Ask ten engineers what the difference is between gauge and absolute pressure and you’ll get nine confident answers and one quiet “…actually, remind me again?” It’s one of the most-asked questions in pressure instrumentation, and getting it wrong at the specification stage is one of the most common reasons a pressure transmitter ends up giving “the wrong reading”, when in reality, the sensor is working perfectly, it’s just referenced to the wrong thing.
This guide breaks down the three pressure reference types you’ll see on every datasheet, gauge, absolute and sealed gauge, explains where each one belongs, and helps you pick the right variant before you order.
First principles: pressure is always relative to something
Every pressure measurement is a comparison between two pressures: the process pressure you care about, and a reference pressure the sensor compares it against. The three reference types are simply three different choices of reference:
- Gauge pressure (g) — referenced to the current local atmospheric pressure.
- Absolute pressure (a) — referenced to a perfect vacuum (0 bar absolute).
- Sealed gauge pressure (sg) — referenced to a fixed, sealed-in atmospheric pressure (typically 1 bar) trapped inside the sensor at manufacture.
That single distinction, what’s on the back side of the diaphragm, is what changes the reading you see.
Gauge pressure: the everyday default
A gauge pressure transmitter has a vented reference, usually through a small capillary in the cable or housing, so the back of the sensing diaphragm always “sees” current ambient pressure.
- Reading is zero when the process is at atmospheric pressure.
- Self-compensates for weather and altitude.
- Best for: open tanks, vented hydraulic systems, mains water pressure, irrigation, general industrial process pressure.
The Titan TPT-SDI Pressure Transmitter with SDI-12 is a typical example, supplied as gauge for most water industry, hydrology and RTU applications where the process is vented to atmosphere. The DMP331i / DMP333i Precision Pressure Transmitter is another commonly specified gauge unit in industrial and laboratory environments.
Watch out for: the vent must stay dry and unobstructed. A blocked or waterlogged vent will make a gauge transmitter drift as ambient pressure changes.
Absolute pressure: when you need a true reference
An absolute pressure transmitter has a sealed, evacuated cavity behind the diaphragm. The sensor measures pressure against a near-perfect vacuum.
- Reading is never zero in normal atmosphere — you’ll see ~1.013 bar a at sea level.
- Doesn’t move with the weather, because the reference is fixed.
- Best for: barometric pressure measurement, altitude, vacuum systems, vapour pressure, leak testing, scientific and aerospace applications.
For barometric and combined measurements, the SBPT Barometric Pressure & Temperature Transmitter is purpose-built around an absolute reference. Process-grade absolute variants are also available in the XMP i Precision Pressure Transmitter and the DMP331i / DMP333i, both available across the wider digital pressure sensors range.
Watch out for: if you specify absolute when you really wanted gauge, your readings will appear “one bar too high”, a classic commissioning headache.
Sealed gauge pressure: the high-pressure compromise
A sealed gauge transmitter has a sealed reference cavity like an absolute sensor, but the cavity is filled with atmospheric pressure (≈1 bar) at the time of manufacture, not vacuum.
- Reads zero (approximately) at atmospheric pressure.
- No vent — so no risk of moisture ingress through a capillary.
- Best for: high pressure ranges, subsea installations, sealed or potted assemblies, and any environment where a vented gauge transmitter would be vulnerable.
This is why most high pressure transmitters above ~10 bar are offered as sealed gauge, at higher pressures, the small error introduced by ambient drift (a few millibar) is negligible relative to the measurement, but a flooded vent would be catastrophic.
The Titan TPTd Digital Pressure Transmitter, the TPT-SDI, and the ceramic-sensor XMP ci Process Pressure Transmitter are all available as sealed gauge for exactly these applications.
A quick decision guide
Ask yourself three questions:
- Is the process vented to atmosphere, and is the pressure below ~10 bar? → Specify gauge.
- Do you need a reading that doesn’t change with the weather (barometric, altitude, vacuum, vapour, leak test)? → Specify absolute.
- Is the pressure high, the environment wet or sealed, or the installation subsea? → Specify sealed gauge.
When it’s still unclear, the simplest check: “If I disconnect this sensor from the process and leave it on the bench, what reading do I want to see?”, zero (gauge or sealed gauge) or local atmospheric pressure (absolute).
Why this matters at commissioning
Mis-specifying the reference type is one of the top three causes of “the sensor is wrong” callouts in pressure instrumentation. Symptoms include:
- A new transmitter reading ~1 bar too high → likely absolute supplied when gauge was wanted.
- A gauge sensor drifting overnight or seasonally → blocked vent or wrong choice for a sealed application.
- High-pressure transmitter slowly losing accuracy in a wet environment → vented gauge specified where sealed gauge was the correct choice.
A five-minute conversation at the enquiry stage saves a multi-day commissioning issue later. If you’re unsure which reference type fits your application, the team at Stork Solutions can review your process conditions and recommend the right variant before you order.
The takeaway
Gauge, absolute and sealed gauge aren’t interchangeable, they’re three deliberate engineering choices that determine what your sensor is actually measuring. Get the reference right, and the rest of the specification (range, accuracy, output) falls into place.
Explore the full digital pressure sensors range or get in touch to discuss the right reference type for your application.
