Hydraulic and pneumatic load cells

Both cells convert force into a fluid pressure, which is measured by a pressure gauge or transducer.

Hydraulic load cell

The force acts on a piston that bears on a liquid (oil) held in a chamber and sealed by a rolling diaphragm. The fluid pressure rises:

$$p = \frac{F}{A}$$

and the pressure gauge, calibrated in force units, shows the load.

      F
      v
   [=======] <- loading plate
   |piston |
   |-------|  diaphragm seal
   | oil   |---> pressure gauge / transducer
   |_______|

• Almost frictionless because of the diaphragm.
• Needs no electric power, so it is suitable for hazardous areas.
• Useful for heavy loads (tank weighing, bridge testing, truck scales) and remote reading.
• Temperature changes affect the oil volume and pressure.

Pneumatic load cell

The force is balanced by air pressure acting on a diaphragm. The force pushes the diaphragm (flapper) toward a nozzle, which restricts the air bleed, so the pressure under the diaphragm rises until the balancing force $pA = F$ is reached. This back pressure is read on a gauge and is proportional to the force.

   Air supply --[restrictor]--+--> gauge
                              |
                      nozzle  |
                     ======== flapper moved by F
                        F v
                   [diaphragm]

• Intrinsically safe, clean and inexpensive.
• Relatively insensitive to temperature, unlike hydraulic cells; no liquid to leak.
• Typical for food and pharmaceutical work and for weighing small loads (up to about 20 kN) with good accuracy.
• Limited by response lag (compressible air, tubing volume) and needs a clean, dry air supply.

Comparison with strain gauge cells