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Land survey precision tape calibration.

Proper use of measuring tapes (we call them chains) for precision work requires temperature correction and specific tension. Basic steel tapes have a coefficient of thermal expansion (CTE) of approx. 12 ppm per deg C.

Calibration tapes are made of Invar, and a similar alloy commercially sold as Lovar. These alloys have a CTE of about 1 ppm (?) per deg C.

Q: what is the true (accepted/published) CTE of 36Ni/64Fe steel (Invar)?

Q2: what is the CTE and composition of Lovar?

I have a K&E Lovar 100 foot 'standard' tape. (It has only 3 marks (0, 50, and 100).)

What I have found re: CTE of Lovar is anywhere from 0.5 to 5 ppm per deg. Some cite better than Invar, some say an economical alternative to Invar, or not as good as Invar, or somewhere in between Invar and steel.

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  • $\begingroup$ Do you need anything like 10 ppm accuracy for surveying? What is the task you're actually trying to accomplish? There are probably laser-based metrology tools that'll work better. $\endgroup$
    – Carl Witthoft
    Jul 12, 2016 at 13:05
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    $\begingroup$ It's perhaps a trip back in time. Yes there are new ($) technologies. But the state of the art, pre-electronics, was easily <10 ppm. The science is mature. So, the computations should address the best known value of CTE. And Lovar is in the 1-2 ppm class. I am looking for 'brochure' numbers on Lovar. It's metrology. And I have a pile of 1950s era, high gear, and want to characterize them, as best I can. $\endgroup$
    – user18317
    Jul 12, 2016 at 13:34
  • $\begingroup$ These alloys are not steels. The only similarity is that they contain some iron. $\endgroup$
    – blacksmith37
    Dec 3, 2019 at 2:14

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Presumably the measured properties here are credible, since NASA was comparing Lovar and Invar "for optical, instrument and electrical applications in particular where dimensional stability is critical" - for example space telescopes.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150018888.pdf

Note the caveat mentioned in the report: the measured properties depend on how the material was manufactured and heat treated, so the NASA data might not be applicable to your tapes.

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I have a K&E LOVAR 100 ft base tape. And the ‘brochure’ specs.

LOVAR per K&E is quoted at 0.44ppm/C. NIST discusses the difference in CTE amongst commercial “alloys of steel” as being only a few percent +/- and negligible. Steel is approximately 12-13ppm/C.

So, 0.44ppm +/- “a few percent” of 0.44ppm I believe is the answer to your question.

In several textbooks, field experience with Invar tapes temperature correction was unnecessary. Error in tension was a greater source of uncertainty. I have made careful measurements at near 0C and sunny conditions with tape temperature of 45+, over 180 m, 600 ft, using a digital load cell for tension. I can neither confirm or deny less than 1ppm. However, I applied 0.44ppm and the residual error was less than the temperature correction.

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By experience, it is not a good idea to take any standard value or manufacturer value. Just check your chain on 10 degrees and on 25 degrees for a very long distance, on the limit of your chain, with no arrow, so on a flat ground. And just make your numbers.

When you need to make a high precition work, you can not just take the values of one specification or one brand. You should check by yourself.

So try to find one element with very small contraction or expansion, like a masonry wall and use this one as your base. Then, check the length of it with one low temperature and with a high temperature. If the wheater conditions are not the same, then record it.

When you have some repetitions with different conditions then you can make your numbers to check the expansion of your chain.

The geodetic spanish base at Madridejos it was check with a 4 meters rod. It is a 14644 meters base... so it took 3 years to measure it in order to check the expansion of the invar rod. The accuracy was 2.5 mm for a +14 km baseline.

One manufacturer is giving this values as reference and as maximum deviation, but it is not the real one.

Anyhow, check This

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  • $\begingroup$ 1. I don't understand your answer. 2. Posting a link with "check this" is not accepted behaviour on any SE site. You should quote anything relevant in your answers, and use the link to back up your answer; no user should need to follow a link that you post. $\endgroup$
    – AndyT
    Sep 26, 2019 at 14:20
  • $\begingroup$ Ok, AndyT. Noted... It is not required to give me a negative vote in each post. I am helping in SO for excel issues and there are many replies like this. One time is enougj $\endgroup$
    – David García Bodego
    Sep 26, 2019 at 14:45

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