I just got to read Rick Brown's eloquent analysis of timing variation. He
is right on the money.
The SCCA did not readdy consider the scientific definition of accuracy or
precision when they wrote the rules. I asked them about this before I ever
produced the first box.
I also forgot to comment on photohead response time. It is indeed the
limiting factor for accuracy. There are only a handfull of commercially
available photoheads that meet the 4 criteria needed for being sold as part
of a JACircuits timing system:
1) be a 'light-operate', current sink interface (many, many available)
2) run on 10-18 VDC (many available), preverrably 8-30VDC (few
available)
3) have 0.001 sec response time (a few available)
4) have 80-100 foot range (a few available)
The Radio Shack photoheads are abysmal for response time. The photoheads
that have a 0.001 sec response time actually run at 8-10 KHz so they can
'vote' on whether they should send a trip signal or not.
One of the West Coast SCCA regions opted for a longer range retroreflective
photohead that had a 0.005 response time. They acknowledged they were
trading off precision for reduced potential damage to the photoheads.
John
JACircuits Autocross Timing Systems
http://www.jacircuits.com
- Timing the SCCA Solo II Nationals since 1985
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