Gauge vs Absolute vs Differential Pressure: Differences, Formulas & When to Use Each

Last Updated: 2026-11-05

Quick Answer: Gauge, Absolute, Sealed Gauge and Differential Pressure in One Table

Gauge pressure is measured relative to the atmosphere around the instrument, absolute pressure is measured relative to a perfect vacuum, sealed gauge pressure is measured relative to a fixed 1 atm trapped inside the sensor, and differential pressure is the difference between two process points. The one formula that connects the first two is:

P_abs = P_gauge + P_atm
P_atm (sea level, ISO 2533) = 101.325 kPa = 1.01325 bar = 14.696 psi = 760 mmHg

If a gauge reads 0 bar(g) at sea level, the absolute pressure is 1.013 bar(a). If a vacuum pump pulls the same line down to -0.8 bar(g), the absolute pressure is 0.213 bar(a), or 21.3 kPa(a). The rest of this guide shows how each reference is built into the sensor, where each one is the right choice, and three worked examples that catch the mistakes we see most often in orders.

What Is Gauge Pressure?

Gauge pressure (spelled “gage pressure” in older US standards and some ASME documents) is pressure measured relative to the ambient atmosphere. The sensor has a small vent, usually through the cable or a breather in the housing, that lets atmospheric pressure act on the back of the diaphragm. The electronics therefore output the difference between process pressure and whatever the atmosphere is doing right now.

That is exactly what most process applications want. A pump discharge of 6 bar(g), a hydraulic line at 250 bar(g), a compressed-air header at 7 bar(g): in each case the operator cares about pressure above ambient, because that is what pushes fluid through the pipe and what the pipe wall has to resist.

Three properties follow from the vented design:

• Zero is “free”. Open the process connection to air and the gauge reads 0. That makes field zero checks trivial.
• Negative readings are possible. A gauge sensor can read down to about -1 bar(g), which is a vacuum relative to the atmosphere. Ranges that cross zero (for example -1 to +9 bar(g)) are called compound ranges.
• The reference moves. A weather front of ±30 mbar or a change in altitude shifts the reference and therefore the reading of a fixed absolute process. For a 10 bar(g) span this is a 0.3% effect; for a 100 mbar vacuum process it is 30%, which is why vacuum work usually needs absolute pressure.

Transmitters like the YL805 are ordered in gauge form by default and cover -100 kPa to 70 MPa, which spans everything from a vacuum-assisted process to high-pressure hydraulics with ±0.5% FS accuracy and a choice of 4–20 mA, 0–5 V, 0–10 V, 0.5–4.5 V or RS485 output.

What Is Absolute Pressure?

Absolute pressure is measured from a perfect vacuum. In the sensor, the reference cavity behind the diaphragm is evacuated and sealed during manufacture, typically to below 0.01 mbar, so the diaphragm always sees process pressure on one side and essentially nothing on the other. The output therefore does not care what the atmosphere is doing.

The absolute pressure formula is the gauge formula rearranged:

P_abs = P_gauge + P_atm          (standard form)
P_gauge = P_abs − P_atm          (converting a datasheet in bara to barg)
P_vacuum = P_atm − P_abs         (how much below atmosphere, always positive)

Absolute pressure can never be negative. Zero is a perfect vacuum, which no real pump reaches; a good rotary-vane pump gets to about 0.1 mbar(a), and a turbomolecular pump to 10⁻⁷ mbar(a). Everything in the normal process world sits between 0 and a few hundred bar(a).

Absolute pressure is the natural quantity in physics and chemistry. The ideal gas law PV = nRT, vapor-pressure tables, boiling points, the speed of sound and the density of a gas all use absolute pressure. Whenever a calculation feeds one of these, you need P_abs, and if your instrument reads gauge you must add the local barometric pressure, which brings us to altitude.

Atmospheric Pressure vs Altitude

The reference for a gauge instrument is not 1.013 bar everywhere. The International Standard Atmosphere (ISO 2533 / ICAO) gives these values; actual daily readings swing roughly ±3 kPa around them with the weather.

The right-hand column is the point: a bottle sealed at sea level and carried to a mine site or an Andean water treatment plant at 3,000 m reads 0.31 bar(g) on a gauge instrument without any change in its contents. If the process logic uses gauge pressure to infer something absolute, the error at 5,000 m is 46 kPa, almost half an atmosphere.

What Is Sealed Gauge Pressure?

A sealed gauge sensor is built like an absolute sensor, with a closed reference cavity, but the cavity is sealed with air at roughly 1 atm instead of being evacuated. At sea level on an average day it reads approximately the same as a gauge sensor. It is used when a vent is impractical or dangerous:

• High-pressure ranges, typically above about 50 bar. At 400 bar full scale, a 30 mbar barometric swing is 0.0075% FS, far below the accuracy class, so the vent buys nothing and only adds a path for moisture. Sealed gauge is the standard build at these ranges.
• Submerged or washdown installations. A vent tube that can flood or ice up is a failure point. Sealed gauge removes it, at the cost of a small barometric error.
• Hermetic or hazardous-area housings where a breather would compromise the enclosure rating.

The trade-off is a barometric and altitude sensitivity that cannot be zeroed out in the field unless the transmitter has a zero adjustment. Our guide to pressure transmitter types covers the sealed gauge build in more detail; the short rule is that sealed gauge is a high-range or wet-location substitute for gauge, not a substitute for absolute.

Transmitters sourced through SIBXSUN that offer all three references on one body include the YL0500 (gauge, absolute or sealed gauge; -0.1 MPa through 0–10 kPa up to 100 MPa; ±0.1%, ±0.25% or ±0.5% FS; 4–20 mA with HART, 0–10 V, RS485 or I²C; on-site LED/LCD display; ≤3 ms response) and the YLEX01 explosion-proof pressure transmitter (-0.1 to 100 MPa, gauge/absolute/sealed, 4–20 mA with optional RS485, 150% FS overload). You choose the reference as an order code; the sensor core is different, so it cannot be changed later in the field.

What Is Differential Pressure?

Differential pressure (DP, ΔP) is the pressure at one point minus the pressure at a second point, measured by a single sensor with two process connections: a high (H) port and a low (L) port. The diaphragm sees one pressure on each side, so what comes out is the difference and nothing else. The common pressure present on both sides, called the static or line pressure, is cancelled.

This is why DP transmitters carry two ratings: a differential span (for example 0–250 mbar) and a static pressure rating (for example 40 bar). A DP transmitter with a 250 mbar span can sit on a 40 bar line and still resolve a 2 mbar change between the ports, something no pair of gauge transmitters can do.

Differential pressure is the workhorse behind three big measurement families:

1. Flow through a restriction. Orifice plates, Venturi tubes and flow nozzles create a ΔP proportional to flow squared (ISO 5167). The DP transmitter is the flow meter’s brain.
2. Filter and strainer condition. A clean filter might show 50 mbar; a clogged one 300 mbar. The absolute value of the line pressure is irrelevant.
3. Level in pressurized or sealed vessels. With the H port at the bottom of a tank and the L port in the vapor space, ΔP equals liquid head regardless of tank pressure.

Low-range DP, below about 10 kPa, is its own specialty: cleanroom and isolation-room pressures (typically 10–30 Pa), HVAC filter monitoring and furnace draft. A micro differential pressure transmitter such as the YL808N is built for these air-pressure spans, while the YL808B differential pressure transmitter and the YL1800 cover liquid and gas process differentials. The YL1800 uses a silicon piezoresistive core with 4–20 mA, 0–5 V, 0–10 V, I²C or RS485 output, IP65 as standard with IP67 on request, and an operating range of -30 to +80 °C.

Suffix Cheat Sheet: psia, psig, bara, barg, kPa(a), kPa(g)

The reference is encoded in the unit suffix. The number alone is meaningless without it, and a surprising number of RFQs we receive omit it.

Two conventions trip people up. First, “bar” written alone on a process gauge is gauge pressure by habit, but “bar” alone in a chemistry text or a gas-law calculation is absolute. Second, Modbus and HART registers carry only a number and a unit code; whether that number is kPa(a) or kPa(g) is defined by the transmitter’s order code, not the protocol. If you need help converting between the units themselves, see our pressure units converter for psi, bar, kPa and feet of head.

How the Sensor Is Built: Where the Reference Comes From

All four pressure types can be made from the same piezoresistive silicon or ceramic diaphragm. The difference is what is on the back side of that diaphragm.

Because an absolute sensor has a full atmosphere of pressure on it even when the process is at ambient, a 0–1 bar(a) absolute transmitter spends its whole life at the top of its range. That is normal and intended. Overrange protection, however, works differently: a 0–1 bar(a) absolute transmitter exposed to 3 bar(g) of process sees 4 bar(a), which is 400% of span. Always specify absolute ranges with the maximum process pressure in mind, not just the operating point.

Three Worked Examples

Example 1: Gauge Reading at Altitude to Absolute

A nitrogen blanket on a storage tank at a plant 2,000 m above sea level is controlled by a gauge transmitter reading 2.00 bar(g). What is the absolute pressure, and what would the same gauge show at sea level?

P_atm at 2,000 m (ISA)  = 79.50 kPa = 0.795 bar
P_abs = P_gauge + P_atm = 2.000 + 0.795 = 2.795 bar(a)

At sea level, the same 2.795 bar(a) blanket would read:
P_gauge = 2.795 − 1.013 = 1.782 bar(g)

The blanket is 0.218 bar lower in absolute terms than the sea-level engineer assumed. If the tank’s vapor-pressure calculation or the supplier’s relief-valve sizing used 2.0 barg plus 1.013, it is off by 7%. The clean fix is an absolute transmitter configured for 0–4 bar(a), which reads 2.795 wherever the tank is installed.

Example 2: Vacuum Pump Reading to Absolute kPa

A vacuum dryer gauge reads -0.80 bar(g) at a coastal plant. Convert to absolute pressure in kPa and mbar, and state how much vacuum this is.

P_abs = P_gauge + P_atm = (−0.800) + 1.013 = 0.213 bar(a)
      = 21.3 kPa(a) = 213 mbar(a) = 160 mmHg

Vacuum below atmosphere = 1.013 − 0.213 = 0.800 bar = 80.0 kPa

At 213 mbar(a), water boils at about 61 °C (steam tables, saturation at 21.3 kPa). Now suppose a cold front drops atmospheric pressure by 25 mbar. The gauge transmitter still reads -0.80 bar(g) because its reference moved with the weather, but the absolute pressure in the dryer is now 188 mbar(a) and the boiling point has dropped to about 59 °C. The gauge instrument cannot see this; a 0–500 mbar(a) absolute transmitter can, which is why vacuum drying, distillation and freeze-drying are specified in absolute units.

Example 3: Two Gauge Transmitters vs One DP Transmitter on a Filter

A duplex strainer on a 10 bar(g) cooling-water line should trigger cleaning when the pressure drop reaches 150 mbar. Option A: two 0–16 bar(g) transmitters at ±0.25% FS, subtract in the PLC. Option B: one DP transmitter with a 0–400 mbar span at ±0.25% FS and a 16 bar static rating.

Option A uncertainty per transmitter = 0.25% × 16 bar = 40 mbar
Worst-case subtraction error         = 40 + 40 = 80 mbar  (RSS: 57 mbar)
Measured 150 mbar could be anywhere from 70 to 230 mbar

Option B uncertainty = 0.25% × 400 mbar = 1 mbar
Measured 150 mbar is 149 to 151 mbar

Option A is 50 to 80 times less certain and will either clean the filter far too early or let it clog. It also requires two process penetrations and two calibrations that must track each other over temperature. The DP transmitter is the correct instrument whenever the differential is small compared with the line pressure, which is the usual case for filters, orifice flow and pressurized tank level.

Which Reference to Specify: Decision Table

For an on/off decision rather than a continuous signal, the same references apply to pressure switches; our pressure switch types and working principle guide explains setpoint and deadband for mechanical, electronic and digital switches.

Common Mistakes We See in Orders and Commissioning

1. Ordering gauge when the process is vacuum. A -1 to 0 bar(g) gauge transmitter looks like a vacuum instrument, and for a rough vacuum of 500 mbar(a) it is acceptable. Below 100 mbar(a), barometric drift of ±30 mbar is ±30% of the reading. Specify 0–100 mbar(a) or 0–1 bar(a) absolute instead.
2. Reading a kPa(a) register as kPa(g) over Modbus. The transmitter sends 101.3 at ambient; the PLC programmer, expecting gauge, adds an offset or flags an alarm. The unit code in the register does not carry the reference. Write the reference in the loop tag and the HMI label.
3. Assuming 1.013 bar everywhere. Any site above about 500 m needs the local value. A plant at 1,500 m that adds 101.3 kPa instead of 84.6 kPa to a gauge reading is 16.7 kPa high on every absolute calculation, enough to miss a vapor-pressure limit.
4. Using a sealed gauge transmitter as a reference barometer. Sealed gauge drifts with weather and altitude exactly like the container in the altitude table. It is a high-pressure convenience, not an absolute instrument.
5. Static-pressure overload on DP. A DP transmitter with a 16 bar static rating connected to a 40 bar line with the manifold equalizing valve closed will be damaged on one side even though the differential span is only 400 mbar. Check the static rating, use a proper manifold, and open the equalizer before the block valves.
6. Field zeroing an absolute transmitter to 0 at ambient. It should read local atmospheric pressure, not zero. “Zeroing” it to 0 at ambient throws away 1.013 bar of range and makes it a badly made gauge instrument. Zero an absolute transmitter only against a reference vacuum or trim it against a calibrated barometer.

For the rest of the selection process, including range, accuracy class, process connection and wetted materials, our 8-step guide to choosing a pressure transmitter walks through each decision, and the pressure sensor vs pressure transmitter comparison covers when a raw millivolt sensor is enough.

Sourcing Notes: What to Put in the RFQ

SIBXSUN supplies pressure transmitters sourced from partner factories in China, and the pressure reference is the single most frequently missing line in the enquiries we receive. A complete request for quotation for any of the transmitters on our pressure sensors and transmitters category should state:

• Reference: gauge, absolute, sealed gauge or differential (plus static rating for DP). Write it in words, not just a suffix.
• Range in a unit with suffix: for example 0–4 bar(a), -1 to +9 bar(g), 0–400 mbar(d) at 16 bar static.
• Site altitude if above 500 m, so we can confirm that a gauge range still makes sense and that any absolute range has the right span.
• Maximum process pressure and vacuum, including upset conditions, so the overrange and burst ratings are checked against the correct absolute value.
• Output and power: 4–20 mA (2-wire, with or without HART), 0–10 V, RS485 Modbus RTU or I²C, with the supply voltage available at the loop.
• Accuracy class: ±0.1%, ±0.25% or ±0.5% FS; absolute references at low spans generally cost more for the same class.
• Hazardous area certification if the location requires an explosion-proof body.

Typical lead times for standard gauge ranges are shorter than for absolute and low-span differential builds, which are usually made to order; sample quantities are available. The pressure transmitter supplier page lists the full YL series with ranges, outputs and lead-time notes.

Frequently Asked Questions

What is the formula for absolute pressure?

Absolute pressure equals gauge pressure plus local atmospheric pressure: P_abs = P_gauge + P_atm. At sea level P_atm is 101.325 kPa (1.01325 bar, 14.696 psi), so a gauge reading of 2.00 bar(g) is 3.01 bar(a). At altitude P_atm is lower, so the same gauge reading corresponds to a lower absolute pressure. A perfect vacuum is 0 bar(a), which equals -1.013 bar(g) at sea level.

Is bar absolute or relative (gauge)?

Bar by itself is only a unit and says nothing about the reference. The suffix tells you: bar(a) or bara is absolute, referenced to a perfect vacuum; bar(g) or barg is gauge, referenced to local atmospheric pressure. Most industrial gauges and transmitters read gauge pressure unless the datasheet says absolute, so an unlabeled bar reading on a process gauge is almost always bar(g).

What is the difference between psia and psig?

psia is pounds per square inch absolute, measured from a perfect vacuum; psig is pounds per square inch gauge, measured from the surrounding atmosphere. The two differ by local atmospheric pressure, which is 14.696 psi at sea level and about 12.2 psi at 1,500 m. A tire at 32 psig is roughly 46.7 psia at sea level. psid denotes a differential between two points.

When should I use an absolute pressure transmitter instead of a gauge transmitter?

Use absolute pressure whenever the process physics depends on true pressure rather than pressure relative to the weather: vacuum drying and distillation, boiling point and vapor pressure control, barometric and altitude measurement, gas mass-flow temperature and pressure compensation, and any vacuum process below about 100 mbar(a). A gauge transmitter drifts with barometric changes of about ±3 kPa, which is 30% of a 100 mbar vacuum setpoint.

Can I measure differential pressure with two gauge transmitters instead of one DP transmitter?

You can, but the error is usually unacceptable for small differentials. Two transmitters with 0–10 bar range at ±0.25% FS each contribute 25 mbar of uncertainty, so a 150 mbar filter differential could read anywhere from 100 to 200 mbar. A dedicated differential pressure transmitter with a 0–250 mbar span at 0.25% gives ±0.6 mbar on the same measurement. Use two gauge transmitters only when the differential is large relative to the line pressure.

Not sure whether your application needs gauge, absolute or differential? Send us the process, range, site altitude and output signal and we will confirm the reference and range before you place the order.