Hi Mathias

We calibrated our 0.4 and 0.1mm pipettes sucking degassed water in and pumping it out again on a vacuum line. The trick here is to have a vessel with degassed water on a precision balance. For each of these steps the weight decrease of the balance is measured (weights stored online as function of time, evaluation of a step function as linear extrapolation...). Advantage is that not the large weight of the pipette is on the balance but the smaller weight of the water vessel (around 60g, allowing for higher precision balances). 100 fillings per day are feasible in automated mode, giving a precision (as standard eviation of several intakes) of better than 0.4% for the large and better than 0.9% for the small pipettes. Of course the uncertainty of the mean is much smaller (around half a permill or better), but I tend to trust more in the standard deviation because I suppose that the filling process itself is not more precise (probably also for gases: mechanical deviations of the position of stamp on seat during each process, gas flow, etc.).

The general principle of this kind of calibration was first described in a diploma thesis (Jörg Glandorf (1991), Department of Environmental Physics, Bremen University), but as far as I know is unpublished yet.

However: five digits in my opinion are really difficult...

Axel

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Axel Suckow
Isotope Hydrologist, Geochronologist
Isotope Hydrology Laboratory
International Atomic Energy Agency
Wagramer Strasse 5
A-1400 Vienna
Austria
Phone: 0043 1 2600 21759 (desk) or 26762 (mass spec)
email: [log in to unmask]
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''
 

> -----Original Message-----
> From: Noble Gas Geochemistry Network 
> [mailto:[log in to unmask]] On Behalf Of 
> Matthias Brennwald
> Sent: Friday, 13 November 2009 08:51
> To: [log in to unmask]
> Subject: Re: air pipette repair/construction
> 
> On Nov 13, 2009, at 1:00 AM, NOBLE-GAS-NETWORK automatic digest system
> wrote:
> 
> > I'd be very happy to learn about the design details for a pipette, 
> > beeing a priory accurate to 5 digits. Can you please share these 
> > information?
> >
> > Thanks
> >
> > Axel
> >
> > (the next noble gas machine to build surely comes...)
> 
> 
> Me, too!
> 
> Actually I just finished volume calibration of three pipettes 
> to be used in our lab. I did this by diluting pure nitrogen 
> gas into the pipette volume, and compared the dilution factor 
> with the dilution I get from a known reference volume. The 
> dilution was measured using a Digiquartz pressure sensor 
> under controlled temperature conditions.  
> This yielded a precision of about 0.5 permille (i.e. four 
> significant digits). The accuracy is determined by the 
> accuracy of the reference volume (also about 0.5 permille in my case).
> 
> Another method is to fill the pipette with (degassed) water 
> and compare the weight of the filled pipette with that of the empty
> (evacuated) pipette. I think this is what many did in the 
> past. There are several drawbacks, however:
> * The pipette itself may be much heavier than the water. 
> Compared to the pipette itself, the weight difference is 
> small and difficult to measure. To circumvent this, people 
> used small and light syringes to fill the pipette with water 
> and compared the syringe weight before and after filling the 
> pipette. This step solves the problem, but adds another 
> source of uncertainty. Also, I was told the syringe weight 
> was found to change during weighing because water evaporates 
> from the tip of the syringe.
> * It can be difficult to find and remove water residuals, 
> e.g. from the inner parts of the valves. Even if the 'outer' 
> part of the valves is dried under vacuum, the water may 
> prefer to stay in there for a while.
> * The procedure is time intensive, which may mean you cannot 
> repeat the measurements to get 'good' statistics. The 
> gas-dilution technique is much quicker. I could easily do 
> about ten measurements in one day.
> 
> I'd be very interested to hear how others do it.
> 
> 
> Matthias
> 
> 
> ----
> Matthias Brennwald
> Eawag, CH-8600 Dübendorf
> Office Eawag BU-C08: +41 44 823 53 05
> Lab Eawag LA-C76: +41 44 823 53 06
> Lab ETHZ CLA A31.6: +41 632 09 36/39
> [log in to unmask]
> 

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