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Open resource1. Purpose and Engineering Decision
Pressure mistakes in compressed-air work are rarely caused by difficult arithmetic. They are usually caused by mixing units, mixing gauge and absolute pressure, or forgetting that local atmospheric pressure is part of the conversion.
The tool must answer three questions at the same time: What is the equivalent pressure in another unit? Is the stated value gauge, absolute or vacuum-related? What atmospheric pressure was used when gauge and absolute values were converted?
User task | |
|---|---|
Convert units only | Equivalent bar, kPa, MPa, psi, kgf/cm2 and optional vacuum units |
Gauge to absolute | Pabs = Pgauge + Patm, with Patm shown |
Absolute to gauge | Pgauge = Pabs - Patm, with Patm shown |
Interpret negative gauge | Shows sub-atmospheric state and equivalent absolute pressure |
Vacuum entry | Converts vacuum depression to absolute pressure when the vacuum convention is explicit |
Feed other tools |
2. Core Pressure References
Reference | Meaning | Zero point |
|---|---|---|
Gauge pressure | Pressure relative to local atmosphere | Local atmospheric pressure |
Absolute pressure | Pressure relative to perfect vacuum | Perfect vacuum |
Vacuum / negative gauge | Pressure below local atmosphere | Must be converted through atmospheric pressure |
Differential pressure | Difference between two pressure points | Not automatically gauge or absolute |
A pressure value without a reference tag is incomplete. The calculator should display gauge values as bar(g), kPa(g), psi(g) and absolute values as bar(a), kPa(a), psi(a). For differential pressure, use a separate label such as kPa differential or mbar differential rather than pretending it is gauge pressure.
3. Fundamental Equations
Pabs = Pgauge + Patm
Pgauge = Pabs - Patm
Pvacuum depression = Patm - Pabs
All values in these equations must use the same unit before addition or subtraction. For unit conversion alone, only a scale factor is required. For gauge-absolute conversion, the local atmospheric pressure becomes an explicit input.
4. Supported Pressure Units and Base Factors
Recommended internal base unit: pascal absolute (Pa). The UI may display engineering units, but all calculation logic should normalize to Pa first.
Unit | Equivalent in Pa | Typical use |
|---|---|---|
1 bar | 100,000 Pa | Compressed-air system pressure |
1 kPa | 1,000 Pa | Pressure loss, instrumentation |
1 MPa | 1,000,000 Pa | Higher pressure engineering |
1 psi | 6,894.757 Pa | Imperial compressor data |
1 kgf/cm2 | 98,066.5 Pa | Legacy plant and equipment data |
1 mbar | 100 Pa | Low differential pressure |
1 mmHg | 133.322 Pa | Vacuum / legacy instrumentation |
1 inHg | 3,386.389 Pa | Vacuum and atmospheric references |
5. Unit Conversion Logic
The unit-conversion layer is separate from pressure-reference conversion. First convert the entered pressure to the internal base unit. Then apply gauge, absolute or vacuum reference logic. Finally convert the normalized result to the selected display unit.
Ptarget = Psource x (source factor in Pa) / (target factor in Pa)
From | To | Approximate factor |
|---|---|---|
bar | psi | x 14.5037738 |
psi | bar | x 0.0689475729 |
bar | kPa | x 100 |
bar | MPa | x 0.1 |
bar | kgf/cm2 | x 1.01971621 |
kgf/cm2 | bar | x 0.980665 |
inHg | kPa | x 3.386389 |
mmHg | kPa | x 0.133322 |
6. Atmospheric Pressure Input
Atmospheric pressure is not a universal constant at every site. It varies with elevation and weather. For high-elevation sites, process design, testing or precision reference conversion, the user should enter a measured or project-approved atmospheric pressure.
Atmospheric-pressure source | Recommended confidence |
|---|---|
Measured barometer / validated plant instrument | High |
Project design atmospheric pressure | High |
Site elevation based estimate | Medium |
Default 101.325 kPa(a) | Preliminary only unless appropriate for the site |
7. Input Model
Input group | Field | Validation / note |
|---|---|---|
Pressure | Value | Numeric; range depends on selected reference |
Pressure | Unit | bar, kPa, MPa, psi, kgf/cm2, mbar, mmHg, inHg |
Reference | Gauge / Absolute / Vacuum depression / Differential | Explicit selection required |
Atmosphere | Patm value | Required for gauge-absolute-vacuum conversion |
Atmosphere | Patm unit | Any supported pressure unit |
Output | Target unit | Any supported unit |
Output | Target reference | Gauge / Absolute / same reference |
Metadata | Source note | Optional tag: gauge, transmitter, compressor display, datasheet |
8. Negative Gauge Pressure and Vacuum
A negative gauge pressure is not a negative absolute pressure. It means the measured pressure is below local atmospheric pressure. For example, -0.20 bar(g) at 1.01325 bar(a) atmosphere corresponds to 0.81325 bar(a).
0.81325 bar(a) = -0.20 bar(g) + 1.01325 bar(a)
A vacuum gauge may instead report vacuum depression as a positive number below atmosphere, for example 0.20 bar vacuum.
Entry | Meaning | At Patm = 1.01325 bar(a) |
|---|---|---|
-0.20 bar(g) | 0.20 bar below atmosphere | 0.81325 bar(a) |
0.20 bar vacuum depression | Atmosphere minus 0.20 bar | 0.81325 bar(a) |
0.20 bar(a) | Absolute pressure | -0.81325 bar(g) |
9. Differential Pressure Boundary
Differential pressure is often used for filters, dryers, separators, pipe sections and pressure-drop diagnostics. It is a difference between two pressures, not a gauge-to-absolute value. A 20 kPa differential remains 20 kPa differential regardless of atmospheric pressure, provided both pressure taps use the same reference basis.
If the user wants upstream and downstream absolute pressures, those pressures must be entered separately.
10. Reference Test Case A - 7 bar(g) to Absolute Pressure
Input: 7.00 bar(g), local atmosphere 1.01325 bar(a).
Pabs = 7.00 + 1.01325 = 8.01325 bar(a)
Output | Result |
|---|---|
bar(a) | 8.01325 |
kPa(a) | 801.325 |
psi(a) | 116.222 |
Gauge input retained | 7.00 bar(g) |
Atmospheric pressure used | 1.01325 bar(a) |
11. Reference Test Case B - 100 psi(g) to bar(a)
Input: 100.0 psi(g), atmosphere 14.7 psi(a).
Pabs = 100.0 + 14.7 = 114.7 psi(a) = 7.9083 bar(a)
The tool must show both the gauge reading and the absolute result because a later FAD or gas-law calculation requires the absolute value, while the plant operator normally works with the gauge value.
12. Reference Test Case C - High-Elevation Site Effect
A compressor operates at 7.0 bar(g). Compare two atmospheric pressures: 1.01325 bar(a) and 0.850 bar(a). The gauge reading is identical, but absolute discharge pressure is not.
Atmosphere | Gauge pressure | Absolute pressure |
|---|---|---|
1.01325 bar(a) | 7.0 bar(g) | 8.01325 bar(a) |
0.850 bar(a) | 7.0 bar(g) | 7.850 bar(a) |
This distinction matters when pressure is inserted into FAD/reference conversion, receiver calculations or compressor-ratio analysis.
13. Reference Test Case D - Vacuum Conversion
Input: -60 kPa(g), atmosphere 101.325 kPa(a).
Pabs = -60 + 101.325 = 41.325 kPa(a) = 0.41325 bar(a)
Equivalent absolute pressure is also approximately 12.20 inHg(a). If the user instead entered 60 kPa as a positive vacuum depression, the same absolute result is obtained only because the convention was explicitly selected.
14. Output Specification
Output | Display rule |
|---|---|
Primary converted pressure | Large result with unit and reference tag |
Gauge equivalent | Shown when mathematically valid |
Absolute equivalent | Always available when Patm is known |
Atmospheric pressure | Displayed beside gauge-absolute conversion |
Vacuum equivalent | Shown only when selected / useful |
Unit set | bar, kPa, MPa, psi, kgf/cm2 plus optional mbar/mmHg/inHg |
Calculation trace | Unit normalization, reference conversion, final unit conversion |
Confidence | High / medium / preliminary based on Patm source |
Warnings | Reference missing, Patm defaulted, impossible absolute pressure, ambiguous vacuum entry |
15. Validation and Guardrails
Condition | Required behaviour |
|---|---|
Absolute pressure < 0 | Block calculation |
Absolute pressure = 0 | Allow only as perfect-vacuum boundary; warn about ideal limit |
Gauge selected without Patm | Require Patm or visibly accepted default |
Target absolute requested from differential pressure | Block; differential is not a reference pressure |
Vacuum value > Patm | Block because calculated absolute pressure would be negative |
Unknown reference type | Do not guess |
Extreme pressure outside configured tool range | Return warning / engineer review flag |
Legacy kgf/cm2 input | Convert numerically but preserve source unit in trace |
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. Prices, tariffs and efficiencies shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by an engineer against the machine data sheet and the site.
Pressure unit and gauge/absolute conversion
Convert between bar, psi, kPa and MPa and between gauge and absolute pressure. A mix-up between gauge and absolute is a classic cause of wrong sizing.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Pressure: 7
- Unit: bar g
Result
- bar gauge: 7
- bar absolute: 8.01
- psi gauge: 101.5
- psi absolute: 116.2
- kPa gauge: 700
- MPa gauge: 0.7
Method. Absolute pressure = gauge pressure + atmospheric (1.01325 bar at sea level). 1 bar = 14.504 psi = 100 kPa = 0.1 MPa. Hyderabad is about 540 m above sea level; atmospheric pressure there is roughly 0.95 bar, so enter the local value in the flow tool if it matters.
What to take from it. A 7 bar gauge system is 8.01 bar absolute. Ratio and flow-conversion work uses absolute pressure.