Common Causes of Zero Drift and Practical Solutions
Anyone working in instrumentation has probably encountered this problem:
Low differential pressure transmitters and micro-pressure transmitters are among the most unstable instruments in industrial sites when it comes to zero drift.
This is especially common in applications such as:
- Furnace draft pressure
- Air duct micro-pressure
- Clean room differential pressure
- Dual remote seal low-level measurement
Once ambient temperature changes, direct sunlight appears, or strong winds occur, the transmitter reading starts drifting.
You re-zero it today, and tomorrow it drifts again.
Many technicians blame the transmitter itself, but in reality, zero drift is usually caused by a combination of installation, environment, process conditions, and instrument limitations.
This article summarizes the most common root causes of zero drift in low-range differential pressure transmitters, along with practical field-proven solutions.
1. Temperature Effects — The Most Common and Hidden Cause
Temperature variation is the number one reason for zero drift.
Common sources include:
- Day/night temperature differences
- Seasonal temperature changes
- Direct sunlight on one side
- Furnace radiation heat
- Excessive heat tracing
These factors cause thermal expansion and contraction of:
- Sensor diaphragms
- Fill fluid
- Capillary systems
- Internal electronic circuits
As a result, the transmitter output slowly shifts.
Dual remote seal systems are particularly sensitive.
A common field symptom is:
- Level appears higher during hot afternoons
- Reading drops again at night
This is typically caused by uneven capillary temperature distribution.
Practical Solutions
- Install sunshades or insulation covers
- Keep both capillaries under identical thermal conditions
- Avoid “one side exposed, one side shaded”
- Do not place heat tracing too close to the transmitter body
- Select transmitters designed specifically for micro-pressure applications with advanced temperature compensation
2. Wind and Atmospheric Disturbance — “Stable When Calm, Drifting When Windy”
Low pressure ranges are extremely sensitive to atmospheric disturbance.
Strong wind blowing directly across:
- Vent ports
- Impulse pipe openings
- Atmospheric reference ports
can create artificial pressure differences.
Typical symptoms include:
- Reading fluctuates only on windy days
- Pressure changes when doors open or close
- Values jump during fan startup/shutdown
In furnace draft systems, wind direction alone can significantly affect readings.
Practical Solutions
- Install wind protection covers for outdoor transmitters
- Route vent ports downward with elbow protection
- Avoid placing micro-pressure tapping points near:
- Doors
- Ventilation outlets
- Fan discharge areas
- Minimize direct airflow impact on the sensing diaphragm
3. Mechanical Installation Stress — Poor Installation Creates Permanent Drift
Mechanical stress is another major but often overlooked cause.
Problems include:
- Twisted mounting
- Uneven bolt tightening
- Rigid impulse piping
- Pipe vibration
- Structural resonance
All these forces can transfer directly to the sensing diaphragm.
Even if the transmitter is re-zeroed, the reading may drift back over time.
Practical Solutions
- Install the transmitter level and stress-free
- Tighten bolts evenly in diagonal sequence
- Leave expansion allowance in impulse tubing
- Avoid rigid pipe pulling
- Use vibration dampers where necessary
4. Impulse Line Problems — Condensation, Blockage, and Asymmetry
Impulse piping issues are extremely common in low DP applications.
Typical problems include:
- Uneven condensate levels
- Partial blockage from dust or oil
- Different pipe lengths
- Different ambient temperatures on each side
These conditions create additional static pressure differences.
Practical Solutions
- Regularly drain condensate
- Keep liquid levels balanced on both sides
- Clean impulse lines periodically
- Route both high and low pressure lines together
- Maintain identical insulation and environmental exposure
5. Instrument Quality Limitations — Cheap Models Drift More
Not all transmitters are designed for micro-pressure stability.
Low-cost models often suffer from:
- Poor temperature compensation
- Low-grade silicon sensors
- Uneven fill fluid distribution
- Weak anti-interference capability
Micro-pressure measurement requires significantly higher sensor stability than standard pressure applications.
Practical Solutions
- Avoid ultra-low-cost transmitters for critical low-range applications
- Choose industrial-grade high-precision models
- Perform periodic calibration and zero verification
- Replace aging transmitters before severe drift develops
6. Electrical Noise and Moisture
Electrical interference can also cause unstable zero readings.
Common issues include:
- Unstable 24VDC power supply
- Signal cables routed with power cables
- Improper shield grounding
- Moisture ingress
- Terminal oxidation
Symptoms often appear as:
- Slow drifting
- Random jumping
- Intermittent instability
Practical Solutions
- Use stable isolated power supplies
- Separate signal cables from power cables
- Ground shield layers at one end only
- Improve waterproof sealing of junction boxes
- Inspect terminals regularly
When Can You Perform Online Zero Adjustment?
Not all instruments can be adjusted directly during operation.
Applications Usually Safe for Online Zeroing
- Furnace draft pressure
- Air duct pressure
- Room differential pressure
- General non-critical micro-pressure systems
Conditions:
- Process must be stable
- No major load fluctuation
- No environmental compliance data involved
Applications That Should NOT Be Zeroed Online
Never casually adjust zero during operation for:
- CEMS systems
- Environmental monitoring instruments
- Gas detection systems
- Safety interlock instruments
- Boiler drum level
- Main steam pressure
These instruments require proper isolation and standard calibration procedures.
Three Common Field Methods to Eliminate Zero Drift
Method 1 — Online Zero Adjustment During Stable Process
Most common field method.
Procedure:
- Ensure stable process conditions
- Keep both pressure sides connected normally
- Verify the actual process pressure should be zero
- Execute local zero calibration
- Confirm stable return to baseline
Advantages:
- No shutdown required
- No tubing removal
- Fast and practical
Method 2 — Atmospheric Equalization Calibration
Industry-standard method.
Procedure:
- Close isolation valves
- Open equalizing valve
- Vent both sides to atmosphere
- Remove all liquid column effects
- Perform zero calibration
- Restore process operation
This method eliminates:
- Mechanical stress
- Liquid head effects
- Installation-related offset
Method 3 — DCS Software Offset Compensation
Used only when field adjustment is impossible.
Procedure:
- Keep transmitter untouched
- Add PV offset inside DCS
- Compensate fixed drift temporarily
Important:
This is only a temporary workaround, not a root-cause solution.
Long-Term Solutions to Reduce Future Drift
The real goal is not repeated zero adjustment.
The goal is reducing future drift.
Recommended Practices
- Stress-free installation
- Symmetrical impulse piping
- Balanced condensate levels
- Proper insulation
- High-precision temperature-compensated transmitters
- Proper grounding and shielding
- Seasonal calibration routines
- Periodic preventive maintenance
Important Field Rules
Never forget these principles:
- Never casually zero safety or environmental instruments online
- Do not calibrate during strong wind or unstable process conditions
- Hardware calibration is always better than software compensation
- Low differential pressure instruments require more frequent verification
Final Thoughts
In many cases, a drifting low differential pressure transmitter is not actually “faulty.”
The real causes are usually:
- Temperature variation
- Wind disturbance
- Installation stress
- Impulse piping issues
- Moisture
- Electrical interference
Stable micro-pressure measurement depends not only on the transmitter itself, but also on proper installation and environmental control.
In real industrial applications:
70% of stability comes from installation and process conditions.
Only 30% comes from the instrument itself.