What a temperature calibrator is
A temperature calibrator produces and measures the electrical signals that stand for temperature, so that a thermocouple, a resistance thermometer or a temperature transmitter can be checked against a known value. It is the reference in the comparison rather than the instrument under test, and it works in the language of the sensor: millivolts for a thermocouple, ohms for a resistance element, and a current or voltage loop where a transmitter is being checked.
The point of the instrument is that it can be carried to the device rather than the device brought to a bath. A plant has temperature instruments welded into vessels, mounted in ducts and buried in machinery, and a calibrator that connects at the terminal head checks them where they stand. That is why the handheld signal calibrator exists alongside the laboratory bath, and why the two are not substitutes for one another.
What decides the choice is the list of signals the work demands. A calibrator built for temperature covers a defined set of thermocouple types and resistance elements; one built around the process loop covers current and voltage instead, and some instruments do both. The list of types, and the accuracy with which each is produced, settle the answer long before the case, the screen or the battery do.
How a temperature calibrator measures
For a thermocouple the calibrator generates a small voltage that corresponds, on the relevant reference table, to a stated temperature. That correspondence is complete only once the temperature of the calibrator terminals is accounted for, because a thermocouple reports the difference between its measuring junction and its cold junction. This is why a temperature calibrator has a cold-junction compensation function: it measures or is told the terminal temperature and adds the equivalent voltage, so that the value it sources represents the temperature at the far end.
For a resistance element the calibrator produces a resistance corresponding to the temperature, which means simulating the element rather than supplying a voltage. The value has to be presented in a form the instrument reading it accepts, and the resistance ranges and resolution of a calibrator decide whether it can simulate a Pt100 at the low end of the scale as well as a Cu50 in the middle of its own.
Measuring and sourcing are separate functions and are normally kept apart. Reading a signal and generating one at the same time, particularly when resistance is being produced, loads the source and shifts the value it presents, so a calibrator is used for one duty or the other. Where a transmitter is being checked end to end, the calibrator sources the sensor signal and a second reading of the output current completes the loop.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy
class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Signal families covered | Thermocouple types, resistance elements, or both, with or without process current and voltage | Whether the instruments on the list can be checked at all, since a calibrator cannot produce a type it does not cover. |
| Thermocouple types | A defined set drawn from the common letters such as K, J, T, E, N, R, S and B | Whether the sensors actually installed are among them, which for specialised or high-temperature types is the first thing to establish. |
| Resistance element types | Pt100 and Cu50, with the resistance ranges that go with them | Whether the resistance thermometers on site can be simulated over the part of the scale they occupy. |
| Cold-junction compensation | Internal measurement of the terminal temperature, or an external reference junction | The validity of every thermocouple value produced, since an uncompensated source represents the junction difference rather than the temperature. |
| Accuracy statement and its form | A figure quoted against full scale or against the reading, stated separately for measurement and for output | The uncertainty at the point being checked, which is why the two forms have to be read rather than compared as single numbers. |
| Sourcing modes | A steady value, an automatic step, or a ramp | How much of a routine is done by hand and how much by the instrument, which on a large instrument list is the difference between a morning and a week. |
| Display and record | A single reading, or a trend of the measurement and the output against time | Whether the evidence of the check is produced automatically or assembled by hand afterwards. |
How to choose a temperature calibrator
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| The sensors to be checked | The calibrator has to cover every type on the list, not most of them | List the thermocouple letters and resistance elements actually installed. |
| The accuracy required | It follows from the tolerance on the device under test and the ratio the quality system demands | Give the tolerance, and state the ratio required between it and the calibrator. |
| Whether a transmitter is in the chain | Checking a transmitter means sourcing the sensor signal and reading the output | State whether the work ends at the sensor, at the transmitter, or at the loop. |
| Cold junction | Thermocouple values are valid only if the reference junction is handled | State whether an external reference junction is used on site. |
| Where the work is done | A bench and a plant location impose different demands on size, power and durability | State whether calibration is a bench activity or is performed in the field. |
| Documentation | A traceable certificate for the calibrator, and a record for each device checked | State what has to be issued, and to what standard. |
| Whether a true temperature is needed | A signal calibrator checks the electrical chain, not the physical sensor and its installation | State whether the sensor itself has to be checked against a real temperature. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a temperature calibrator usually turns out to be, and the
point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a temperature calibrator |
| Pressure calibrators | Generate and measure pressure as the reference quantity. Temperature and pressure are different physical problems and neither instrument replaces the other, although the electrical half of a transmitter check is common to both. |
| Multifunction calibrators | Cover temperature, pressure and electrical signals in one instrument, which suits a team working through a mixed instrument list. The trade is usually a less specialised temperature specification at a given cost. |
| Electrical calibrators | Produce and measure electrical quantities without temperature tables or cold-junction handling. Where the work is a loop check rather than a sensor check, the electrical calibrator is the instrument and the temperature functions are not needed. |
| Temperature calibration equipment | Provides a real, controlled temperature - a dry block or a bath - into which the sensor itself is placed. That checks the whole chain including the sensor and its installation, which a signal calibrator cannot; the two are used together rather than instead of one another. |
| Reference standards | Carry the traceable reference from which a calibrator is itself checked. They are the top of the chain rather than the working instrument, and a calibrator is only as good as the standard it is compared against. |
The commonest error here is buying a temperature calibrator for a job that is really a loop check. Where the sensor and its installation are in question, a signal calibrator will pass a chain that a real temperature would fail.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry
carries these. Where a figure is not known, send the duty description and we will
confirm what it implies.
- The thermocouple types and resistance elements to be covered
- The tolerance on the devices under test, and the accuracy ratio required
- Whether transmitters have to be checked, and whether the loop has to be powered
- Whether an external reference junction is used
- Whether the work is on a bench or in the field
- Whether stepping or ramping output is required for routine work
- What certificates or records have to be issued
- Quantity, destination port and required delivery date
