Every reading on a control room screen has already been converted at least twice before anyone sees it.
Here is how a physical quantity turns into a signal, and then into the number an operator actually reads.
Field Instrument Unit Conversion is the chain of steps that turns a physical measurement into a signal, and that signal back into a number an operator can trust.
This is not the scaling formula itself, that is already covered in our sensor scaling article, this piece focuses on the actual unit conversions engineers do at each layer in daily fieldwork.
The Three Layers of Field Instrument Unit Conversion
A single measurement passes through three distinct representations before it reaches a screen, and each one uses its own units.
The actual linear formula that converts between these layers, along with a full worked example, is covered in depth in our sensor scaling article, and the PLC specific raw count ranges for major brands are covered separately in our PLC analog scaling guide.
This article focuses on something different, the actual unit conversions engineers do every day at each of these three layers, pressure units, temperature units, and signal to engineering unit conversion specifically.
Converting Pressure Units for Transmitter Ranging
Pressure is the unit that trips up new engineers most often, since a single transmitter datasheet, a customer specification, and a calibration bench standard can each use a different pressure unit.
| From | To | Multiply By |
|---|---|---|
| psi | kPa | 6.895 |
| bar | kPa | 100 |
| kg/cm2 | kPa | 98.07 |
| inH2O | kPa | 0.2488 |
| atm | kPa | 101.325 |
A more detailed conversion table and worked examples specifically for atmosphere to kPa conversion are already covered in our atm to kPa pressure conversion article, worth checking whenever the source unit is atmospheres rather than psi or bar.
The safest field habit is to convert every pressure value to one common unit, usually kPa, before it goes anywhere near a range configuration screen, rather than converting mentally at the point of data entry.
Converting Temperature Units and Thermocouple Ranges
Temperature conversion looks simple, degrees Celsius to Fahrenheit, but a thermocouple adds a second layer since its millivolt output is not linear across its full range the way a pressure transmitter's signal usually is.
F = (C times 9 divided by 5) plus 32
Example: 150 degrees Celsius
F = (150 times 9 divided by 5) plus 32
F = 270 plus 32 = 302 degrees Fahrenheit
Different thermocouple types, J, K, T, and others, each have their own millivolt to temperature reference table, and the exact usable range varies by type, covered fully in types of thermocouples and ranges.
Converting a thermocouple's raw millivolt signal into an actual temperature value cannot use one simple multiplier the way pressure or level often can, since the relationship curves slightly across the sensor's range.
Converting a 4 to 20 mA Signal to Engineering Units
This is the conversion engineers do most often in the field, translating a live current reading on a multimeter directly into the process value it represents.
Example: Transmitter ranged 0 to 250 degrees Celsius (LRV = 0, URV = 250)
Measured signal = 14.2 mA
EV = ((14.2 minus 4) divided by 16) times 250, plus 0
EV = (10.2 divided by 16) times 250 = 159.4 degrees Celsius
LRV is the lower range value and URV is the upper range value, both set during calibration and both printed on the transmitter's configuration sheet, not something to guess from the process itself.
A dedicated calculator and additional worked examples for this exact conversion are available in 4 to 20 mA signal conversion calculator and 4 to 20 mA signal conversion, useful for checking a manual calculation against a second source.
Common Field Instrument Unit Conversion Mistakes
Most conversion errors in the field come from the same small set of habits, all of them avoidable once an engineer knows to check for them.
A live zero offset, and the zero and span errors that come from getting this wrong, are covered in full detail in offset error, zero point error and span error, worth reviewing whenever a converted reading looks consistently off by a small fixed amount.
Field Instrument Unit Conversion Questions Engineers Ask
Related Articles on This Site
- PLC Analog Scaling and Raw Count
- Atm to kPa Pressure Conversion
- Types of Thermocouples and Ranges
- 4 to 20 mA Signal Conversion Calculator
- Offset Error, Zero Point Error and Span Error
External References
What We Learn Today
- Field Instrument Unit Conversion happens across three layers, physical unit, signal unit, and digital raw count.
- Pressure, temperature, and 4 to 20 mA conversions each have their own formula and their own common mistakes.
- Most conversion errors come from an outdated range assumption, not from the math itself being wrong.