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We compared three steel tapes on 23 April 1998:
Mike Wickiser's 60m IRWIN,
Pete Riegel's 30m Brazilian STANLEY,
Mike Sandford's 50m RABONE SILVERLINE.
Inspection of the Irwin showed that although the tape had many very small patches of paint removed at the edges of the blade, this was largely cosmetic and there was no evidence of thinning caused by dragging the tape along the ground which would only be serious if large areas of paint and the metal underneath were removed. The paint on the other two tapes was in very good condition.
PK nails were placed at the following approximate separations: 10m, 30m, 50m. All the tapes were stretched with a tension of 10lbs measured by a Salters 11lb spring scale. The temperature was 19C in the shade and 21C in the weak sun. For each measurement two readings were taken with Pete and Mike interchanging roles. The greatest difference between our readings with the same tape was 0.5mm, the reading resolution. Pete's Stanley Tape had not been used since its calibration at the Stanley factory in Nov 1996 (see MN Jan 1997 page 5). The factory calibration report showed it to be 9.7mm long, and the error to be uniformly distributed along the tape.
Two nails about 30m apart:
Stanley (PR) = 29.8645m after correction (+9.7mm) as per factory calibration = 29.8742m
Irwin (MW) = 29.873 m
Silverline (MS)= 29.872 m
All tapes agree to within 2.2 mm. The Irwin is only 1.2mm different from the Stanley which was factory calibrated in Dec 1996.
Two nails about 50m apart:
Irwin (MW) = 49.8655 m
Silverline (MS)= 49.864 m
The difference between these tapes is now 1.5 mm in 50m. It was 1 mm in 30m. The two differences are consistent with a very slight change of scale between the two tapes.
We realised that the final 10m of the Irwin had not been checked. We therefore carried out the following between nails about 10m apart:
Irwin 50m to 59.976m = 9.976m
Stanley 0m to 9.9742m = 9.9742m after correction (+2.8mm) as per factory calibration =9.977m
Over this 10m interval the tapes were tensioned by feel since we did not have a convenient attachment point for the spring scale, so this comparison may be slightly less accurate but nevertheless it shows that the final 10m of the Irwin tape is not significantly stretched. Noting the 200 foot point on the reverse of the tape was just beyond the 60m point and might have been used at some time by Mike Wickiser, we checked with the foot scale between 200 foot and 167ft 3.25in = 32ft 8.75in = 9.9758m, which agrees with the metric scale on the other side of the Irwin. We concluded that the Irwin was pretty much spot on. Mike S thanks Mike W for a good tape! Discussing the results, we felt it was probable that the Irwin tape had been inside spec when manufactured and was unchanged in length since manufacture. We regard the debate on [the stretching with time of] steel tapes as closed, at least as far as it affects calibration courses.
Mike Sandford and Pete Riegel. From Measurement News Forum 25 April 1998