.
  • The transmitter uses a pressure balance principle where the output pressure of the transmitter is feedback to restore the balance of the device, in this case the relative position of the nozzle and baffle
  • This transmitter is of reverse action, the output increases as the level decreases. In the figure, the valve is draining the tank so that the level falls. The same transmitter can be used for the direct control of a replacement water valve that feeds a boiler feed tank because we could change the internal valves

Stuck in Pneumatics level transmitter

  • When a level measuring device remains “stuck” at a measurement value of roughly 40% despite fluctuations in the process liquid level between 20% and 80% of the range
  • The pneumatic transmitter’s cover is taken off by an instrument technician, who then briefly presses the baffle against the nozzle. The level indication in the control room is fixed at roughly 40% and makes no movement at all.
  • Following the results of the technician’s test, rate the likelihood of each potential fault in the table below, indicating whether the fault is “probable” (worth considering as a cause of this system’s trouble) or “unlikely” (either completely ruled out as a cause, or just not worth considering at this point in the diagnosis):

List of Defects in Pneumatic Level Transmitter

Determine whether the list of defects below are probably to happen or not.

  • Obstructing in isolation valve and equalizer valve
  • Replace the liquid that was lost in the “wet” leg.
  • Low air supply pressure
  • Clogged transmitter restrictor 
  • Stuck in indicator pointer
  • Blocked transmitter nozzle
  • Plugged 3-15 PSI signal tubing
  • Transmitter was not calibrated.


Instrumentation Engineer

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