Calibration & Metrology · Temperature Calibrators

Handheld Temperature Calibrator

Handheld temperature calibrator that measures and sources thermocouple and resistance signals, with cold-junction compensation, step or ramp output and a colour trend display.

OHR-C01 (3 models) ·

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Key specifications

Models
A process loop version, a temperature-only version, and a multifunction version covering both
Thermocouple types
S, B, E, K, R, J, T and N, each with its own range and accuracy
Resistance elements
Pt100 and Cu50
Resistance ranges
0 to 400 ohm, 0 to 1,000 ohm and 0 to 10 kilo-ohm
Accuracy
Stated as 0.05 percent of full scale on the measurement side for most types, with output accuracy stated separately and less tightly
Cold junction
Manual or automatic, with an external compensation input on the lower edge of the body
Output modes
Automatic step and ramp, as well as a steady value
Display
A colour screen able to show the measurement trend and the output trend at the same time
Charging
Type-C, from a phone charger
Body
48 mm wide at the upper section, 132.2 mm high and 27 mm thick, with a silicone keypad

A temperature instrument on a plant is rarely somewhere convenient. It is welded into a vessel, mounted in a duct or buried under lagging, and the only place the loop can be reached is at the terminal head. A calibrator earns its place by being small enough to take there and by speaking the language of the sensor once it arrives, so that the loop can be checked where it stands rather than dismantled and brought to a laboratory.

That is a different job from the one a bath does, and the two are frequently confused. A bath provides a real temperature; a signal calibrator provides the electrical equivalent of one, and the difference decides what each of them can and cannot prove.

Product overview

The range covers three handheld models. One is built for the process loop, with direct voltage and current measurement and output, a transmitter output and a loop supply. One is built for temperature alone, covering thermocouple types, resistance elements and resistance. The third carries both sets and adds frequency measurement and output.

They share a body, a colour screen, a rechargeable battery charged over Type-C and a set of five terminals distinguished by colour.

How the instrument produces and reads a temperature

For a thermocouple the instrument generates a small voltage corresponding, on the relevant reference table, to the chosen temperature. That correspondence is only complete once the temperature of the terminals is accounted for, because a thermocouple reports the difference between its measuring junction and its cold junction. The instrument therefore applies cold-junction compensation, either measuring the terminal temperature itself or accepting a reference from the external socket on the lower edge, and adds the equivalent voltage to what it sources.

For a resistance element the instrument simulates the element instead, producing the resistance that corresponds to the temperature. The same circuits work in reverse when a device is being read, so the instrument can both source a value and measure one, at accuracies that are stated separately and are not the same.

How to select the right model

Start with the instrument list rather than with the calibrator. If the work is temperature only, the temperature model covers it. If the work is a loop that has to be powered and read in current or voltage, the loop model is the one, and the temperature functions are not needed. Where both appear on the same list, the multifunction model saves carrying two instruments, at the cost of one instrument that has to be understood properly before it is used.

Then confirm the signal types actually installed. A calibrator covers a defined set of thermocouple letters and resistance elements, and a sensor outside that set cannot be checked with it at all, however accurate the instrument is on the types it does cover.

Measurement limitations and selection considerations

The most useful constraint in the material is procedural: measurement and sourcing are advised against at the same time, particularly when resistance is being produced, because the load shifts the value being sourced. A loop check is therefore a sequence rather than a simultaneous operation.

Two things the page does not claim. It makes no statement about ingress protection, which the material does not give for an instrument intended for plant use. And it does not treat the nominal accuracy figure as the accuracy on every range: the measurement and output tables differ, type by type, and the page states both rather than the more flattering one.

Features

  • Temperature measurement and signal output in one handheld instrument, so a loop can be checked from the sensor terminals through to the reading
  • Cold-junction compensation on the measurement and the output side, without which no thermocouple figure is complete
  • A resistance output that simulates Pt100 and Cu50, so a resistance thermometer loop can be checked without a real temperature
  • Automatic step and ramp output for routine work, instead of setting each value by hand
  • A colour screen showing the measurement trend and the output trend together, which makes a drifting loop visible rather than only a number
  • A rechargeable battery charged over Type-C, with the compensation socket and the charging socket separated so both can be used at once

Technical specifications

Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.

Temperature and resistance signals
Thermocouple types and rangesS and R from minus 50.0 to 1768.0 degrees Celsius, B from 400 to 1820, E, K and N from minus 270.0 upwards, J from minus 210.0 to 1200.0 and T from minus 270.0 to 400.0Standard
Thermocouple measurement accuracy0.05 percent for all types except T, which is stated as 0.1 percent, at an ambient of 23 degrees Celsius plus or minus 2Standard
Thermocouple output accuracyStated per type and looser than the measurement figure, from 0.05 percent on type E to 0.3 percent on type N, again at 23 degrees Celsius plus or minus 2Standard
Temperature coefficient25 ppm per degree Celsius on most types and 50 ppm on R and NStandard
Input impedance1.8 megohm on the measurement sideStandard
Resistance elementsPt100 from minus 200.0 to 850.0 degrees Celsius at 0.05 percent on measurement, and Cu50 from minus 50.0 to 150.0 at 0.2 percentStandard
Resistance ranges0 to 400 ohm at 0.01 ohm, 0 to 1,000 ohm at 1 ohm and 0 to 10 kilo-ohm, all on measurementStandard
Resistance output0 to 400 ohm onlyStandard
Accuracy outside the stated ambientGiven as a temperature coefficient only; no worked figure at other ambient temperatures is suppliedInformation required
Process signals by model
Direct voltage measurement110 mV, 1 V and 30 V ranges; on the loop version and the multifunction version, not on the temperature versionStandard
Direct voltage output110 mV, 12 V and 24 V; on the loop version and the multifunction versionStandard
Direct current measurement20 mA, 100 mA and 1 A; on the loop version and the multifunction versionStandard
Direct current output24 mA, with a passive transmitter output at the same figure and a 24 V loop supplyStandard
Frequency measurement and outputMeasurement to 50 kHz and output at 1 kHz and 10 kHz; on the multifunction version onlyStandard
Temperature version scopeThermocouple, resistance element and resistance measurement and output only; no process current or voltage, and no frequencyStandard
Reading accuracy on the process rangesNot stated separately in the materialInformation required
Body, power and connections
Dimensions48 mm at the upper body width, 64 mm across the screen area with its frame, 77.8 mm at the lower body width, 132.2 mm high and 27 mm thickStandard
TerminalsRed and green for measurement, yellow and blue for output, black for the common, with legends for the measurement positive, the common and the output positiveStandard
Charging and compensation inputsA Type-C charging socket and an external cold-junction compensation socket, both on the lower edgeStandard
PowerAn internal rechargeable battery, chargeable from a phone charger over Type-CStandard
Power savingA backlight power-saving modeStandard
Weight and battery lifeNot stated in the materialInformation required
Ingress protectionNot stated in the materialInformation required

Available models

Selection table
Model Configuration Key ranges Availability
OHR-C01 Process loop calibrator: direct voltage and current measurement and output, a passive transmitter output at 24 mA and a 24 V loop supply, without temperature functions Voltage measurement at 110 mV, 1 V and 30 V; voltage output at 110 mV, 12 V and 24 V; current measurement at 20 mA, 100 mA and 1 A; current output at 24 mA The form for a loop that has to be checked and powered, where the sensor signal is not part of the work
OHR-C02 Temperature calibrator: thermocouple, resistance element and resistance measurement and output only Thermocouple types S, B, E, K, R, J, T and N; Pt100 and Cu50; resistance 0 to 400, 0 to 1,000 ohm and 0 to 10 kilo-ohm; resistance output to 400 ohm The form for temperature work alone, where no process current, voltage or frequency is needed
OHR-C03 Multifunction calibrator carrying both the process signal set and the temperature set, with frequency measurement and output added Everything in the two versions above, plus frequency measurement to 50 kHz and output at 1 kHz and 10 kHz The form for a mixed instrument list, at the cost of one instrument that has to be understood before it is used

Customization

  • The model, which decides whether process signals, temperature signals or both are covered
  • The thermocouple type and the resistance element selected for the duty, entered at the keypad rather than ordered as a variant
  • The output mode, steady, stepping or ramping
  • The cold-junction method, manual or automatic, and whether the external compensation input is used
  • The language of the interface, which can be switched between English and Chinese on the instrument

Certification and compliance

Certification documents not yet supplied

Certifications are only published once a certificate copy can be produced, so this section is deliberately empty rather than populated with assumed approvals. Tell us which approvals your application requires and we will confirm what is available for this instrument.

Parameters still to be confirmed

The following are required for a complete specification but are not stated in the documentation supplied to us. They are listed rather than estimated:

  • The official model designation, where the codes in the material carry a prefix that belongs to a third-party brand rather than to the supplier
  • The reading accuracy on the process current and voltage ranges, which is not stated separately
  • Weight and battery life, neither of which is given
  • The ingress protection of the body, which is not stated for an instrument intended for plant use
  • The temperature coefficient to apply when the ambient is outside 23 degrees Celsius plus or minus 2, and a worked figure at a stated ambient
  • Whether a traceable calibration certificate can be issued for the instrument, and to which standard
  • The resistance output accuracy, which is not given on the same basis as the resistance measurement figures
  • Whether the English interface is available on every model or only on some
  • The part number and length of the standard test leads, and whether the external compensation probe is included
  • Whether the reported output accuracy figures are quoted against full scale or against the reading

Frequently asked questions

Does a signal calibrator check the sensor itself?

No, and the distinction matters when a tolerance is in question. The instrument generates the millivolts or the resistance that a correct sensor would produce at a stated temperature, so it verifies the wiring, the transmitter and the indicating instrument. It cannot say whether the sensor still has the characteristic it was made with, nor whether it is installed where it can measure the process. Confirming the sensor needs a real temperature - a dry block or a bath - and that is a separate exercise with separate equipment.

Why do the measurement and output accuracies differ?

Because they are different circuits. The measurement side presents a high impedance across the source and its accuracy depends on that input stage; the output side drives a load and its accuracy depends on the output stage and the current it has to deliver. On this instrument the two are stated separately and the output figures are looser, from 0.05 percent on one thermocouple type to 0.3 percent on another. Buying on the headline figure alone would therefore misstate what the instrument can do as a source.

Can measurement and output be used at the same time?

The material advises against it, and the reason is electrical rather than procedural. Sourcing a resistance while measuring at the same time loads the output and shifts the value being presented, so the reading and the output are both compromised. Where a transmitter has to be checked end to end, the work is done in the sequence the loop imposes: source the sensor signal, read the output current, rather than attempting both at one instant.

How is the cold junction handled in the field?

Either internally or with an external compensation input on the lower edge of the body. Internal compensation measures the terminal temperature and applies the equivalent voltage automatically, which is adequate when the instrument has been at the site temperature long enough to be stable. The external input takes a reference junction instead, which is the arrangement used where the compensation has to be defined rather than measured. The material also notes that a large change in ambient temperature should be allowed to settle before either method is relied on.

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