State primary standard and test scheme for means of measuring coefficients of linear expansion for solids in the range 90?1800�K

Abstract

It has been necessary to revise GOST 8.018-75 and to devise a new state primary standard (SPS) for the unit of linear expansion coefficient (LEC) on account of the extending temperature ranges used with materials, the general use of low-expansion materials, and tightening requirements on accuracy in measuring LEC. The new test scheme of [I] covers the range 0.05-25'10 -6 K -: and temperatures of 901800~ and it contains four grades of standard means of measurement. The new standard has the following changes relative to GOST 8-08-75: the upper temperature limit of SPS has been raised to 1800~ the range in LEC measured with the SPS has been extended at the lower end; the accuracy of the SPS has been improved by a factor five in the range 300-450~ and by 1.5 in the range 450-II00~ comparison standards and working standards have been introduced; and the number of grades in the standard facilities has been increased. The latter change has occurred because the accuracy of third-class standard measures according to GOST 8.018-75 is insufficient for checking precision dilatometers of various types. In order to relieve the load on the SPS, the new test scheme had introduced first-class standard dilatometers for certifying LEC standard measures for checking precision dilatometers. The SPS heads the test scheme, as it provides for reproducing the unit of LEC with a standard deviation (SD) for the average LED in a i00 ~ range with three independent measurements of 0.20-1.5"i0 -e K -l with a residual systematic error (RSE) not exceeding 0.03-30' I0 -e K -l, in accordance with the temperature and LEC. The primary standard is a standard suite of means of measurement serving to reproduce and store the LEC unit and to transmit it to secondary standards [2]. The LEC unit is reproduced on standard dilatometers by using standards for the units of length and temperature and is transmitted to secondary standards (LEC measures) by absolute joint measurement of the extension of the measure and change in temperature causing it, with subsequent calculation of the LEC. The joint measurements are processed on an algorithm for defining the parameters in the approximating function and the variances of them, as well as the LEC (average and differential) and the variances of these [3]. The SPS consists of the following means of measurement: dilatometers type DIS-7, DI-8, and DIV-6 correspondingly for the ranges 90-450~ 300-1100~ and 600-1800~ together with LEC measures for monitoring the stability of the standard dilatometers. An interference principle is used to measure the extension in all the dilatometers, which enables one to measure this in the secondary standards at the wavelengths of :*aCd and He--Ne lasers. The measure is placed between interference plates, which form a Fizeau interTABLE i