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accuracy of stop watch
3

accuracy of stop watch

accuracy of stop watch

(OP)
I have an odd question that is bugging me and I really need some help.  We are timing a particular process that is very important by using a stopwatch and visually watching this equipment move from one point to another in only 2-2.3 seconds.  I want to change this to something more high-tec, but I need proof of course that this is inaccurate so as to determine the money is well spent.  Is there anywhere I can find the average visual accuity of a person to be able to measure this by hand, and the stopwatch accuracy.  Would anyone out there trust doing something like this for an important peace of equipment??  I never had but the crack engineering staff here finds it normal.  

RE: accuracy of stop watch

I never thought my father (the the BMOC jock history major) would be of any use in an engineering discussion, but I've been proven wrong. . . (he'll be thrilled!)

I recall a discussion with my dad about this--he's a former college sprinter and high school track coach. He had stated (I don't know his source) that 100 m times when hand-timed can be off by as much as 0.2 seconds. I also remember reading an article in Sports Illustrated about football scouting in which this same same time was stated as the error due to hand-stopwatches in clocking players' 40 yard times, so I presume that my father's source was reasonable.

Given that in both cases, the person with the stopwatch is the one doing the starting, this essentially boils down to a visual accuity issue in determining when the runner passes the end line. Your problem appears to involve a timing of both the beginning and end, so the error may be even worse.

Don't hold me to it, but based on these facts, I would say +/- 0.2 seconds of error (I would say this is a fixed error, and largely independent of total time scale). For something which takes a few minutes, that may tolerable, but for ~2 seconds . . .?

I agree with your initial feeling that they may need better equipment.
Brad

RE: accuracy of stop watch

You may need to do a "Gauge Reliability and Repeatability" study. This should be able to identify the true accuracy of your measuring process. We were briefly taught this as part of a course on Statistical Process Control, so that'd be a good place to start.

Cheers

Greg Locock

RE: accuracy of stop watch

2
Both my Traffic Accident Reconstruction and Human Factors Handbook give a mean time of 0.2 seconds reaction for an expected single event.  For a more complex event - starting and stopping the stop watch - the variance in from 0.35 to 0.4 seconds.  This would give you a +/- 15 to 20% error in your measurements just from timing, not to mention other sources of error.  I just hope your not working with something toxic.  Seriously, I would look at better timing equipment of some type.  I have used electric eye timers with great success when measuring to less than 1/100th of a second, and they are not that expensive or hard to use.  

One way to make your argument for better timing is to perform an error analysis on the total test system.  This will tell you the total possible error of the test and give you an idea of the significance of the timing error relative to the total.

RE: accuracy of stop watch

(OP)
Thanks... I am printing up these replies to maybe help coax people here to loosen some purse strings.  I just started here and was amazed! at the low consistency in the process here.  Thanks again!

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