Why is it that when I check my pipettes for accuracy they are out! Its more complicated than that. Firstly let me state fro the outset. I do know how to pipette and both forward and reverse (in fact I also teach the newly graduates when they first start in the lab). Our pipettes have new o-rings and seals replaced every 12 months (a bit of an overkill but its in our budget to do so). I also know how to fix and adjust them correctly.
Now the problem. When I go to measure the volumes gravimetrically, I find that they are spot on at lets say 20ul but when I crank it up to 200ul they show that I am measuring considerably less - in as much as 20ul for one pipette (and its a relatively new one only 2 years old). All pipettes are in pristine working order and the temp of the water etc is correct so please do not question the calibration checking technique. (sorry I am a bit anal about how I do it as I know its correct).
So I then take the pipette and measure this:
at 20ul I get 20ul
at 50ul I get 48ul
at 100 I get 96ul
at 150 I get 143ul
at 200 I get 189ul
(The measurements are done at 23deg all uniform in every way. That is the speed of the pipetting, the orientation of the pipette, the time of equalisation of the balance (approx 20 per measurement). Yes this example above is typical of what it happening. I am sure that I have eliminated every measuring factor. No leaks, clean etc. The balance is correct.
How can a pipette do this. Its basically a syringe with a spring. Its a simple instrument. I am hesitant to send the pipettes off to be checked and calibrated when I am sure that they are correct. I would say that 5-6 of them are doing this. I am wondering if I should calibrate them at the midpoint and then accept the rest. This has started to creep in at the moment. I am following NATA guidelines with respect to the volumes that need to be checked on a pipette: 10% of the max volume and the Max volume of the pipette is the minimum requirement.