Simple cell count summary

Dear colleagues,
Thank you for you replies. It is delightful to learn and here is my
take:
1.	Determination of absolute cell number by volumetric measurements
is problematic due to inaccuracy in volume assessments in general,
sheath back flow, and unspecified volume for flushing the previous
sample. 
2.	However, combination with time, which yields flow, opens
possibilities. Flow rates are very precise. Therefore, one can measure
the flow with beads (I wish flow cytometers had flow meters) and then in
the rest of samples use time elapsed to get the volume. There are
caveats: the counting should not be too terribly fast because flow
cytometers do not see events coming closer than few microseconds apart,
and also time measurement accuracy is a factor. 
Best regards,
Sergei