Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

accuracy of stop watch

accuracy of stop watch

accuracy of stop watch

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.

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.


Greg Locock

RE: accuracy of stop watch

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

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!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


eBook - Integrating the Engineering Ecosystem
Aras Innovator provides multiple options for integrating data between systems, depending on the scenario. Utilizing the right approach to meet specific business requirements is vital. These needs range from authoring tools, federating data from various and dissimilar databases, and triggering processes and workflows. Download Now
Research Report - Simulation-Driven Design for SOLIDWORKS Users
In this engineering.com research report, we discuss the rising role of simulation and the paradigm shift commonly called the democratization of simulation. In particular, we focus on how SOLIDWORKS users can take advantage of simulation-driven design through two analysis tools: SOLIDWORKS Simulation and 3DEXPERIENCE WORKS. Download Now
White Paper - Industry 4.0 and the Future of Engineering Education
With industries becoming more automated, more tech-driven and more complex, engineers need to keep their skills and knowledge up to date in order to stay on top of this wave—and to be prepared for the Industry 4.0 future. The University of Cincinnati offers two online Master of Engineering degree programs designed specifically for practicing engineers. Download Now
eBook - The Design Gridlock Manifesto
In this eBook, you’ll learn 6 ways old CAD technology slows your company down and hear how design teams have put those problems to rest. “The Design Gridlock Manifesto” shares first-hand modern CAD experiences from 15 companies around the world. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close