Stop Watch Calibration Reality Check

gpainter

Quite Involved in Discussions
Time study departments not using calibrated stop watches should have their rates be declared invalid! If you ever dealt with them, this could be very funny!
 
K

Ken K

Ok, after reading through the Guide I tried the Direct Comparison Method on a couple of the stop watches. As per the NIST recommendation I used the 10800 second interval. This was selected to to keep the uncertainty of human reaction time (me) insignificant and to exceed the 0.02% requirement recommended.

On the first stop watch the time after completion was 10800.02 s. So for a
24hr period I can assume the time would be 86400.16 or +0.16 s / 24hrs which is well within tolerance.

My question is how many 3hr cycles are needed or recommended to assure the item is calibrated? Would one cycle be enough or would you average multiple cycles?
 
S

skappesser

Since you are gaining familiarization with this procedure, I would recommend that you repeat it a few times to gain confidence with it, just to verify that your uncertainty window is what you estimated. All your measurands should reside within your uncertainty value. Once you are convinced that you have a solid calibration method, then one cycle per DUT should suffice.
It has been my experience that one cycle is enough, unless there is a special requirement requested by the user (customer). I don't think the NIST procedure specifies more than once, unless I missed something.
 
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K

Ken K

Thanks for the reply skappesser. I was thinking 3 per timer and reporting the average.

And as you said, just to verify that your uncertainty window is what you estimated.
 
S

skappesser

Hershal - I agree about the NIST procedure. Have you ever been to their Measurement Museum? It is so cool. If you are ever in the Maryland area on an assessment, you should give them a call to see if you can gain access. Their security is tight - not sure if you still need a sponsor to get on the campus these days.

I like to suggest that the GPS frequency standard systems are a great money-saver for organizations that have a lot of basic time/frequency test and measuring equipment. A good used GPS system can be bought for like $5k, and can be used in-house to cal / verify almost anything with a time base.

All our electronic counters and synthesizers at our facility source a GPS system as the main time base. Saves a lot in outside calibration costs, at the same time maintaining low uncertainties as a high quality primary frequency standard. Sure is easier and more affordable than a Cesium or Rubidium system...
 
S

skappesser

I love it Jim! How about an official metrology joke de jour?

Q: How many metrologists does it take to replace a light bulb?

A: None, they make a calibration technician do that...!

HEEEE-HAWWWWWWWWWW!
(Lord, I apologize for that...never met a cal tech I didn't like!)
 
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Hershal

Metrologist-Auditor
Trusted Information Resource
My favorite is from my days as a bench tech (yes, I really was one.....), which went:

We do precision guesswork.

Course that was the good ol' days before MU.....

Hershal
 

RCW

Quite Involved in Discussions
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:rolleyes:

I have a customer who has found it cheaper to buy a new NIST traceable stopwatch rather than have the previous one calibrated at a lab. However by not having the previous stopwatch calibrated at a lab, he does not know if the stopwatch was previously out of cal at anytime since the original calibration.

Any comments on solving his dilemma?

Looks like it is better to review the NIST calibration procedure posted in this thread and calibrate it yourself. This is still probably more expensive though than just buying an inexpensive (aka cheap) new one.
 

BradM

Leader
Admin
I have a customer who has found it cheaper to buy a new NIST traceable stopwatch rather than have the previous one calibrated at a lab. However by not having the previous stopwatch calibrated at a lab, he does not know if the stopwatch was previously out of cal at anytime since the original calibration.

Any comments on solving his dilemma?

Looks like it is better to review the NIST calibration procedure posted in this thread and calibrate it yourself. This is still probably more expensive though than just buying an inexpensive (aka cheap) new one.

Good morning!

I don't understand how they can justify buying new ones, when all you need is a telephone to verify the existing timers. I can do five or six timers (not at the same time; I stagger them by 1 minute) in an afternoon.
Now, there is the caveat of the process and how accurate they need the timers to be. I usually check to a process requiring +/-3 seconds or more. If it is a tight tolerance, there may need to be a different procedure.

The cost of buying new ones, purchase orders, shipping charges, getting new ID numbers, etc. and then disposing, seems to not be a total cost not to be taken lightly at every interval.

Also, how good is the calibration they get with the new one? Does it give them true confidence?

Too, timers generally seem pretty reliable. You may can extend the interval a bit.
 
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