1. Overview
Atmospheric pressure is the force exerted by the weight of air above a given point. It decreases with altitude and varies with weather. All gas law calculations require absolute pressure—using gauge pressure is a common and costly error.
Absolute Pressure
Pabs (psia)
Referenced to perfect vacuum. Required for gas laws.
Gauge Pressure
Pgauge (psig)
Referenced to local atmosphere. What field gauges read.
Vacuum
Pvac (in Hg)
Pressure below atmospheric. Pabs = Patm − Pvac
Barometric
Patm (psia)
Local atmospheric pressure. Varies with elevation and weather.
Key Equations
Gauge ↔ Absolute Conversion:
Pabsolute = Pgauge + Patmospheric
Pgauge = Pabsolute − Patmospheric
Example: 100 psig at sea level = 100 + 14.7 = 114.7 psia
⚠️ Critical: Gas laws (PV = nRT), density, and compressibility calculations require absolute pressure. Using gauge pressure causes 10–15% errors at typical conditions.
2. Standard Conditions
Different industries use different "standard" base conditions. The difference matters for custody transfer—verify contract specifications.
| Standard | Pressure | Temperature | Use |
|---|---|---|---|
| AGA / API 14.3 | 14.73 psia | 60°F | US natural gas custody transfer |
| API (Liquids) | 14.696 psia | 60°F | Oil, NGL measurement |
| ISO 13443 | 101.325 kPa | 15°C | International gas/LNG |
| ISA / ICAO | 14.696 psia | 59°F | Atmospheric modeling |
| EPA | 14.7 psia | 68°F | Emissions reporting |
Volume Correction
Flow at Standard Conditions:
Qstd = Qactual × (Pactual / Pbase) × (Tbase / Tactual) × (Zbase / Zactual)
T in absolute units (°R = °F + 459.67). Zbase ≈ 1.0 at standard conditions.
💰 Contract Impact: 14.696 vs. 14.73 psia = 0.23% difference. On 100 MMscfd, that's 230 Mscfd or ~$420,000/year at $5/MMBtu.
3. Altitude Corrections
Atmospheric pressure decreases with elevation. Use actual Patm for gauge-to-absolute conversions at elevated sites.
-->ISA Barometric Formula
ICAO Standard Atmosphere (valid to 36,089 ft):
P = 14.696 × [1 − (0.0000068756 × h)]5.2558
h = elevation in feet. Reference: ICAO Doc 7488, US Std Atmosphere 1976
Quick Approximation (±2% to 10,000 ft):
Patm ≈ 14.7 − (h ÷ 2,000)
Denver (5,280 ft): P ≈ 14.7 − 2.64 = 12.06 psia (actual: 12.10)
Reference Table
| Elevation | Patm (psia) | in Hg | % Sea Level |
|---|---|---|---|
| Sea level | 14.696 | 29.92 | 100% |
| 1,000 ft | 14.17 | 28.86 | 96% |
| 2,500 ft | 13.42 | 27.32 | 91% |
| 5,280 ft (Denver) | 12.10 | 24.63 | 82% |
| 7,500 ft | 11.13 | 22.65 | 76% |
| 10,000 ft | 10.11 | 20.58 | 69% |
Weather Variation
Sea Level Barometric Range:
High pressure: 30.2–30.9 in Hg (14.9–15.2 psia)
Normal: 29.5–30.2 in Hg (14.5–14.9 psia)
Low pressure: 28.5–29.5 in Hg (14.0–14.5 psia)
Hurricane: < 27.9 in Hg (< 13.7 psia)
⚠️ Weather Station Caution: Reported "barometric pressure" is often corrected to sea level for comparison. At Denver, weather may report 30.0 in Hg when actual station pressure is only 24.6 in Hg. Verify before using.
4. Unit Conversions
Common Conversions
| From | To | Multiply By |
|---|---|---|
| psi | kPa | 6.89476 |
| psi | bar | 0.06895 |
| bar | psi | 14.504 |
| psi | in Hg | 2.036 |
| in Hg | psi | 0.4912 |
| psi | in H₂O | 27.68 |
| ft H₂O | psi | 0.4331 |
| atm | psi | 14.696 |
Standard Atmosphere Equivalents
1 atm (exactly) =
14.696 psia = 101.325 kPa = 1.01325 bar
29.92 in Hg = 760 mm Hg = 760 Torr
33.9 ft H₂O = 407 in H₂O = 1013.25 mbar
Gauge ↔ Absolute
Ppsia = Ppsig + Patm
Pbara = Pbarg + 1.01325 × (Patm/14.696)
PkPa abs = PkPag + 101.325 × (Patm/14.696)
Use local Patm for elevated sites, not sea level value.
📋 International Note: "2 bar" is ambiguous—could be gauge or absolute. Use barg (gauge) or bara (absolute). Similarly: psig/psia, kPag/kPa abs.
5. Engineering Applications
-->Gas Density
Real Gas Density:
ρ = (Pabs × MW) / (Z × R × T)
Pabs in psia, T in °R, R = 10.73 psia·ft³/(lbmol·°R)
At 5,000 ft: 100 psig = 112.2 psia (not 114.7). Using sea level Patm overestimates density by 2.2%.
Compressor Ratios
Compression Ratio:
r = Pdischarge,abs / Psuction,abs
Example at 5,000 ft (Patm = 12.2 psia):
Suction: 50 psig → 62.2 psia
Discharge: 200 psig → 212.2 psia
r = 212.2 / 62.2 = 3.41
If sea level Patm used incorrectly:
r = 214.7 / 64.7 = 3.32 (2.6% error → affects power calc)
Flow Measurement
Orifice meters (AGA 3 / API 14.3) require absolute pressure for both density (ρ) and expansion factor (Y):
ρ = Pabs × MW / (Z × R × T)
Y = 1 − (0.41 + 0.35β⁴) × ΔP / (κ × Pabs)
Using gauge pressure causes 1–3% flow error.
Vacuum Systems
Pabsolute = Patm − Pvacuum
10 in Hg vacuum at sea level:
Pabs = 14.7 − (10 × 0.491) = 14.7 − 4.91 = 9.79 psia
Hydrostatic Head
Pbottom = Psurface + (ρ × h / 144)
50 ft water column, open tank:
P = 14.7 + (62.4 × 50 / 144) = 14.7 + 21.7 = 36.4 psia
Best Practices
✓ Do
- Always clarify psig vs. psia
- Use site-specific Patm for elevated locations
- Verify base conditions in custody transfer contracts
- Use absolute temperature (°R, K) in gas calculations
- Specify bara/barg or psia/psig explicitly on P&IDs
✗ Avoid
- Assuming 14.7 psia at elevated sites
- Using gauge pressure in gas law equations
- Using weather-reported "sea level corrected" pressure as actual
- Ambiguous units like "psi" or "bar" without g/a suffix
Ready to use the calculator?
→ Launch CalculatorReferences
- ICAO Doc 7488-CD, Standard Atmosphere, 3rd ed., 1993
- US Standard Atmosphere, 1976 (NOAA/NASA/USAF)
- API MPMS Ch. 14.3 / AGA Report No. 3
- 43 CFR 3175 (Federal gas measurement)