Stop Watch Calibration Reality Check

[Marc]

I thought this was good. You can guess the gist of the previous 'advice' on calibrating a stopwatch....

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Date: Thu, 4 Feb 1999 09:00:00 -0500
From: "Klouse Kenneth P." KlouseKP MSHA.gov
To: Greg Gogates
Subject: RE: Re1: Stop Watch Cal (fwd)

Unless I'm missing something, anyone who spends more than 15 minutes calibrating a stop watch, or claims accuracies of .001%, better smell what their peddling, get a life, or start measuring the reflexes of the stop watch operators. I'm not saying that stop watches so not need to be calibrated, I'm saying that one should look at the accuracy needed for the measurement, and calibrate the instrument accordingly. As Jeff Gust so eloquently put it, " As you would note, the random error from the person dominates the uncertainty budget, and in fact is 41 times greater than that of the unit under test tolerances. " If that person hasn't had his coffee that morning or he didn't get enough sleep, his reaction time could be all over the map.

As an assessor of product certification schemes, it is enlightening to read responses from different people on these varied subjects. But sometimes I think we have to step back and see the practicality of what we are doing or requiring.

 

[Marc]

I sorta figured this also could fit here. Another reality check. What if there is no 'standard'?

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Date: Sat, 20 Feb 1999 05:54:03 EST
From: Rcveale
To: iso25
Subject: Re: Linear Displacement Potentiometer

Pierre,

There are no American nor ISO standards for linear displacement potentiometers. There might me a military standard that I am not aware of.

A group was formed several yars ago to write a standard for linear transducers, and a draft was written; however, it was never finished nor published as a standard. A couple of years ago there was an attempt to revive the Group, but there appeared to be little interest or need for such a standard.

If a need can be shown to exist and someone is willing to put together a group to do the work, there is a procedure in place for producing the standard.

Ralph Veale, Chairman
ASME B89.1 Subcommittee for Length

 

[Ken K]

Ah yes...stopwatches and timers

We have 8 timers and 4 stopwatches whose calibrations are due shortly. In the past we have just tossed them into the garbage and replaced them. During our last audit, the auditor asked why we don't just use the internet to calibrate them. OK...I'll bite

So, I'm looking at the calibration certificates that came with them. It states:

The maximum error of this Digital Timer at the time of calibration did not exceed 0.01%.

As measured: +/- 0.367 seconds per 24hrs.

Expanded measurement uncertainty at K=2: +/- 0.13 seconds per 24hrs.

So then I went to thise site: http://www.time.gov/timezone.cgi?Central/d/-6/java

I assume this is the site the auditor meant. So now what? Check each one against "The Clock"? For how long? How would one document the results? How could one prove this was actually performed without a certificate?

 

[BradM]

Probably there won't be a certificate. But you could accept World Time as a "Naturally Accepted Standard".

Verifying stopwatches is an interesting concept. One thing I have seen is a test rig. This "starts" the timers and the standard timer at the same time. It does minimize some of the human error.

BTW... Has anyone ever seen a stopwatch be wrong ?

 

[skappesser]

Attached is NIST's procedure calibrating stopwatches/timers - it's is freely available from them. I picked up a copy at the Measurement Museum during my last visit there. I doubt anyone could reasonably question the validity of this. Hope this helps!

 

[BradM]

Thank you for the NIST document! I'll try to read it in the next couple of days.

 

[afuller]

If you try and calibrate a timer using the internet wouldn't you have to consider the speed of the internet connection as well as the computer speed?
How can you verify what you are seeing on the internet is really accurate?

 

[skappesser]

You are welcome Brad.
Most of the cal labs I have worked in over the years had plenty of homegrown cal procedures for these devices, some of which were pretty good and others that were pretty hokey. If I remember correctly, our uncertainty for this measurement was like 0.3 or 0.4 seconds (K=2), mainly due to the human reflex component. Some procedures had electronic pick-ups attached to the stopwatch circuitry - gating electronic counters. I never failed an electronic stopwatch. There was an old mechanical timer I failed once..."back in the day".
When I heard that NIST published this one, I encouraged the large commercial calibration company I worked for at the time to adopt it. All the calibration techs really liked it.

 

[skappesser]

If you try and calibrate a timer using the internet wouldn't you have to consider the speed of the internet connection as well as the computer speed?
How can you verify what you are seeing on the internet is really accurate?

The NIST procedure I attached earlier addresses your question nicely...

 

[Hershal]

The NIST guide provided has a lot of good information, and is written by well known Metrology professionals.

I have assessed timer calibrations using GPS-sync'd time bases and that often results in MU under 0.1 seconds.

Hershal