Pressure-gauge calibration

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cycleruk
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Re: Pressure-gauge calibration

Postby cycleruk » 6 Jul 2013, 10:44am

Brucey wrote: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.

cheers


Not when lorry tires can go upto 250 PSI. :shock:
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Brucey
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Re: Pressure-gauge calibration

Postby Brucey » 6 Jul 2013, 10:57am

I'd argue that truck tyres are atypical for most tyre fitters, and I'd also question how accurately truck tyre pressures are set, and therefore how accurate the gauges need to be.

cheers
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reohn2
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Re: Pressure-gauge calibration

Postby reohn2 » 6 Jul 2013, 8:43pm

FWIW I have a Topeak Joeblow Sport track pump,with a dial guage,a Schwalbe digital guage and a Topeak digital guage.I checked them against each other earlier today they're within 3psi of each other.That's close enough for me :)
If anyone asks how do know they're all not way out? I don't :? :mrgreen:
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Mick F
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Re: Pressure-gauge calibration

Postby Mick F » 6 Jul 2013, 9:16pm

Three all agreeing is a good start indeed. :D

I have a "pencil' thingy in the boot of the car. You know the sort ..... a plunger comes out and you read off a scale. It's only Shader and applicable for cars, so no use for my bike's Presta at circa 110psi, though perhaps I could test it with my Shader valve trailer tyres to some degree. The track pump and the digi gauge are Shader/Presta/Woods of course. I could get the trailer tyres up to 50 or 60 psi without problems I would think - I'd have to read what the tyres say on their sides.

Worth a test tomorrow, but if two don't agree, I'm back to square one .................
Mick F. Cornwall

MartinC
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Re: Pressure-gauge calibration

Postby MartinC » 7 Jul 2013, 7:50pm

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


Probably neither. Next question? :D

Brucey
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Re: Pressure-gauge calibration

Postby Brucey » 7 Jul 2013, 8:15pm

pencil gauges are least likely to be accurate, espcially little-used pencil gauges; a slightly sticky piston or a leak somewhere will completely scupper their accuracy IME.

Most pencil gauges don't read over 50psi, so note also that any calibration check at this pressure may be meaningless if you are really interested in the accuracy of the other gauges at ~100psi.

cheers
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Binkyboy
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Re: Pressure-gauge calibration

Postby Binkyboy » 10 Jul 2013, 1:09pm

Does it really matter?
Before we were affluent and could afford gauges, we just used to pump the tyre up until it was "hard", and were quite happy with the result.

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meic
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Re: Pressure-gauge calibration

Postby meic » 10 Jul 2013, 1:22pm

Back when I was a motorcycle courier, I could detect when my motorcycle tyres were out by even a couple of psi, then reset them checking with a pencil gauge.

Now it didnt matter if they really were the marked 30psi or not, if the gauge said 30psi then they were the right pressure, whatever that was. The gauge did need to be precise enough for it to show the pressure to around a psi and it did manage that.

My gauge on my Topeak Road Morph will jerk about 1 bar at a time, so it isnt that precise or accurate but it still gives satisfactory results as I am happier to ride with a much broader range of pressures on a cycle.
Yma o Hyd

Brucey
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Re: Pressure-gauge calibration

Postby Brucey » 10 Jul 2013, 1:27pm

I used to think that the absolute pressure didn't matter, and (to an extent) I still do with supple-walled tyres; you can tell instantly if they are too soft.

However I have found that with some puncture resistant bicycle tyres, I just can't tell if the tyres are hard enough or not. This wouldn't be a big problem except that they are just vulnerable to damage if used when underinflated.

cheers
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Mick F
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Re: Pressure-gauge calibration

Postby Mick F » 10 Jul 2013, 1:51pm

My problem isn't that my tyres aren't hard or that they're too soft. It's just that it's like the old Chinese proverb:
Man with a watch knows the time. Man with two watches never knows the time.

I plan on a ride to Bedford and back in late August and I'll be taking my small Lezyne pump and separate digi pressure gauge. If I'm happy at 120psi as recorded on my track pump, I'll have to be happy with the "same pressure" of 110psi as measured by my digi gauge.
Mick F. Cornwall

MartinC
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Re: Pressure-gauge calibration

Postby MartinC » 10 Jul 2013, 5:06pm

That's exactly it. 'Your' pressure is x - that's 120psi on one gauge and 110psi on the other. If you want the absolute value of x you'll need to find a pump that's been calibrated to the accuracy you want. If you're not sure whether x is the right pressure or not then you can experiment to find out what it should be. Your only problem is if someone's told you what x should be and you want to pump to that (and being slightly OCD, in the nicest possible way :) ).

Brucey
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Re: Pressure-gauge calibration

Postby Brucey » 20 Jul 2013, 10:39pm

I checked the calibration of some car tyre gauges today.

I got the following results;

34.5psi - digital gauge (new, from a major tyre manufacturer)
35 psi - balloon (pencil) gauge, (new but cheap)
37.5psi - new digital gauge(*)
37.5 psi - garage type pcl inflator with built-in gauge
38 psi - balloon gauge, (new but cheap)
39 psi - CAT branded Accugage (new, unused)
40 psi - Accugage (not brand new, but little-used, kept for calibration checks... :shock: )
40 to 41psi - balloon gauges (4-off, old but good quality)
42 psi - CAT branded Accugage (old, pretty beat up)

(*) I got some odd readings with this gauge, see comments below.

I was able to check all the gauges several times on the same car tyre without letting so much air out that the pressure changed. I confirmed this using several of the gauges a second or even third time. I suppose that you would be happy to say that the tyre was probably 39psi +/- 1.5 psi but that still leaves several outliers. I can forgive the Accugages to some extent because they are calibrated at mid-scale (i.e. 80 psi on a 160psi gauges like these, so might be (say) +/- 1.5 psi through much of the range). Car tyre pressures vary anyway with ambient temperature and when running hot; 5psi variations are not ususual because of this. The specified pressures are normally 'cold' pressures. Bicycle tyres normally tend not to run hot in the same way but can still increase in pressure considerably if the rim gets hot during braking.

I tried a few of the gauges on a 50psi bicycle tyre, and if anything, they were in better agreement. Very odd.The balloon gauges mostly only go to 50psi so I couldn't try them at higher pressures.

The digital gauge that appeared to be the more accurate (of the two I tried) in fact gave the most erratic readings of any gauge. However after having used it five times, it appeared to become more consistent. I have no idea why.

I may try all my bike pump gauges yet. The last time I did this IIRC I got over 10psi variation at ~100psi.

cheers
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Mick F
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Re: Pressure-gauge calibration

Postby Mick F » 21 Jul 2013, 12:05pm

Thanks Brucey! :D
Excellent bit of work.

It looks like you can't believe any gauge.
Mick F. Cornwall

reohn2
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Re: Pressure-gauge calibration

Postby reohn2 » 21 Jul 2013, 12:12pm

Mick F wrote:My problem isn't that my tyres aren't hard or that they're too soft. It's just that it's like the old Chinese proverb:
Man with a watch knows the time. Man with two watches never knows the time............


Old Lancashire proverb say,"man with two watches has one watch too many" :mrgreen:
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profpointy
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Re: Pressure-gauge calibration

Postby profpointy » 21 Jul 2013, 1:17pm

Doesn't matter a jot if gauge is correct; but does matter that it's consistent.

By experience you will determine what reading is ptimum for your combination of bike, tyre, weight, conditions, then use that as your setting. You don't care if the gauge says 100psi, 90pso or 30 bongos