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15 replies

Why do we post-calibrate?

duanerussellduanerussellMeasurer
If we do our pre-calibration when the temps are lower than when we measure, why do we post-calibrate?

We calibrate, arrive at the course, and do our first measurement. The course is 54,000 clicks long. We measure the second time, and the course is 53,957 - 54,043 clicks. All is good, as it indicates we did not lose air during the rides. Or, if we did, we were still within tolerance, and adjusted the course accordingly. The course is slightly longer than if we had our second ride closer to 54,000 clicks.

If we calibrated, and had a less-slow leak, then our second ride would not be within tolerance, most-likely. So, we would re-measure. Still not within tolerance. So, we would need to start over.

If we calibrated when it was hot, then had a rain squall cool the pavement, our measurements, while consistent, would not really reflect an accurate measurement, since our tires shrunk due to the drop in temp. So, the course is actually short. If it warms up before we do our post-cal, everything (falsely) appears to be in order.

A leak at the end of the second measurement would invalidate the measurement, due to the post-cal being bad, even if both measurements were within tolerance.

If the counter is not functioning properly, we likely would not get two consistent measurements. And, as my ride in Houston shows, my calibrations were consistent, but the counter skipped during the measurement ride. Had I done two rides, instead of a team measurement, I likely would have had inconsistent rides.

So, is there really a good reason for post-cal rides? Wouldn't problems show during the two rides? The exception is when the pre-cal is hotter than the measurement.
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AdminAdminMember
It's not unusual for me to get applications where the post-cal is larger than the pre-cal, because they didn't do the measurement until the afternoon and didn't post-cal until the early evening.

That being said, I would be fine if they did just one post-cal ride if it came up 5 or more counts smaller than the pre-cal average. That happens to me quite often on my first post-cal ride, and it is a total waste of time to ride it 3 more times when there is zero chance the average will be larger than the pre-cal.
duanerussellduanerussellMeasurer
Mark, that demonstrates what I was thinking about the post-cal being done when pavement (and air) is cooler than during the measurement. Do the measurers then go back and adjust the course?

Measuring at first-light when it will not be cooler/colder than my pre-cal is one good reason for riding during the best part of the (warm) day - early morning! Post-cal never makes me go adjust the course.
duanerussellduanerussellMeasurer
Jim, the slow leak would show in your measurements. If they are within tolerance, then the leak was inconsequential. No need for post-cal.

If the leak was consequential, the two measurements would not be in tolerance, and one would have to re-do the entire process. Again, post-cal only shows further deflation after acceptable measurements, which really makes the post-cal irrelevant.
AdminAdminMember
Yes, the measurers do have to return to the course and lengthen it if the post-cal ends up being bigger. That's why those of us who calibrate at home are careful to avoid measurement situations where the post-cal will be colder than the pre-cal.

Measurer calibrates at home, drives 2 hours to the course, and completes his two measurements of the 5k in 30 minutes. The slow leak in his tube was enough to significantly change his constant during his drive to the course, but not enough to cause his rides to being outside tolerance with each other. Ha! :-)
duanerussellduanerussellMeasurer
Mark, you are again making my point (regarding the leak): If there is enough of a leak to be detected at a post-cal, it would also show on the two rides. The two counts would not be in-tolerance, and the measurer would try one more. That, also, would be out of tolerance, so the measurements would have to be discarded. Post-cal would not be necessary, since the measurements were invalid.

Further, if one set an on-site calibration check course, they could verify good measurements. Not sure why anyone would not use an on-site check course, to verify no problems before leaving the course.
AdminAdminMember
I think you missed the distinction I was making Duane.
After calibrating at home there is a long drive to the course. Long enough so that tire leaks enough to make the calibration inaccurate. Once he gets to the course he does the two 5k measurements in 30 minutes. Not enough time for the slow leak to cause the two measurements to be out of tolerance.
oscarwagneroscarwagnerMember
Experimenting with airless tires, I put on a fat one and rode it around my 681 m block once. Next day, I calibrated early, measured a 2 mile course and recalibrated at a warmer temperature later in the day. The tire evidently settled into the rim or mooshed. I had to go back and add 9' to the course. It did it on one more course and I "retired" the tire. Start air temp was 94 F and finish was 122 F.

An on site check ride course would probably have caught it. But then what do you do?
duanerussellduanerussellMeasurer
Mark,

Yes, I missed that point. That is a valid point. That is the best reason for doing a post-cal for every measurement. Now, I can rest easy, knowing why we do something that rarely has an impact. (But, that is also why I ride airless tires.)

Oscar,

The problem is the "fat" tires. They do squish more in heat than thin tires. However, I have a measuring acquaintance that used to be quite large. He tried 700c x 20 airless tires. If it was "cold" (never below 45° where he measures), they were fine. But, when it got 75° or more, the tires squished under him. He quit using them. His measuring partner, though, a petite lady, had no problems with the thin tires. I don't, either. But, I measure almost exclusively in first-light conditions, so I don't get into hot measurements. I do one every couple of years, but that is it. The early measurer gets the best conditions. Wink
Last edited by duanerussell
B bobthurstonMember
Interesting discussion, I've been reticent to pipe up while I pondered whether my reaction was just an unreasonable attachment to how we've done it over many years. I still don't know, but I still favor post- as well as pre-calibration. My thoughts:
-- not every measurer will be able to reason these things through as Duane suggests
-- as a certifier, seeing the post-calibration gives me an additional look at the measurer's consistency, and sometimes thought process, to see how they treat the post-cal numbers
-- also, as a certifier, I've come across quite a few submissions where the measurer made fairly minor mistakes, resulting in a course that is technically short according to their working constant. If I see the post-cal results I can consider the measurement results using the average constant, and avoid sending the measurer out on another long drive to make a small adjustment.
-- as a measurer, I prefer to use the average constant in most cases, and along with that I nearly always use the "SSS", or sum of shortest splits, method to analyze results. I feel this is a good way to have a solid course that is not short and has a minimal amount of "cushion". (I don't encourage newer measurers to use the average constant, but having the information needed to find the average constant can always be helpful.)

I also agree with Mark's reason. And I definitely favor the "temporary unofficial calibration course" as Duane has explained. I've had enough "unfortunate" post-cal results (and yes, have gone back to fix courses) that I vote for anything to avoid having to "re-do".

My two cents
nathanporchnathanporchMember
I agree with Bob on all his points but particularly the first. As a Certifier, I can glean a lot of information about the measurement and the Measurer if I'm not familiar with their work. The overall presentation of the measurement data is to me, an indicator of quality of work.
If I receive sloppy data, it's a red flag to me that the Measurer may have an "it's close enough" mentality and possibly an inaccurate course. Having post cal data helps complete the overall snapshot of the measurement day.

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