Calculation & Calibration of Pressure Transmitter (4–20 mA Loop)
Important Definitions
1. URL (Upper Range Limit)
The highest value of the calibrated measurement range that the transmitter can measure.
2. LRL (Lower Range Limit)
The lowest value of the calibrated measurement range that the transmitter can measure.
3. LRV (Lower Range Value)
The process value corresponding to the LRL, representing 0% of span.
4. URV (Upper Range Value)
The process value corresponding to the URL, representing 100% of span.
Example – Pressure Transmitter (0–20 psi)
| Parameter | Value |
|---|---|
| LRL | 0 psi |
| LRV | 0 psi |
| URL | 20 psi |
| URV | 20 psi |
Span = URL − LRL = 20 − 0 = 20 psi
Pressure Calculation When 4–20 mA is Given
Given:
-
Current range = 4–20 mA
-
Pressure range = 0–20 bar
Calculate the Span
-
Pressure span = 20 − 0 = 20 bar
-
Current span = 20 − 4 = 16 mA
Scaling Factor
Convert Current → Pressure
Formula
Examples
| Current (mA) | Pressure (bar) |
|---|---|
| 4 mA | (4–4)×1.25 = 0 bar |
| 8 mA | (8–4)×1.25 = 5 bar |
| 12 mA | (12–4)×1.25 = 10 bar |
| 16 mA | (16–4)×1.25 = 15 bar |
| 20 mA | (20–4)×1.25 = 20 bar |
Current Calculation When Pressure Range is Given (0–20 bar)
Given
-
Pressure range = 0–20 bar
-
Current range = 4–20 mA
Determine the Scaling Factor
Convert Pressure → Current
Formula
Examples
| Pressure (bar) | Current (mA) |
|---|---|
| 0 bar | 4 mA |
| 5 bar | 8 mA |
| 10 bar | 12 mA |
| 15 bar | 16 mA |
| 20 bar | 20 mA |
Note on Negative LRL
When the LRL is negative, always remember to subtract correctly:
Example:
-
LRL = −500 bar
-
URL = +500 bar
Total Span=URL−LRL=+500−(−500)=1000 bar
Negative ranges are very common in differential pressure measurement (vacuum to positive pressure).