Calibration Constant Change in Water

Mike,

Interesting situation. I hope you rode SPR and didn't go off road. Please post your results when things dry out.

And I thought it was only the postman who went through, no matter the weather.

Paul, It was the shortest way home! Also I would be able to get a good measure from 8k to the finish and even with a reduced accuracy 7k to 8k interval I am now able to let the race director know quite closely where the start/finish line needs to be.

Gene, The SPR is a very good point. I was following the race director on his bike so I knew that I was not going to need to actually swim. However he rode closer to the road centre than me. I took a line as close to the hedge as I deemed prudent, but I was worried that there might be a ditch or some other problem hidden in the brown water, and I remember picking a line just a little further out to give me some margin to ensure I was not pitched off the bike into the water. You can see some grass of the verge in the picture and I may be 1 metre outside the SPR and so riding a longer distance, fortunately the total angle of the bend in road is not very large over the flooded section, so I have still got a reasonable overall length measurement, assuming of course that the constant changes by less than the 1% change which occurs when I get off the bike and push it.

1% constant change would give 2m over 200m, and so my provisional start/finish result is probably within the SCPF. But has anyone ever taken a course under water calibration?

Mike,

Thanks for the fun!

While analyzing the effect of riding through the water, don't forget to account for the tire size change due to the hydrostatic pressure of the water covering the bottom of the tire.

This could cause serious inaccuracy, and further research is clearly necessary.

It is still raining and floods are worse than they were 3 weeks ago, so I have not yet returned to see if it is dry and I can get a check measurement between 7k and 8k to determine the effect of the flood on the calibration constant.

Pete asks about the the effects of hydrostatic pressure squeezing the tyre. We have the pressure coefficient for a tyre reported by Neville here. His figures suggest about 0.2% for 1 bar pressure change, at least for his tyre. But 1 foot of water would have only given me 0.03bar pressure on the tyre under the water. So it looks to me that the hydrostatic pressure effect should be negligible.

To go along with Pete's hydrostatic pressure change, don't forget to factor in the (assumed) water temp being lower than the air you had been riding through. That will reduce pressure in your tire, also, resulting in further inaccuracy.

As with Pete, said with tongue in cheek.

You could calibrate the bicycle in a 50 meter swimming pool...

I don't know about special procedures for laying out a cal course underwater, but Bob Letson and I checked the length of a calibration course in Korea once during a pouring rain, when the standing water was up to curb height on top of the course. I can't remember what we used for intermediate markings-- clearly the masking tape method was out of the question-- other than that everything was the same, and we used a thermometer immersed in the water to assess the expansion of steel. Of course we were lucky that the water was just a few inches deep, no snorkeling needed.

The calibration course checked out. After that I remember hours of toweling off the tape so it wouldn't rust, while Bob told stories of measuring in the early days under Ted Corbitt's guidance.

We were there, we thought, to check the measurements for the Olympic marathon to be held in Seoul the following summer. That check never happened, but that's a whole other story.

The course has dried out, and I did a complete remeasurement on Sunday

For 7k reference point to 8k reference point:
when flooded 968.4m
when dry 966.9m

Increase when 200m under water = 1.5m
So it looks as if the constant increases when under water by approx 1.5/200= 0.75%

However caution is required since there could easily be errors in lining up with the reference points at 7k ( a tree trunk 5m away across the road) and at 8k (an electric pole about 2m away). In fact I could imagine it possible that allowing for worst case errors would give the increase as 1.5m +/- 1.5m. So my measurement is not accurate enough to be really useful, except to say that a 200m stretch of flood wont impact on a measurement of a 10k very significantly- it is small enough to be accommodated within the SCPF, if there were no other problems on the measurement.

Of course I would really needed two nail-marked reference points either side of a flood in order to make a more accurate estimate. At various times in the last 7 weeks there has been a 20m long flood on my cal course at home. I did consider going out in the rain, nailing it, and trying to compare cals on that 20m with something else not under water, but I decided that 20m would be too short to get an sufficiently accurate calibration.

All the roads have pretty well dried out now, but now knowing what local areas are prone to flooding in extreme conditions, I suppose I could prepare by marking out with nails a cal course which is at risk of flooding and a close by course which would always be dry. I would not need the exact lengths, just the ratio of cal counts when both are dry and then I would just have to wait for another flood. My only worry is that they say this has been the wettest winter at least since 1995 and possibly even since records began. I could have a long wait.

Continuing this line of informal research, I was measuring a course in Bermuda when a huge rainstorm came through. Lots of water but the storm passed fairly quickly, and I went to calibrate. There were two huge puddles, each over 1 foot deep for significant portions.
https://plus.google.com/photos.../6003281655414395217
I would guess these large puddles covered about a fifth of the total calibration course length, and I also predicted that they would significantly affect calibration (raising the calibration constant).

Immediately following this aquatic ride I rode over to another calibration course for comparison. Results: "water ride", 2829.5 counts over 250 meters; "dry ride", 2830.13 counts over 250 meters. Not enough difference to talk about; the "dry ride" course is a bit hard to follow as the railway trail snakes just a tad compared to the straight line measured.

If you're wondering why we're using 250 meter cal courses, it's because it is really hard to find a straight-line 300 meters in Bermuda. Laid one out 4 years ago, but one end of that one is now inside a high-security gate near the airport.

In summary, I think that the amount of water I rode through (I just did 2 rides on that occasion!) did not significantly affect the calibration. But what if the whole cal course were under water?? I guess, as Pete and others already said, "further research is needed."

One delightful finding: the Jones-Riegel counter can take a huge plunge and keep on ticking!