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.
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.
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.