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
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