Sealed Gauge Pressure (PSIS) Explained
PSIS (Sealed Gauge Pressure) is a pressure measurement referenced to a fixed, sealed “atmospheric” pressure trapped inside the sensor at the factory, rather than being continuously vented to the current ambient atmosphere (as in PSIG).
In plain terms:
- PSIA (Absolute): referenced to vacuum (true zero)。
- PSIG (Vented gauge): referenced to current ambient atmospheric pressure, via a vent path.
- PSIS (Sealed gauge): referenced to a sealed-in atmospheric pressure (fixed reference), with no vent hole to ambient.
PSIS vs PSIG vs PSIA: the key difference is the reference
1) PSIA — Absolute pressure
Absolute pressure uses vacuum as its reference, so it never goes negative. If you expose an absolute sensor to ambient air, it reads approximately local atmospheric pressure (~14.7 psia at sea level).
2) PSIG — Gauge (vented) pressure
Gauge pressure is referenced to real-time ambient atmosphere. A vented gauge sensor will read ~0 when its pressure port is exposed to ambient air, regardless of day-to-day weather changes (because the reference tracks ambient).
A common relationship:
Pabs=Pg+Patm
3) PSIS — Sealed gauge pressure
Sealed gauge pressure is referenced to a sealed-in (fixed) “atmospheric-like” pressure, typically near 1 bar / 14.7 psi at the time of sealing. There is no vent path to track current atmospheric changes.
A practical way to think about it:
Ppsis≈Pabs−Psealed
where (P_{sealed}) is the fixed trapped reference, not today’s ambient atmosphere.
The most important PSIS behavior: “open to air” may NOT read zero
This is where many field misunderstandings happen.
- With PSIG (vented gauge): open to air → ~0 (because the reference is the same air).
- With PSIS (sealed gauge): open to air → may show an offset, because today’s atmospheric pressure may differ from the sealed reference pressure。
Some manufacturers describe PSIS as essentially an absolute sensor with an offset (often ~14.7 psi) so it behaves “like gauge” under standard conditions—while noting readings can vary slightly with barometric/weather changes.
Why use PSIS instead of PSIG?
1) When you cannot (or should not) vent to ambient
A sealed gauge reference is often used when a vent path is impractical or undesirable—classic example: depth sensors in submersible equipment that cannot run a vent tube to the surface.
2) When venting introduces reliability risks
Vented references can suffer from moisture ingress, condensation, contamination, clogging, or packaging complexity. Sealed gauge avoids the vent hole by design.
3) When barometric variation is “small enough” for the system
If your measurement range is large (e.g., high-pressure hydraulics), the atmospheric variation may be negligible relative to your required accuracy—so sealed gauge can be acceptable (depending on specs and tolerances).
When PSIS is a poor choice
PSIS is often not ideal 什么时候：
- You need pressure referenced to current ambient with minimal offset at low ranges (e.g., very low-pressure measurements where barometric changes are a big fraction of full scale).
- Your application requires a true vacuum reference or vacuum engineering conventions—use PSIA。
Typical applications for PSIS
- Depth / level / submersible systems where venting is impractical
- Sealed enclosures (IP-rated devices) where you want to avoid vent paths
- Rugged industrial sensing where the environment can contaminate vent tubes and filters
常问问题
Is PSIS the same as absolute pressure (PSIA)?
No. PSIA is referenced to vacuum; PSIS is referenced to a sealed-in atmospheric-like pressure。
Why does my PSIS sensor not read 0 when open to air?
Because today’s ambient atmospheric pressure may not equal the sensor’s sealed reference pressure; that difference appears as an offset.
What’s the main advantage of vented gauge (PSIG) over PSIS?
A vented gauge sensor will nominally read zero at pressure release regardless of atmospheric changes—very useful in low-pressure ranges.
