In the field of industrial instrumentation, pressure transmitters often require adjustments to their measurement range, especially when installed in varying physical positions relative to the pressure tapping point. This process is known as zero migration, which includes zero elevation, zero suppression, and no migration. Understanding and properly adjusting for these conditions is essential to ensure accurate measurements in liquid level and pressure applications.
1. What Is Zero Migration in Pressure Transmitters?
Zero migration refers to the shift of the transmitter’s zero point due to installation position or external pressure influences, such as the pressure from a filled capillary line. If a transmitter’s zero reading is not zero when no pressure is applied, it indicates the presence of zero migration.
Zero Suppression (Negative Migration): Occurs when the transmitter is installed above the pressure tapping point.
Zero Elevation (Positive Migration): Happens when the transmitter is installed below the tapping point, or when there is positive pressure in the impulse line at zero process pressure.
No Migration: Applies when the transmitter and tapping point are at the same elevation.
2. Classification of Zero Migration
| Type of Migration | Condition | Application Example |
|---|---|---|
| Zero Elevation | Differential pressure > 0 (PT > PE) | Capillary lines or wet leg measurement with elevated tap |
| Zero Suppression | Differential pressure < 0 (PT < PE) | Transmitter installed above the tapping point |
| No Migration | Equal elevation (PT = PE) | Transmitter and tapping point at same level |
3. Typical Scenarios
A. No Migration Scenario
If the pressure tapping point and the transmitter are on the same horizontal level, no migration is required. The output corresponds accurately to the input pressure, starting at 4 mA when pressure is zero.
Example:
A differential pressure transmitter installed level with the pressure ports on a horizontal pipeline.
B. Zero Elevation Scenario (Positive Migration)
Occurs when the transmitter is below the tapping point or when capillaries filled with silicon oil exert pressure even at zero process pressure.
Explanation:
The filled capillaries or elevation difference causes the transmitter to sense a pressure at the zero liquid level. To compensate, the zero point must be shifted upward (elevated), so that a 4 mA signal corresponds to the actual 0-level condition.
Application:
Double-flange differential pressure transmitters measuring liquid levels from an upper tapping point.
C. Zero Suppression Scenario (Negative Migration)
Occurs when the transmitter is installed above the pressure tapping point. Due to gravity, the hydrostatic pressure acts in reverse, causing the transmitter to detect a negative pressure when the actual process pressure is zero.
Solution:
Adjust the zero downward (suppress it), so that 4 mA corresponds to the actual zero pressure condition in the process.
Application:
A liquid level transmitter installed higher than the bottom tapping point of a storage tank.
4. Example: Adjusting for Negative Migration
Let’s say we have a pressure transmitter originally calibrated for 0–40 kPa, and we now need it to measure from -10 to +10 kPa.
This requires:
A negative migration of 10 kPa
A reduction of the span from 40 kPa to 20 kPa
Adjustment Steps:
Initial Span Adjustment:
First, reconfigure the transmitter to measure a new range of 0–20 kPa.Power Down and Hardware Configuration:
If the migration is large (e.g., 10 kPa), disconnect power, remove the amplifier board, and move the shorting jumper to the negative migration position.Zero Adjustment:
Reconnect power, apply a pressure of -10 kPa, and adjust the zero potentiometer until the output current is 4 mA.Full Scale Validation:
Apply +10 kPa pressure and confirm the output is 20 mA. If deviation exists, fine-tune the span (gain) potentiometer.
5. Special Case: Remote Seal or Capillary Transmitters
For remote or capillary-equipped differential pressure transmitters (e.g., for tank level applications):
Always require zero suppression or elevation, depending on the fluid column in the capillary lines.
Even if the transmitter is installed between the flanges, zero migration still applies due to filled silicon oil or static head pressure.
6. Summary
Properly handling zero migration ensures that pressure transmitters provide accurate readings across diverse installation scenarios. Whether you’re measuring fluid levels in tanks, pipelines, or complex systems with capillaries, understanding the principle of zero elevation and suppression is vital for correct calibration and optimal process control.
If you’re interested in sourcing reliable and easy-to-calibrate pressure transmitters, Dalian Zero Instrument Technology Co., Ltd offers a range of solutions tailored to your installation needs.