Level Transmitter Scaling Calculator
Convert a 4–20 mA level transmitter signal into engineering units, or calculate the expected transmitter current from a measured level.
Level
How Level Transmitter Scaling Works
A typical 4–20 mA level transmitter represents a configured measurement range using 4 mA for the Lower Range Value (LRV) and 20 mA for the Upper Range Value (URV).
For a linear transmitter, the measured level can be calculated directly from the transmitter current.
Level = LRV + ((mA − 4) ÷ 16) × (URV − LRV)
The reverse calculation determines what current should be produced for a particular level:
mA = 4 + ((Level − LRV) ÷ (URV − LRV)) × 16
4–20 mA Level Reference
| Current | Percent of Span | Level |
|---|---|---|
| 4 mA | 0% | LRV |
| 8 mA | 25% | 25% of span |
| 12 mA | 50% | 50% of span |
| 16 mA | 75% | 75% of span |
| 20 mA | 100% | URV |
Example
Suppose a level transmitter is configured for:
- LRV = 0 ft
- URV = 20 ft
- Measured signal = 14 mA
The transmitter is 62.5% of the way through its 4–20 mA span.
Level = 0 + ((14 − 4) ÷ 16) × 20
Level = 12.5 ft
Therefore, a 14 mA signal corresponds to approximately 12.5 ft for this transmitter configuration.
Field Notes
Check the transmitter configuration
The LRV and URV used by the controller must match the transmitter's configured range. A transmitter can be physically measuring the correct level while the controller displays the wrong value if the scaling ranges do not match.
4 mA does not always mean an empty tank
The transmitter's LRV may represent a level other than zero. For example, a transmitter might be configured for 5–25 ft, making 4 mA equal to 5 ft and 20 mA equal to 25 ft.
Check the signal at the transmitter and controller
When troubleshooting a level indication problem, compare the current measured at the transmitter with the current arriving at the control system input. A difference can indicate wiring, termination, grounding, or other loop problems.
Level measurement technology matters
Different level technologies—including differential pressure, radar, ultrasonic, guided wave radar, and other sensors—can have different installation and configuration considerations. This calculator addresses the 4–20 mA scaling relationship rather than the underlying measurement technology.