Air Temperature Metrology - ATM

Project Description

Overview
This EURAMET project includes two main activities:

Perform a pilot study (in the form of an interlaboratory comparison) to explore issues around calibration in air of temperature sensors;
Feed into a guidance document the findings from the pilot study.

Background and need for the project
Air temperature is measured for a multitude of purposes. Atmospheric air temperature is the key variable in indoor climatisation, in meteorological observations and climate studies. It is also critical to precision dimensional and mass measurements. Understanding and fully evaluating measurement uncertainty for air temperature measurements is an open scientific and technical issue now motivating research efforts and discussion both at the CIPM CCT and in WMO expert teams. While calibration of temperature sensors in liquid is well characterised, the calibration of thermometers in air still requires definition of procedures and guidelines. The problem of the lack of a guide for the calibration of thermometers in air was underlined at the EURAMET TC-T meeting of 2017 as a result of a specific scientific workshop on the matter. The analysis of the air temperature measurement procedures and uncertainty evaluation, including the aspects and contributions due to the calibration of sensors is also a task of the CCT Working Group on Environment and is included in the strategy roadmap of CCT.

Objective 1 - Interlaboratory comparison of techniques for calibration of thermometers in air in the range of atmospheric temperature
An interlaboratory comparison will be performed as a pilot study for the evaluation of a best procedure to be adopted for the calibration of thermometers in air. The comparison will be performed in the main range of atmospheric air temperature, from -80 °C to + 60 °C in three loops with the possibility for participants to choose the lower limit to -40 °C or -80 °C. A number of representative different thermometers will be collected for the circulation among participants, for calibration in each laboratory.

One of the conditions for joining the pilot comparison is for all participants to have the capability to cover the whole selected temperature range. A comparison protocol will be agreed by all participants prior to the start of the comparison. The study will take into account the findings of the previous EURAMET P1061 intercomparison, carried out to investigate the reliability and equivalence of calibration methods used by NMIs in calibrating air thermometers. The effect of self-heating depending on the air velocity and a study on radiative heating will be included in the investigation.
At the end of the comparison, the pros and cons of the different procedures used by the participating partners will be jointly analysed, reported and used for defining the best practice. This will be based on the capabilities to make measurements with low uncertainty, by reducing the effect of quantities of influence and on the possibility to include a wide range of sensors so the procedure would have quite general applicability.

Objective 2 - EURAMET Guidelines for calibration of thermometers in air
Based on the best practice defined and evaluated during the ILC, the participants will draft a proposal for a EURAMET guide on calibration of thermometers in air. The document will contain a full documented procedure for performing the calibration, together with definition of uncertainty budget components and methods for their evaluation. The guide will cover a range of sensor types; i.e. those usually involved in air temperature measurements for applications such as in industry, indoor ambient, meteorology and climate.
The proposed guide will include prescriptions on how to evaluate and minimize where possible, the effect of heat biases in the calibration accuracy, due to radiative and convective effects. The document will be submitted to TC-T Working Group on best practices (guides) for discussion and approval.

Impact
The main benefit of this project will be the preparation of a EURAMET guide for the calibration of thermometers in air. This will provide best practice which is currently lacking. The work will also be presented at the CCT for possible adoption by other RMOs and as input to the WG ENV.
This proposal is also expected to bring valuable input to the following:
WMO CIMO Expert team on operational in situ technologies - for the definition of best practice and sustained performance classification, GCOS task team on Global Surface Reference Network - for inclusion in the requirements of reference grade air temperature measurements, GRUAN - for the general aim at documented traceability for radiosondes temperature profiles.
A detailed description of the project is reported in the Annex I to this project protocol.

List of acronyms
CCT = Consultative Committee for Thermometry
CIPM = the International Committee for weights and measures Comité International des Poids et Mesures
GCOS = Global Climate Observing System
GRUAN = GCOS Reference Upper Air Network
ILC = Interlaboratory comparison
NMI = National Institute of Metrology
RMO Regional Metrology Organisations
WMO = World Meteorological Organisation

Progress Report 2024-04-08

Objective 1:
Completed

ILC
The first task of the project was in terms of conducting an Interlaboratory comparison, among the participants, with the main objective of to identify unwanted influences, errors and contributions to the uncertainty on air temperature measurements.

ILC
The ILC was organised in three loops: NSAI, CEM and INRiM have been pilots. The measurements at JV provided the linkage between loops. All participants measured the resistance of all circulating thermometers in air at nominal temperatures -40 °C, -20 °C, 0 °C, 20 °C, 40 °C and 60 °C. In loop 1 additional measurements are carried out at -60 °C and -80 °C.

Loop 1: Participants: NSAI (Pilot), NPL, LNE, INTA, GUM, INTiBS, JV (Link)
- Range: -80 °C to 60 °C. JV only measures down to -40 °C, but participates to provide linkage with the other two loops
Loop 2: Participants: CEM (Pilot), DTI, RISE, VTT, UME, EIM, MBM, MIRS, JV (Link), DMP (added)
- Range: -40 °C to 60 °C
Loop 3: Participants: INRiM (Pilot), BFKH, SMD, PTB, CMI, SMU, BEV, JV (Link)
- The measurement range is -40 °C to 60 °C.

The ILC was concluded in 2023 and results published as final report on the open access repository Zenodo, The report from interlaboratory comparison of air thermometer calibration procedures can be downloaded here>>.

ILC results:

From the discussion oft the ILC results, at TC-T 2023 in Bratislava, it emerged the need to study a standardised method to minimize radiative effects on sensors under calibration in climatic chambers. Also in the case of air chambers in liquid baths a study is required to reach common design and performances. This was the objective of one PRT submitted to the 2023 EPM Green Deal Call.

Objective 2:
EURAMET Guide on the calibration of thermometers in air. This task is not yet started, waiting for the result of the further study on standard sub chambers.

Link with the BIPM CCT

This project was presented at the meetings of the BIPM CCT Task Group on Air temperature (CCT- TG AirT), which was formed in 2021, also thanks to the result of this EURAMET ATM 1459.
The CCT TG AirT started the work considering the results of the ATM ILC and promoting a further world ILC with an already standardised prototype of sub chamber designed.

Meetings: Project meetings are organised as side events at every TC-T.

Conference: At the 2023 “Metrology for Meteorology and Climate – MMC2023” Conference, a specific session on “Air temperature measurements” was organised.

Progress Report 2020-05-19

Meeting
A project meeting was held in April 2019 during the EURAMET TC-T meeting 2019. All project participants attended the meeting, which was focused on starting the ILC protocol. An invited talk from a representative of the length community (A. Balsamo - Chair CCL WG MRA) brought the needs of dimensional metrology for accurate air temperature values.
Pilots and partners presented their laboratories.
The ILC protocol and the reporting sheets, prepared by JV in close collaboration with the pilots, have been presented, discussed and agreed.
The main objective of the comparison is to identify unwanted influences on air temperature measurements, and to provide a basis for recommendations to laboratories that offer air temperature calibrations, or in some way use air temperature measurements as important auxiliary information during other calibrations.

Thermometers
INRiM coordinated the collection of thermometers taking part in the ILC loops.
The circulating instruments are PT 100 thermometers. At the start of the ILC six different models are available, each with three units (one for each loop). The models are sourced from different manufacturers and represent diverse applications. Most of the sensors can be placed in a TPW cell.
ILC
The ILC is organised in three loops: NSAI, CEM and INRiM are pilots. The measurements at JV provide the linkage between loops. All participants measure the resistance of all circulating thermometers in air at nominal temperatures -40 °C, -20 °C, 0 °C, 20 °C, 40°C and 60 °C. In loop 1 additional measurements are carried out at -60 °C and -80 °C.
Loop 1
The participants in loop 1 are: NSAI (Pilot), NPL, LNE, INTA, GUM, INTiBS, JV (Link).
The temperature range is -80 °C to 60 °C. JV only measures down to -40 °C, but participates to provide linkage with the other two loops.
Loop 2
The participants in loop 2 are: CEM (Pilot), DTI, RISE, VTT, UME, EIM, MBM, MIRS, JV (Link), DMP (added)
The measurement range is -40 °C to 60 °C.
Loop 3
The participants in loop 3 are: INRiM (Pilot), BFKH, SMD, PTB, CMI, SMU, BEV, JV (Link)
The measurement range is -40 °C to 60 °C.
Schedule
The schedule assumes on average 1 week for transportation between laboratories and 3 weeks for measurements. Pilots have another week to complete the measurements to account for measurements in liquid baths. Dates for the circulation in each loop are detailed in the protocol.
Progress
In May 2019 the ILC started. Participants performed the measurements in due time, as also the shipping followed the schedule. In 2019 the ILC progressed according to the plan. The capabilities of all partners and their usual participation in EURAMET projects and comparisons allowed such constant progress of the activities.

Progress Report 2019-03-15

After a launch meeting during the TC-T in Borås (SE) in 24 May 2018, the project started with presentation of the two main tasks: the research intercomparison and the preparation of the guide on calibration of thermometers in air. At the meeting the activities preliminary to task 1 have been planned and during the year the advances have been the following:

a) The comparison has been defined and will be performed within the limits of near surface atmospheric air temperature, from 80 °C to + 60 °C in two different ranges:

Range A: -80 °C to 60 °C

Range B: -40 °C to 60 °C

Participants choosed one of the two ranges considering that a condition for joining the pilot comparison is for all participants to have the capability to cover the whole temperature range for the selected option.
Two sub tasks were created.

Sub Task 1.1 Liquid – air comparison of calibration results (to be confirmed)

Sub Task 1.2 INRiM – IMBiH bilateral comparison

A task 0 was added, for collecting thermometers
A task 3 was added to describe impact activities

b) the advisory board (AB) members have been identified and the AB group made operative. the first task has been the discussion and drafting of the ILC protocol.
c) The protocol has been completed and is now ready for discussion and approval by all participants at the project kick off meeting in Torino, TC-T April 7 2019. At that date the ILC will start. It prescribes the technical and data analysis features for the ILC, together with loop dates.
d) an excell file with different sheets has been prepared to collect ILC data
e) after presenting the project to manufacturers, a representative number of thermometers has been collected in groups of three identical (each one for taking part in one loop in parallel).
At the time of preparing this report, the discussion is opened within the AB to consider the opportunity and technical aspect to make the ILC valid for CMC requests by participants.

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