Calibration check of manual pipettes

3 posts / 0 new
Last post
Nic
Nic's picture
Calibration check of manual pipettes

Hello,
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.

Thanks
Nic

Guest (not verified)
Guest's picture
The expectations based on

The expectations based on time are misleading. The pipette should last the certain number of measurements. In average usage, it may take a year. In heavy usage, it may be much shorter time.

I would also focus on users.  Do they often drop pipettes on the floor? When they increase the volume, do they go over the volume and back to desired volume? This is recommended by manufacturers to save rubber seals. Are pipettes used in with corrosive chemicals? Using tips with the protective barrier may help.

I do not worry about this much. I just check the calibration regularly and fix the ones that are off.

It may be a good idea to talk to manufacturer customer service about your concerns. The companies often do know their bussiness.

Nic
Nic's picture
Owczarzy,

Owczarzy,
Thankyou for your reply
Our pipettes are certainly not over used, compared to other sections of our lab.  I did say in my initial post that when I checked the pipettes for correctness ........ all the factors were done correctly.  Pipettes are used on peripheral bloods and antibodies.  No need for barrier tips for the work we are doing.  We do not drop pipettes and they are stored upright.

So how do you fix this problem with the pipette.  If I adjust the volume at 200ul then the volume at 20ul is way above its volume.  My initial thought is to set the correct calibration volume at the 50% volume (ie 100ul on a 200ul pipette.
I am hesitant to mention the brand of pipette we use but suffice to say that they are a common and well respected brand on the market.
My last resort is to contact the manufacturer but I would like to understand why this is occuring.  As an adjustable pipette should deliver either all high or all low (or the desired volume) of the volume.
The only factor I can think of is that at the low end (20ul) we are at the lower end of our balance limit. (we are using 4dp balance).