7 - Calibrations and standards  pp. 238-283

Calibrations and standards

By J. A. McLean and G. Tobin

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Standardisation of basic measurements

Many instruments for basic physical measurements can be purchased with accuracy certified by the National Physical Laboratory in the UK, the National Bureau of Standards in the USA, or from equivalent authorities in some other countries. Instruments such as mercury-in-glass thermometers, mercury barometers, certified weights and resistance boxes should maintain their accuracy indefinitely; they may be used for regular calibrations of thermistors, thermocouple- or resistance-thermometers, manometers or pressure sensors, electronic balances, and resistors. Other electronic instruments require to be checked against a reliable standard at least once a year and often more frequently; this can be done by some suppliers who are approved by their national standards authority. A high-quality, multirange digital voltmeter, regularly serviced by an approved supplier, is now an essential piece of laboratory equipment for calorimetry, and has virtually replaced the old standard cell and potentiometer. It may be used as a secondary voltage standard for the calibration of potentiometric recorders as well as of a great many instruments which provide output in voltage analogue form. For direct calorimetry a high quality wattmeter is equally essential.

Calibration of gas analysers

Introduction

Accurate calibration of gas analysers provides one of the greatest challenges in calorimetry; it is an area whose importance is not always fully appreciated even amongst some workers engaged in calorimetry. The calibration methods used and the care with which calibration is carried out, are often inadequate to achieve the appropriate level of accuracy for their experiments.