1. Basic Concepts of Shift
1.1 What is Shift?
Shift refers to adjusting the output characteristics of a pressure transmitter to adapt to specific measurement requirements under particular working conditions. Shift operations modify the transmitter's zero point (Zero) or span (Span) to ensure accurate pressure measurement even in complex environments.
1.2 Zero Shift
Zero shift occurs when the measurement reference point (zero point) of the transmitter deviates. For example:
Standard output: 0 bar → 4 mA
After zero shift: 2 bar → 4 mA (zero point shifted up by 2 bar)
Common causes:
Static pressure effects due to installation position (e.g., liquid column pressure)
Sensor drift caused by temperature changes
Component aging after prolonged use
1.3 Span Shift
Span shift refers to changes in the measurement range (Span) of the transmitter, altering the slope of the output signal. For example:
Standard span: 0–10 bar → 4–20 mA
After span shift: 0–10 bar → 5–21 mA (slope changed)
Common causes:
Variations in sensor sensitivity
Structural deformation due to overpressure
Medium characteristics (e.g., corrosive fluids)
2. Classification and Principles of Shift Methods
2.1 No Shift
Definition: The transmitter operates according to standard characteristics without adjustment.
Output formula:
Applicable scenarios: Ideal conditions with no additional environmental influences.
2.2 Negative Shift
Definition: The zero point is adjusted downward, setting the measurement starting point below the standard zero.
Example:Standard: 0–10 bar → 4–20 mA
Negative shift: -5–5 bar → 4–20 mA (zero point lowered by 5 bar)
Mathematical expression:
Common causes:
Measuring vacuum or negative pressure environments
Liquid level measurement in sealed vessels (compensating for gas-phase pressure)
2.3 Positive Shift
Definition: The zero point is adjusted upward, setting the measurement starting point above the standard zero.
Example:Standard: 0–10 bar → 4–20 mA
Positive shift: 5–15 bar → 4–20 mA (zero point raised by 5 bar)
Mathematical expression:
Common causes:
Pressure measurement in high-pressure pipelines
Compensating for static pressure in liquid level measurement (e.g., tank bottom pressure)
3. Key Parameter Relationships and Calculations
3.1 Measurement Range vs. Span Range
3.2 Shift Value Calculation
Shift value (ΔP) calculation formula:
Example:
Original measurement range: 0–10 bar
New measurement range: 3–13 bar
Shift value ΔP = 3 bar (positive shift)
Span range remains 10 bar (13 bar - 3 bar)
