Pressure Transmitter Scaling Calculator
Convert a 4–20 mA pressure transmitter signal into a pressure value using the transmitter's calibrated range.
Calculate Pressure
How Pressure Transmitter Scaling Works
A typical pressure transmitter converts a measured pressure range into a 4–20 mA signal. The transmitter's Lower Range Value (LRV) corresponds to 4 mA, while the Upper Range Value (URV) corresponds to 20 mA.
The receiving PLC, controller, or meter uses the same range information to convert the measured current back into pressure.
What the Variables Mean
- LRV: Pressure represented by 4 mA
- URV: Pressure represented by 20 mA
- mA: Measured transmitter output current
- 16: The 16 mA span between 4 and 20 mA
4–20 mA Pressure Reference Points
| Current | % of Span | Example: 0–100 PSI |
|---|---|---|
| 4 mA | 0% | 0 PSI |
| 8 mA | 25% | 25 PSI |
| 12 mA | 50% | 50 PSI |
| 16 mA | 75% | 75 PSI |
| 20 mA | 100% | 100 PSI |
Worked Example
Suppose a pressure transmitter is configured for:
- LRV = 0 PSI
- URV = 100 PSI
- Measured signal = 12 mA
First determine how far the signal is through its 4–20 mA span:
The signal is therefore at 50% of the transmitter's range.
Result: 12 mA represents 50 PSI.
Gauge vs. Absolute Pressure
Pressure transmitters can be configured to measure different pressure references. This matters when interpreting the value reported by the transmitter.
- Gauge pressure: Measures pressure relative to atmospheric pressure.
- Absolute pressure: Measures pressure relative to a vacuum reference.
- Vacuum / negative gauge: Represents pressure below atmospheric pressure using negative gauge values.
Understanding LRV and URV
LRV and URV are sometimes confused with the physical limits of the transmitter. They are actually the configured measurement range.
For example, a transmitter capable of measuring a much larger pressure range could be configured for:
URV = 50 PSI
In that case:
- 4 mA = 10 PSI
- 12 mA = 30 PSI
- 20 mA = 50 PSI
The PLC must use the configured range, not the transmitter's maximum possible pressure rating.
Field Application
This calculation is commonly used when commissioning or troubleshooting pressure transmitters connected to PLCs, DCS systems, building automation systems, and other control systems.
A useful troubleshooting sequence is:
- Verify the transmitter's configured LRV and URV.
- Measure the actual loop current.
- Calculate the expected pressure from the current.
- Compare the calculated value with the controller display.
- Check whether the PLC scaling uses the same range.
- Verify the pressure reference: gauge or absolute.
Troubleshooting
Current is correct but the PLC pressure is wrong
If the measured loop current is correct but the displayed pressure is incorrect, the problem may be in the controller's scaling. Verify the PLC's analog input configuration and engineering-unit range.
4 mA does not equal the expected pressure
Check the transmitter's configured LRV. Do not assume that 4 mA always represents zero pressure.
20 mA does not equal the expected pressure
Check the transmitter's configured URV and verify that the PLC uses the same upper range value.
Pressure is consistently offset
Check transmitter zero calibration, impulse lines, installation conditions, and whether the expected pressure reference matches the transmitter configuration.
The transmitter reads correctly but the PLC does not
Measure the loop current directly. If the current corresponds to the expected pressure but the PLC display does not, investigate analog input configuration, raw-count scaling, engineering-unit conversion, and signal wiring.
Common Mistakes
- Using the transmitter's maximum range instead of its configured URV.
- Assuming 4 mA always means zero pressure.
- Using gauge pressure values with an absolute-pressure transmitter.
- Using the wrong pressure unit in the PLC.
- Forgetting that a negative LRV is valid.
- Scaling the PLC with different LRV/URV values than the transmitter.
- Diagnosing a PLC scaling problem without first measuring the loop current.