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? reference chain of calibrations measurement uncertainty International Vocabulary for Metrology (VIM) definition: “property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty”. 1. Confidence in the 2. Quality of my measurement and 3. Equivalence to other similar measurements

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

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.

The unit definition and its practical realization 7 base units The kelvin, unit of thermodynamic temperature, is the fraction 1/ of the thermodynamic temperature of the triple point of water. Many derived units The pascal, unit of pressure or stress, is the force applied per unit area. Derived quantityNameSymbol Expressed in terms of other SI units Expressed in terms of SI base units pressure, stresspascalPaN/m 2 m –1 kg s –2 Signed by 17 countries on 20 May 1875 Today 56 Member States and 37 Associates USA is member since 1875

Discrete reference scale: Fixed point cell Interpolation instrument: Standard platinum resistance thermometer Thermometry practical reference standards

ITS 90

Unbroken chain of calibrations for temperature measurements Measurement in lab. and in liquid For measurements in air see for example the calibration of climatic chambers guide:

Traceability scheme for resistance thermometry measurements Traceability/uncertainty calculation  start from the mathematical model Traceability for the measurand Y = traceability for all the input quantities x i

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

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, …

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) mass volume Humidity = mass of water vapor in a unit volume of moist air temperaturepressure at a given temperature and pressure. water vapor pressure enhancement factor uncertainty due to formulae calculations Humidity primary standards

Humidity primary standards

Traceability scheme for humidity measurements

Traceability scheme for humidity measurements

Primary standards in pressure measurement m p A Temperature reference Important influence Mass reference Length reference Laser interferometric manometer Piston gauge

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

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.

Questions? Ask your National Metrological Institute!