Gauge, absolute and differential
- Gauge (g): relative to the local atmosphere. A tyre gauge reads 0 at sea level and at the top of a mountain.
- Absolute (a): relative to perfect vacuum. Needed for vacuum services, vapour pressure, gas density calculations and steam tables.
- Differential (d): the difference between two points. Used for flow, level, filter and pump monitoring.
P_abs = P_gauge + P_atm (P_atm ≈ 1.01325 bar at sea level)
Units you'll meet
| Unit | Equals |
|---|---|
| 1 bar | 100 kPa = 14.504 psi = 1.0197 kg/cm² = 10 197 mmH₂O |
| 1 psi | 6.8948 kPa = 68.948 mbar = 27.68 inH₂O |
| 1 mmH₂O | 9.80665 Pa (water at 4 °C) |
| 1 inH₂O | 249.09 Pa (at 4 °C; some standards use 20 °C or 60 °F) |
| 1 atm | 101.325 kPa = 760 mmHg = 760 torr |
Convert any of these with the Unit Converter.
Pressure gauges
Most local gauges use a Bourdon tube (a curved tube that straightens under pressure); low ranges use diaphragms or capsules. Good practice:
- Select the range so that normal operating pressure sits at about 50–75 % of full scale.
- Use liquid-filled gauges on vibrating or pulsating services, plus a snubber on pump discharges.
- Use a siphon (pigtail) on steam, so condensate protects the element from live steam.
- Use a solid-front, blow-out-back case for safety on high-pressure gas.
Pressure transmitters
Smart transmitters use capacitive, piezoresistive or resonant sensors and output 4–20 mA with HART, or a fieldbus. Key specification points:
- URL vs calibrated range: a sensor with URL 100 bar can be ranged 0–10 bar (10:1 turndown), but accuracy worsens at high turndown. Check the vendor's formula.
- Overpressure and static pressure limits, especially for DP cells in high-pressure lines.
- Wetted materials (316L, Hastelloy, Monel, tantalum) and fill fluid for the process chemistry.
- Hazardous-area certification (Ex d / Ex ia) to match the area classification.
Diaphragm (remote) seals
A seal isolates the transmitter from process fluids that are corrosive, viscous, crystallising, very hot or sanitary. A capillary filled with fill fluid transmits the pressure. The trade-offs are slower response and temperature errors from the fill fluid. Keep capillaries as short as possible, protected from the sun, and of equal length on both sides of a DP cell (a balanced system).
Valve manifolds
A 2-valve manifold (block and vent) serves gauge pressure transmitters. DP transmitters use 3-valve (two blocks plus an equaliser) or 5-valve (adds two vents) manifolds.
Typical sequence: putting a DP transmitter into service
- Check that all valves are closed, then open the equalising valve.
- Slowly open the HP block valve. Both sides are now pressurised equally, so the cell sees no overload.
- Close the equalising valve.
- Slowly open the LP block valve.
To take it out of service, reverse the order: close LP, open the equaliser, close HP, then vent. With the equaliser open and both blocks closed, the transmitter should read zero DP, which is a quick zero check.
Installation tips
- Liquids: transmitter below the tap, so gas bubbles rise back to the process.
- Gases: transmitter above the tap, so condensate drains back.
- Steam: transmitter below the tap with filled legs (condensate), so hot steam never reaches the sensor.
- Slope impulse lines at least 1:12 (about 8 %), avoid loops that trap gas or liquid, and keep both legs of a DP pair at the same temperature.
- Heat-trace lines that can freeze or solidify, but don't overheat transmitters beyond their limits.