Pressure-gauge calibration

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Mick F
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Pressure-gauge calibration

Postby Mick F » 4 Jul 2013, 4:02pm

I have a digital pressure gauge, and there's an analogue dial on my track pump.

They don't agree. Digi says 100psi, dial says 110psi. 10% difference!

Which is right?
How can I tell?
Mick F. Cornwall

Valbrona
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Re: Pressure-gauge calibration

Postby Valbrona » 4 Jul 2013, 4:07pm

But the rest of us dunna worry about 10psi.
I should coco.

ChrisButch
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Re: Pressure-gauge calibration

Postby ChrisButch » 4 Jul 2013, 4:50pm

10% variation a lot less than you find in garage forecourt airline gauges!

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cycleruk
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Re: Pressure-gauge calibration

Postby cycleruk » 4 Jul 2013, 4:51pm

Mick F wrote:I have a digital pressure gauge, and there's an analogue dial on my track pump.

They don't agree. Digi says 100psi, dial says 110psi. 10% difference!

Which is right?
How can I tell?


You can't unless one of them was bought with a test calibration certificate.
Calibrations are done to a known traceable national standard and are usually checked against a "standard test gauge" or with a "dead weight tester".

You could ask around to see if any where has one or maybe your local tyre shop has a calibrated set up.
I doubt that garage forecourt pumps are any more accurate.
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reohn2
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Re: Pressure-gauge calibration

Postby reohn2 » 4 Jul 2013, 5:13pm

Valbrona wrote:But the rest of us dunna worry about 10psi.

Speak for yourself.
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Re: Pressure-gauge calibration

Postby Brucey » 4 Jul 2013, 5:35pm

you can check the calibration of a gauge against a gauge of known calibration. Your gauge will then be some kind of secondary (or tertiary) standard.

If you wish to keep a gauge as a reference for your own use, a traditional gauge with a good quality bourdon tube in it, stored correctly, used infrequently, is as good as anything. You can check the calibration of your track pump against the reference gauge on a regular basis. If the reference gauge is stored damp, overpressured, dropped etc etc then it will need to be checked again.

Although they are pretty good, I don't think you can rely on any electronic gauge to definitely hold calibration in the long term; too many things can go wrong IMHO.

Of readily available tyre gauges, 'accu-gage' ones are pretty reasonable and come with a calibration cert.

http://www.getagauge.com/faq.cfm

they do a gauge PR160BX for bicycle tyres up to 160psi with presta valve fitting; calibration should be +/- 2psi at 80psi. Dunno where you can get one in the UK though.

A simple DIY method of checking calibration is to use a dead weight on a pump handle, having measured the piston diameter accurately. However I expect you to find that the piston sticking friction will create an error that is - or + depending whether the load is increasing or decreasing.

IME very many track pump gauges over-read rather than under-read. I have no idea if this is commonly the case or indeed why it might be, but it holds true for my motley collection of gauges.

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Mick F
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Re: Pressure-gauge calibration

Postby Mick F » 4 Jul 2013, 7:42pm

Brucey wrote:A simple DIY method of checking calibration is to use a dead weight on a pump handle, having measured the piston diameter accurately.
If I blocked off the outlet of my track pump and put a weight on the handle then noted the dial's reading, it would be an accurate check? Sounds a bit too simple, but it seems to make sense.

Of course we have to consider the scale errors inherent in spring-loaded dials across their range .................
Mick F. Cornwall

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Re: Pressure-gauge calibration

Postby cycleruk » 4 Jul 2013, 8:39pm

Mick F wrote:
Brucey wrote:A simple DIY method of checking calibration is to use a dead weight on a pump handle, having measured the piston diameter accurately.
If I blocked off the outlet of my track pump and put a weight on the handle then noted the dial's reading, it would be an accurate check? Sounds a bit too simple, but it seems to make sense.

Of course we have to consider the scale errors inherent in spring-loaded dials across their range .................


In theory Yes.
If your piston is 1 sq,inch and you put a 1lb weight on then that is 1psi.
A pressure gauge should be tested at 3 places on the dial. Say for instance 5%, 50% & 100%.
Some gauges have a stop at the zero position which may give a false reading if the zero is negative.
Testing at 3 places is the minimum that will show up a linearity error. (ie it may be correct at 5% and 100% but out in the middle.)
Other errors can be "zero", which is corrected by adjusting the pointer.
"Range" which can only be corrected by adjustment inside the gauge workings.
"Linearity" is the awkward one and is usually caused because the internal workings are not "square" at mid range or the tube has been damaged/over-pressurised.

The little spring inside a gauge is there only to take up the "play" in the gearing.
The "spring" in the pressure tube is the action that actually returns the pointer to zero.
The other error is known as "hysteresis" and is simply the gauge reads different going up against coming down. Usually caused by gummy gearing or a tube that has been over pressurised.

And you thought it was just simply a pressure indicator. :lol:
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Re: Pressure-gauge calibration

Postby Bogawski » 4 Jul 2013, 9:26pm

That's good, my track pump, (a Wrench Force) can read 100psi after a couple of pumps, very unpredictable. My digital one now has a problem with display so I bought a dial type from Evans which is ok for the moment though whose calibration can you trust?

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[XAP]Bob
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Re: Pressure-gauge calibration

Postby [XAP]Bob » 5 Jul 2013, 7:39am

All very complicated - a manometer is what you need. Trouble is that 100 psi is how many bar?

Hmm, a 250 foot manometer. No problem, you live on a hill don't you Mick?
Or you could get some mercury, and go for a 20 foot one, that would fit up the side of most houses...
Last edited by [XAP]Bob on 5 Jul 2013, 7:51am, edited 1 time in total.
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Re: Pressure-gauge calibration

Postby Brucey » 5 Jul 2013, 7:47am

Bogawski wrote:That's good, my track pump, (a Wrench Force) can read 100psi after a couple of pumps, very unpredictable. ...


a track pump gauge measures the pressure in the hose, which is not the same thing as the pressure in the tyre unless the tyre valve is open. If the valve -or any other check valve in the line- is closed (or at least not yet open) when you start pumping, the pressure can rise very high in the hose and on the gauge.

A pump with a Schrader connector should read the tyre pressure as soon as the pump is connected because (unlike a presta connection) the connector positively opens the valve. With a presta type, the valve is opened by differential air pressure.

However, presta-type valves do tend to stick slightly; many years of inflating tyres has ingrained a habit of freeing the valve and releasing a little air before connecting a pump and starting to inflate a tyre. If you don't do this it can take ~50psi excess pressure to open the valve.

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Mick F
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Re: Pressure-gauge calibration

Postby Mick F » 6 Jul 2013, 7:09am

My question still remains.
Which one is correct?

I know the theory of calibrating measuring devices, but it's not worth the effort to build a manometer! I have enough to do without finding a 250ft hose pipe. :wink:

Garage forecourts are notorious for poor tyre gauges, but I wonder if a professional tyre fitting company have their equipment calibrated? I could take a tyre there and check the pressure then compare my readings.
Mick F. Cornwall

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Re: Pressure-gauge calibration

Postby [XAP]Bob » 6 Jul 2013, 7:25am

Mick F wrote:My question still remains.
Which one is correct?

I know the theory of calibrating measuring devices, but it's not worth the effort to build a manometer! I have enough to do without finding a 250ft hose pipe. :wink:

Garage forecourts are notorious for poor tyre gauges, but I wonder if a professional tyre fitting company have their equipment calibrated? I could take a tyre there and check the pressure then compare my readings.

at least 500 feet, need both sides after all :)
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
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Re: Pressure-gauge calibration

Postby Brucey » 6 Jul 2013, 7:33am

just buy a decent gauge with a calibration cert, and only use it as your reference. They are (as per the link I posted previously) not necessarily expensive.

Any thought that a car tyre fitters will have an accurate gauge is (IME)

a) hilarious, and

b) irrelevant; they are typically measuring pressures 1/3rd of your range of interest.

Any gauge that is used (and abused) on a daily basis is liable to go out of calibration, hence keeping a gauge for use only as a reference.

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Re: Pressure-gauge calibration

Postby cycleruk » 6 Jul 2013, 10:36am

Mick F wrote:My question still remains.
Which one is correct?


Both could be wrong. :?
I've tested a few thousand gauges over the years of both mechanical and electronic versions and there is just no way of knowing without a standard to check against.
There's no such thing as a tailwind.
It's either a headwind, or you're going well.