Pressure References and Sensing

The three references

• Gauge pressure — measured relative to local atmospheric pressure, through a vent in the transmitter. The usual choice for process pressure. Note that its reading moves with barometric pressure and with altitude, and that the vent must be kept clear — a blocked or waterlogged reference vent produces a slowly drifting error that is very hard to diagnose.
• Absolute pressure — measured relative to a sealed vacuum reference. Required for vacuum service, for gas calculations, for compressor performance work and anywhere a gauge measurement's dependence on the weather is unacceptable.
• Differential pressure — the difference between two process connections. The basis of orifice flow measurement, hydrostatic level, filter and strainer monitoring, and pump differential monitoring.

Sensing technologies

• Capacitive — a diaphragm moves between capacitor plates. Very stable, excellent for differential measurement, the traditional workhorse.
• Piezoresistive (strain gauge) — resistance changes with diaphragm strain. Compact, wide range, good performance, the most common modern technology.
• Resonant silicon — a vibrating element whose frequency changes with strain. Excellent long-term stability and very high accuracy.
• Mechanical — Bourdon tube, diaphragm and bellows gauges. Still ubiquitous for local indication, and still the thing a technician actually looks at when troubleshooting.

Protecting the sensor

• Diaphragm seals (chemical seals) — isolate the transmitter from corrosive, viscous, crystallising or hot process fluids using a fill fluid in a capillary. They introduce their own errors: temperature effects on the fill fluid, capillary length and head effects, and slower response. Keep capillaries short, equal and shaded, and select a fill fluid appropriate to the temperature range.
• Siphons and pigtails — cool steam before it reaches the sensor.
• Pulsation dampeners and snubbers — protect against rapid pulsation from reciprocating equipment, which destroys sensors and gauges.
• Overrange protection — transmitters have a stated overrange limit; exceeding it damages the sensor permanently, which is why manifold operating sequences matter.

Selection points

Range chosen so normal operation sits in the middle portion, not at the bottom of the scale; wetted materials compatible with the process including sour service requirements; process connection and any seal arrangement; area classification and certification; and the accuracy the duty actually needs, since a custody transfer measurement and a pump discharge indication have very different requirements.