Kelvin redefinition

Presented byDr Efrem K Ejigu

Test and Measurement conference _16 Sept 2019Misty Hills, Muldersdrift

Contents

• Introducing Kelvin

• The need for redefinition

• The Redefinition of Kelvin

• The Redefinition impact on temperature measurement

• African perspective

Introducing Kelvin

• The kelvin is the fundamental unit of temperature.

• At the beginning gauging temperature used no fixed scale and no degrees.

• Later on there were different scales used and creates a confusion

• The confusion was about to be settled when a scale which is based on the fundamental physics of temperature is created

• The minimum temperature that is theoretically possible to achieve is termed as absolute zero and this is used in the absolute temperature scale concept

• This was coined by lord Kelvin and the scale bears his name

• CGPM adopts Kelvin (K) as the base unit for thermodynamic temperature

• Kelvin is defined as the fraction 1/273.16 of the thermodynamic temperature at the triple point of water

• The triple point of water was chosen as the reference temperature as it can easily be realized in a glass cell and its reproducibility is extraordinary

The need for redefinition

• Thermodynamic temperature is an absolute measure of the average total internal energy of an object

• In principle the natural unit of thermodynamic temperature is Joule and a separate base unit for temperature is not required

• But for practical reasons Kelvin remains the base unit

• Kelvin determination is slightly different from set up to set up due to the difficulty of creating exactly a water mixture with exactly the same isotopic content

• The appropriateness to meet the demands of the century and beyond

• The need to link to other SI units as it was defined on arbitrarily chosen temperature

• Create room for advancement in temperature measurement in high and low thermodynamic temperature measurements

The Redefinition of Kelvin

• A major revision on the four SI unit bases is done on 16November 2018 at the BIPM, around 60 NMI directors voted

• The redefinition came into effect on May 20, 2019

• Kelvin was defined by triple point of water by choosing it as afundamental fixed point and assigning a temperature value of273.16 K

• K is a fraction1

273.16of the thermodynamic temperature of the

TPW

• Kelvin is now redefined by assigning a value to the Boltzmannconstant, kb

• The new definition fundamentally links the Kelvin to the unit ofenergy and hence to the units of mass, length and time

CCT Conditions

• To define Kelvin using Boltzmann constant (kb) the following two condition were set by the CCT.

• kb will be given exact fixed value in Joules per kelvin• The value obtained by at least one of the method for kb has a relative

standard uncertainty of less than one part in million

• At least one fundamentally different type of measurement yields similar values with a relative standard uncertainty of less than 3 parts per million

• Acoustic Gas Thermometry (AGT), Dielectric Gas Thermometry (DCGT), Johnsen Noise Thermometry (JNT), and Doppler Broadening Thermometry (DBT) methods were considered

Acoustic Gas Thermometry (AGT)

• One of the methods that contributed towards the re-determination of the Boltzmann constant.

•1

2𝑚𝑣𝑅𝑀𝑆

2 =3

2𝑘𝐵𝑇

• 𝑣𝑅𝑀𝑆2 =

3

𝛾0𝑤02

• 𝑘𝐵 =𝑤02𝑀

𝑇𝛾0𝑁𝐴• 𝑣𝑅𝑀𝑆 - root-mean square velocity of a molecule

• M – average molar mass of the gas

• 𝑁𝐴- Avogadro constant

• Through the accurate determination of speed of sound

• Accurate theoretical relationship b/n Speed of sound in resonator and its frequencies

• Relative uncertainty of <1ppm meeting one of CCT criteria (LNE & NPL)

• NIST, LNE,NPL,INRIM & NIM

Dielectric Gas Thermometry (DCGT)

• One of the methods that contributed towards the re-determination of the Boltzmann constant.

• 𝑝 = 𝑘𝑏𝑇𝜀−𝜀0

𝛼0

• 𝑘𝑏 =𝑝

𝑇

𝛼0

𝜀−𝜀0• p – Gas pressure

• 𝜀 - electrical permittivity of a gas

• 𝛼0- static electric dipole polarizability

• 𝜀0- Electric constant

• Through the determination of electrical permittivity of the gas

• Relative uncertainty of <3ppm obtained meeting the CCT criteria

• PTB

Johnsen Noise Thermometry (JNT)

• One of the methods that contributed towards the re-determination of the Boltzmann constant

• 𝑉2 = 4𝑘𝑏𝑇𝑅∆𝑓

• 𝑘𝑏 =4𝑇𝑅∆𝑓

𝑉2

• 𝑉2 - mean square noise voltage• R – frequency independent resistance• ∆𝑓 – band width of the detection system

• Through the determination of the mean-square noise voltage developed in a resistor

• Relative uncertainty of <3ppm obtained meeting the CCT criteria

• NIST & NIM

• The Kelvin is the SI unit of the thermodynamic temperature its magnitude is set byfixing the numerical value of the Boltzmann constant (kb) to be equal to exactly1.380649*10-23 when expressed in the SI units of s-2m2kgK-1 (JK-1), where thekilogram, metre and second are defined in terms of h, c and ΔνCs.

• h = planck constant, c = speed of light in vacuum, ΔνCs = caesium frequencycorresponding b/n hyperfine levels of the unperturbed ground state of the 133Cs atom(stable isotope of Cs atom).

• In terms of defining constants, 1 K =1.380649×10−23

(6.62607015×10−34)∗(9192631770)hΔνCsk𝑏

• One Kelvin will be equal to the change of thermodynamic temperature T that results ina change of thermal energy kbT by 1.380649*10-23 J

• Kelvin is now defined by SI derived unit of energy, Joule

• Kelvin is defined in terms of average amount of energy that the atoms and moleculesof a substance possess at a given temperature

The Redefinition impact on temperature measurement

• Water triple point will loss its special status and become just another fixedpoint. It will still be the reference fixed point for ITS90

• The relative uncertainty of kb , 3.7 * 10-7 will be transferred to thetemperature of TPW

• After redefinition the best estimate of TPW will remain the same 273.16 K,but instead of this value being exact the standard uncertainty of WTP will be0.1 mK and this value will propagate to all thermodynamic temperatures

• In practice the change mainly affect thermodynamic temperaturemeasurements close to TPW as uncertainties well away from TPW is muchbigger

• There is no immediate impact on the status of ITS90 or PLTS-2000(covering temperature 0.65 K to 1357.77 K)

• Have more benefit to temperature measurement below 20K and above 1300K

• Primary thermometry may offer lower uncertainty that was available withdefined scales. Like Spectral–band radiometric, acoustic gas, polarizing gasand Johnson noise thermometry

• Fixed point temperatures assigned in ITS90 or PLTS-2000 are fixed. Hence theredefinition does not have any effect on the temperature value or uncertainties

• A definition free of material and technological constraints enables thedevelopment of new and more accurate techniques for making temperaturemeasurements traceable to the SI, especially at extremes of temperature.

• After the redefinition, the guidance on the practical realization of the kelvin willsupport its world-wide dissemination by describing primary methods formeasurement of thermodynamic temperature and equally through the definedscales ITS-90 and PLTS-2000

• Encourage the use of both thermodynamic and ITS90 temperatures.

• Have impact on uncertainties in thermodynamic temperature measurements

• In long term, primary methods evolve and achieve lower uncertainties they willbecome more widely used and gradually replace ITS90

• Though Kelvin is no longer defined by the triple point of water it will remain areliable calibration point owing to its unchanging physical property in a well-defined experimental setup.

• Global regulation of the realization and dissemination of Kelvin will be done by by MeP-K-19 and any subsequent updates

• TG-SI of the CCT Report to CIPM , 2007

Impact highlight

Short term:

• Kelvin definition is independent of any material property

• No favoured fixed point

• No favoured measurement method

Medium to long term

• Thermodynamics measurement & ITS90 coexist

• Thermodynamics temperature measurement is superior in temperature range of

<20K and >1300 K

• With improvement of primary thermometry thermodynamic measurements may

replace ITS-90

• ITS90 determination through radiation thermometry will eventually be

replaced by absolute/relative primary thermometry (radiometry)in the high

temperature range.

• For temperatures <1 K variants of JNT may be able to provide sufficiently low

uncertainty

• For temperature points between 0,65 to 1 K low temperature primary

thermometry may will replace the ITS90/PLTS2000

African perspective

• African NMIs provide traceability through IST90 realization

• Most AFRICAN countries obtain traceability from NMISA

• At NMISA Temperature traceability is provided through ITS90 realization using range of fixed point cells

• NMISA can serve all the industries with its ITS90 realization facility

• ITS90 thermodynamic temperature determination is not affected by the new redefinition (at the moment)

• Since there is no thermodynamic temperature determination other than ITS90 at NMISA its temperature traceability will not be affected

• This opens new development opportunities for NMISA to provide traceability in the ultra low and very high temperature range by investing in new technologies

• For frequently asked questions

regarding SI redefinition please

check

https://www.bipm.org/en/

measurement-units/

rev-si/faqs.html

Thank you