Two Pressure Sensors Disagree by About One Bar? Check the Reference
Pressure values are incomplete without their reference. Gauge and absolute sensors can disagree while both measure correctly. Check the specified reference before recalibrating a replacement that appears roughly one atmospheric pressure away from the old value.
Illustrative pressure arithmetic; calibration requires appropriate equipment and procedures.
What is happening?
Gauge pressure is relative to prevailing ambient pressure; absolute pressure is relative to vacuum. Converting between them requires the appropriate ambient reference. Atmospheric pressure varies, so a fixed one-bar correction is only an approximation.
Check these five things
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Read gauge, absolute, or differential designation from both instruments.
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Check units and the pressure reference expected by PLC calculations.
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Compare instruments under a known documented condition.
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Review any existing conversion or offset in the signal path.
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Validate replacement suitability before accepting the configuration.
Worked example
In a simplified example with ambient pressure exactly 1.00 bar absolute, a vessel at 3.00 bar absolute is at 2.00 bar gauge. The difference is intentional. At another ambient pressure, the relationship changes accordingly.
Quick diagnostic reference
| Quantity | Reference |
|---|---|
| Absolute | Vacuum |
| Gauge | Ambient atmosphere |
| Differential | A second pressure port |
The mistake to avoid
Adding a fixed offset to make displays agree can corrupt calculations that require absolute pressure, especially where ambient conditions change or vacuum operation matters.
How to verify the fix
Verify reference and units through the sensor, PLC, HMI, alarm thresholds, and calculations. Record the accepted reference explicitly in tag names or descriptions.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
WIKA: Gauge and absolute pressure differences. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.