Gauge vs absolute — two zeros
Every pressure needs a reference point, and instrumentation uses two. Absolute pressure measures from perfect vacuum — its zero is the total absence of pressure, and it can never be negative. Gauge pressure measures from whatever the local atmosphere happens to be — its zero is "the air around us", so an open tank reads 0 and a vacuum reads negative. The two scales are the same ruler shifted by one atmosphere:
The suffix convention marks the reference: psig / barg / kPag for gauge, psia / bara / kPaa for absolute. Most process transmitters and gauges are gauge-referenced; absolute references appear in vacuum service, boiling-point calculations, gas-law work and altitude-sensitive applications.
barg to bara — the everyday conversion
This is the same relationship written in the units most process plants actually use on their datasheets:
| barg | bara | psig | psia |
|---|---|---|---|
| 0 | 1.013 | 0 | 14.70 |
| 1 | 2.013 | 14.50 | 29.20 |
| 2 | 3.013 | 29.01 | 43.71 |
| 5 | 6.013 | 72.52 | 87.22 |
| 10 | 11.01 | 145.0 | 159.7 |
| −0.5 | 0.513 | −7.25 | 7.45 |
Note the last row: negative gauge pressure is a partial vacuum, and it still maps to a positive absolute pressure. Absolute pressure has nowhere below zero to go.
Where this bites: steam tables, gas law calculations and compressor work are all defined in absolute pressure, while plant gauges read gauge. Feeding 10 barg into a steam table expecting 10 bara gives a saturation temperature 4 °C off — see the saturated steam table.
Altitude correction
Atmospheric pressure is not a constant — it falls with elevation. This tool estimates it with the standard barometric formula:
At sea level that gives 101.325 kPa (14.696 psi); at 1000 m, 89.9 kPa; at 2000 m, 79.5 kPa. Day-to-day weather adds roughly ±3 kPa on top, which is why absolute-critical work uses a barometer reading, not a formula.
Worked example
A compressor discharge gauge in Chennai (sea level) reads 7.0 barg. The process simulation needs absolute pressure:
- Atmospheric ≈ 1.013 bar at sea level
- P_abs = 7.0 + 1.013 = 8.013 bara
The same reading at a plant 1500 m up would be 7.0 + 0.846 = 7.846 bara — a 2% difference in gas density calculations, from altitude alone.
Field notes
- Read the datasheet suffix. Ordering an absolute-range transmitter for a gauge application (or vice versa) is a real and expensive procurement error.
- Vacuum arithmetic: −40 kPag at sea level = 61.3 kPaa. If your absolute result comes out negative, the gauge reading is below full vacuum — physically impossible, so recheck the numbers.
- Sealed-gauge instruments (psis) reference a factory-sealed atmosphere rather than ambient — common on hydrostatic level sensors. Treat them as gauge with a fixed offset.
Frequently asked questions
What is the difference between psig and psia?
psig measures pressure relative to local atmospheric pressure; psia measures relative to perfect vacuum. psia = psig + atmospheric pressure (about 14.7 psi at sea level).
What is 0 barg in absolute pressure?
At sea level, 0 barg is approximately 1.013 bara — an open vessel at atmospheric pressure reads zero on a gauge-referenced instrument.
How do I convert barg to bara?
Add atmospheric pressure: bara = barg + 1.01325 at sea level. So 5 barg is 6.013 bara. Going the other way, bara = barg − 1.01325, meaning 6 bara is 4.987 barg.
How do I convert barg to psia?
Two steps: add atmospheric pressure to get bara, then multiply by 14.5038. For 5 barg — 5 + 1.01325 = 6.013 bara, then × 14.5038 = 87.2 psia.
How many mmWC is 1 barg?
1 barg equals 10,197 mmWC gauge. The unit conversion factor is the same whether the pressure is gauge or absolute — only the reference point differs.
Does altitude change the conversion?
Yes. Atmospheric pressure falls roughly 1.2% per 100 m of elevation near sea level. At 1500 m altitude it is about 84.6 kPa instead of 101.3 kPa, shifting every gauge-absolute conversion by the same amount.
Can gauge pressure be negative?
Yes — a vacuum reads negative on a gauge scale. Full vacuum at sea level is about −101.3 kPag (−14.7 psig), which equals 0 kPa absolute. Absolute pressure can never be negative.
Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.