Absolute pressure and gauge pressure use different reference levels. A conversion needs the atmospheric pressure that applies to the measurement.
Label both quantities
Absolute pressure is referenced to vacuum, while gauge pressure is referenced to atmosphere. For compatible measurements at the relevant condition, absolute pressure equals gauge pressure plus atmospheric pressure. The pressure datum belongs beside the unit; “bar” alone is not enough.
Check an invented positive-pressure example
An illustrative gauge reading is 200 kPa and the corresponding local atmospheric pressure is 95 kPa. The absolute value is 295 kPa. Replacing the local atmosphere with 100 kPa would give 300 kPa, introducing a 5 kPa difference in that calculation.
Record the atmospheric observation or approved reference method and its applicability in time and location. This example concerns datum conversion, not a vacuum percentage or compressor performance claim. The converted result also retains the uncertainty and conditions of its inputs.
Prevent hidden assumptions in software
Use clearly named fields for gauge, atmosphere and absolute values. Preserve the equation and datum definition in any export. Ask the reviewer whether a nominal atmosphere is acceptable for the task rather than embedding it without explanation. Do not add atmospheric pressure to a reading already defined as absolute.
Primary reference: Manufacturer technical reference.
Instrument evidence worksheet
This blank worksheet is for your own project. It contains no H M machine trial result.
| Review question | Evidence to retain |
|---|---|
| What datum does the input use? | Gauge or absolute definition |
| Which atmospheric value applies? | Location, timing and method |
| What transformation is implemented? | Named equation and units |
Customer Questions
Is “kPa” enough to identify the datum?
No.
Should atmosphere be added to an already absolute reading?
No.
Does the example establish a site atmospheric value?
No. Its 95 kPa is invented for the calculation.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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