Pressure-gauge calibration

[Brucey]

BTW if you want to reset the pointer, the correct point is not just to the left or on top of the stop. It is usually about 1/8" to the right of the stop at 0 psi. (This is one reason why I suggested that you wind the quadrant back rather than remove the pointer; you can have no idea where to put it back exactly...)

If you fit it there, then lift it over the stop (or move the dial around the pointer, then replace it), you should find that the gauge is better calibrated than at present, but some experimentation may be required to get it right.

If the pointer is sprung hard against the stop, it stops the gauge from rattling itself to death when not in use or when it is in transit.

When checking/calibrating, remember also that in any event an inexpensive bourdon tube gauge isn't likely to be super-accurate except in the middle third of its range.

cheers

[Ayesha]

I used to use a device called a Ruska 'Dead weight' pressure gauge calibrator. The pressure was measured by spinning weights on a vertical piston. When the weights span freely half way up the piston travel, the pressure was more or less correct.
‘More or less’ correct because there is always daily fluctuations of atmospheric pressure. Tiny, tiny weights could be used so it agreed with a mercury column barometer.

For calibrating the pressure transducer, the Druck ‘Absolute’ transducer was firstly subjected to as close to vacuum as possible with a pump. It was ‘Zeroed’ and then subjected to 1 bar abs air pressure with the Ruska. In this situation, it was ‘Spanned’.

A multipoint curve was recorded to calculate the Druck transducer’s characteristic to a fifth order polynomial. This ‘poly’ was entered into my ‘linearization’ Vbasic equation.

Not for bike tyres mind. For ‘Inlet manifold depression’.

For my bike tyres, I pump them up and then put my weight on the bike to observe the tyre's deformation.

[MartinC]

Yes, the real requirement is for a tyre deformation gauge that measures the tyre drop when you're in your normal riding position!

[mig]

is there anything that anyones does to a track pump to maintain it generally? any sort of lubrication etc? or do they just get left in the shed?

[Mick F]

the actual tyre pressure drops when the pump is reconnected, because you have to 'inflate' the hose from the limited volume of the tyre, thus dropping the pressure.

Oh yes, I agree with that, and it's obvious really, but disconnecting also removes some pressure.

I have a Topeak head on my pump, I couldn't get on with the dual head unit that came with the pump, it never seemed to fit properly.

This is the one I use now:

Topeak.jpg (3.86 KiB) Viewed 658 times

This was the original:

Dual.jpg (1.6 KiB) Viewed 658 times

When the pump is connected, the tyre valve is depressed. When disconnecting, because you have to unlock the head and pull it off against the rubber seal grip, there's a finite time that it all takes, and therefore you loose air. Because I have narrow tyres with not much tyre volume, it affects the pressure easily. I can't quantify this, but I reckon a few percent.

Conversely, using a small pressure gauge, it fits on and comes off in a jiffy with minimal air release.

Therefore I cannot compare apples with apples.
This is the nub of my problem.

[MartinC]

Mick, it's Schrodingers Cat. You can't measure the pressure without changing it.

[Mick F]

Yup.
Quantum Theory too. Just looking at a particle changes it.

Stand fast on my earlier comment today.
I've just spent a happy half-hour looking very closely at the way the head fits onto the Presta valve.

I took the head off the hose and fitted a short length of hose to the head, then bent it right over and secured it with a ty-wrap to seal it off. I fitted the head to the valve and removed it a few times and could hear there was minimal - if not nearly zero - air escape.

Or should I say that there was air escaping on release until I learned something!

What I have been doing all these years, is pushing the head on too far! I have been pushing the head right on as far as it would go. The head then depresses the valve fully, and it is a pain to pull right off nice and smartly. By only pushing it to where the valve-cap screws on, the head removes easily ........ and as a plus, the valve is not depressed mechanically but only through positive air pressure from the pump. Consequently, there is no air-loss on disconnection.

This still doesn't answer my problem of calibration or knowing which gauge is best, but at least I'm part way there.

[ianr1950]

That's a pretty fair amount of posts just to do with the accuracy of cycle tyre pressure gauges. I now know why I love reading some of the topics.
Does it really matter though for the general cyclist.
I know how important it is in an elite cyclists sphere but not for the general guy surely.

[Mick F]

No, it doesn't matter. Nor should it matter.

It's just that if you have a gauge and want to get the pressure correct, you should know that the gauge is ok. Maybe it's just good to know that the gauge gives constant results.

[Brucey]

Normally it doesn't matter too much, but there are some tyres and some operating conditions that mean that you might be in danger of running a tyre close to its maximum or minimum pressure. [In the event of a tyre problem, nothing beats any accusation (from the tyre maker/supplier) of over or underinflation like 'I checked it using my calibrated gauge'....]

Obviously when comparing tyre pressures with other folk, it helps if you are speaking the same language, too.

cheers

[jb]

I imagine when a tyre manufacturer puts the max. tyre pressure on their tyres they are pretty much aware of the average Joe's tyre gauge accuracy and adjust to suit.
A tyre exploding at 1 Pascal over stated Maximum pressure above atmospheric as measured by the NPL wouldn't get too far past the Judge.

[Brucey]

but you would still need to have measured it to know that....

cheers

[jb]

Well somebody somewhere has the wonderful job of deliberately blowing tyres (& other things) up to destruction in order to know that.
He'll be the guy with the big earplugs & callibrated gauge.

[ianr1950]

Some people have a lot of time on their hands to spend this much time on trying to confirm the accuracy of basic pressure gauges when it isn't necessary.
I can understand in a technical situation which I have to do working in university civil engineering labs and need accredited apparatus but for my cycling as long as I pump my tyres up to the same reading on the gauge every time and they stay pumped up what does it matter in the general cycling world that the majority of us live in.
5, 10 psi is not going to affect me one way or another.

When I used to track race I did have calibrated gauges but that was a different situation.

[Brucey]

....as long as I pump my tyres up to the same reading on the gauge every time and they stay pumped up what does it matter in the general cycling world that the majority of us live in.
5, 10 psi is not going to affect me one way or another.

Really? Very many tyre failures are caused by under or overinflation. I myself had a very nasty car accident because I was stupid enough to believe a garage forecourt pressure gauge. Since then I've carried one that I check for accuracy from time to time.

The same considerations can go for bicycle tyres, too. Some tyres have an operating window a mile wide, but others don't. On just one page of the Schwalbe catalogue I find a 35mm tyre with a minimum pressure of 55psi and fatter tyre with a maximum pressure of 55psi. 5 or 10 lbs the wrong way could result in tyre failure with either sort.

When I used to track race I did have calibrated gauges but that was a different situation.

yes, different indeed. Your chances of going under an HGV because of a tyre failure were zero.