What Is a Process Calibrator? Types and Selection Criteria
A process calibrator is a handheld test device that measures and generates 4-20 mA, voltage, resistance and temperature sensor signals to test field instruments.
A process calibrator is a handheld test device that both measures and generates process signals such as 4-20 mA current loops, DC voltage, resistance and temperature sensors, and is used to test the accuracy of field devices. It is used in commissioning, periodic verification and troubleshooting of instruments such as transmitters, PLC analog input cards, indicators, recorders and valve positioners.
Types
| Type | What it does | Model in the catalog |
|---|---|---|
| Temperature calibrator | Tests temperature transmitters, indicators and controllers by generating or measuring a temperature sensor signal | UNI-T UT701 |
| Current and voltage (loop) calibrator | Generates and measures mA signals and measures DC voltage; tests 4-20 mA loops | UNI-T UT705 |
| Multifunction process calibrator | Combines several signal types in one device; temperature- or loop-focused types exist | UNI-T UT714 (temperature), UT715 (loop), UT725 |
Compare which signals each model generates and measures in the specification list on the product page.
Source and simulation modes of the current output
- Source: The calibrator generates the loop current itself and sends it to a receiver without its own supply (PLC analog input, indicator).
- Simulation (SIM): The loop is powered by an external supply; the calibrator takes the place of a two-wire transmitter and sets the loop current.
- Measure: The transmitter output or the loop current is read. Some models also supply loop voltage to power the transmitter while measuring; check this in the datasheet.
Selection criteria
- Signal types: The calibrator must be able to generate and measure the input and output signals of the devices to be tested: mA, V or mV, Ω, thermocouple type and RTD (e.g. Pt100).
- Accuracy: The calibrator should be clearly more accurate than the tolerance of the device under test; common practice is at least a fourfold margin. Accuracy is given as ±(percentage of reading + digits); some models in the catalog state ±(0.02% + 2) for DC voltage and current functions.
- Current mode: If the loop is externally powered, SIM mode is used; if not, source mode. If you will measure a transmitter in the field without its own supply, choose a model that provides loop power.
- Temperature signal: Cold junction compensation in thermocouple simulation and 2-, 3- or 4-wire connection support in RTD measurement directly affect the result.
- Field use: Step and ramp outputs speed up valve and indicator tests. The catalog has models powered by a 9 V (6F22) battery and by a rechargeable AA battery pack.
Common mistakes
- Using a multimeter instead of a calibrator: A multimeter reads loop current but does not generate a signal; the scaling and alarm points of an analog input cannot be tested with a known signal.
- Connecting source mode to a powered loop: The loop then has two sources and the reading becomes meaningless; use SIM mode on a powered loop.
- Using plain copper wire for a thermocouple connection: Without compensating cable matching the thermocouple type, the connection points add error to the measurement.
- Neglecting the calibrator's own calibration: For valid results the calibrator must have a current, traceable calibration certificate.
Related standards
- IEC 60584-1: Voltage-temperature tables and tolerances of thermocouple types.
- IEC 60751: Resistance-temperature relationship of industrial platinum resistance thermometers such as Pt100.
Models in the catalog
All 5 active products in the Process Calibrators category are Unit branded: UT701, UT705, UT714, UT715 and UT725. All can be ordered directly. For measurement-only work see Digital Multimeters; for thermometers and temperature meters see the Temperature and Heat Control Instruments category.
Frequently asked questions
What is the difference between a process calibrator and a multimeter?
A multimeter only measures current, voltage and resistance. A process calibrator measures these signals and also generates them at known values; transmitters and analog inputs are tested with this signal.
What is the difference between source and simulation (SIM) mode?
In source mode the calibrator generates the loop current from its own supply. In SIM mode an external supply powers the loop and the calibrator only sets the current, behaving like a two-wire transmitter.
Can a HART transmitter be adjusted with a process calibrator?
The calibrator shows the error by applying a known signal to the transmitter input and measuring its output. Changing parameters over HART requires a device that communicates via HART; this communication usually needs a 250 Ω resistor in the loop. Check the model's HART support on the product page.
How often should a process calibrator be calibrated?
The interval is set by the manufacturer's recommendation and your quality system; common practice is once a year.