7/28/2019 Cuadro picnmetro

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Laboratrio Central de Metrologia

REFERENCES

1. ISO 3507 - Laboratory glassware - Pyknometers, Genve 1999;

2. ISO 4787 - Laboratory glassware - Volumetric glassware - Methods for use and testing of capacity;Genve, 1984;

3. M.G.Cox, The evaluation of key comparison data, Metrologia, 2002, Vol. 39, 589-595.

ABSTRACT

The EUROMET comparison Volume Calibration of a 100 ml Gay-Lussac Pycnometer, between fourteenNational Metrology Institutes (NMIs), was performed with the gravimetric method procedure. This paperdescribes the volume instrument, the method and formula for volume determination, the calibrationprocedure, the experimental conditions and the measurement results with the associated uncertainties.This comparison was expected to be representative for all type of laboratory glassware.

ELSA BATISTA, EDUARDA FILIPE

Portuguese Institute for Quality (IPQ)

Rua Antnio Gio, 2 Caparica - Portugal

STATISTICAL DATA

The determination of the statistical parameters: reference value, uncertainty of the

reference value and a chi-square test were performed according to M.G.Cox [3].

Consistency statistical test - Chi-square test

To identify the inconsistent results a chi-square test can be applied to all results.

degrees of freedom: = N-1

Regard the consistency check as failing if:

The obtained weighted mean using the 14 laboratories is: y = 100,0917 ml with au(y) = 0,0006 ml for k=2

The chi-square test gives the following results: ;

The consistency test fails. The value for one of the laboratories SLM:

>

The volume value for the SLM is then removed from the weighted mean calculation and anew consistent test is performed. The results now are the following:

;

In conclusion the results are consistent and a new value for the weighted mean wasobtained: xref = 100,0914 ml, u(xref) = 0,0006 ml with k=2.

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Determination of the referencevalue

Determination of the referencevalue uncertainty

Figure 3 Laboratory results with reference value

Figure 4 Mean average of presented uncertianties

INTRODUCTION

Volume measurement is critical in many laboratories and industries. In order to identifyand reduce possible errors in intensive liquid handling process it is necessary to calibratethe used volumetric glassware equipment.

The majority of the National Metrology Institutes (NMIs) performs this type of calibration.With the purpose of comparing the experimental calibration procedure and uncertainties aEUROMET 692 comparison was performed with 14 participants NMIs. The calibration ofGay-Lussac pycnometer of 100 ml, representative for all type of laboratory glassware, wascarried on and IPQ, as the pilot laboratory, provided the pycnometer volume standard

(figure 1).

The main reasons for choosing this type of pycnometer were:

Easy handling;

The volume cannot be changed unless breaking the instrument;

Easy cleaning;

Possibility to observe air bubbles.

The Instrument

A pycnometer is an instrument used forthe measurement of the density ofdifferent liquids from water to highviscosity paints. For a correct densitymeasurement, the volume of thepycnometer must be obtained bycalibration using a gravimetric method.

Figure 1 Gay-Lussac pycnometer of 100 ml

The suggest method to perform thepycnometer calibration was the gravimetricmethod, were the calibration liquid isdistilled water at a reference temperatureof 20 C. The following formula describedin ISO 4787 can be used for the calculationof the contain volume:

CONCLUDING REMARKS

This comparison involved 14 laboratories and lasted one and a half year. One of the majorrisks was to break the glass pycnometer and this occurred after 5 measurements.Replacing the pycnometer and adding a correction to the first 5 volume results solved theproblem.

Globally the results are quite satisfactory, the maximum and minimum reported volumesdiffer less than 0,01%. With the exception of two participants, the laboratories volumeresults were quite consistent with the reference value and with each other. The uncertaintybudgets are very similar and the major uncertainty component to the final uncertainty was,for the majority of the participants, the water density.

UNCERTAINTIES

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Components taken in account: massdetermination, mass pieces density,air density, water density, expandedcoefficient, temperature and othersmaller and specific components.

As can be seen the major source ofuncertainty is the water densityfollowed by the mass determination.

Figure 3 Laboratories results compared withreference value

THE METHOD

Correction of the measured results

During the comparison it was necessary to use two Gay-Lussac pycnometers because thefirst one (with the number 62) was broken after the participation of five laboratories andreplaced by a new one with number 144.To compare the two groups of results a correction was applied to the results of the firstpycnometer.The correction was obtained averaging the difference of the values obtained by thelaboratories that performed the calibration of both pycnometers, PTB and IPQ.

Table 1- Correction of the volume

The value 0,1118 ml was then added to the determined volume of the five laboratories thatperformed the calibration of the pycnometer n 62.

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Figure 2 Weighing scale and F2 mass

Difference (ml) Average

(ml)

Uncertainty

(ml)

IPQ 0,1109IPQ1 - IPQ2

0,1118 0,0016

PTB0,1120

PTB1 - PTB2

The suggest method to perform thepycnometer calibration was the gravimetricmethod, were the calibration liquid isdistilled water at a reference temperatureof 20 C. The following formula describedin ISO 4787 can be used for the calculation

of the contain volume: