Pete:
Thanks for the apology for using the word diddling. However, your revised expression,"messing about", still seems unfair with regards to my efforts to maintain pressure throughout the measuring process to within 1psi."Accurately maintaining constant" would seem to be a more accurate and less inflammatory expression.
Dale:
Mechanical pressure gauges seem to be fairly rugged. I have a large dial 3-60 psi and a small dial 6-160 psi that are 15 years old. At a certain pressure I found the former read 54 psi and the latter 57 psi whereas some factory-checked gauges I purchased recently read 55 psi.
However, unlike a steel rule it does not matter for our application if pressure gauges do not read accurately in standard units. All that is necessary is that for a given pressure any one gauge gives exactly the same reading for at least one day.
I suggest one way to see if a given gauge is up to the job is to measure a course of about 300 meters with tire pressure at a particular gauge reading. Let a lot of air out of the tire, restore the initial pressure, and re-measure the course. Both course measurements should be the same.
If the calibration factor is always measured at the same pressure, any shift outside the usual tight range might alert to a shift in gauge characteristics.
Mechanical pressure gauges seem to be fairly rugged. I have a large dial 3-60 psi and a small dial 6-160 psi that are 15 years old. At a certain pressure I found the former read 54 psi and the latter 57 psi whereas some factory-checked gauges I purchased recently read 55 psi.
However, unlike a steel rule it does not matter for our application if pressure gauges do not read accurately in standard units. All that is necessary is that for a given pressure any one gauge gives exactly the same reading for at least one day.
I suggest one way to see if a given gauge is up to the job is to measure a course of about 300 meters with tire pressure at a particular gauge reading. Let a lot of air out of the tire, restore the initial pressure, and re-measure the course. Both course measurements should be the same.
If the calibration factor is always measured at the same pressure, any shift outside the usual tight range might alert to a shift in gauge characteristics.
While I am not a fan of meddling with tire pressure during a measurement, I can think of one advantage.
If you know the pressure at calibration, and you get a flat, you can repair it and pump back up to the previous pressure. I've found that this yields a constant that is very close to what it was before the puncture.
I do not recommend this, but thought I'd mention it.
If you know the pressure at calibration, and you get a flat, you can repair it and pump back up to the previous pressure. I've found that this yields a constant that is very close to what it was before the puncture.
I do not recommend this, but thought I'd mention it.
Pete:
I had added this advantage to my list in my post of 6 May.
In a simulated puncture, I had let all the air out of a tire, and found that on restoring the pressure, I got back the same calibration factor within the limit of the error of determination.
I had added this advantage to my list in my post of 6 May.
In a simulated puncture, I had let all the air out of a tire, and found that on restoring the pressure, I got back the same calibration factor within the limit of the error of determination.