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National Measurement Institute

Temperature Capabilities

The unit of temperature is the Kelvin, symbol K, defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. It is impractical to perform calibrations directly in terms of thermodynamic temperature. Instead they are made in terms of the International Temperature Scale of 1990 (ITS-90) which was adopted internationally on 1 January 1990. The scale has been constructed in such a way that any temperature value expressed in terms of this scale is a close numerical approximation to the thermodynamic temperature. The International Temperature Scale is based on easily realised phase transitions of pure materials, called fixed points. Suitably defined interpolating thermometers and interpolation formulae establish the full range of the scale, extending from 0.65 K to the highest temperature practically measurable in terms of the Planck radiation law. Further details of the ITS-90 may be obtained from NMI.

NMI maintains a range of fixed points, the values of which are typically known to a few mK. These fixed points are used to calibrate NMI's interpolating thermometers which, together with the interpolating formulae specified in ITS-90, allow the calibration of clients' instruments.

Thermometers conforming to the requirements of the ITS-90 and to certain dimensional requirements stated below may be calibrated directly in terms of the ITS-90. Those, that are not suitable for this type of calibration are calibrated by intercomparison with appropriate working standards maintained by NMI.

For historical reasons, the Celsius scale is also used to express temperature, where the degree Celsius (°C) is equal in magnitude to the Kelvin (K). In this text temperatures above 273.15 K (i.e. 0 °C) are quoted in degrees Celsius, and those below in Kelvin, except where convenience dictates otherwise. Differences in temperature are usually quoted in K.

Calibration services are available for fixed point cells, resistance thermometers, thermocouples, radiation thermometers, liquid-in-glass thermometers, AC resistance bridges and calibrators. Click here:

Before consigning an instrument for calibration always consult us to discuss your requirements. For further information contact

Fixed Point Cells

Fixed point enclosures are used to define the temperature scale by realising the equilibrium state of triple points and the phase transition of certain substances which occur at highly reproducible temperatures. The triple point of a material is the temperature at which the solid, liquid and the vapour phase coexist in thermal equilibrium at the vapour pressure of the material. The freezing and the melting points of a material occur at constant temperatures at which a substance undergoes a phase transition from liquid to solid and from solid to liquid respectively.

NMI maintains primary and secondary fixed points used to define the ITS-90 temperature scale for temperature range 83.805 8 K to 960 °C, and thus is equipped to calibrate customer's fixed points for the same ITS-90 temperature range by direct intercomparison, with the following uncertainties:

Intercomparison with NMI
Ar triple point using SPRT 83.805 8 K 2 mK
Hg triple point using SPRT 234.315 6 K 0.3 mK
water triple point using SPRT 273.16 K 0.1 mK
Ga melting point using SPRT 29.764 6 °C 0.3 mK
In freezing point using SPRT 231.928 °C 0.8 mK
Sn freezing point using SPRT 419.527 °C 0.3 mK
Cd freezing point using SPRT 321.068 5 °C 0.4 mK
Zn freezing point using SPRT 419.527 °C 0.5 mK
Al freezing point using SPRT 660.323 °C 1.5 mK
Ag freezing point using SPRT 961.78 °C 7 mK

Resistance Thermometers

The term resistance thermometer covers a wide range of instruments. The sensitive element may be a pure metal such as platinum, an alloy or a semiconductor. The shape of the complete instrument and the nature of its sensitive element are dictated by its intended use and the temperature range and accuracy over which the instrument is to be used. Metal thermometers must be of the potential-lead type.

Standard Platinum Resistance Thermometers (SPRT)

Platinum resistance thermometers satisfying the requirements of the ITS-90 may be calibrated at the appropriate fixed-points of that scale in the range 83 K to 962 °C. To fit NMI's calibration equipment the element must be mounted and sealed in a sheath so that the distance from the tip of the element to the bottom of the head of the thermometer is at least 550 mm, and the diameter of the sheath should be less than 8 mm.

The results of the calibration are given as a table of constants for the ITS-90 equation for the appropriate temperature range.

The uncertainty of a calibration is of the order of a few mK over the entire range. Typical uncertainties are 0.1 mK at the triple point of water, 1 mK at the tin and zinc points, and 2 mK at the argon point.

In order to use platinum thermometers to their maximum accuracy it is necessary to determine the linearity of the measuring instrument. Advice on the specifications of most instruments or on the appropriate method of determining their linearity can be obtained from NMI.

Platinum resistance thermometers can be very susceptible to mechanical strain caused by vibration or mechanical shock. The most precise thermometers should be delivered to NMI by hand and returned in the same manner. If it is essential that the thermometer be tested in the 'as received' condition this should be made clear in the request for test. When no information is received with a thermometer, NMI will use its discretion on the treatment given. In such a case the thermometer is taken at least to its highest temperature of use several times before calibration.

Equipment and technique
Temperature range
From Ar triple point to Hg triple point 83.805 8 K to –38.834 4 °C 2 to 0.5 mK
Hg triple point to Zn freezing point –38.834 4 to 419.527 °C 0.5 mK
Zn freezing point to Al freezing point 419.527 to 660.323 °C 0.1 to 1 mK
Zn freezing point to Ag freezing point 419.527 to 961.78 °C 0.1 to 7 mK

Industrial Platinum Resistance Thermometers

Potential-lead-type platinum resistance thermometers designed for use as lower grade standards than those satisfying the requirements of the ITS-90 are calibrated by comparison with NMI's working standards.

Results of the calibration are given in the most suitable form for the particular thermometer. This may be in the form of a interpolation equation relating the resistance of the thermometer to temperature.

Resistance thermometers in which the resistive element is a metal other than platinum, is an alloy, or is a semiconductor, such as a thermistor, may be calibrated by arrangement. New types of thermometers and new types of associated measuring equipment are continually being developed. Potential clients are requested to contact NMI to see if the instruments are suitable for calibration.

Digital Thermometers

Digital thermometers combining sensor and display into an instrument can be calibrated by comparison with NMI's working standards.
Equipment and technique
Intercomparison with SPRT in flow Cryostat –200 to –80 °C 25 to 15 mK
Intercomparison with SPRT in alcohol stirred enclosure –80 to 0 °C 15 to 5 mK
Intercomparison with SPRT in oil and alcohol stirred liquid enclosures –40 to 250 °C 5 mK
Intercomparison with SPRT in stirred oil and salt liquid enclosures 40 to 540 °C 5 mK
Intercomparison with SPRT in a stirred salt enclosure and heat pipes 540 to 962 °C 7 to 20 mK


NMI calibrates thermocouples for applications that require the best available accuracy, as indicated in the table below. The table contains typical values of calibration uncertainty as issued by NMI for thermocouples of good quality. Thermocouples that do not need calibration to this accuracy will not be calibrated by NMI unless the test is warranted by special circumstances.

The thermocouple wires submitted for calibration should be in as-new condition and, in most cases, at least 1 m in length. Any rare-metal thermocouple that has been used prior to calibration is first scanned for thermoelectric inhomogeneity. If the scan shows the thermocouple to be unfit for calibration, a charge for the scan will be made, otherwise the cost of the scan is included in the appropriate fee for that calibration.

NMI may conduct tests by arrangement on thermocouples or associated equipment of unusual or special design or requiring special tests (e.g. determination of Seebeck coefficient or a calibration at the melting point of platinum, 1 769 °C).

Equipment and technique
Thermocouple type
Intercomparison with SPRT in flow cryostat and stirred liquid enclosures Base-metal
–200 to 200 °C
1 µV + 0.01%E
Intercomparison with SPRT in stirred liquid enclosures and heat pipe + Cu point Base-metal 0 to 1 100 °C 10 µV +0.14%E
Intercomparison with SPRT in intercomparison enclosures Rare-metal 0 to 1 100 °C 0.31 µV to 0.011% E
Intercomparison with SPRT in intercomparison enclosures Rare-metal 0 to 1 550 °C 0.31 µV to 0.014% E
Homogeneity test Rare-metal 450 °C (0.01%E) µV
Au/Pt thermocouple using ITS-90 fixed point​ Gold-platinum​ 0 to 960 °C​ 30 mK​

Radiation Thermometers

All objects emit thermal radiation. At low temperatures objects emit mainly infra-red radiation (radiant heat), whilst at higher temperatures, objects begin to glow (emitting visible light). The quantity of thermal radiation emitted increases with temperature, and the average wavelength decreases (from IR to NIR to visible to UV) as the temperature is increased.

Radiation thermometers, also known as pyrometers, measure the thermal radiation emitted by an object and calculate its temperature. Radiation thermometers are available for temperatures ranging from –50 to 3 000 °C. There are several main types of radiation thermometers:

  • IR usually 7 to 14 µm devices, typically covering –50 to 1 000 °C
  • narrow band visible or NIR devices, e.g. 650 nm devices, covering 900 to 3 000 °C
  • ratio radiation thermometers, using two measurement wavelengths in an attempt to overcome emissivity problems

The quantity of radiation emitted by a hot object depends on its emissivity (think of the fact that a shiny metal teapot stays hot longer than a painted teapot!). As a result, radiation thermometers will indicate different temperatures on objects with different emissivities, even when they are at the same temperature. Therefore, when calibrating radiation thermometers we need to have a source with a very well known emissivity. This is usually achieved using blackbody sources. These are long, deep cavities in a block of uniform temperature. At NMI we have blackbody sources covering the temperature range –40 to 2 900 °C, with emissivities as high as 99.9%. We calibrate clients radiation thermometers by measuring these sources with NMI standards and the clients device.

One common problem with many lower grade radiation thermometers, usually low cost 7 to 14 µm devices, is that they collect radiation from well outside the nominal target region specified by the manufacturer. This size-of-source effect results in an indicated temperature which depends on the size of the object being measured. We assess this effect and include it in the uncertainty of calibration. For devices with a very large size-of-source effect, a graph of the temperature vs target size will be included in the report. It will usually result in a greatly increased calibration uncertainty.

Equipment and technique Range Uncertainty
PRTs and stirred fluid bath blackbodies –40 to 250 °C 0.1 to 0.3 °C
Standard radiation thermometers and heat-pipe blackbodies 300 to 1 100 °C 0.5 to 2 °C
Standard radiation thermometers and graphite tube furnace 900 to 2 900 °C 2 to 4 °C

Radiation Sources

Blackbody sources are used for the calibration of radiation pyrometers. NMI calibrates these blackbody sources by measuring their spectral radiance temperature at wavelengths from 0.65 to 14 µm as a function of the temperature indicated by a contact thermometer embedded in the wall of the blackbody.

Equipment and technique Range Uncertainty
Measurement of radiance temperature using NMI filter radiometers –40 to 2 900 °C 1 to 3.5 °C

Strip Lamps

Equipment and technique Range Uncertainty
Vacuum 800 to 1 550 °C 0.1 °C
Gas filled 1 200 to 2 200 °C 2 °C

Liquid-in-glass Thermometers

NMI does not provide a routine service for the calibration of liquid-in-glass thermometers. Services for these instruments are provided by NATA-accredited calibration laboratories. Information regarding these instruments can be provided by NMI on request. However, in special circumstances, the calibration may be undertaken by NMI if prior arrangements are made.

Equipment and technique Range Uncertainty
Intercomparison with SPRTs in flow cryostat –200 to –50 °C 0.3 °C
Intercomparison with SPRTs in stirred liquid baths –50 to –10 °C 25 mK
Intercomparison with SPRTs in stirred liquid baths –10 to 50 °C 5 mK
Intercomparison with SPRTs in stirred liquid baths 50 to 100 °C 15 mK
Intercomparison with SPRTs in stirred liquid baths 100 to 250 °C 40 mK
Intercomparison with SPRTs in stirred liquid baths 250 to 300 °C 0.15 °C

AC Resistance Bridges

The instruments used to measure the resistance of a resistance thermometer are calibrated by determining their linearity because the resistance ratios not the absolute values are required in the calibration of resistance thermometry.

It is usually preferable that the instrument be calibrated by the user but, in special circumstances, the measurement may be undertaken by NMI if prior arrangements are made.

Equipment and technique
Linearity measurement using ratio transformers 16 to 400 Ω 0.1 ppm


Surface Temperature Probes/Calibrators

Equipment and technique Range Uncertainty
Measurement by comparison with NMI surface temperature blocks 20 to 300 °C 0.2 to 0.7 °C

Dry Block Calibrators

Dry block calibrators are calibrated using NMI reference thermometers. The dry block calibrator should be supplied with an insert. Measurement of the temperature and temperature profile are made using NMI reference thermometers. The uncertainty in the calibration of the device is dominated by the temperature uniformity of the well. Customers should be aware that manufactures' accuracy claims do not generally include well uniformity and are thus usually very optimistic.

Equipment and technique
Using NMI reference thermometers –200 to 660 °C device dependent


The water content of a gas may be expressed in various ways, the most common being the relative humidity and the dew point temperature. For water vapour in air, NMI has adopted the precise definitions of these quantities given in the Technical Regulations of the World Meteorological Organisation, Volume 1, Appendix D.

Psychrometers with Mercury-in-glass Thermometers

Any fully ventilated psychrometer is acceptable for calibration. The client should arrange to have the thermometers calibrated at a NATA-accredited laboratory and submit the certificate together with the instrument for calibration.

The covering for the wet element is replaced if necessary and the reassembled instrument is compared with a reference psychrometer under one set of conditions to test for any significant departure of the psychrometer coefficient from the accepted value. If the results of the psychrometer intercomparison are satisfactory, the report states this fact.

Electronic Psychrometers

Electronic psychrometers contain electric thermometers and usually incorporate a processor so that the relative humidity can be calculated from the dry and wet element temperatures and displayed. In order to calibrate these instruments, it must be possible to calibrate the thermometer elements while they are still attached to the electronic readout. Also, it must be possible to read both the dry and wet element temperatures.

If the relative humidity is displayed, it is usually possible to provide a table of corrections to be applied to the displayed relative humidity readings.

An electronic psychrometer is calibrated in a similar way to an Assman psychrometer, involving a full calibration in which the thermometers are checked and the psychrometer coefficient is calculated. The thermometer elements should then be checked yearly and a full calibration repeated if the thermometers correction drifts.

Dew Point and Frost Point Hygrometers

For a dew or frost point hygrometer, the inbuilt thermometer is not removed and the instrument as a whole is calibrated using a humidity generator. Standard dew or frost point calibrations are within the range of –30°C to 80 °C. An extended frost point range down to –70 °C is possible.

Relative Humidity/Temperature Meters

NMI accepts a range of relative humidity/temperature meters for calibration. The relative humidity probe is compared against a humidity generator which passes a sample of moving air through a test vessel, into which the relative humidity probe is inserted. The standard range for relative humidity calibrations is 10 to 90% relative humidity at room temperature (22 °C). Typical uncertainties over this range are 0.2 to 0.9% relative humidity; however these are dependent on the stability of the client's instrument. Relative humidity calibrations can also be conducted over a temperature range of 5 to 80°C. Some calibrations over this extended range require the use of other standards and will be quoted on application.

Temperature calibrations are carried out in air in a temperature controlled chamber. The calibration range for the temperature sensor is –20 to 80 °C and is usually carried out at three points selected by the client. Additional temperature calibration points are charged accordingly.

Relative humidity/temperature meters are only adjusted if they are outside the manufacturer's specifications and if requested by the client.

Hygrometers of many other types may be acceptable for calibration. The stability of the instrument, the temperature dependence of its performance, the magnitude of any hysteresis effects and the inherent precision of the hygrometer are factors which are taken into account in determining its suitability for calibration. Information concerning calibration procedures and fees are available on application.