Diaphragm for Pressure Measurement

A Diaphragm is a flexible membrane which expands and contracts according to the process pressure. Its deflection is proportional to the process pressure. In other words, it is a sensor which converts process pressure to mechanical displacement. It requires another element to convert this displacement to analog or digital readable form and calibrated in terms of pressure units. It uses a comparison type of measurement, i.e. the process pressure is compared with the external pressure. A diaphragm sensor is shown below:


Pressure Sensor

As shown in the image above, the diaphragm is subjected to external pressure. It thus expands and contracts as the pressure changes. The displacement of the diaphragm result in motion of the gear which moves the indicating needle on a scale. The scale is calibrated in terms of pressure units. In this case, we have used a mechanical assembly. We can also use electrical displacement sensors like LVDT or Piezoelectric crystal, which can measure the displacement of diaphragm with higher accuracy.
Diaphragms can be used only for pressures below 400 psi.

Material of Construction:
  • Phosphor: For non-corrosive process.
  • Brass : For low pressure measurements.
  • Beryllium Copper: For medium range process.
  • SS316: For corrosive fluids and high pressure measurements.
  • Depending upon pressure ratings and fluid compatibility, various other materials like monel, tantalum, titanium, various other SS grades are also used.
Advantages:
  • Good Linearity
  • Small size
  • Moderate Cost cost
  • Seals the process. Therefore, no leakage problems.
Drawbacks:
  • Limited to low pressure.
  • Difficult to repair.
  • Susceptible to cracking.

Applications:
  • Processes where purity is important. Diaphragm isolates the sensor whose metal parts can contaminate the fluid with ionic reactions. e.g Food, pharmaceuticals, chemicals, etc.
  • Pneumatic processes where proper sealing is necessary.

Diaphragms used for pressure measurement are sometimes also known as diaphragm seals, since they isolate the secondary transducer from the process fluid.