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Q: where t buy calibrated steel tape measure or similar ( No Answer,   2 Comments )
Subject: where t buy calibrated steel tape measure or similar
Category: Science > Instruments and Methods
Asked by: scienceparknews-ga
List Price: $30.00
Posted: 31 Jul 2004 23:18 PDT
Expires: 07 Aug 2004 13:11 PDT
Question ID: 381908
I need to buy a device so I can make accurrate distance measurements
in the range of a few inches up to 25feet. Ideally this will be a
steel tape measure or laser distance measurment meter that comes with
a signed certificate confirming that it has been calibrated and the
calibration is traceable to NIST.
If NIST traceable calibration is not available, I still need a signed
certificate saying that it has been calibrated by some metrology lab,
preferably at the manufacturer, since this will save time (sending a
tapemeasure to an outside lab for calibration/certification is
possible, but will add to both cost and elapsed time. I would
like to order and receive the device well before the end of August 2004.
The device, including calibration, should be less than $200. My
absolute top price limit is $600 for the calibrated device.

So far I have checked the NIST web site--they don't sell tape
measures. And suppliers of surveyors equipment--they sell the tape
measures and laser distance measurement devices, but their web pages
don't offer any calibration certificate, but I need a certificate.
Probably if I called the surveying equipment manufacturers they could
advise me. I know boat builders and machine tool makers and lots of
companies need to make accurate distance measurements. I look forward
to your help on this problem! thank you.

Request for Question Clarification by pafalafa-ga on 01 Aug 2004 04:23 PDT
Could you possibly tell us a bit more about WHY you need such a device?

It's hard for me to believe there is such a thng as a calibrated steel
tape measure.  NIST standards are accurate out to something like nine
or ten decimal places, but an off-the-shelf tape measure is going to
expand and contract with temperature changes, and with use, and
probably from other factors as well.  It's good for one or two
decimals places, tops!

A laser might be more stable and more accurate, but even this will
vary according to the operating conditions, and won't be consistently
precise at the level of exactitude used by NIST and other
standard-setting bodies.

Perhaps if you explained a bit more about just what you need, and why,
we might be able to assist you.



Clarification of Question by scienceparknews-ga on 01 Aug 2004 12:16 PDT
I calibrate geiger counters and other radiation detectors for a
living. In previous years, the government inspector has asked to see
the calibration certificate for the NIST traceable gamma radiation
source that I use, (no problem, I have that.) and the nist traceable
certificate for the pulse generator I use, again--no problem. This
year he wants to know how I can prove the accurracy of the distance
measurement between source and radiation sensor (20inches to 25ft). He
is being quite rasonable in his request since the radiation field is
determined by the distance and the source strength. My calibrations
need to be accurate to plus or minus 5 percent, so if I have a
distance measurement that is accurate to a tenth of an inch, I'm ok.

The inspector does NOT want to hear that everything SHOULD be ok
because I bought a tape measure or laser device from a repudable
manufacturer, he wants to see a calibration certificate! I am
perfectly willing to do temperature correction calculations. If the
manufacturer will put a serial number on the tape measure or etc. and
will provide a signed certificate from his in-house or out-of house
metrology lab, stating that (at such and such a temperature)it is
accurate to plus or minus 0.1inch (or 2mm-- a metric scale device is
ok)that will satisfy my requirements. The reason I mentioned NIST is
that, to certify the tape measure, the metrology lab has to carefully
check it against a STANDARD, it is usual to describe the STANDARD on
the calibration certificate. That standard may or may not be "NIST

Request for Question Clarification by pafalafa-ga on 01 Aug 2004 12:46 PDT
Thanks for the's a big help.

I'll keep looking, and let you know if I find anything.  In the mean
time, though, What does the government inspector have to say about all
this?  How do others in your situation calibrate their distance
measurments?  Are there any government guidelines, best practices,
etc. that come into play here?

I can't believe you're the only person this inspector is dinging for
distance certification.  The inspector should be able to provide
additional guidance, I would think.

Let me know if you've explored this option with your friendly G-man (or G-gal!).


Clarification of Question by scienceparknews-ga on 02 Aug 2004 23:22 PDT
If I can't solve this myself, then I will have to ask the g-man for
any suggestions he may have. However, I would much rather solve it
without his help. I agree with your statement that there must be lots
of people who need to make accurate and VERIFIABLE distance
measurements in this range--I hope to find out how others do it.
There is no answer at this time.

Subject: Re: where t buy calibrated steel tape measure or similar
From: crythias-ga on 01 Aug 2004 12:34 PDT
You said:
"My calibrations need to be accurate to plus or minus 5 percent"

5% of 25 feet is 15 inches. 
5% of 20 inches is 1 inch. 

Regardless of variations in steel with weather, 5% is a HUGE
variation. Anything you get is fine. Measure first with any standard
measure tape, then collaborate it with a laser measuring device. Take
the average, you've got your accuracy. The worst that could happen is
both measurements are off significantly. I doubt any house contractor
would be willing to risk 5% accuracy at 25ft with his Stanley tape
Subject: Re: where t buy calibrated steel tape measure or similar
From: crythias-ga on 01 Aug 2004 12:54 PDT
Accuracy 99.5% +/- 1/4"(1cm)

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