Comparison on Uncertainty Calculations in Flow Calibration

Project Description

Earlier flow inter-comparisons have shown differences in uncertainty judgements not only in size but also in fundamental understanding which aspects should be covered by an calibration uncertainty statement and which not. Since a few years we have the Guide to the Expression on Uncertainty in Measurement. But the older ISO-Standard 5168 also still exists. However it seems appropriate to work for a harmonisation in this field. Such a task could be performed in different ways. We propose to let the interested laboratories make an uncertainty evaluation on a given calibration result.

We suggest to start with an actual example of a gravimetric calibration of a flow meter at our laboratory. Together with a short description of the flow rig and the way the calibration was performed all the raw data and the calculation of the final values are given. The participating laboratories are then asked to start an evaluation of the uncertainty for this calibration. In doing so it will probably be necessary to ask for additional information. We then intend to answer incoming questions concerning different aspects of the work done or the test equipment used.

As a result of the exercise each participating laboratory presents not only the final uncertainty figures but also a detailed presentation of the evaluation. Our laboratory will then put together the different contributions and analyse similarities and differences. On the basis of this we both could learn from each other and form the basis for a common approach.

It is certainly necessary to discuss the outcome of such a comparison. However it is quite time consuming to meet. Further it is most probable that more than one person from each laboratory would be engaged in this discussion. A new way of administrating such a discussion could be to start a bulletin board on internet where everyone from the participating laboratories could both give his view and read everyone else.

If the participants will find this exercise useful it could be proceeded by  putting forward a somewhat different calibration example involving for  instance a volumetric example or using a master meter and a two stage calibration technique. It also would be possible for other laboratories not  engaged from the beginning to join later on. Further on even more  complicated calibration situations could be analysed of those interested.

Final Report 2000-03-13

The purpose of the suggested project was to compare and harmonize flow measurement uncertainty budgets. The way to go was to let all participants perform an analysis on a distributed calibration example (gravimetric hot water calibration).

At the Euromet delegates meeting in Vienna (March 1999) a certificate on a flow meter calibration in water was handed out to the interested delegates together with detailed information of the meter (Woltman meter), photos of the rig and the calibration object as well as a sketch and a description of the test bench, the calibration procedure and all the raw data. Based on this information the collaborating laboratories were asked to deliver their solution to an uncertainty analysis and budget for this calibration result. The piloting laboratory (SP) offered to answer questions related to the calibration that the other participant might ask for providing their solution. A date, end of June 1999, was set out to send in suggestions to the project coordinator for an uncertainty analysis concerning the described calibration. Besides SP’s analysis only one answer from BEV (Austria) was received. And no further contribution arrived after that.

This small project progress was reported during the following Euromet meeting 2000 in Warsaw (point 3.8 of the minutes). There also the SP-analysis was presented and a short comparison with the BEV budget was given. The SP-analysis was distributed with the minutes and was later on send to laboratories that asked for this. But the interest to go on to repeat this exercise with a different calibration technique and a new situation, as was originally planned, was very cool and it was decided not to go on. Thus a comparison report between individual budgets was never written.