Neville -
Thanks for your comments. I appreciate all the work you and the Committee do to make my measuring easier.
I first marked my rim per your web site's directions; they were very helpful in getting it set up. I decided to use 40 marks per rim because it would give me differences of about 2 inches. Only one side of the wheel is marked; this way, it's obvious if the wheel is mounted backwards. As the bike rolls forward, the numbers increase (I have the '0' spoke set to the one just before the valve for the tube.).
Per an earlier post of yours, I tried a Bell computer originally when measuring, so had to use 2 magnets because of its limitations in setting the circumference. When the Bell died earlier this year (corroded contact), I installed the Protege (purchased late summer of 2007), carefully following the directions for sensor positioning relative to the magnets, but kept the same circumference of 5000mm.
Now, as to your analysis:
>>If you repeat a ride over the same mile course your rim reading should be within 0.1 rev of that in the first ride whether you have a Protege on board or not. Clearly this is not the case with your results, and you therefore have at least one problem unrelated to operation of the Protege.
==>For the smaller variances, the course is rather challenging in sections, with many slow, tight turns near curbs, traffic stops and restarts, sudden pedestrian appearances (necessitating more stops and restarts), lengthy rough brick surfaces in places, etc. By my arithmetic, .08% works out to about 0.6 revs per mile, so variances of 0.2 revs seem to be well within the usual tolerances.
>>It's possible that you may have sometimes lost a count by letting the meter go to sleep and moved off without awakening it.
==>For each mile segment, I restarted the computer at 0.00, as indicated in Other Details.
>>I am not clear as to why you do not estimate actual rim readings, but just go to the nearest division. Also, to quote down to one thousandth of a revolution is incorrect.
==>Each rim marking is 0.025 (half a nickel), so 0.65 = 26 marks, and 0.35 = 14 marks. I'm just converting the marks to decimal, and so use 3 places to report it. If you prefer, I can post it using fractions. I don't estimate the rim readings because I don't think the extra work gets me a more accurate course. The stripe of paint I used for each mile is about 1/3 of the distance between 2 rim segments, so I am not confident that I'm accurate to within, say, 1/5 of a rim segment during a remeasure.
===================Finally=========================
It's the large variances that concern me. Clearly, the magnets are not always detected as they pass the sensor, and either one could be missed (sometimes more than once) during a particular mile segment. How is a measurer to know if one is missed or not without using a mechanical counter to double-check it? How is a certifier supposed to know how a magnet and sensor are mounted on the measurer's bicycle, and that they are working correctly?
After my rides, I do not have confidence in the accuracy of the magnet / sensor combo for measuring courses to certification standards. However, I love it for laying out courses more quickly.
Thanks again for taking the time to respond to my post, and for all that you do.
- Alan