Question on dead times

robert ashcroft cytomat at
Fri Aug 7 08:22:20 EST 1998

Dear Joseph,

Measurement dead-time does cause errors in machine count vs the haemocytometer 
The simple reason is that the machine misses measuring cells due to its blind 
intervals from dead-time, whereas the slide count doesn't miss them.

There is a simple way to see what the machine's dead-time actually is, by doing 
a kind of dose-response curve.

Once you do that, it is simple enough to predict your loss-rates from 
dead-time, at any cell flow-rate. Thios involves using a model of cell-flow, 
agreed as the Poisson distribution.

Basically what you do, is increase the concentration of consecutive samples, 
and keep increasing it until the registered flow-rate on you machine tops out. 
The top-out rate converts to the dead-time, by the inverse of the maximum 
registered flow-rate, the one you get, despite an increase in the concentration 
of the loaded sample.

After that, we can talk.

Best regards,
-----Original Message-----
From:	Josep M. Gasol [SMTP:pepgasol at]
Sent:	Thursday, August 06, 1998 6:51 PM
To:	Cytometry Mailing List
Subject:	Question on dead times

Dear friends,

Following some recent comments I would like to ask if anybody knows for
sure what's the "true" rate of particles that the electronics of a
FacsCalibur can see without losing events due to them passing in the dead
time zone. My experience counting natural planktonic bacteria suggests that
a flow rate above 500 particles per second underestimates true
concentration (measured with an alternative method). This can certainly be
due to two bacteria coming in the same drop through the laser, but could
also be due to the electronics not being able to handle that many events
per second. By the way, anyone has an idea of a way of checking for two
particles going in the same drop ? Increasing bead concentration and
relating doublets to singlets could be a way. Anyone ever tried ?

A further related question: If I am gating what I see with a "software
gate", (I mean not an electronic gate), I imagine that I should be
concerned with the "total flow rate" (particles in my gate and those
outside the gate) and not with the "acquired data rate". Am I right ?

Thanks for your help !

Josep "Pep" Gasol

Josep M. Gasol
Institut de Ciencies del Mar, CSIC
Pg. Joan de Borbo, s/n
E-08039 Barcelona, CATALUNYA
Phone: 34 93 221 6416
Fax: 34 93 221 7340
email address: pepgasol at
Department's web page:   <>

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