8/10/2019 Verification of pipette calibration

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8/10/2019 Verification of pipette calibration

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Figure 1: Box plots comparing the volumes delivered by Gilson and volumetric pipe es, expressed as a percentage of theselected volume. Data pointsfor singlealiquot and10 aliquot trials areshownas blue crosses andredpluses, respec vely.

Table 2: Measurement uncertainty es mated by the GUM method for the P1000 pipe e.

Uncertainty Source Value (

) (L) Distribu on Divisor Standard Uncertainty (L)

Room temperature ( 2 C) 0.4 Normal 1 0.4Standard error on mean of data 0.20 Normal 1 0.20

Barometric pressure ( 10 kPa) 0.1 Normal 1 0.1Read-out resolu on (0 .1 g) 0.05 Rectangular 3 0.03

Combined Standard Uncertainty - Normal - 0.46

Expanded Uncertainty - Normal, k=2 - 0.92

Table 2 presents an es mate of uncertainty by the GUM (Guide to Uncertainty in Measurement) method.With a 95 % condence interval, the expanded uncertainty for the P1000 pipe e is 0 .92 L. Similar uncertaintyanalysis for the P100 and P20 pipe es gives expanded uncertain es of 0 .12 L and 0 .07 L, respec vely, withthe uncertainty from the effect of room temperature and barometric pressure on the Z-factor diminishing atlower volumes. The volumetric pipe e has an expanded uncertainty of 28 L.

4 DiscussionUncertainty can be reduced by accoun ng for the effect of temperature and barometric pressure on the

Z-factor. Since this effect scales linearly with mass, this reduc on in uncertainty will have greatest effect onmeasurements of large volumes. The Z-factor varies by 2 10

4 Lmg 1 C 1 for temperatures around 20 C,and by 1 10

5 Lmg 1 kPa 1 for pressures around 100 kPa. For the P1000 pipe e, measuring the temper-ature to 0 .5 C would reduce the corresponding uncertainty to 0 .1 L, represen ng a reduc on of 75 %.

As can be seen in Figure 1, the data points for P20 are split into 3 discrete values; this is due to the limitof the read-out on the analy cal balance. The balance reads to 0 .1 mg which equates to 0 .5 % discrimina onfor the P20 pipe e. The uncertainty of the read-out dominates for small volumes, but this can be reducedthrough the use of a 5-gure balance, which would give a standard uncertainty of 0 .003 L.

For the volumetric pipe e, contamina on of the internal surface at the p led to high reten on. However,this reten on was sporadically overcome by the contact of the p against the side of the beaker, dependingon the exact angle of contact. Since the angle of contact had some varia on, this led to large random errorswith a bias that lowered the mean value, causing a large systema c error.

In conclusion, the P20, P100 and P1000 pipe es demonstrated reasonable calibra on, whilst the 5 .00mLvolumetric pipe e was shown to be unreliable and inaccurate, likely due to contamina on.

References[1] Gilson, Inc., 3000 Parmenter Street, P.O. Box 620027, Middleton, WI 53562-0027, USA. Verica on Proce-

dure for Accuracy and Precision , August 2007.

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