Published on August 22, 2026 at 6:26 AM
In almost every plant, measurement values travel through the cables as current, not voltage — and the range starts at 4 mA rather than 0. Both have solid reasons, and both explain why a process calibrator is such a useful tool.
Current instead of voltage, because current is the same everywhere in a series loop. A voltage signal loses amplitude over a hundred metres of cable; a current signal does not — the measured value arrives at the control system exactly as the transmitter sent it.
And 4 mA instead of 0 mA, because otherwise you couldn’t tell two completely different states apart: „the measured value is zero“ and „the line is broken“. With a live zero, 4 mA marks the start of the measuring range — and 0 mA means fault. This distinction is the reason the standard looks the way it does.
What it is
PRC30 is a multifunction process calibrator that both generates and measures signals:
- 0 to 24 mA (corresponding to −25 to 125 % of range) for reading and sourcing
- 24 V DC loop power for 2-wire current loops
- 0 to 20 V DC as a voltage source
- Eight thermocouple types — J, K, T, E, C, R, S and N — with cold junction compensation, displayed either as millivolts or as temperature according to the thermocouple table
- Up to five freely adjustable presets, zero adjustment and automatic zeroing when switching modes
It also has standard banana jacks and mini thermocouple connectors, a large backlit dot-matrix display, battery operation with an external power adapter for continuous use, and a hard-shell case with test leads, alligator clips and cables.
What problem it solves
The key point is that the device handles both directions — so a chain can be broken open at any point you choose.
As a source, it replaces the sensor: you disconnect the transmitter, feed in exactly 4, 8, 12, 16 and 20 mA instead, and check what the controller, display and control system make of it. If the display doesn’t show 50 % at 12 mA, the fault lies downstream of the sensor — regardless of whether the process is currently running.
As a meter, it replaces the control system: you connect it to the transmitter’s output and read what it is actually sending. If that matches the physical condition, the sensor is fine.
Together, the two operating modes answer the question that comes first in every process fault and usually stays open the longest: Is it the sensor, the wiring or the evaluation? Without a calibrator, people guess and swap parts; with one, the chain is split at two points and the answer is there in minutes.
The 24 V loop power is the function you only miss when it isn’t there. A 2-wire transmitter draws its energy from the same loop over which it sends its signal — without power, it is dead. With its own source, you can test it on the bench or in the field without using the plant supply or having to put anything back into operation.
The thermocouple simulation covers the other major signal family. The same logic applies as with the current signal — and the same limitation: what is simulated is the signal, not the sensor. A controller that displays the simulated value correctly has been verified; the test says nothing about the condition of the thermocouple in the plant.
Three typical applications
- Commissioning: Check loops before the process starts up — run through the signal path, scaling and limit values once.
- Troubleshooting: Narrow down faults between sensor, wiring and evaluation without swapping parts on suspicion.
- Recurring checks: Verify and document controllers, displays and recorders against known input values.
Why it pays off
The calculation is one of time. A process fault with an unclear cause means downtime — and the usual approach without a calibrator is to replace the transmitter and hope. If the fault is actually a loose terminal or incorrect scaling in the controller, you’ve replaced a working part and end up right where you started.
Then there is the value of documentation. A loop that has been tested and documented with known input values is a verifiable state — if a deviation appears later, you know since when it no longer holds. Without these check points, the uncertainty always reaches back to commissioning.
