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Gauge vs Absolute vs Vacuum Pressure Explained with Examples
Quick answer: gauge pressure is measured against the atmosphere around you, so an open pipe reads zero. Absolute pressure is measured against a perfect vacuum, so the same open pipe reads about 101.3 kPa at sea level. Vacuum is pressure below atmospheric pressure, shown as a negative gauge number such as -60 kPa. The conversion is simple: absolute = gauge + atmospheric. Most industrial gauges, pumps and compressors use gauge pressure. Absolute pressure matters for vacuum work, altitude, gas calculations and some sensors. If you pick the wrong type, your reading will be off by about 100 kPa. This guide explains each type with worked examples and a simple table for choosing.
Gauge pressure
Gauge pressure uses the local atmosphere as zero. A tyre at 220 kPa gauge has 220 kPa more pressure inside than the air outside. An open water tank shows zero gauge pressure at the surface. A gauge exposed to the air with no connection reads zero. This is the type you will meet most often: pressure gauges on pumps, compressors, boilers and hydraulic systems all normally show gauge pressure.
Gauge pressure can be negative. A reading of -40 kPa gauge means 40 kPa below atmospheric pressure, which is a vacuum. A gauge that covers both vacuum and positive pressure is called a compound gauge.
Absolute pressure
Absolute pressure counts from zero pressure, a perfect vacuum, and can never be negative. Atmospheric pressure at sea level is about 101.3 kPa absolute, and it varies with the weather and with height above sea level. Absolute pressure is used when the answer depends on the true amount of gas, not on the weather. Examples include vacuum processes, gas laws, altitude measurement, boiling point calculations and barometers.
Absolute sensors have a sealed reference chamber with a vacuum inside, so they ignore changes in atmospheric pressure. An open-to-air absolute sensor reads around 101 kPa, which surprises people who expected zero.
Vacuum
Vacuum is the condition of being below atmospheric pressure. It can be described in two ways:
- As a negative gauge pressure: -60 kPa.
- As an absolute pressure: 41.3 kPa absolute (101.3 − 60).
Deeper vacuum is often quoted in absolute terms (mbar, Torr or microns), because the gauge scale only goes down to about -101.3 kPa. For details of vacuum units and instruments, see vacuum gauges and digital vacuum gauges.
Differential pressure
Differential pressure is the difference between two pressures. Both could be gauge or both absolute and the difference is the same. See differential pressure explained.
The conversion table
| Situation | Gauge (kPa) | Absolute (kPa, sea level) |
|---|---|---|
| Open to the atmosphere | 0 | 101.3 |
| Tyre at 220 kPa | 220 | 321.3 |
| Compressor receiver at 800 kPa | 800 | 901.3 |
| Pump suction at -40 kPa | -40 | 61.3 |
| Vacuum chamber at -90 kPa | -90 | 11.3 |
| Perfect vacuum | -101.3 | 0 |
In bar, the same pattern applies: absolute (bar) = gauge (bar) + 1.013.
Worked examples
Example 1: a pressure vessel rated in absolute terms
A specification says a vessel operates at 5 bar absolute. On a gauge-pressure instrument that is 5 − 1.013 = 3.987 bar, or about 399 kPa gauge. If you mistakenly set a 5 bar gauge-pressure alarm, you would be running at 6.013 bar absolute, which is over the limit by more than a whole bar.
Example 2: altitude and the weather
A gauge pressure instrument is zeroed against the local atmosphere, so a pump reading 500 kPa gauge reads the same at the beach or in the mountains. At higher altitude, the atmosphere is thinner (for example, about 90 kPa at roughly 1,000 metres), so the absolute pressure for the same gauge reading is lower. Where results depend on the true amount of gas, such as a gas flow calculation, absolute pressure is the correct input.
Example 3: a sensor ordered by mistake
A buyer orders an absolute pressure sensor, range 0 to 10 bar, for a hydraulic circuit with a working pressure around 6 bar. The sensor works, but reads 1 bar high at rest and shows about 7 bar when the system is at 6 bar gauge. The control system was set up for gauge pressure, so alarms and displays are wrong. The fix is to order a gauge-pressure sensor. The WIKA Model A-10 is available as gauge, absolute and compound, so always state which one you need. See the sensor, transducer and transmitter guide.
Which type do you need?
| Application | Usual type |
|---|---|
| Pumps, compressors, hydraulics, boilers | Gauge |
| Suction lines and mixed vacuum or pressure | Compound gauge |
| Vacuum chambers, freeze drying, packaging under vacuum | Absolute or vacuum |
| Weather and altitude | Absolute (barometric) |
| Gas flow calculations | Absolute, often with temperature |
| Filter and flow monitoring | Differential |
Common mistakes
- Ordering an absolute sensor for gauge pressure work, or the reverse.
- Reading a vacuum on a positive-only gauge. It will sit on the stop.
- Comparing gauge and absolute numbers without converting.
- Forgetting that atmospheric pressure changes with weather and altitude.
- Specifying a range in absolute pressure but buying a gauge-pressure instrument.
Reading the unit labels
Datasheets often use letters to show which type of pressure is meant. Learn these and you avoid most mistakes.
| Label | Meaning |
|---|---|
| kPa(g), barg, psig | Gauge pressure, relative to the atmosphere |
| kPa(a), bara, psia | Absolute pressure, relative to a perfect vacuum |
| kPa(d), bard | Differential pressure |
| mmHg, Torr, inHg | Usually vacuum or absolute; check the datasheet |
Atmospheric pressure changes with altitude and weather
Atmospheric pressure is not a fixed number. It falls as you go higher, and it also changes by a few percent with the weather.
| Height above sea level | Approximate atmospheric pressure |
|---|---|
| 0 m | 101.3 kPa |
| 500 m | 95.5 kPa |
| 1,000 m | 89.9 kPa |
| 1,500 m | 84.6 kPa |
| 2,000 m | 79.5 kPa |
These are standard atmosphere values; the real pressure varies with the weather. This is why an absolute pressure of 5 bar means something different from 5 bar gauge, and why a gauge-pressure instrument gives the same reading on a pump at the beach and in the mountains, while an absolute instrument does not. For many industrial jobs this difference is small compared to the working pressure, but for vacuum and gas calculations it matters.
Sealed gauge pressure
A sealed gauge sensor is referenced to a fixed pressure sealed inside the sensor, close to normal atmospheric pressure, instead of the changing atmosphere. It is used for high ranges, where small atmospheric changes do not matter, and it avoids the need for a vent to the atmosphere. Compared with a vented gauge sensor it can read slightly differently from zero when the weather changes, which is normal and small relative to a high range.
A pre-order checklist
- State whether the specification is in gauge, absolute or differential terms.
- Check the units on the datasheet and your controller.
- If the system can go into vacuum, say so.
- For sensors, confirm the pressure type in writing with your supplier.
- Test the zero on arrival: a gauge sensor should read zero open to the atmosphere, an absolute one around 101 kPa.
Practice questions
Try these to check your understanding. The answers follow.
- A tank gauge reads 150 kPa. What is the absolute pressure?
- A process runs at 30 kPa absolute. What does a gauge-pressure instrument read?
- A 2 bar absolute sensor is open to the air. What does it read?
- A vacuum gauge reads -0.5 bar. What is the absolute pressure?
Answers. (1) 150 + 101.3 = 251.3 kPa absolute. (2) 30 − 101.3 = -71.3 kPa gauge, which is a vacuum. (3) About 1.013 bar, because the air presses with atmospheric pressure. (4) 1.013 − 0.5 = 0.513 bar absolute.
Converting a datasheet requirement
A specification says “maximum 7 bar absolute”. Your pump gauge reads gauge pressure. The equivalent limit is 7 − 1.013 = 5.987 bar gauge, so set your alarm at about 5.9 bar gauge. If the specification was in gauge pressure and you treated it as absolute, you would allow a pressure that is a full bar too high. This type of mix-up is common when equipment is specified overseas, and the cure is to write the type of pressure next to every number.
Psi versions of the same idea
| Situation | psig | psia |
|---|---|---|
| Open to the air | 0 | 14.7 |
| A tyre at 32 psi gauge | 32 | 46.7 |
| A compressor at 116 psi gauge (8 bar) | 116 | 130.7 |
| Perfect vacuum | -14.7 | 0 |
Why this matters for safety
Relief valve settings, vessel design pressures and alarm limits are normally stated in gauge pressure for industrial plant, but vacuum equipment, gas calculations and some foreign specifications use absolute pressure. A misread can put a vessel above its design limit or switch an alarm at the wrong point. Whenever you transfer a number from a drawing, a datasheet or a manual, write the type of pressure next to it (g, a or d), and confirm it with the person who wrote the specification if there is any doubt. A minute spent on this check is far cheaper than an incident or a wrongly ordered sensor.
Frequently asked questions
What is the difference between gauge and absolute pressure?
Gauge pressure is relative to the atmosphere. Absolute pressure is relative to a perfect vacuum. Absolute equals gauge plus about 101.3 kPa.
Can gauge pressure be negative?
Yes. A negative gauge pressure is a vacuum.
Why does my absolute sensor read about 1 bar when disconnected?
Because it measures from a perfect vacuum, and the atmosphere is about 1 bar absolute.
Which do pressure gauges show?
Almost always gauge pressure. See what is a pressure gauge.
How do I convert psi gauge to absolute?
Add about 14.7 psi.
Next steps
Read about range selection, then browse our pressure sensors or contact us to confirm the pressure type you need.
