The Kelvin and temperature measurements

Journal of Research of the National Institute of Standards and Technology, Jan-Feb, 2001 by B.W. Mangum, G.T. Furukawa, K.G. Kreider, D.C. Ripple, C.W. Meyer, G.F. Strouse, W.L. Tew, M.R. Moldover, B. Carol Johnson, H.W. Yoon, C.E. Gibson, R.D. Saunders

The International Temperature Scale of 1990 (ITS-90) is dcfined from 0.65 K upwards to the highest temperature measurable by spectral radiation thermometry, the radiation thermometry being based on the Planck radiation law. When it was developed, the ITS-90 represented thermodynamic temperatures as closely as possible. Part I of this paper describes the realization of contact thermometry up to 1234.93 K, the temperature range in which the ITS-90 is defined in terms of calibration of thermometers at 15 fixed points and vapor pressure/temperature relations which are phase equilibrium states of pure substances. The realization is accomplished by using fixed-point devices, containing samples of the highest available purity, and suitable temperature-controlled environments. All components are constructed to achieve the defining equilibrium states of the samples for the calibration of thermometers. The high quality of the temperature realization and measurements is well documented. Various research efforts are desc ribed, including research to improve the uncertainty in thermodynamic temperatures by measuring the velocity of sound in gas up to 800 K, research in applying noise thermometry techniques, and research on thermocouples. Thermometer calibration services and high-purity samples and devices suitable for "on-site" thermometer calibration that are available to the thermometry community are described. Part II of the paper describes the realization of temperature above 1234.93 K for which the ITS-90 is defined in terms of the calibration of spec-troradiometers using reference blackbody sources that are at the temperature of the equilibrium liquid-solid phase transition of pure silver, gold, or copper. The realization of temperature from absolute spectral or total radiometry over the temperature range from about 60 K to 3000 K is also described. The dissemination of the temperature scale using radiation thermometry from NIST to the customer is achieved by calibration of blackbody sources, tungsten-strip lamps, and py rometers. As an example of the research efforts in absolute radiometry, which impact the NIST spectral irradiance and radiance scales, results with filter radiometers and a high-temperature blackbody are sunimarized.

Key words: acoustic thermometry; blackbody sources; calibrations; gas thermometry; Johnson noise thermometry; Kelvin; pyrometers; radiation thermometry SPRTs; thermocouples

Available online: http://www.nist.gov/jres

Introduction

This paper gives a brief review of the realization of the kelvin at the National Institute of Standards and Technology (NIST) and of current research and other activities in thermometry. (From 1934 to 1988, NIST was known as the National Bureau of Standards (NBS), and from 1903 to 1934 it was known as the Bureau of Standards (BS); from 1901 to 1903, it was known as the National Bureau of Standards.) The paper is in two parts. Part I concerns contact thermometry and the realization of the International Temperature Scale of 1990 (ITS-90) (1) at temperatures below 1235 K. Part II concerns non-contact (radiation) thermometry and the realization of the ITS-90 at temperatures above 1234 K.

NIST has been involved in the field of thermometry since shortly after the creation of NBS, and laboratory notebooks detailing calibrations of liquid-in-glass thermometers date back to 1904. Similarly, notebooks concerning calibrations of thermocouples date to 1909 and work on platinum resistance thermometers dates back to 1907. Thus, temperature, one of the SI quantities for which NIST has the responsibility for disseminating its measurement unit--the kelvin--to U.S. industry, has been a feature of the NIST work throughout most of the existence of the organization.

Part I. Contact Thermometry

1. Introduction

The quantity that is designated thermodynamic temperature is defined by the laws of thermodynamics; it is indicated by the symbol T, and has the unit kelvin, symbol K. The unit of thermodynamic temperature is defined to be the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. It common practice to express temperatures in terms o their differences from 273.15 K, the value for the ice point. A thermodynamic temperature T expressed in this manner is known as a Celsius temperature t, which is defined by the equation

t/[degrees]C = T/K - 273.15. (1)

The unit of Celsius temperature is the degree Celsiu, symbol [degrees]C. The magnitude of the degree Celsius defined to be the same as that of the kelvin. Measures of temperature that are defined to be consistent with the laws of thermodynamics are said to be thermodynamic temperatures. Thermodynamic temperatures, however, are very difficult to measure precisely and accurately Consequently, internationally-agreed scales of temper tare, with temperatures on the scale as close to thermodynamic temperatures as possible at the time the scales are approved, are used to approximate the thermodynamic temperature. These international temperature scales are defined in terms of fixed points, vapor pressures of some liquefied gases, thermometers that can be measured very precisely and fairly easily, and equations that relate measurements of these thermometers to temperatures of the scale.