PROPOSED USE OF TIRE PRESSURE TO IMPROVE CALIBRATION

I observed the same sort of relationship between temperature and pressure in my five year study of a pneumatic tire, reported in Measurement News, Number 127, September 2004.

In spite of seeing how well one can predict tire size knowing temperature, I'm reluctant to abandon post-measurement calibration. Unless one rides all the time with a pressure gauge attached, which is not easy, one does not know how the tire is changing. Does it have a slow leak? A postcal will tell you.

As for using remote calibration courses, if the calibration course is so far from the race course that it is inconvenient to return for a final adjustment, one should simply lay out a 300 meter or 1000 foot calibration course on site. This is always a good idea when a measurement is away from home.

In 1999 a group of measurers descended on Pittsburgh to do a group ride of the US 2000 Olympic Trials marathon course. I was the data taker, and did not ride. Results were reported in Measurement News, November 1999, Issue #98.

Part of the data I took was tire pressures at calibration time. In order to not disturb the tires’ calibration, they were initially pumped up to a desired level. Then I used a digital pressure gauge (Smart Gauge by Topeak) to read the pressure. Once this was recorded the bicycle was calibrated. The tire pressures were again checked after postcalibration was done.

One thing I noticed was that if I was not extremely careful, the seal between gauge and tire valve did not always get made or broken instantly. A short hiss indicated that a shot of air had escaped. When this happened the pressure was reduced, and the original pressure was forever unknown. Although I tried hard, I had enough bad tries that I judged that my pressure data was too flawed to be meaningful.

Using a pressure gauge on a tire always causes a small amount of air to escape from the tire to fill the void within the gauge. Thus any gauge reading will be lower than the actual pressure before the gauge was applied, depending on the amount of air that is allowed to escape.

I think making adjustments based on tire pressures is unjustified simply to avoid recalibration.

Pete:
Thank you for the kind compliments.
I have read your report covering the calibration factors derived from a tire over five years, but find that your conclusion is opposed to one derived from my study. I assume that you pumped your tire to 125 psi, took the temperature, and measured the calibration factor from the calibration course. Although your results show a lot of scatter, you were surprised that there was a trend to a lower calibration factor with increasing temperature. You had expected since you always pumped up the tire to the same pressure, that it would have the same size regardless of temperature. Actually my results meet your expectation, and I am proposing we use this very finding to improve the accuracy and ease of certification.
The method of constant tire pressure obviously works very well with the moderately high performance wheel I use, but I am hopeful that further investigation will show that it is of general applicability.
Change in tire size can be monitored by tire pressure, but I do not see why you believe it is necessary to attach a gauge to the wheel for continuous readout.
It is true that postcalibration can detect a leak, but pressure monitoring can detect it much earlier and avoid much wasted measurement effort.
You recommend setting up a calibration course next to the race site. However to make the most accurate calibration course you must be able find a long straight section of traffic-free road, and lay it out at close to 68 F on a overcast day with a helper. This is usually not possible at the race site. Even if I was able to find a suitable location, it would take me twice as long to set up a calibration course as to do the actual measurement of the race course. The point is though why do this when a constant pressure method gives better results?

Pete:
I should like to respond to new points you raise in your revised comments.
Although you do not state what bicycle valves you were dealing with in your difficulties in measuring tire pressures, I am pretty sure they must have been Schraders. Although I have not tested these valves yet, I was very much aware of the problems you describe.
One solution might be to use the threaded connector sold by Silca that never allows any air to escape from the tire except that which enters the gauge. Another might be to convert the rim for Presta valves using an adapter that costs less than a $1. If my pump hose adapter is used, there would be no additional cost.
The Presta gauge I describe in my report also never allows any air to escape from the tire except that which enters the gauge. The amount must be negligible because successive readings give identical results as far as can be discerned.
Still whether the amount of air that enters the gauge is small as with the Presta or large as with the Schrader is not important, because their use allow the same pressure to be used for the calibration as for the race-course measurement.
I am proposing the constant pressure method not simply to avoid doing the postcalibration, but to improve the accuracy of certification.
I shall probably do some further tests on three wheel types and the Schrader to see how at constant pressure size varies throughout a day of extreme temperature variation.

Here is how I understand Neville’s proposed system to work:

1) Sufficient experiments are done to establish a predictable relationship between wheel size and tire pressure.

2) The bicycle is calibrated after a note of its internal pressure is made.

3) The course is measured.

4) The tire pressure is checked. If the pressure has risen, this is an indication that the tire is larger than at the beginning, and counts per km has declined. Thus the initial calibration may be used as official.

5) If pressure has declined, the new wheel size may be determined from (1) and adjustments made.

All this seems straightforward and scientifically correct. There are a few concerns that come to mind:

1) How much pressure/calibration information is needed to establish reasonable accuracy, and how is it to be maintained? If I sit around all winter, and go out in spring to measure, what do I use to support my use of pressure information? Last fall’s data? Do I need to re-establish a pressure/size baseline?

2) Neville refers to “pressure monitoring.” How is this done? Each time pressure is checked a little air is lost from the tire. How is this accounted for? Misgiving over this was the reason for my wondering whether the gauge was always connected to the tire during all phases of the measurement. If loss is minimal, error, as Neville says, would be negligible. But a slip of the hand can release more than a minimum.

3) How will varying weight on the front wheel be accommodated? I wear more in winter than in summer, and sometimes carry a backpack. Also, my own weight varies. As a result my wheel circumference, measured over 60 times over 5 years on different calibration courses, immediately after pumping to 125 psi, varied from 261.42 to 262.19 cm per revolution, or from 11060 to 11092 counts per kilometer. My tube has a Schrader valve. I find this pretty good agreement but not quite enough to use as reliable calibration information.

4) The purported purpose of using pressure as an adjusting factor seems principally to avoid a long trip to and from a remote calibration course. Neville seems to have a perception that it is difficult to lay out a calibration course. This used to be true when a half-mile or kilometer was required. With 300 meters or 1000 feet it is almost always possible to find enough straight road near the race course, and it takes less than a half hour to lay down the course, with one helper. I have laid out a calibration course by myself in an hour. Moreover, it need not be done on a cloudy day at 68F. It can be done under any condition. I always use a remote calibration course when a measurement is more than 30 miles from home.

5) How will the information be presented to a certifier in a way that is uniform, credible and understandable? As a certifier I’d prefer not to have to treat each measurement as a special case. A certain degree of uniformity in submissions makes the process easier. Also, the process must be understandable to the certifier. Picture Albert Einstein sending in a measurement complete with arcane mathematical symbols and telling the certifier that it’s logical and correct. After all, the guy’s Einstein – doesn’t this mean he’s right? Maybe, maybe not. Whoever he is, he’s got to convince me in terms I understand.

The whole justification for the proposed method originally rested on Neville’s erroneous premise that it is hard to set up an on-site calibration course. This is not the case. Is there another reason why this method should be considered aside from fun with science? Neville now cites “to improve the accuracy of certification.” Whether such improvement exists remains to be seen. Precal-measure-postcal is simple and direct. Eliminating the postcal in favor of pressure adjusting doesn’t seem to me to lead to accuracy.

I share some of Pete's concerns about this proposed measurement technique; however, I don't agree with all of his points.

I do not see the significance of what happens from one season to the next since, as I understand the method, the data used were all gathered on the same day. Pressure and circumference are taken at the start of calibration and throughout the day until post-measurement calibration.

Perhaps a compromise on this issue would be to allow the method to be used but still require a post-measurement calibration (PMC). In this way readings would have been taken before calibration, before and after each ride, and before PMC. In my opinion this would result in a more accurate measurement as it would adjust wheel circumference during the mesurement procedure to assure that the constant remains more valid.

It can be very difficult to find a suitable site for laying out a calibration course in some areas (more because the potential of parked cars on a straight stretch of road)and a helper is not always available. While it can be done solo, it is very time consuming and has a greater potntial for calculation errors since there are more calculations. The other question I always have when using race personnel to help lay out a calibration course is what exactly is the person on the other end of the tape doing?

My personal experience shows one example where this system would have been very helpful. I measured a 5 km course about 70 miles from home, using my home calibration course. The weather was fairly constant up until the PMC. While driving home, the temperature dropped and there was a heavy rain. Consequently I encountered a higher PMC constant requiring adjustment of 8' to the course. As I understand Neville's system, this need for course adjustment would not have been necessary since I would have adjusted pressure (and therefore circumference) prior to my PMC.

The biggest potential problem I see with this method is variation in equipment and the competence of the person using the technique.

As a certifier, I have less concern with understanding what has been done with this system than I do with someone who is new to the system using the electronic counter system (perhaps this is due to the fact that I have yet to try the electronic system).

As a final note, I too applaud Neville for his continuing quest to make our task easier and more accurate. Nothing was ever improved without someone "pushing the envelope".

Paul Hronjak
hronjak@simflex.com

Part of the problem here is that Neville is thinking and writing in terms of wheel circumference, while almost all of his potential readers are accustomed to thinking in terms of counts per kilometer. The two are readily equated, but only with extra mental effort.

Also part of the problem is that we have not yet seen a simple, step-by-step description of the process. I took a stab at it but may have it wrong.

As I see it, pressure vs circumference data (or counts if you will) are used to determine the final constant. While this may approach the accuracy of a direct calibration, it is not quite as accurate. It may well be good enough, but we lack experience to judge this.

I’d be inclined to accept a measurement with pressure-determined postcal, but only in a case of dire need. If a remote calibration course is to be used, it’s likely one close to the measurer’s home. This given, there is no reason not to do a direct postcalibration. If an adjustment is required, this can be conveyed to the race director by telephone. I’d be disinclined to drive 80 miles to move a start line mark 16 feet.

I think the proposed method is ingenious but unproven and risky.

Last minute brain spasm: Couldn't the method be used to replace precalibration as well? If it's good enough for postcal, why not precal?

My bicycle has presta valves. When I measure tire pressure or pump up tires, some air escapes. I doubt that I could keep tire pressure uniform.

I don't pump up my bicycle tires on the day of a measurement, because compressing air heats it and the tires might loose pressure as they cool. Should I be concerned about this?

I have measured calibration courses when the temperature was significantly colder than 68 degrees F. You can adjust for the difference in temperature.

My calibration courses have always been within 2 miles of the race start or finish. I worry that my bicycle tire pressure would change, if I carried my bicycle far in a car.

Dale Summers

Dale:
It has been one of my pet hates for many years that commercial pressure gauges and pumps badly leak air from the tire, but I now have solved the problem.See my report:

http://home.earthlink.net/~caverhall/pressandcalib.htm

I will have a lot more to report in the near future.

Also, your tire pressure worries during race-course measurement would end if you were able to adopt the constant-pressure method described in the report.

Of course you can tape a course at any temperature, but accuracy will not be as good as if you do it at close to 68F under a cloudy sky.Measurement with correction for temperature deviation is not as good as that done without.Also on a hot intensely sunny day you never really know the temperature of the tape.

Dale:
I have extensively elaborated on my report and have included a detailed account of my design for a Schrader gauge that vents no significant air from the tire.
Even if you do not use my hose and gauge connections, I disagree that you cannot keep your tire pressure constant as required in the method. Note that air losses from the tire will only occur at connection and never at disconnection, when your floor pump gauge will show the precise pressure of the tire. Thus, you will be able to keep the same pressure before every course ride as you had before calibration.
You can make a simple check to see whether or not the method will work for you. Mark a fairly straight course and ride it several times over a period of days and temperature extremes keeping of course the weight on the bike and the pressure constant. You will be surprised as to how stable the total counts will be.

The constant-pressure method can give good results if used with extreme care, but there are some things that influence its accuracy:

(1) Calibration values change constantly during a ride, as portions of the course may be in hot sun, while others may be shaded. If the pressure in the tire is adjusted to match that of the precalibration, it may be necessary to be sure that the conditions are the same.

(2) While it may be possible to measure, say, a 5 km course in 20 minutes, and while the pressure may stay relatively constant during that time, it is more common for a 5 km measurement to take a few hours, as it rarely comes out right the first time, and requires adjustment. During this time the pressure is going to change. What is to be done? Take a reading every half hour, and release air or pump it in to maintain a constant pressure? This is risky business.

(3) If the tire pressure is not adjusted during the measurement, but is checked afterward, it will be found to be different than at the start. What is to be done with this information? If my tire pressure is 125 at the start and 122 at the end, what do I do about it?

Given the above I can see no reason why the pressure method is in any way easier or better than the method we use now. The fact that it can be used by a knowledgeable individual with custom pressure-measurement equipment does not make it a better method. The precal-measure-postcal method frees the measurer from any concern about his tire pressure and allows him to focus on the measurement itself. This makes mistakes less likely.

As the sole benefit of the pressure method is to avoid postcalibration I believe the cure is worse than the disease.

Pete:
I should like to respond to the points you raised in your last post.

(1) Calibration values change constantly during a ride, as portions of the course may be in hot sun, while others may be shaded. If the pressure in the tire is adjusted to match that of the pre-calibration, it may be necessary to be sure that the conditions are the same.

I am not quite clear as to what you mean here. However, my results so far seem to indicate that it can be cool and shady at the calibration and hot and sunny at the race-course measurement, but if the same pressure is used, the calibration constant changes little.

(2) While it may be possible to measure, say, a 5 km course in 20 minutes, and while the pressure may stay relatively constant during that time, it is more common for a 5 km measurement to take a few hours, as it rarely comes out right the first time, and requires adjustment. During this time the pressure is going to change. What is to be done? Take a reading every half hour, and release air or pump it in to maintain a constant pressure? This is risky business.

It should only be necessary to monitor pressure during the few minutes spent on the critical rides not on the many hours usually spent at the site.

(3) If the tire pressure is not adjusted during the measurement, but is checked afterward, it will be found to be different than at the start. What is to be done with this information? If my tire pressure is 125 at the start and 122 at the end, what do I do about it?

I think this is a minor detail that would be easy to get everybody’s agreement on. Any increase in pressure could be ignored as this would give a longer course. An adjustment for any decrease could be made by adding 0.5 m/psi for a 5K. In the example you give (which would be rather extreme for a 5K) the average fall would be 1.5 psi and 0.75 m would be added.


Given the above I can see no reason why the pressure method is in any way easier or better than the method we use now. The fact that it can be used by a knowledgeable individual with custom pressure-measurement equipment does not make it a better method. The precal-measure-postcal method frees the measurer from any concern about his tire pressure and allows him to focus on the measurement itself. This makes mistakes less likely.

The traditional method does not free me from concern that measurements taken as temperatures peak mean an overly long course.

As the sole benefit of the pressure method is to avoid postcalibration I believe the cure is worse than the disease.

I see the following benefits of pressure monitoring:

(i) Improved accuracy.

(ii) Post-calibration not needed.

(iii) A temporary calibration course does not have to be set up.

(iv) Frequent recalibrations during course measurement not needed.

(v) Return to the race site to make adjustments in the measurements not needed.

(vi) Early alert of a slow leak in the tire so less wasted efforts.

(vii) Temperature measurements not needed.

(viii) Easier scheduling of measurements (eg do not have to get back for post-calibration before dark and rapid drop in temperature).

(ix) Calculations usually simplified.

(x) Tire puncture during measurement just a minor problem.

I have some comments on points raise a few posts ago.
My basic proposal is that better measurements can be made by monitoring tire pressure to keep it the same or slightly above that use in calibration, so that the caibration constant is more nearly constant. I am not proposing to calculate the calibration constant from pressure measurements.
Post-calibration is unnecessary in the method, but Paul's suggestion to retain it might be a good interim measure to provide method validation. Also it could be used as a check that a measurer was making correct pressure measurements.

Right, well some of the discussion of tire pressure is very interesting, but somewhat misses the pont. The pont is that it’s not just the tire pressure, but the bike that is a variable that may change during the day. That’s why we do calibration rides before and after.

A will accept that the outside diameter of the wheel will change with the internal pressure of the air in the tire. But it’s not the only variable!

Although you may have a constant amount of air in the tire, as measured by your pressure gauge, while the wheel is static in the back of your van, that does not mean the external diameter of the wheel is constant.

Temperature will change the diameter. Ride a course when it’s dry on a hot day, and then after a tropical thunderstorm do it with cool standing water on the course. Your counts will change a lot although you may have lost no air from the tire, and after you have put it back in your van the gauge may read the same as the start.

Temperature variation is caused by the obvious, sun, shade and puddles. Puddles, in my experance make a big difference. There is also heat generated by work, riding fast over the ground makes tires hot. I ride my bike a mile or two before running it on the calibration course to get it warmed up. Otherwise my first two runs are one or two counts off.

A quick burst of rain can change the temperature of the black top from more than 110 to 65.

The difference that temperature affects the diameter is not just the expansion or contraction due to the temperature of the gas in the tube, but also the expansion or contraction of the rim and spokes. The bimetal thermal compensator made from a rack of rods, like the one that’s built into a good grandfather clock pendulum, is one way to overcome the affects of temperature change, but impractical to implement in bike wheels.

Big nubbly tires on mountain bikes generate more rolling resistance, and therefor more heat in the tube. Mountain bike tires are normally inflated to lower pressures and have more surface aria that expands more readily. So small variations in temperature or barometric pressure may varied relatively big changes in diameter.

We can have a go at calculating the expansion coefficients of the gas, the elastic surfaces and the metal components, the angle of the sun, the depth of the puddles, the rotational speed of the wheel as it sheds the water from the puddle, the .......

Or we can take the unit as a whole, and refer it back to some form of constant reference, like a calibration course.

I agree that monitoring the bike to make sure that you are not losing a significant amount of pressure during the days measurements is a good idea. But even if you don’t, the lost pressure will show up in the post measurements calibration rides.

The last thing we want is people “Adjusting” the pressure between the pre-and post rides. This can only lead to a larger variation in errors, both from pressure and from operator error.

I have a professional foot pump with a plunger handle that is worked with both ands and held steady by the feet. It has a good quality pressure gauge on it. I use it before I start my first calibration ride. By inflating the tire to a known mark, 100 psi, I always start with about the same pressure.

Once I have warmed my bike up by doing a mile or two first the rides on my standard course are often very close to each other. But they are not the same, or in the same range as rides on a previous occasions. On different days the temperature, wether and wind seem to have more of an affect than starting tire pressure.

P.S. Should we have a space on the application form for the riders, before and after, blood pressure and alcohol level?

While James does raise a number of valid points I will take issue with the listing of variables being separate from tire pressure. True, these issues will all have an effect on tire diameter; however, most of them do so by resulting in a raising or lowering of the internal pressure of the tire ... primarily those which result in temperature change will directly result in pressure change.

The concern expressed earlier about air loss when taking pressure readings was one that I shared until I witnessed Neville's system in action. The pressure guage he uses does not result in any air loss and a magnifying glass showed no difference in readings after multiple readings.

I agree with James. While Neville may succeed in designing and building specialized pressure-testing equipment which serves his purpose, expecting the person on the street to get it right would be ridiculous. Diddling with the tire pressure during a measurement is, in general, a terrible idea.

While a postcalibration may be done as a check, what meaning does it have when pressure has been trifled with?

Just because something CAN be done does not mean that it SHOULD be done. This method is too unsound for general application.

James:
You do not seem to have grasped the concept of my new proposal, and I hope my responses below help clarify things for you. Perhaps after reading my current and future reports you will give it a valid try.

Right, well some of the discussion of tire pressure is very interesting, but somewhat misses the pont. The pont is that it’s not just the tire pressure, but the bike that is a variable that may change during the day. That’s why we do calibration rides before and after.

Only variation in the front wheel is important. The rest of the bike is unimportant as long as total weight and weight distribution are kept constant.

A will accept that the outside diameter of the wheel will change with the internal pressure of the air in the tire. But it’s not the only variable!

Although you may have a constant amount of air in the tire, as measured by your pressure gauge, while the wheel is static in the back of your van, that does not mean the external diameter of the wheel is constant.

Your physics is incorrect here. The amount of air in the tire is a function of volume, temperature, and pressure not just pressure alone. Most readers of this bulletin board realize that even with a constant amount of air in the tire, diameter will vary, but I am not sure what point you are making.

Temperature will change the diameter. Ride a course when it’s dry on a hot day, and then after a tropical thunderstorm do it with cool standing water on the course. Your counts will change a lot although you may have lost no air from the tire, and after you have put it back in your van the gauge may read the same as the start.

Temperature variation is caused by the obvious, sun, shade and puddles. Puddles, in my experance make a big difference. There is also heat generated by work, riding fast over the ground makes tires hot. I ride my bike a mile or two before running it on the calibration course to get it warmed up. Otherwise my first two runs are one or two counts off.

A quick burst of rain can change the temperature of the black top from more than 110 to 65.

The difference that temperature affects the diameter is not just the expansion or contraction due to the temperature of the gas in the tube, but also the expansion or contraction of the rim and spokes. The bimetal thermal compensator made from a rack of rods, like the one that’s built into a good grandfather clock pendulum, is one way to overcome the affects of temperature change, but impractical to implement in bike wheels.

There is no need to belabor the point that temperature changes the wheel diameter. The novelty of my finding is that through actual experimental results, the predominant effect of temperature on wheel diameter appears to be through its effect on air pressure.When pressure is kept constant no significant diameter change is seen. See for example the following results for circumference from my report, which were all obtained at 118 psi:

Day 0.0 at 22 deg C = 209.31 cm
Day 1.0 at 10 deg C = 209.28 cm
Day 8.1 at 38 deg C = 209.30 cm
Day 8.4 at 29 deg C = 209.33 cm

Big nubbly tires on mountain bikes generate more rolling resistance, and therefor more heat in the tube. Mountain bike tires are normally inflated to lower pressures and have more surface aria that expands more readily. So small variations in temperature or barometric pressure may varied relatively big changes in diameter.

I think that what you are saying is that mountain-bike tires are quite different, and in this you may be right because I have not tried them.

We can have a go at calculating the expansion coefficients of the gas, the elastic surfaces and the metal components, the angle of the sun, the depth of the puddles, the rotational speed of the wheel as it sheds the water from the puddle, the .......

Or we can take the unit as a whole, and refer it back to some form of constant reference, like a calibration course.

You should note that I am not advocating esoteric new calculations, but actually a reduction in the current ones.

I agree that monitoring the bike to make sure that you are not losing a significant amount of pressure during the days measurements is a good idea. But even if you don’t, the lost pressure will show up in the post measurements calibration rides.

Post-calibration usually detects major air leaks when it is too late to avoid wasted effort.

The last thing we want is people “Adjusting” the pressure between the pre-and post rides. This can only lead to a larger variation in errors, both from pressure and from operator error.

I agree incompetent measurers might not do a good job with the proposed method, but they are probably not doing a good job with the traditional method either. Retention of post-calibration through an interim period would be a good check as to whether measurers were getting it right.

I have a professional foot pump with a plunger handle that is worked with both ands and held steady by the feet. It has a good quality pressure gauge on it. I use it before I start my first calibration ride. By inflating the tire to a known mark, 100 psi, I always start with about the same pressure.

Once I have warmed my bike up by doing a mile or two first the rides on my standard course are often very close to each other. But they are not the same, or in the same range as rides on a previous occasions. On different days the temperature, wether and wind seem to have more of an affect than starting tire pressure.

I think the point you are making here is that you have tried a constant pressure method and it does not work. Unfortunately, your results are not valid.
I think it is possible to reproduce a pressure with a floor pump, but it really depends are your hose connection and your technique of disconnection. Even if you always get the same presure on hose disconnection, you ride your bike to “warm it up” for various distances and temperatures before going to your calibration course. Thus, you really have not much idea of the actual pressure when you start your calibration. This probably accounts for your observation that the calibration factor is very dependent on air temperature.
Although you do not give any quantitative data, I am guessing you have noticed a significant variation of the calibration factor over many months where conditions may have varied wildly. However, my method works on the premise that at constant pressure the factor is quite constant over the few hours that measurements are made not over many months. Thus your experience is totally irrelevant.

How trustworthy are tire pressure gauges? Neville has observed that tire circumference remains almost constant, when air pressure as measured by his pressure gauge is held constant. Would this be true for other pressure gauges assuming no operator error? How does the quality of tire pressure gauges compare with that of surveyor’s steel tape? What should a measurer look for in a tire pressure gauge? Would a measurer need to test his pressure gauge as Neville has tested his, before trusting it? If a steel tape breaks, you see it. How would you know that your tire pressure gauge is failing?

I see two applications for the constant tire pressure method of measurement by bicycle. (1) For some races I have spent as much time finding a route that was approximately the correct distance as in doing the certification measurement. (2) Sometimes a race director wants a race route that is close the the correct distance, but doesn’t want it certified. Neville’s method should work well in either of these cases.

Dale Summers

Pete:
You describe my method as diddling or trifling with the pressure. Diddling means cheating and trifling means acting without serious purpose and I am at loss as to how you can believe these things.
You are right that when the method is fully validated and measurers are confident with it, post-calibration will be superfluous. In the meantime, post-calibration will serve as a check that it is working, i.e. if the measurer gets the same pre- and post-calibration factor, this is a good sign that he is doing things correctly.
You contend that I am “building specialized pressure-testing equipment” for my own purpose. To the contrary, my goal is to develop a method that any measurer can use with a cheap, simple, and readily-available gauge to improve the accuracy and efficiency of his measurements. I am pleased to say I will shortly report that I seem to have met my goal.

Neville -

I regret the inaccurate use of the word "diddling." Please substitute "messing about."

You have got a solution for a non-problem here.

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.

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.

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.