तापमान तथा आर्द्रता मानक

1. Objective:

The unit of temperature at present is defined as a fraction 1/273.16 of the thermodynamic temperature of triple point of water (TPW). The primary standards of temperature based on TPW and other fixed points called the thermal equilibrium states are well established at NPL as per International Temperature Scale of 1990 (ITS-90) in the form of sealed cells of high purity materials. The temperature scale at NPL from -189 °C to 962 °C, (FPs Ar, Hg, Sn, Zn, Al, Ag) is realized by SPRT with uncertainty varying from ±0.17 m°C at TPW (0.01 °C) to maximum ± 6 m °C at Ag point (961.78 °C). In the thermocouple thermometry, the range from 0 °C to 1554 °C is realized by standard Type-S & R TCs with uncertainty varying from ±0.32 °C at indium point (In,156.5985 °C) to ± 0.5 °C at Palladium point (Pd,1554 °C). The radiation temperature scale has been established by installing photoelectric linear pyrometer, LP4 and realizing the silver point (Ag, 961.78 °C) and copper point (Cu, 1084.62 °C) in the form of metal-in-graphite blackbodies with an uncertainty better than ± 0.5 °C. The high temperature range from 800 °C to 2200 °C is thus maintained by transfer standard lamps using LP4 pyrometer with uncertainty varying from ± 0.9 °C to ± 2.5 °C while the total radiation infrared thermometry is available in the range from 50 ºC to 3000 ºC. The apex level calibration in temperature metrology is available in the laboratory in the range from -200 °C to 3000 °C.

Realization of Triple Point of Water (TPW) & Set-up for
Primary Standard Fixed Points by using SPRT

Set-up for Realization of Ag (961.78 °C) & Cu (1084.62 °C) Points
by Photoelectric Radiation Pyrometer, LP4

Set-up for calibration of noble metal Standard Thermocouples (Type-S, R & B) (By Fixed Points and Comparison methods)

Measurement Set-up for Co-C and Fe-C Eutectic

Fixed Points for High Precision Thermocouples

High Temperature Black
Body Source up to 3000 °C

2. Existing Apex Calibration Facilities:



Temperature Range

Primary Standards Fixed Points and SPRTs

SPRTs with Fixed points of TPAr, TPHg, TPW, Ga, In, Sn, Zn, Al, and Ag with high Precision Resistance Bridge

-189 °C to 0.01 °C

0.01 °C to 660 °C and

660 °C to 962 °C

Primary Standards Fixed Points and Spectral Photoelectric Radiation pyrometers

Fixed point blackbodies of Ag and Cu with Spectral Photoelectric Linear Pyrometer, LP4 and heat pipe Blackbody Source

962 °C & 1084 °C

800 °C to 2200 °C

50 °C to 1300 °C and

600 °C to 3000 °C

Thermocouple Thermometry (TC)

(Noble metal TCs), Fixed Points & Comparison

Fixed points of  In, Sn, Zn, Al, Ag, Cu and Pd with Type-S & R & B TCs and Precision DVM using high stability Furnaces

0 °C to 1554 °C

0 °C to 1200 °C and

0 °C  to 1600 °C

Liquid-in-Glass Thermometry (LIGT)

SPRTs with Precision Resistance Bridge and High stability Liquid baths/Fluidized Bed Bath

-90 °C to 300 °C and

300 °C to 500 °C


2500-Thunder Scientific Humidity Generator

10 % to 95%

3. Highlights of Achievements:

  • High temperature facility has been created in the fields of eutectic fixed point of Co-C and Fe-C using pure metal thermocouple at a temperature of 1400 °C. Fixed point cells of high purity graphite have been designed and fabricated and developed in the laboratory for realization of the fixed point using precision thermocouples.
  • Calibration set-up for infrared thermometers and thermal imagers has been established by procuring a large temperature range blackbody source operated for temperature range from 600 °C to 3000 °C. This unique facility has been set up first time in the laboratory to support industries and laboratories for apex calibration on non-contact means of temperature calibration.
  • Primary standards for high temperature calibration by spectral radiation pyrometry has been set up by installing a photoelectric radiation linear pyrometer, LP4 developed by m/s KE Technologies, Germany. Fixed points of silver (Ag, 961.78 °C) and copper (Cu, 1084.62 °C) in the form of metal-in-graphite black bodies of these materials have been realized with significant precision of the order of ±0.05 °C. The facility is ready to calibrate high temperature tungsten lamps and optical pyrometers in the range from 800 °C to 2200 °C.
  • Calibration facility for hygrometers has been set-up by procuring new humidity generator of Thunder Scientific-2500 based on temperature-pressure parameters with significant stability of better than 0.1% and capable of providing calibration in the range 10% to 95% RH with an uncertainty of 0.3% RH.

4. Participation in the International Key Comparisons:

S. No.

Key Comparison

Parameter & range



SADC-Met, 2000-02

LIGT, 0 to 50 °C

Satisfactory completed


APMP-S-T7, 2006-07

S-TC, 0 to 1100 °C

Satisfactory completed


APMP-KC-3, 2000-01

Hg, TPW, Ga, Zn by SPRT

Ga qualified, Results awaited


APMP-TK-4, 2005-06

Al and Ag by SPRT

Final draft 2013


APMP-T-Pt/Pd, 2012-13

Pt/Pd TC, Co-C point   (1324 °C)

Under comparison



LIGT, 0-250 °C

Under process

5. Traceability of Temperature & Humidity Standards:

The Temperature & Humidity Standards Group maintains the primary and secondary standards as per procedures of the International Temperature Scale of 1990 (ITS-90) and establishes the mutual compatibility of temperature standards within the country through international comparisons of APMP regional programs. Various proficiency testing programs (PT-Program) were organized by the Group in LIGTs and TCs in collaboration with NABL to disseminate the traceability of the laboratory to the NABL accredited laboratories and user industry as a part of metrology service to the nation.

6. CMCs Published in the KCDB, BIPM France:






Cover. Factor,


Confid. Level

NMI Service provider




Ga Point

29.7646 °C

1.36 mK




March 2011


LIGT, 0.05 °C

Water Bath

5 to 95 °C

0.03 °C




March 2011


LIGT, 0.05 °C

Oil bath

95 to 200 °C

0.04 °C




March 2011



Oil bath & TPW cell

01 to 100 °C

4-15 mK




March 2011

7. Specific and Significant facilities for users:

1. Calibration Facilities
Apex level calibration facilities are available in the temperature and humidity standards for specific instruments like SPRTs, noble metal Thermocouples, precision LIGTs, Optical total radiation and infrared pyrometers, Tungsten lamps, thermal imagers in the range from -200 °C to 3000 °C and hygrometers in the range 10% to 95% RH.

2. Consultancy Service to Industry:

    1. Characterization of thermal comparison sources

    2. Establishing calibration laboratories.

संपर्क जानकारी