Pressure type usually means what reference you measure against (absolute vs gauge vs differential, etc.). In engineering discussions, it can also include physical pressure concepts (static, dynamic, hydrostatic, vapor pressure…). This article covers both, so you can spec sensors correctly and avoid common field errors.
Topic map
- Reference-based pressure types: absolute, gauge (relative), differential, sealed gauge
- Key formulas: Pabs = Pg + Patm, Pg = Pabs − Patm, ΔP = P1 − P2
- Vacuum/compound measurement and when to use them
- Vented vs sealed reference (when you cannot run a vent tube)
- Real application mapping: tires, tanks, filters, HVAC ducts, vacuum pumps, barometric measurements
1) Reference-based pressure types (the most important “pressure types” for sensors)
WIKA summarizes it well: the main types are differentiated only by the reference pressure (absolute, gauge/relative, differential).
1.1 Absolute pressure (Pabs)
Definition: pressure referenced to a (near) perfect vacuum as the zero point.
Key behaviors
- A true vacuum is 0 on an absolute scale.
- If you open an absolute sensor to the atmosphere, it reads roughly the local atmospheric pressure (varies with weather/altitude).
Common use cases
- Barometric pressure, altitude compensation, sealed chambers, vacuum systems, boiling/cavitation calculations, any application where you need a constant reference independent of weather.
Common labels
- kPa(a), bar(a), psia
1.2 Gauge pressure / Relative pressure (Pg)
Definition: pressure referenced to the prevailing ambient atmospheric pressure.
WIKA provides the core relationship:
Pg=Pabs−Pamb
Key behaviors
- When the measured point equals ambient air, a gauge sensor reads ~0.
- Gauge readings shift if atmospheric pressure changes (that’s normal, because atmosphere is the reference).
Common use cases
- Tire pressure, pumps, compressors, open tanks, most “everyday” industrial pressure readings.
Common labels
- kPa(g), bar(g), psig
1.3 Vacuum pressure (negative gauge) and vacuum gauges
In practice, “vacuum” often means pressure below atmospheric, expressed relative to atmosphere (gauge reference), sometimes shown as negative gauge pressure or as “vacuum in inHg”.
When absolute is better: if you need unambiguous low-pressure values, use absolute pressure (because gauge vacuum can be ambiguous without context).
1.4 Differential pressure (ΔP)
Definition: the difference between two pressures:
Δp=p1−p2
Key behaviors
- Both sides can be varying; the instrument reports only the difference.
- Widely used in process systems and HVAC because it “does the subtraction for you.”
Common use cases
- Filter monitoring, cleanroom/duct static pressure, flow measurement across orifice/venturi, liquid level in closed tanks, heat exchangers.
Common labels
- Pa DP, kPa ΔP, psid
1.5 Sealed gauge pressure (PSIS) / Sealed reference
Definition: like gauge pressure, but the atmospheric reference is trapped (sealed) inside the sensor instead of vented to ambient.
Why it exists
- Many installations cannot run a vent line (submersible equipment, harsh environments, compact enclosures). A sealed gauge avoids venting but uses a fixed trapped reference.
Key behavior to know
- Because real atmospheric pressure changes with weather and altitude, a sealed gauge sensor may not read exactly “0” when exposed to current ambient (its reference is the sealed-in atmosphere).
Common labels
- psis, sealed gauge, sealed relative
1.6 Compound pressure (compound gauge)
Compound measurement displays both positive pressure and vacuum (negative relative to atmosphere) on the same dial/scale—useful when a system can swing from vacuum to pressure.
2) The “must-know” formulas (reference conversions)
From WIKA’s reference relationships:
Absolute from gauge
Pabs=Pg+Patm
Gauge from absolute
Pg=Pabs−Pamb
Differential
Δp=p1−p2
3) Quick selection table (which pressure type should you spec?)
| Pressure type | Reference | Best for | Typical examples |
|---|---|---|---|
| Absolute | Vacuum (0) | Weather/altitude-independent reference; vacuum systems | Barometric, sealed chambers, vacuum pumps |
| Gauge (vented) | Local atmosphere | Most industrial “above atmosphere” readings | Compressors, pumps, open tanks, tires |
| Differential | P1 − P2 | “Pressure drop” or “two-point” monitoring | Filters, ducts, flow elements |
| Sealed gauge | Sealed-in atmosphere | When venting is impossible | Submersible/remote sealed devices |
| Compound | Atmosphere (±) | Systems that cross vacuum ↔ pressure | HVAC service, leak testing |
4) Physical pressure concepts (often also called “pressure types”)
These are not “reference modes,” but they matter in fluid systems and can affect sensor placement.
4.1 Atmospheric (barometric) pressure
Standard atmosphere is commonly referenced as 101.325 kPa (absolute) at sea level (nominal), but local values change with weather and altitude.
4.2 Hydrostatic pressure (fluid at rest)
Pressure due to a fluid column:
p=p0+ρgh
This is why elevation differences can create measurable pressure differences in liquids (tanks, level measurement, pipelines).
4.3 Static, dynamic, and stagnation (total) pressure
- Dynamic pressure relates to fluid motion and is tied to kinetic energy per volume; it is part of Bernoulli’s equation.
- Stagnation pressure (total pressure) combines static + dynamic in incompressible flow.
These concepts matter for pitot tubes, airflow measurements, and when pressure taps are placed poorly in high-velocity streams.
4.4 Vapor pressure
Vapor pressure is the pressure of a vapor in equilibrium with its liquid/solid phase at a given temperature—important for boiling points, evaporation, and cavitation margins.
4.5 Partial pressure (gas mixtures)
In gas mixtures, total pressure is the sum of component partial pressures (Dalton’s law).
This matters for oxygen sensors, gas blending, humidity, and some process safety calculations.
5) How to write pressure specs correctly (so buyers & engineers don’t misread them)
5.1 Always include the reference in the unit
Good:
- 0–10 bar(g)
- 80–120 kPa(a)
- 0–500 Pa ΔP
Avoid: “0–10 bar” (missing reference).
5.2 Use consistent suffixes
Common field conventions:
- psia / psig / psid
- kPa(a) / kPa(g) / kPa ΔP
Wording guidance often emphasizes stating the reference explicitly to avoid ambiguity.
6) Common mistakes (and how to avoid them)
Using gauge when you need absolute (altitude/weather creates apparent “drift”).
If the process is sealed or you need a constant reference, choose absolute.Forgetting sealed gauge isn’t “true gauge”
Sealed reference can’t track real-time barometric changes, so “zero” can shift.Treating vacuum gauge readings as absolute
Vacuum values can be ambiguous unless the reference is stated.Specifying ΔP but installing a single-port gauge sensor
Differential requires correct plumbing (two ports, correct high/low side).
FAQs
What’s the simplest way to explain absolute vs gauge pressure?
Gauge pressure is referenced to atmosphere; absolute pressure is referenced to vacuum. WIKA defines gauge as .
When should I choose a sealed gauge pressure sensor?
When you can’t vent the reference side to ambient (submersible or enclosed devices).
What is differential pressure used for most often?
Monitoring pressure drop (filters), measuring flow across restrictions, and duct/room pressure control.
What is a compound gauge?
A gauge that indicates both positive pressure and vacuum (negative relative to atmosphere).
Why does a sealed gauge not always read exactly 0 in open air?
Because its reference is the trapped atmospheric pressure at sealing time, while real atmosphere changes with weather and altitude.
How do I convert gauge to absolute?
Use .
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[…] or inches of water column (in. w.c.). It is not automatically an absolute pressure reading. Our pressure-types guide explains the difference between gauge, absolute and differential […]