http://economie.fgov.be

Miruna DOBRE – SMD (Belgium)Stephanie BELL – NPL (United Kingdom)

Dolores del CAMPO – CEM (Spain)Martti HEINONEN - MIKES (Finland) Domen HUDOKLIN – LMK (Slovenia)

Giuseppina LOPARDO , Andrea MERLONE – INRIM (Italy)

METROLOGICAL TRACEABILITY

FOR METEOROLOGICAL SENSORS

ILLUSTRATED THROUGH

EXAMPLES

Why do I need traceability?

International Vocabulary for Metrology (VIM) definition: “property of a measurement result whereby the result can be related to a referencereference through a documented unbroken chain of calibrationschain of calibrations, each contributing to the measurement uncertaintymeasurement uncertainty”.

1. Confidence in the

2. Quality of my measurement and

3. Equivalence to other similar measurements

http://www.bipm.org/en/publications/guides/vim.html

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Maintained by National Measurement Institute

( unit realization)

Maintained by National Measurement Institute

( unit realization)

Compared with primary standard (calibration at NMi)

Compared with primary standard (calibration at NMi)

Compared with secondary standard

(calibration at accredited or final user’s lab)

Compared with secondary standard

(calibration at accredited or final user’s lab) Compared with

working standard (in-field or lab.

calibration)

Compared with working standard

(in-field or lab. calibration)

International agreement

(CGPM decision)

International agreement

(CGPM decision)

The unbroken chain of calibrations

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A measurement has metrological traceability if:

1. An unbroken and documented chain of calibrations links the

instrument to an internationally agreed reference;

2. Each chain link has documented uncertainty estimations;

3. The final user has procedures to ensure that the instrument

maintains its calibration with associated uncertainties over

time;

4. The influence factors affecting the measurement are

quantified and a total measurement uncertainty is calculated,

documented and reported with the result.

http://economie.fgov.be

The unit definition and its practical realization

7 base unitsThe kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

Many derived unitsThe pascal, unit of pressure or stress, is the force applied per unit area.

Derived quantity Name SymbolExpressed in terms of other SI units

Expressed in terms of SI base units

pressure, stress pascal Pa N/m2 m–1 kg s–2

www.bipm.org

www.bipm.org

• Signed by 17 countries on 20 May 1875• Today 56 Member States and 37

Associates

USA is member

since 1875

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Discrete reference scale: Fixed point cell

Interpolation instrument: Standard platinum resistance thermometer

http://www.bipm.org/en/publications/mep_kelvin/

Thermometry practical reference standards

http://economie.fgov.be

http://www.nmij.jp/english/library/units/temperature/

ITS 90

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Unbroken chain of calibrationsfor temperature measurements

Measurement in lab. and in liquid

For measurements in air see for example the calibration of climatic chambers guide:

www.dkd.eu/dokumente/.../dkd_r_5_7_e.pdf

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Traceability scheme for resistance thermometry measurements

nxxxfY ...,, ,21Traceability/uncertainty calculation start from the mathematical model

Traceability for the measurand Y = traceability for all the input quantities xi

rWWfT

XRR

CR

TRW

s

01.090

http://www.bipm.org/en/publications/guides/gum.html

http://economie.fgov.be

Humidity quantities -what is the measurand?

Humidity is a general term encompassing:

•Vapour pressure (partial pressure) of water (in units of pressure)

•Dew-point temperature [and frost point] (in units of temperature)

•Composition (dimensionless)

•Relative humidity (dimensionless) but symbol %rh used (also %hr,

%ur, %rF, etc) - not very standardised!

•And other less-used parameters

http://economie.fgov.be

Humidity primary standards

• By measurements: gravimetric hygrometer

• By realizations: a system that can produce a gas stream of known humidity

By saturation at defined pressure(s) and temperature(s)

By gas blending, …

http://economie.fgov.be

Absolute humidity generator operation:

1.Saturate the gas with water at a known temperature and pressure. The mole fraction x of water vapor in the gas is

2.For 2-p expand till atmosphere, for divided flow mix with known amounts of dry gas (traceable flowmeters)

ssss PPTfTex ,

Humidity = mass mass of water vapor in a unit volumevolume of moist air at a given temperaturetemperature and pressurepressure.

water vapor pressure enhancement factor

uncertainty due to formulae calculations

Humidity primary standards

http://economie.fgov.be

Humidity primary standards

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Traceability scheme for humidity measurements

http://economie.fgov.be

Traceability scheme for humidity measurements

http://economie.fgov.be

Primary standards in pressure measurement

Hgp

m

p

A

Agmp

Temperature reference

Important influence

Mass reference

Length reference

Laser interferometric manometer

Piston gauge

http://economie.fgov.be

Other effects affecting measurement result:

•dilatation of piston and cylinder due to temperature and pressure;•surface tension effects around piston;•air buoyancy over masses;• liquid column head,•….

Primary standards in pressure measurement

http://economie.fgov.be

A measurement has metrological traceability if:

1. An unbroken and documented chain of calibrations links the

instrument to an internationally agreed reference;

2. Each chain link has documented uncertainty estimations;

3. The final user has procedures to ensure that the instrument

maintains its calibration with associated uncertainties over

time;

4. The influence factors affecting the measurement are

quantified and a total measurement uncertainty is calculated,

documented and reported with the result.

http://economie.fgov.be

http://www.bipm.org/en/practical_info/useful_links/nmi.html

Questions?

Ask your National Metrological Institute!