Steel Tape Correction Assumptions

It's been a long time, but I seem to recall, 68°F (20°C).

As I read the old textbooks, the tape was originally calibrated with some standard (non-zero) tension even when the tape is supported on a flat surface.

You can find formulas for the tension constant from cross-sectional area or weight per 100 feet. If that isn't in your books, I'll find it in mine.

Temperature constant is pretty standard (same in multiple references) for an iron tape. Standard was most often 68 F, I think.

Is there anywhere you can take your tape for comparison to a known distance?

In my opinion just laying the tape flat on the floor will probably introduce more error in a measurement than any temperature correction. It would probably be different if the tape would actually lay perfectly flat, but it probably wont due to age, storage, minor kinks, etc.

You really don't need to pull the hell out of it...just put some tension on the tape as you make your measurements, it will be a lot more accurate. 😉

> Is there anywhere you can take your tape for comparison to a known distance?

Ya, but it's under 2' of snow right now and it was 14 below this morning. Not interested.

Sheesh. Nothing in this business seems absolute.

> You really don't need to pull the hell out of it...just put some tension on the tape as you make your measurements, it will be a lot more accurate. 😉

OK. 5lbs? 10lbs? There's no sag, so I don't think I can technically use the tension formula. If I can't find any mfgr's recommendations, I'll just keep it taut and go from there. I have a new Komen fiberglass tape to compare it to, but that didn't come with any specs either on how much tension it needs at 100' to be correct.

Maybe I can measure using various random tensions, with both tapes, and load all the measurements into Starnet along with the EDM measurements I'll be taking, and let it deal with the uncertainty.:-D

Sheesh. Nothing in this business seems absolute.

> OK. 5lbs? 10lbs? There's no sag, so I don't think I can technically use the tension formula.

2 to 5 pounds on each end would be more than enough to get a tape (that has spent most of its life wrapped round and tight) to lay flat. If you're using the tape posted in the pic, I can see at least two minor kinks.

Just trying to help you get the tape as flat as you can.

You have email!! I have a pdf that should help. Also I would suggest you check for nationally standardized base lines in your area. The NGS page EDMI Calibration Base Line Program should give you a start. Although they are designed for EDM check/calibration many also have a 100 foot monument for tape standardization.

EDIT: The inclusion of a 100 foot monument is noted in the monument description section of the base line information. Bolton CBL near Columbus, Ohio includes the following:

>THE 100-FOOT TAPE CALIBRATION STATION IS STANDARD NGS CALIBRATION BASE LINE DISK STAMPED "100 FT 1992" SET IN THE TOP OF A 30 CM (12 IN) DIAMETER CONCRETE POST FLUSH WITH THE SURFACE OF THE GROUND. IT IS 58.2 M (190.9 FT) EAST OF THE CENTERLINE OF NORTON ROAD, 24.0 M (78.7 FT) NORTH OF, AND ABOUT 1.0 M (3.3 FT) HIGHER THAN, THE CENTERLINE OF JOHNSON ROAD, AND 20.1 M (65.9 FT) NORTH- NORTHEAST OF WOODEN UTILITY POLE # 191-19.

Sheesh. Nothing in this business seems absolute.

As I recall the standard was/is 15lbs at 68 degrees F for Lufkin highway tapes.

> You have email!! I have a pdf that should help. Also I would suggest you check for nationally standardized base lines in your area. The NGS page EDMI Calibration Base Line Program should give you a start. Although they are designed for EDM check/calibration many also have a 100 foot monument for tape standardization.
>
Thanks. That's perfect. I split the difference and pulled it to 17.5 lbs. The tape was flat. The stretch difference for the extra 2.5 lbs is barely visible to the eye, and probably less than the .0025 prick punch marks I put in the "monuments", aluminum bars chemically anchored in the concrete floor. I'm good to go.

As for the NGS baseline: That'll have to wait until spring.

Steel Tape Correction

After laying it flat, fully supporting it, pulling lightly on it and keeping it on a straight line, maybe even correcting for temperature, you have no idea if that tape is 99.95 feet, 100.000 feet, or 100.07 feet long (It looks like a 200' tape to me). That's the number one reason you need to calibrate it, especially if you're going to calibrate your EDM with it.

Maybe a silly question for me at this point as I'm now retired.
But it always bothered me. My books are in deep storage..somewhere.
I never remember anyone saying that you needed to correct a distance
(measured or laid out) made with a tape flat on the ground vs being held
at ends unsupported with a sag at set tension.
Maybe it wasn't a significant amount in the days of chaining.
Normally, you would only be going shorter distances (say maybe 25').

But intuitively you felt there MUST be a difference to correct for.
But how much??

One incident I remember was that we were going to measure vertically
up a Bilby Tower to establish an elevation on the Instrument Base.
A 100' HI so to speak. How to hold it relative to surface monument...in a handhole.
Nobody had any idea if we needed to hold some tension or how much,
what correction was needed if any. etc.
We pulled standard tension and figured that at the distances they were
going to be sighting it didn't make a difference anyway.
Plus this is Horizontal Control.

So, academic question, is there a correction and if so how much? Formula?

PS we had a 100' baseline (two steel brackets on a wall)in the basement
of a large office building. Think we held standard temperature
corrected for basement temperature and varied
the tension until it matched 100.000'. That was the value attached to the chain.

One of the pleasurable, pride building physical skills missing nowadays is learning to be a good CHAINMAN.

Thanks for any info

The Coefficient Of Expansion Varies With The Steel Type

An Invar Steel tape has 1/10 to 1/30 the expansion of a steel tape. It is used for baseline surveying and for calibrating steel tapes. It has to be handled much more carefully than steel. Coefficient of Expansion varies with the other alloys in the steel.

The tape you display with a reel may or may not be Invar Steel. Meticulous surveyors would take or send their invar tape to labs for checking. It would come back with a certificate. Without a certificate you do not know what you have. Any number anyone gives you may be completely erroneous.

You are looking for accuracy and precision which do not come cheap. However you do not buy accuracy and precision off the shelf. Anything you buy is rather useless without knowledge and experience.

Paul in PA

Temperature correction is almost universally 6.45 x 10^-6 per degree F, which means that 15 F change in temperature makes about 0.01 ft in 100.

One must distinguish between a tension stretch correction and a sag correction.

The tension stretch will be small. One textbook example for a medium-weight tape calculated 0.013 ft difference per 100 ft between 10 and 30 pound tension.

A vertical tape would have no sag, and its tension stretch behavior would be approximately as the average tension of the two ends, which of course differ by the weight of the tape. So measure the pull at the top and subtract half the weight or measure at the bottom and add half the weight, if it matters.

When the tape is supported horizontally at end points, the sag correction also depends on tension, and is much larger and variable. See your text book forumulas.

If the flat length is very near 100.000, then there is some tension that will balance the sag with the stretch to give exactly 100.000 feet, although it may not be a good value to use in the field.

A different text gave a sample cal certificate showing one tape's values
Horizontal flat surface
10 pound tension 1 to 100 ft measured 99.999 ft
11.5 pounds 1 to 100 ft measured 100.000 ft
Supported 0 and 100 : 36 pounds for 100.000 ft

As others have mentioned actual tape length, temperature, tape sag and tension all need to be determined and if necessary corrected to get a good measurement. The National Institute of Standards and Technology Dimensional Metrology Group (683.01) performs calibrations of metal measuring tapes.

Although this may appear to be overkill I have worked on projects that required one tape to be standardized and retained strictly for use in checking all other tapes used on the projects. In the 1990s this was a standard part of some USACE surveying contracts.

The Coefficient Of Expansion Varies With The Steel Type

From Lufkin Catalog 201 dated June 1976

Lufkin Steel Tapes are made to government standards. Our bench standards for the United States are certified accurate by the United States Department of Commerce, National Bureau of Standards (Now NIST), Washington DC. Tapes for Canada we make to Canadian government standard.

Temperature standard for steel tapes is 68 degrees F which equals 20 degrees C.

Coefficient of expansion of steel tapes is 0.00000645 per degree F; 0.0000116 per degree C. This amounts to 0.000645 foot or 0.00774 inch per degree F on a 100 foot tape and one-half as much on a 50 foot tape.

(Leaving out information about correction for long runs of pipe and explanation of temperature correction graduations which were an option on long steel tapes)

Standard tension for steel tapes 25 to 100 feet long (10 to 30 meters), supported entire length on horizontal flat surface, is 10 pounds (4.5 kg); for all tapes over 100 feet, 20 pounds. For steel tapes suspended horizontally and supported only at ends, there can be no set or standard tension for accuracy because of weight and other variations.

The following listings are averages of many tests made by the Bureau of Standards of Lufkin steel tapes supported throughout and supported ends only. These averages are taken from experience of tests and should only be used as a working basis only and not to replace the test of any individual tape.

1/4 inch x 100 ft chain tape 10 lbs fully supported, 24lbs ends only
1/4 inch x 100 ft Engineers (.012-inch) Tapes 10 lbs supported, 18 lbs ends
1/4 inch x 100 ft Engineers Extra Heavy Tape 10 lbs supported, 24 lbs ends

This information presented in table format. First column header - Tension of Accuracy supported throughout, Second column header - Tension of Accuracy supported ends only

Note: Temperature and tension need to be taken into account only on very precise work. We recommend government test of tapes to be used for such work or for use as master standards.

And from the description of "Minvar" tapes. Lufkin's brand for invar.

Lufkin Minvar low expansion base line tapes are made from a special alloy. They have an extremely low coefficient of expansion that is 1/30 that of the high quality steel normally used in fine tapes.

and

Standard procedure for comparing or establishing accurate measurements requires a temperature of 20 degrees C (68 degrees F) with 9 kg (20 lbs) tension (tennsion omitted in print) when supported on a horizontal flat surface.

Interesting note is the standard offering of the Minvar 100 ft tape seems to only have had graduations at 0, 50 and 100 ft

Letting Starnet do its thing

Well, if I remember correctly, with angles, I adjusted the Instrument Standard Error settings until the Error Factor approached unity (for the angles).

So, would I do the same thing for distances? If I tighten up the .006562 (2mm) published number for the instrument, I can get the error factor up, but is that the proper way to appraise what the instrument is actually doing?

Or is this just a fools errand, in that I have prisms involved too. When I did this same test last fall with the same instrument on a slightly different range, AC was also .005 greater than the sum of AB and BC.

Back in the early 80's doing Florida DOT contracts we used to compare our Lufkin Highway Tapes to an invar Babbit(?) tape at "assumed" 68F with tension scale at 15lbs supported at 33' and 66'. We liked chain reels when we had them but some new guy would break it and we went back to throwing ours. Some guys can double throw, I'd like to see that.

I still have my Lufkin 6" wood combination correction scale.It gives scales for 50' and 100' tapes The markings work out to about .01' per 15F. I think it odd that the scale starts at 52F and goes up. I wonder what they used in N Dakota.
I've blown up a photo to highlight the markings on mine. If you wish to see it, I can e-mail it.:plumbbob:

I forgot to congratulate you on having 100' indoors:good:

This is what we used