Rick Tarara mentioned timing errors when using photogate timers.
Although the context in which this came up is not exactly the same
context I want to tell you about, I think members of PHYS-L might enjoy
and find useful a paper I wrote about 20 years ago about errors in
photogate timing, and a way to fix them.
At one time I thought I would submit the paper to The Physics Teacher,
but the paper is 7.5 pages long, and I never had the energy to see if it
could be condensed. Actually, it is essentially two papers, one dealing
with timing, and another that gives a lab example with the timing method
we use.
The first part of the paper deals with a timing flags I developed to use
with homemade photogate timers I made that would resolve to the nearest
0.1 ms and would allow the timer to be started and stopped with signals
from any two events whether they occurred in the same photogate or
separate photogates in any order.
The lab is a very instructive lab, and at Bluffton University over 2000
general-education students have performed it.
The paper can be found at...
<www.bluffton.edu\~edmistonm\double flag timing.pdf>
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu