Isotech Gold-Platinum Thermocouple and the DC Measurement Requirements free download

File name Gold-Platinum Thermocouple and the DC Measurement Requirements.pdf

GOLD/PLATINUM THERMOCOUPLE AND THE DC MEASUREMENT REQUIREMENTS by John P. Tavener Isothermal Technology Limited Pine Grove, Southport, Merseyside, PR9 9AG, England E-mail [email protected] Internet www.isotech.co.uk NOVEMBER 2000 INTRODUCTION Thermocouples are nasty complicated ill understood things that measure temperature differences badly and should be avoided at all cost. Calibration of thermocouples - if possible at all - is a topic fraught with measurement problems. This author has avoided them as far as possible for the past 30 years and refuses to calibrate most types of thermocouple in Isotech's UKAS Laboratory. A great friend of mine once said of a fellow scientist "what a pity that such a great man should have devoted so much of his life to such an inferior thing." ITS-90 removed them in favour of the Standard Platinum Resistance Thermometer. The limiting factor with all normal thermocouples is that one or both of the thermo-elements is an alloy and an alloy can not be produced that is homogeneous. This means that, because the Emf of a thermocouple is generated along the wire where there exists a thermal gradient, if the thermal gradient is moved along the thermocouple the Emf will change. This limits the best thermocouple accuracy to about