Gauge vs Absolute Pressure, and the Units Boiler Plants Use
A pressure gauge measures how far a pressure is above the atmosphere around it. Steam tables, physics and many data sheets use absolute pressure, measured from a perfect vacuum. The two differ by one atmosphere, and mixing them is the most common error in steam calculations.
You'll learn
- Explain gauge and absolute pressure
- Convert between kg/cm², bar, MPa and psi
- Convert gauge readings to absolute and back
- Recognize the error a mix-up causes
Before this: Saturation pressure and temperature
Two places to start counting from
The air around us presses on everything at about one atmosphere. A pressure gauge on a boiler ignores that: it reads zero when the boiler is open to the air and shows only the pressure above atmosphere. That is gauge pressure.
Absolute pressure counts from a perfect vacuum instead, so it is always gauge pressure plus atmospheric pressure. Physics, steam tables and gas calculations use absolute pressure, because water and steam respond to the total pressure on them, not to the part a gauge happens to show.
Indian plants write gauge pressure as kg/cm²(g), or sometimes kg/cm²g or “kg/cm² gauge”. Absolute is kg/cm²(a). Always say which.
Converting
| Symbol | Meaning | Unit |
|---|---|---|
| Standard atmosphere: 1.0332 kg/cm² = 1.01325 bar = 0.101325 MPa | — |
| Unit | In MPa | Where you see it |
|---|---|---|
| 1 kg/cm² | 0.0980665 | Indian boiler nameplates and gauges |
| 1 bar | 0.1 | European data sheets |
| 1 MPa | 1 | IF97 and engineering standards |
| 1 psi | 0.0068948 | American equipment |
1 kg/cm² is 0.981 bar: close enough to confuse, different enough to matter.
- A boiler gauge reads 10 kg/cm².
- Add atmospheric pressure:
- In MPa: 11.033 × 0.09807 = 1.0820 MPa(a). That is the number to look up in an absolute-pressure steam table.
What a mix-up costs
Look up 10 kg/cm²(g) in an absolute table without converting and you read the saturation temperature at what is really 8.97 kg/cm²(g): 179.0 °C instead of 183.3 °C. A 4.3 °C error looks entirely plausible and will not be caught by inspection; it shifts latent heat, specific volume and every sizing figure that follows.
The error is largest at low pressure, where the saturation curve is steepest. At 1 kg/cm²(g), confusing gauge and absolute is the difference between 120.1 °C and 99.1 °C.
Vacuum gauges and compound gauges add a third convention: vacuum is often shown as a negative gauge pressure, or as “mm Hg vacuum”. Convert everything to absolute before doing any arithmetic. In GyanTosh’s own steam code, every pressure variable carries its datum in its name for exactly this reason.
Key takeaways
- Gauge pressure is measured from the atmosphere; absolute from a vacuum.
- Absolute = gauge + 1.033 kg/cm² (one standard atmosphere).
- 1 kg/cm² = 0.0980665 MPa = 0.981 bar.
- Treating 10 kg/cm²(g) as absolute understates saturation temperature by about 4 °C.