Key points
- Gauge: most process and utility pressure.
- Absolute: vacuum processes, barometric-sensitive measurements, gas density.
- Differential: filter condition, pump ΔP, flow (orifice) and level (DP).
- Sealed gauge avoids vent moisture problems at high ranges.
The three references
| Type | Reference | Reads at open air | Typical uses |
|---|---|---|---|
| Gauge (G) | Local atmosphere | 0 | Pump discharge, line pressure, CDU loop pressure |
| Sealed gauge (S) | Fixed internal reference | ≈0 (changes with weather) | High ranges, wash-down or submerged sites |
| Absolute (A) | Vacuum | ≈101.3 kPa | Vacuum distillation, condensers, steam density |
| Differential (D) | Second process port | 0 when equal | Filter ΔP, pump ΔP, orifice flow, DP level |
Examples from liquid cooling and HVAC
- CDU loop pressure: gauge, SUP-PE100.
- Filter clogging: differential, or two gauge sensors subtracted in the controller.
- Chilled-water pump control: differential at the remote coil.
- Steam density compensation: absolute or gauge plus barometric, see vortex on steam.
One DP or two gauge sensors?
For small differentials on high static pressure (for example 20 kPa ΔP at 1 MPa), a dedicated DP transmitter such as Supmea's SUP-2051 is far more accurate: two 0.5%-class gauge sensors on a 1.6 MPa range could each carry ±8 kPa error. Where ΔP is large relative to static pressure, two compact gauge sensors are a cost-effective alternative.
Background: pressure measurement, 4–20 mA current loop.
Related questions
What does 'psig' vs 'psia' mean?
psig is gauge pressure; psia is absolute. psia ≈ psig + 14.7 at sea level.
Can a gauge transmitter measure vacuum?
Yes, compound ranges such as −100 kPa to +X kPa cover vacuum relative to atmosphere; absolute is better when the true vacuum level matters.
Sources and further reading
Product specifications quoted here are summarised from Supmea datasheets. Confirm the figures for your exact model before final design.