drummstikk wrote:Gahr wrote: But if you have a crappy one, you can reduce your error by making several small dilutions. So, instead of diluting 10 uL in 1 mL to get a 100x dilution, dilute 1 mL into 2 mL seven times for a serial dilution of 128x. You're just repeating a small dilution over and over until you get the large dilution you're looking for. You'll have to use more of your slurry (1 mL vs. 10 uL), but it will be worth it for the accuracy.
That works only if you ability to measure 1 mL is appreciably better than your ability to measure 10 uL. Each time you do a diliution you make 2 errors - first in measuring the aliquot and second in making up to the new volume. These errors accumulate. If, for example, you make a 1% error (standard deviation) in measuring out the aliquot and a 1% error in making up to the diluted volume and repeat this 10 times the stadard deviation in diluted cell count will be 2*sqrt(10)*1 = 6.32%. If your 10 uL pipetter is off by 10% clearly this puts you ahead of the game but if you micropipetter is off by 5% it doesn't. I just pulled out these numbers for convenience. I'll note that Class A glassware is good to about 0.1% and a Hach TenSet pipet good to about half a percent at 1 mL.
All this is mooted if the error levels from the number of cells counted (see previous posts) are an order of magnitude higher than these numbers.