What Is Gauge Pressure? PSIG vs PSIA, barg and Vacuum on Pump Spec Sheets
HCEM Engineering TeamShare
Gauge pressure is pressure measured relative to the surrounding atmosphere: a gauge open to the air reads zero. Absolute pressure is measured relative to a perfect vacuum, so it is gauge pressure plus atmospheric pressure (about 1.013 bar, 101.3 kPa or 14.7 psi at sea level). That is all PSIG and PSIA mean: psi gauge and psi absolute. The trouble starts on datasheets, where the same pump can be described as "−92 kPa", "93 mbar abs", "690 mmHg" or "−0.092 MPa", and a compressor as "7 bar", "7 barg" or "8 bar(a)".
This guide explains gauge and absolute pressure the way an OEM engineer meets them: on the pressure line of a compressor or vacuum pump specification. It gives the formulas, conversion tables in real units, the notations used on UK, German and Chinese datasheets, and worked examples using values from our own oil-free piston pumps.
What is gauge pressure?
Gauge pressure is the difference between the pressure in a system and the atmospheric pressure around it:
pgauge = pabsolute − patmospheric
Most mechanical pressure gauges work this way. A Bourdon tube or a diaphragm sees system pressure on one side and the atmosphere on the other, so it shows only the difference. Disconnect it and it reads 0, even though the air around it is pressing at about 1 bar.
Gauge pressure is positive when the system is above atmospheric (a compressor outlet, a receiver, a pneumatic line) and negative when it is below (a vacuum chamber, a suction cup, the inlet of a vacuum pump). A negative gauge reading is what most datasheets call vacuum.
Gauge pressure is the natural number for most working jobs because it is what the hardware feels. A hose, a tank wall or a seal is loaded by the difference between inside and outside, not by the absolute pressure inside.
What is absolute pressure, and what is the formula?
Absolute pressure is measured from a perfect vacuum, where the pressure is zero. It can never be negative. The formula for absolute pressure is simply the gauge formula turned round:
pabsolute = pgauge + patmospheric
For vacuum, where the gauge reading is negative, the same formula gives a value between zero and atmospheric. A pump inlet at −92 kPa gauge on a day when the atmosphere is 101.3 kPa is at 101.3 − 92 = 9.3 kPa absolute, or 93 mbar abs.
The same idea gives the textbook formula for pressure under a liquid: pabsolute = patmospheric + ρgh, where ρ is the liquid's density, g is 9.81 m/s² and h is the depth. The ρgh term on its own is the gauge pressure. For an aerator diffuser 1.5 m under fresh water, ρgh = 1000 × 9.81 × 1.5 = 14,715 Pa, about 147 mbar gauge (2.1 psig), or 116 kPa absolute. The air pump feeding it has to deliver more than 147 mbar gauge before any air leaves the diffuser, plus whatever the pipe and the diffuser membrane cost.
The standard atmosphere used in most conversions is 101.325 kPa, which is the same as 1.01325 bar, 1013.25 mbar, 14.696 psi, 760 mmHg or 29.92 inHg. Real atmospheric pressure varies with weather and altitude, which matters for vacuum (see below).
PSIG vs PSIA: what do the letters mean?
PSI is pounds-force per square inch. The suffix states the reference:
- psig: psi gauge, measured from the local atmosphere.
- psia: psi absolute, measured from a perfect vacuum.
- psi with no letter: usually gauge on compressors, air tools and pneumatics, but it is an assumption, not a statement.
- psid: psi differential, the difference between two points, such as across a filter.
At sea level, psia = psig + 14.7. The table converts the pressures you meet most often on compressor datasheets (1 bar = 100 kPa = 14.504 psi):
| Gauge (bar) | Gauge (kPa) | psig | Absolute (bar abs) | psia |
|---|---|---|---|---|
| 0 | 0 | 0 | 1.013 | 14.7 |
| 2 | 200 | 29.0 | 3.013 | 43.7 |
| 4 | 400 | 58.0 | 5.013 | 72.7 |
| 6.2 | 620 | 90.0 | 7.213 | 104.6 |
| 7 | 700 | 101.5 | 8.013 | 116.2 |
| 8 | 800 | 116.0 | 9.013 | 130.7 |
| 10 | 1000 | 145.0 | 11.013 | 159.7 |
Note that "7 bar(g)" and "8 bar(a)" are almost the same pressure. A datasheet that rounds 8.013 bar abs to 8 bar is not quoting a stronger compressor than one that says 7 bar gauge.
What do barg, bar(a), kPa and MPa mean on a datasheet?
Metric datasheets carry the same gauge-or-absolute choice, only written differently. Strictly, SI style puts the reference in the words ("gauge pressure 7 bar") rather than in the unit symbol, but datasheets everywhere add it to the unit. The table shows how the same two numbers appear in the three documents an OEM buyer in the UK or Germany is most likely to compare: a British or European English sheet, a German sheet and a Chinese factory sheet.
| Meaning | UK / English sheet | German sheet | Chinese sheet |
|---|---|---|---|
| Gauge, above atmospheric | 7 bar g, 7 barg, 7 bar(g), 700 kPa, 101.5 psig | 7 bar Überdruck, 7 bar ü, pe = 7 bar | 表压 0.7 MPa, 700 kPa |
| Absolute, from perfect vacuum | 8 bar abs, 8 bar(a), 116 psia | 8 bar absolut, pabs = 8 bar | 绝对压力 (绝压) 0.8 MPa |
| Vacuum as negative gauge | −92 kPa, −920 mbar g, −13.3 psig | Unterdruck −920 mbar, −0,92 bar | −92 kPa, −0.092 MPa |
| Vacuum as absolute | 93 mbar abs, 9.3 kPa abs, 70 Torr | Enddruck 93 mbar (abs.) | 绝压 9.3 kPa |
| Vacuum as depth below atmospheric | 690 mmHg, 27.2 inHg vacuum, 91% vacuum | rarely used | 真空度 92 kPa (no sign) |
Three habits cause most of the mix-ups. Chinese datasheets and gauges often use MPa: 0.7 MPa is 7 bar, and a vacuum gauge scaled "−0.1 to 0 MPa" covers the whole range from perfect vacuum to atmospheric. A Chinese 真空度 ("degree of vacuum") figure may be printed without its minus sign, so "92 kPa" can mean −92 kPa gauge, not 92 kPa absolute. And German sheets write the decimal comma, so "−0,92 bar" is −0.92 bar. When a figure has no suffix and no sign, ask which reference it uses before comparing it with anything.
How is vacuum pressure written on a vacuum pump datasheet?
Vacuum pressure is any pressure below atmospheric. Because it can be expressed from either end of the scale, it has more notations than positive pressure. Industrial pump makers, our own included, usually give the ultimate vacuum as negative gauge pressure in kPa. Laboratory and high-vacuum suppliers usually give absolute pressure in mbar, hPa (1 hPa = 1 mbar), Pa or Torr. North American suction products often use inches of mercury.
Here are the vacuum levels typical of oil-free piston and diaphragm pumps, converted. The absolute columns assume the standard atmosphere of 1013.25 mbar.
| Gauge (kPa) | Absolute (mbar abs) | Absolute (kPa abs) | psig | psia |
|---|---|---|---|---|
| 0 | 1013 | 101.3 | 0 | 14.70 |
| −50 | 513 | 51.3 | −7.25 | 7.44 |
| −86 | 153 | 15.3 | −12.47 | 2.22 |
| −90 | 113 | 11.3 | −13.05 | 1.64 |
| −92 | 93 | 9.3 | −13.34 | 1.35 |
| −98 | 33 | 3.3 | −14.21 | 0.48 |
And the same levels in the "depth" units you see on gauges and older datasheets (1 mmHg = 133.3 Pa; 1 inHg = 3.386 kPa; 1 Torr ≈ 1 mmHg):
| Gauge (kPa) | Vacuum (mmHg) | Vacuum (inHg) | Absolute (Torr) | % vacuum |
|---|---|---|---|---|
| −50 | 375 | 14.8 | 385 | 49.3% |
| −86 | 645 | 25.4 | 115 | 84.9% |
| −90 | 675 | 26.6 | 85 | 88.8% |
| −92 | 690 | 27.2 | 70 | 90.8% |
| −98 | 735 | 28.9 | 25 | 96.7% |
The mmHg column is a depth below atmospheric, not an absolute pressure, even though Torr and mmHg are nearly the same size. "690 mmHg" of vacuum is about 70 Torr absolute. "% vacuum" is the gauge vacuum divided by the atmosphere: 92 ÷ 101.3 = 90.8%. For other values, our CFM to L/min converter handles the flow side of the same datasheet.
The gauge-or-absolute choice also sits behind a different question: whether the vacuum on the datasheet is one your machine will ever work at. The ultimate vacuum is the end of the pump's curve, measured with the inlet closed; your working vacuum is a point on that curve where useful flow remains. We cover that in ultimate pressure vs working vacuum, and the mechanism behind it in how vacuum pumps work.
When should you use gauge pressure and when absolute?
A simple rule covers almost every calculation: differences and forces use gauge; ratios and gas laws use absolute.
Use gauge for forces and pressure drops
A suction cup is pushed onto the part by the atmosphere outside it, so its holding force depends on the gauge vacuum: F = Δp × A. A 50 mm cup has an area of π × 0.025² = 0.00196 m². At −60 kPa gauge the theoretical force is 60,000 × 0.00196 ≈ 118 N, about 12 kg, before any safety factor for acceleration, part orientation or leakage. The absolute pressure in the cup (41 kPa abs) does not give the force directly.
Pressure losses are differences too. A drop of 0.3 bar along a compressed-air line is the same 0.3 bar whether you measure the two ends in gauge or absolute. When a pressure-drop formula asks for the line pressure itself, as our pressure drop calculator does, check whether it expects gauge or absolute.
Use absolute for ratios, gas laws and pump-down time
Anything that multiplies or divides pressures needs absolute values, because zero gauge is not zero pressure.
Compression ratio. Our HC580 oil-free air pump is rated at 60 L/min at 700 kPa. A compressor's discharge rating with no suffix is conventionally read as gauge, so the outlet is at 700 + 101.3 = 801.3 kPa abs. The compression ratio is 801.3 ÷ 101.3 = 7.9, not 7. For the HC1100, rated at 100 L/min at 800 kPa, it is 901.3 ÷ 101.3 = 8.9. The compression ratio is what drives discharge temperature, and the free-air vs compressed-volume conversion is explained in CFM meaning.
Volume of the compressed air. If the HC580's 60 L/min is free air at the inlet, which is how compressor flow is normally stated, the same air takes up 60 × 101.3 ÷ 801.3 ≈ 7.6 L/min once compressed to 7 bar gauge. Using 700 instead of 801.3 would give 8.7 L/min, about 14% too high. Our guide on how to size an oil-free air pump starts from the delivered flow at your working pressure.
Pump-down time. The first-estimate formula t = (V ÷ S) × ln(p0 ÷ p1) only works with absolute pressures: put gauge values in and the start pressure is zero. Take a 10 L chamber, the single-stage HC250 at its listed ≥75 L/min, and a target of −80 kPa gauge (213 mbar abs). Then t = (10 ÷ 75) × ln(1013 ÷ 213) = 0.133 × 1.56 ≈ 0.21 min, about 12.5 seconds. Treat that as a lower limit: −80 kPa is not far from this version's −90 kPa maximum vacuum, so its real speed there is well below 75 L/min. Our evacuation time sizing guide takes the calculation further.
Why does the same vacuum read differently on another day, or at altitude?
Because a gauge reading is relative to the atmosphere, and the atmosphere moves. At sea level it typically drifts by a few tens of millibars with the weather. Suppose a chamber is held at exactly 93 mbar abs:
- on a high-pressure day (1030 mbar), the gauge reads −(1030 − 93) = −937 mbar, or −93.7 kPa;
- on a low-pressure day (990 mbar), it reads −(990 − 93) = −897 mbar, or −89.7 kPa.
Nothing in the chamber changed, but the reading moved by 4 kPa. If a vacuum switch in your machine trips at −90 kPa gauge, that is the difference between a pass and a fault.
Altitude makes the effect larger. At 1,500 m the standard atmosphere is about 84.6 kPa. A gauge there can never read deeper than −84.6 kPa, because that would be below zero absolute. A pump rated at −92 kPa at sea level cannot show −92 kPa in such a location, whatever its condition. If your equipment ships to sites at altitude, state the vacuum requirement in absolute terms and read our guide to altitude and ambient conditions in pump selection.
The same logic applies to sensors. A gauge (vented) sensor follows the atmosphere; an absolute sensor has a sealed vacuum reference and does not. For a process that depends on the gas left in the chamber, such as degassing, drying or a leak test, an absolute sensor gives the repeatable number. For a gripper that depends on holding force, a gauge sensor reads the quantity that matters. Our guide on how to test a vacuum pump covers which gauge to use when checking a pump's ultimate vacuum.
How do you read the pressure line on HCEM datasheets?
Our datasheets give compressor ratings as a flow at a discharge pressure in kPa, and vacuum ratings as negative gauge pressure in kPa, sometimes with mmHg alongside. Here are four models from our parameter tables, with the other notations worked out:
| Model | Type | As stated | Gauge | Absolute (approx.) |
|---|---|---|---|---|
| HC580 (HC580A-60/7, HC580B-60/7) | Oil-free air pump, AC | 60 L/min @ 700 kPa | 7 bar g, 101.5 psig | 8.0 bar abs, 116 psia |
| HC1100 (HC1100A-100/8, HC1100B-100/8) | Oil-free air pump, AC | 100 L/min @ 800 kPa | 8 bar g, 116 psig | 9.0 bar abs, 131 psia |
| HC250A/B | Twin-cylinder oil-free piston vacuum pump | single-stage ≥75 L/min, −90 kPa; dual-stage ≥37.5 L/min, −98 kPa | −90 or −98 kPa | 113 or 33 mbar abs |
| HC480 (HC480A, HC480B) | Oil-free vacuum pump, AC | max. open flow 95 L/min (220 V/50 Hz) or 100 L/min (110 V/60 Hz), maximum vacuum −92 kPa (690 mmHg); listed alternatively as 47.5 or 50 L/min with maximum vacuum −98 kPa (735 mmHg) | −92 or −98 kPa | 93 or 33 mbar abs |
Three points to take from the table. The model codes of the two air pumps carry the rating: 60/7 and 100/8 match the stated flow in L/min and the pressure in bar. The air pump pressures have no suffix; as above, read them as gauge, and if your design depends on the absolute value, ask us to confirm it in writing. The vacuum figures are negative, so they are gauge by definition, and the mmHg values beside them (690 and 735) are the same depth expressed in millimetres of mercury, not absolute pressures.
When you compare these with a laboratory pump quoted at, say, "100 mbar ultimate", convert both to the same basis first: −92 kPa gauge is about 93 mbar abs, so the two are almost the same depth. Browse the full ranges of oil-free vacuum pumps and AC air compressor pumps, and use our vacuum pump specification checklist to state your own requirement unambiguously.
Common questions
Is psig the same as psi?
In most compressor and pneumatic literature, "psi" without a letter means psig. It is a convention, not a rule. Where the difference matters, as in gas calculations or vacuum, write psig or psia explicitly. A plain "psi" scale on a dial gauge is psig: it reads 0 open to air.
Can gauge pressure be negative?
Yes. Any pressure below atmospheric is negative in gauge terms; that is how most industrial vacuum pumps state their vacuum. Absolute pressure can never be negative, and gauge pressure can never be lower than minus the local atmospheric pressure.
How do you convert barg to bara?
Add the atmospheric pressure: bar abs = bar g + 1.013 at sea level. 7 barg is 8.013 bar abs. For a quick estimate, add 1. For psi, add 14.7.
Is a vacuum given in mbar gauge or absolute?
Usually absolute, especially from laboratory and high-vacuum suppliers: "10 mbar" means 10 mbar abs. Industrial suppliers who give vacuum in mbar sometimes mean negative gauge, written as "−900 mbar". The minus sign is the clue; without it, ask.
What does −0.1 MPa mean on a vacuum gauge?
It is −100 kPa gauge, the bottom of a typical Chinese vacuum gauge scale. Because the atmosphere is about 101.3 kPa, −0.1 MPa is close to a perfect vacuum; a pump rated at −0.092 MPa reaches −92 kPa, about 93 mbar abs.
If you are comparing our datasheet with another supplier's, or converting a requirement for an OEM project, send us the pressure or vacuum level with its reference (gauge or absolute), the flow you need at that point, supply voltage and duty cycle, and our engineers will recommend a model, including OEM versions built for your machine.