How do you really know what the temperature is? Michael de Podesta

Transcription

1 How do you really know what the temperature is? Michael de Podesta TECO: Madrid September 2016

2 GOLDEN RULE OF TALKS One talk: one thing This Talk: TWO THINGS!

3 Thing 1: The definitions of the SI units of temperature (K and C) are changing! Thing 2: The International Temperature Scale of 1990 is slightly wrong! Neither change will significantly affect meteorology

4 Measurement is Quantitative Comparison Unknown Standard of an unknown quantity with a standard quantity

5 The Current International System of Units s second kelvin ampere A K The results of ALL physical experiments are referenced to combinations of these base units m kg metre kilogram mol cd mole candela

6 The Current International System of Units s K 1 part in per year m A kg mol cd

7 The Current International System of Units s second kelvin ampere A K m metre The kelvin, the unit of thermodynamic temperature, is the fraction 1/ of the thermodynamic temperature of the triple point of water kilogram kg mol cd mole candela

8 Measurement is Quantitative Comparison Unknown Standard of an unknown quantity with a standard quantity Our standard temperature is the temperature of the triple point of water

9 A triple point of water cell Glass vessel K exactly 0.01 C exactly In practice ± K ± C 1. Fill with water 2. Remove the air 3. Seal 4. Water Vapour fills the space 5. Chill the middle 6. Leave to equilibrate Liquid Water Water Vapour Solid Water All co-exist in equilibrium

10 Is this really what we mean by a kelvin? Or is it just a convenient stable temperature?

11 Measurement is Quantitative Comparison Unknown of an unknown quantity with a standard quantity Standard Historically, unit definitions have included a preferred method of realisation

12 Definitions and Realisations Traditionally unit definitions include some part of their preferred mode of realisation Definition Realisation If we separate the definition from the realisation we allow realisations to evolve over decades and centuries in ways we cannot anticipate

13 Definitions and Realisations How many joules of energy do molecules possess at a particular temperature? Definition The definition What we mean by one kelvin Realisation Realisation How we translate that meaning into practical standards

14 Definitions and Realisations and Uncertainty How many joules of energy do molecules possess at a particular temperature Definition Uncertainty The definition What we mean by one kelvin Realisation Realisation How we translate that meaning into practical standards

15 The kelvin Currently: T TPW = K exactly Proposed: k B = joules per kelvin exactly Recognises the statistical mechanical nature of temperature Concept of one degree no longer tied to T TPW

16 Every temperature measurement is a quantitative comparison of the level of molecular jiggling in a target with The the level SI Joule of (defined molecular in terms jiggling of the fundamental in a triple-point constants) cell From 2018 T TPW will still be a useful reproducible temperature It will still have the same value. But it will no longer define what we mean by one degree

17 Thing 1: The definitions of the SI units of temperature (K and C) are changing! Thing 2: The International Temperature Scale of 1990 is slightly wrong!

18 How do you know if these thermometers are reading correctly? correctly? What do we mean by correctly? Would they agree with other thermometers anywhere on Earth? Would the number they produce match the T used in the equations of Physics? Image: from Reading Atmospheric Observatory

19 Science Molecular Thermometer Motion How do we relate the number produced by a thermometer (e.g. 20 C) to the basic physics describing the jiggling of molecules?

20 Calibration User 1 User 2 User 3 Calibration Lab INRiM CEM NIST NPL PTB LNE-CNAM NIM International Bureau of Weights and Measures, BIPM

21 The International Temperature Scale of 1990 Temperature Fixed Points Interpolating Devices

22 Temperature Fixed Points of ITS-90 This is defined T(K) T (ºC) Triple point of hydrogen Triple point of neon Triple point of oxygen Triple point of argon Triple point of mercury Triple point of water Melting point 1 of gallium Freezing point 1 of indium Freezing point of tin Freezing point of zinc Freezing point of aluminum Freezing point of silver Freezing point of gold Freezing point of copper Where do these other numbers come from? Where do these other numbers come from?

23 Temperature Fixed Points of ITS-90 This is defined T(K) T (ºC) Triple point of hydrogen Triple point of neon Triple point of oxygen Triple point of argon T(K) T (ºC) Triple point of mercury Triple point of water Melting point 1 of gallium Freezing point 1 of indium Freezing point of tin Freezing point of zinc Freezing point of aluminum Freezing point of silver Freezing point of gold Freezing point of copper

24 Interpolation: sprts Photographs Resistance typically ~25 Ω Uncertainty of Measurement Resistance: ~10 µω Temperature:~ few 10 s of µk

25 Interpolation: sprt Temperature ( C) Reference Function W ref for Platinum ln T or Resistance Ratio ln W ref A0 Ai i ref ( t90) C0 Ci i 1 In ITS-90, calibration involves 0.8 Measuring at the triple point of Hg water Measuring at another fixed point 0.6 Finding out the differences between actual the resistance ratio and that of an ideal thermometer 350 i / Temperature (K) 1. Ga W ( t 481) 481 i

26 Interpolation: sprt Temperature ( C) Reference Function W dref for Platinum or Resistance Ratio ln W ref A0 Ai i ln T Hg Ar / In Ga i Sn Zn W Temperature (K) Al Ag ( t 481) ref ( t90) C0 Ci i 1 i

27 The International Temperature Scale of 1990 Temperature Fixed Points Interpolating Devices

28 International Temperature Scale of 1990 Temperatures estimated according to ITS-90 are called T 90 Answers the needs of international comparability Practical, Precise, Reproducible Also T 90 is close to thermodynamic temperature T Based on best understanding in 1990

29 How big is the difference between T and T 90?

30 T T C

31 T T 90 showing only uncertainty in T Uncertainty in T 90

32 Absolute Error

33 The size of the kelvin Just about every thermodynamic function measured in this range is wrong by ~1 part in Slope = 0.1 mk/k

34 Thing 1: The definitions of the SI units of temperature (K and C) are changing! Thing 2: The International Temperature Scale of 1990 is slightly wrong! Neither change will significantly affect meteorology

35 Precision Measurement brings the world into focus

36 More details The National Physical Laboratory is operated by NPL Management Ltd, a wholly-owned company of the Department for Business, Energy and Industrial Strategy BEIS).