Thermocouple calibration

Thermocouple calibration - calibration and supply of platinum-rhodium thermocouples (0 °C to 1600 °C)

Platinum-rhodium thermocouples

NPL offers a UKAS-accredited thermocouple calibration service (laboratory number 0478) for platinum-rhodium thermocouples using freezing points of zinc, silver and copper (up to 1100 °C) and the melting points of gold and palladium (up to 1600 °C).

NPL also supplies calibrated platinum-rhodium thermocouples from stock. These are made from wires assembled into insulators and calibrated up to either 1100 °C or 1600 °C. The wires are 0.5 mm in diameter and 1500 mm long, and the thermocouples can be purchased with or without cold-junction attachments to copper extension leads, and an alumina protection tube.

Thermocouples are widely used in industry to provide temperature measurement and control for numerous applications. They are popular because they are small and rugged, suitable for use to very high temperatures and are relatively low-cost. They also provide a convenient electrical output for data logging and control. Platinum-rhodium thermocouples (types R, S and B) are suitable for use as standards for the calibration of other base-metal thermocouples, and for high-temperature applications.

Calibration using new metal-carbon fixed points

As a result of extensive research and development of high temperature metal-carbon eutectic fixed points, NPL now has a new service supplying calibrated Type R thermocouples with UKAS certificates with significantly reduced uncertainties of measurement.

These uncertainties are achieved by employing the cobalt-carbon eutectic fixed point (1324°C) or palladium-carbon eutectic fixed point (1492 °C) as the highest temperature fixed point in the calibration. The Co-C and Pd-C fixed points have an inherently low uncertainty in comparison with the wire bridge fixed points.

NPL supplies thermocouples from stock, calibrated using the new Co-C or Pd-C fixed points.

Ranges and uncertainties

Calibration uncertainties (95% confidence limits) for type R, S and B thermocouples:

Calibration Range	Calibration uncertainty (k = 2)
Calibration Range	Types R and S	Type B
0 °C to 1100 °C*	< ± 0.3 °C above 500 °C (± 0.5 °C from 0 to 500 °C)	< ± 0.5 °C above 800 °C (± 1 °C from 400 °C to 800 °C)
0 °C to 1600 °C*	± 1 °C up to 1100 °C ± 1.5 °C from 1100 °C to 1600 °C	± 1.5 °C from 400 °C to 1600 °C
	Type R and S
0 °C to 1330 °C†	± 0.3 °C from 0 °C to 1100 °C, rising linearly to ± 0.55 °C at 1330 °C
960 °C to 1500 °C‡	± 0.3 °C from 0 °C to 1100 °C, rising linearly to ± 0.7 °C at 1500 °C

* Type B thermocouples are calibrated from 400 °C
† Calibration at zinc, silver, and Co-C fixed points, type R or S thermocouples only
‡ Calibration at zinc, silver, and Pd-C fixed points, type R or S thermocouples only

Purchase of calibrated NPL platinum-rhodium thermocouples:

Thermocouple type R, S or B
Calibrated from 0 °C to 1100 °C*	Prices on application
Calibrated from 0 °C to 1600 °C*
Calibrated from 0 °C to 1330 °C (new Co-C service)† Calibrated from 0 °C to 1500 °C (new Pd-C service)†
Cold junction attachments
Protective alumina tube (400 mm long, 6 mm outer diameter)