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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]