Pete and Matthew:
Since I made the above posts I have obtained some very nice results, but unfortunately my assumption of responsibilities as a validator has delayed my issuing a full report.
I will say though that I have ridden with the Presta version of the gauge in place for over 100 miles at speeds up to 40 mph and it has continued to perform flawlessly. Seal is perfect even over many weeks. Changes in the wheel calibration factor (so called constant!) are detected down to 0.02 rev/km.
There are many ways in which the pressure- monitoring method can be applied, but I currently favor the use of the pressure coefficient. This is determined by simply riding a calibration course at two tire pressures. For my wheel the coefficient is negative 0.011 rev/km/kPa of pressure change. Note that this coefficient is independent of changes in bicycle weight and temperature, and seems to remain valid for many months. If weight and pressure are kept constant, the calibration factor itself is quite constant.
A typical application of the coefficient method is as follows. The night before a measurement I pump up the tire and fit the gauge to find a pressure of 590 kPa. I put the bicycle outside for a cold night and find in the early morning that pressure has dropped to 540 kPa. By the time I do the calibration though at around 10:15 am, temperature and pressure have risen to 3 deg C and 570 kPa, respectively, and the calibration is 480.00 rev/km. The 5-km course is a 90-min drive away, and I do not get started with the measurement until 1:30 pm, when temperature and pressure have risen again to 14 deg C and 580 kPa. I therefore use 479.89 rev/km (480 – 0.011 x 10) as the calibration factor to calculate the distance for the ride. It is unusual for pressure to change significantly over a 5-km course, but in this case I find that pressure has dropped to 570 kPa. I therefore determine that the average pressure was 575 kPa and extend the ride by 0.275 rev [(-0.011) x (-5) x 5].