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After participating in Pete's quiz on cal course temperature compensation, I wondered how well it works in practical application.

So this morning I went out in the cold on my street to measure a 100m section of my calibration course. I have measured this 100m section many times, a couple of them when the temperature was close to 68 degrees, so I know it is pretty accurate.

It was about 39 degrees when I did the taping, one length of my 100m tape. There was bright sunshine, but it was low in the sky and the course is partly shaded, so I think that had little effect.

My measurement was 100.025m. Applying the temperature compensation gives 100.006m.

I plan to repeat this measurement later this year when it is 80-90 degrees.
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Since accuweather said it was 97 degrees this afternoon, I decided it was a good day to complete this experiment.

My measurement of the 100m section in front of my house gave 99.978m. With the temperature compensation for 97 degrees the final distance is 99.997m.

So the difference in the cold and hot measurements after temperature compensation was 9mm.
Devil's Advocate here. (By the way, Mark, thanks for the effort to supply this info!)

1) The difference between the corrected measurements is 9mm, or less than a cm., with a temperature variation of 58°, on a 100m course. Take that out to a 300m cal course, which is common, and we have 2.7 cm difference.
2) The length of my Jones click is 8.7 cm
3) The adjustment due to temp. is 31% of the length of a Jones click
4) Most measurers only acknowledge half of a click during calibration, if they don't use even click readings.
5) I'll continue to do temperature adjustments, but, if the adjustment due to temp. is less than the margin of error in click-lengths on a calibration ride, why do we do temperature adjustments if we create a course with the temp. above 35° and below 95°? I would expand that range, but those would be normal temps during the taping of a cal course.

Just looking at the basis for the process.
Temperature correction of calibration course layout distance has its origins in the Corbitt era. Subsequently written instructions have continued its requirement.

Years ago I proposed elimination of the requirement to do it, but it did not meet with general approval. I remain convinced that it would do little harm to make it optional, not required.

I’ve gotten it backwards several times, and corrected later, and from what I’ve seen it’s not uncommon for people to get it wrong.

The short course prevention factor would likely cover any small error here.

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The difference between my two measurements at two different temperatures was 9mm AFTER the temperature compensation. Before the temperature compensation it was 47mm for 100m, or 141mm (5.5 inches) for a 300m cal course. So it is not insignificant.

But I would agree that with the current temperature compensation instructions it is very easy to make a mistake and end up subtracting distance from a course when you should be adding distance. So it might be better to eliminate the temperature compensation from the procedure.

A third option though, would be to just simplify the instructions:

If the temperature is above 68F, leave the course as measured.
If below 68F, add
(68-temp) x 0.00000645 x (course length)
to the course.
When the required length of a calibration course was reduced in the late eighties, the instructions were pretty much as Mark has suggested: If the temperature is above 68 degrees F, don't adjust. If it's below 68, add length to the calibration course.

Usually a count on my bicycle works out to be about 9 cm. The constant for my tape is 0.0000093 per degree C, so for the adjustment to be more than half a count, the temperature has to be above 36 or below 4 degrees C. I seldom encounter the former, and try to avoid measuring in the latter as much as possible.

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