In field maintenance, engineers often encounter this situation:
The zero point cannot be adjusted back to 4 mA
The full-scale output cannot reach 20 mA
Mid-range readings show serious deviation
Many technicians keep trying to adjust zero and span repeatedly.
However, once the deviation exceeds the transmitter’s internal adjustment range, calibration will no longer solve the problem. At that point, the issue is usually hardware-related.
So how much deviation is considered “beyond correction”?
Let’s analyze it from an engineering perspective.
1. Zero Deviation Beyond ±5% to ±10% of Full Scale
Zero output is the signal when no pressure (or lower range limit) is applied.
For a standard 4–20 mA transmitter, the zero output should be 4 mA.
Example:
For a 0–1 MPa pressure transmitter:
Expected zero output: 4 mA
Measured output: 2 mA or 6 mA
The deviation is ±2 mA.
Since the full-scale current span is 16 mA (20–4 mA), this represents ±12.5% of span.
Most transmitters only allow zero adjustment within ±5% to ±10% FS.
If the deviation exceeds this range, zero trimming will not bring the signal back to 4 mA.
Common Causes:
Permanent diaphragm deformation due to overpressure
Strain gauge aging
Excessive electronic drift
Internal sensor damage
In such cases, the transmitter hardware is compromised.
2. Span (Full-Scale) Deviation Beyond ±10% to ±20% of FS
Span refers to the output signal at upper range pressure (normally 20 mA).
Example:
For a 0–10 MPa transmitter:
Expected full-scale output: 20 mA
Actual output: 15 mA
The deviation is –5 mA, which equals ±31.25% of the 16 mA span.
This far exceeds the typical span adjustment range (±10% to ±20% FS).
Span calibration will not restore accuracy.
Common Causes:
Severe overpressure damage
Diaphragm rupture
Amplifier gain failure
Incorrect range configuration
When span deviation is excessive, replacing the sensor module or the entire transmitter is usually required.
3. Severe Non-Linearity (Mid-Range Deviation ±3% to ±5% FS or More)
Even if zero and full scale can be adjusted, significant mid-range deviation indicates a deeper issue.
Example:
For a 0–1 MPa transmitter:
0 MPa → 4 mA (correct after calibration)
1 MPa → 20 mA (correct after calibration)
0.5 MPa → Expected 12 mA
Actual output: 10 mA or 14 mA
The deviation is ±2 mA (±12.5% of span).
This indicates severe non-linearity.
Typical Causes:
Damaged sensing element
Capacitive diaphragm failure
Temperature compensation circuit malfunction
Internal mechanical stress
Non-linearity cannot be corrected by zero or span adjustment.
It is a hardware fault.
4. Key Principle: Always Check the Adjustment Range in the Datasheet
Every manufacturer specifies:
Zero adjustment range (e.g., ±5% FS)
Span adjustment range (e.g., ±15% FS)
If the measured deviation exceeds these limits, calibration is ineffective.
This indicates irreversible damage to:
The sensing diaphragm
The strain element
The signal conditioning circuit
At that point, replacement is required.
5. Practical Field Diagnosis Procedure
Before concluding that the transmitter is defective, follow this quick checklist:
Verify stable 24 VDC power supply
Check loop resistance and wiring integrity
Confirm no external pressure leakage
Perform 3-point calibration (0%, 50%, 100%)
Evaluate linearity
If deviation remains beyond adjustment limits, hardware replacement is necessary.
Conclusion
A pressure transmitter cannot be corrected by zero and span adjustment when:
Zero deviation exceeds ±5% to ±10% FS
Span deviation exceeds ±10% to ±20% FS
Severe non-linearity appears in mid-range
In such cases, calibration will not solve the problem because the core sensing element is damaged.
Understanding these limits helps engineers avoid unnecessary recalibration attempts and make faster maintenance decisions.