20 °C—A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements - PMC

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J Res Natl Inst Stand Technol<br>. 2007 Feb 1;112(1):1–23. doi: 10.6028/jres.112.001

20 °C—A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements

Ted Doiron<br>Ted Doiron

1Precision Engineering Division National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8211

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1Precision Engineering Division National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8211

Accepted 2006 Nov 11; Issue date 2007 Jan-Feb.

The Journal of Research of the National Institute of Standards and Technology is a publication of the U.S. Government. The papers are in the public domain and are not subject to copyright in the United States. Articles from J Res may contain photographs or illustrations copyrighted by other commercial organizations or individuals that may not be used without obtaining prior approval from the holder of the copyright.

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PMCID: PMC4654601  PMID: 27110451

Abstract

One of the basic principles of dimensional metrology is that a part dimension changes with temperature because of thermal expansion. Since 1931 industrial lengths have been defined as the size at 20 °C. This paper discusses the variety of standard temperatures that were in use before that date, the efforts of C.E. Johansson to meet these variations, and the effort by the National Bureau of Standards to bring the United States to the eventual world standard.

Keywords: dimensional metrology, gage blocks, gauge blocks, reference temperature, C. E. Johansson, S. W. Stratton

1. Introduction

While most dimensional metrologists know that the reference temperature for dimensional measurements is 20 °C,1 very few know how or why that temperature was chosen. Many people have thought it was, in some sense, arbitrary. In actuality, the decision was the result of 20 years of thought, discussion, and negotiations that resulted in the International Committee for Weights and Measures (CIPM)2 unanimous adoption of 20 °C as the reference temperature on April 15, 1931.

In cleaning up my office I found a pack of correspondence and documents that shed considerable light on how this decision was made by the Bureau of Standards, propagated through U.S. industry, and finally brought to closure at CIPM. It is an interesting piece of history that shows the high level of technical sophistication of metrologists of 100 years ago, as well as the deep commitment of the Bureau’s first Director, Dr. S. W. Stratton, to internationally based standards of metrology [1].

2. The Meter

The U.S. participation in the metric system began much earlier than most people realize, being one of the original signatories of the Convention of the Metre in 1875 which set up the International Committees and the International Bureau of Weights and Measures in Paris. The U.S. was also an early advocate of expanding the role of the Convention beyond its original scope of mass and length to cover a much wider range of measurements. The amendment of the original Convention was ratified in 1921, and since that time CIPM has become the primary authority for nearly all physical units.

The original Convention chose to base length on the French meter bar, which was an end standard. This end standard was replaced with a line scale, and then further replaced in 1889 with the Platinum-Iridium Meter of the Archives that was the standard until replaced with the wavelength of light in 1960. Thus, until 1960 the meter was defined as the distance between two lines on the Meter of the Archives at the temperature of the melting point of ice when supported by two cylinders of diameter at least 1 cm placed symmetrically under the bar and 571 mm apart.

The choice of the melting point of ice seems, today, like a very awkward choice for the reference temperature. It is a difficult temperature at which to actually make measurements, both because of human comfort and the fact that the dew point is generally above this temperature, and some sort of humidity control would be needed. Temperature was, and still is, a major complicating factor in...