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  1. #1
    Senior Member ScottieDog's Avatar
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    Tire Pressure Gauge

    How reliable is a pressure gauge which is incorporated into a pump ?

    I suppose it depends on the pump manufacturer and how good they are, my pump was 50€ (sale price 30€) so I assume its a reasonable one, it´s a floor standing one you pump down onto whilst holding it with your feet, pressure gauge is a standard round clock face type with PSI and both Presta and Schrader (My 700c x 37 tyres are Presta).

    Is it worthwhile getting a stand alone pressure gauge ? Anything recommended ? Maybe digital is better for this type of thing, and accuracy ?

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    Senior Member commo_soulja's Avatar
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    Between the two floor pumps I have, a Wrench Force and a Park Tool, the readings are around minus 3 to 7 lbs off from actual. To get a true reading I use a Accu Gage before I go out on a ride on a bike with a tubeless tire/wheel set up. And yes, a 3 to 7 psi difference does make a difference in tubeless tires.
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    Senior Member MudPie's Avatar
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    Quote Originally Posted by ScottieDog View Post
    How reliable is a pressure gauge which is incorporated into a pump ?

    Is it worthwhile getting a stand alone pressure gauge ? Anything recommended ? Maybe digital is better for this type of thing, and accuracy ?
    1. It'd be a good experiment to tee two pressure gauges together on the same line to verify. I've never done it but seems like to only practical way to verify two gauges accurately.

    2. Using a separate pressure gauge will most likely release air as it is being used, thus you'll never know the true measurement. This is not so critical in large volume automobile tyres, but with small volume bicycle tires, a small bit of air easily affects pressure.

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    There are two measures of accuracy; absolute accuracy, or how close to the true measure they read, and repeatability, or how consistently they're read the same value on multiple readings. Of the two, repeatability is the more important.

    A gauge can be accruate to ± 3psi, yet read (at a true 100psi) 97 one day, and 103 the next. Or a gauge can be accurate to ±6psi and read anywhere from 94 to 106, but if it has good repeatability it'll read the same every day. That's far more important because the true pressure is only a number, and what's best for your weight and riding conditions may vary, but once you decide what works best you want to duplicate it reliably time after time. It doesn't matter what the value is as long as it's consistent.

    Most floor pumps come with Bourdon tube gauges, which when new are generally accurate to about ±2% of the scale, at the center third of the scale, meaning that a 0-200psi pump will be accurate to ±4psi between 70 and 130psi, with the accuracy falling off as you move away to the ends. The nice thing is that Bourdon tube gauges have excellent repeatability.
    Last edited by FBinNY; 09-18-11 at 03:00 PM.
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    Senior Member ScottieDog's Avatar
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    Quote Originally Posted by FBinNY View Post
    , but if it has good readability it'll read the same every day. That's far more important because the true pressure is only a number, and what's best for your weight and riding conditions may vary, but once you decide what works best you want to duplicate it reliably time after time. It doesn't matter what the value is as long as it's consistent.
    Thanks FBinNY, great advice all round. I think the above point sinks in more than others though !

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    jur
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    Confucius says man with gauge knows his tyre pressure. Man with two gauges is never sure.

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    Senior Member ezdoesit's Avatar
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    Just bought this pump yesterday and checked it accuracy out on both tires and it came out dead on.
    I run Vittoria Randonneur 700 X 32 at their recommended pressure of 75 psi and both tires checked out 75.
    Hope this helps you out.
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    Senior Member MudPie's Avatar
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    Quote Originally Posted by jur View Post
    Confucius says man with gauge knows his tyre pressure. Man with two gauges is never sure.
    Quite true. As I say, measure once and believe it. Or measure it 1000 times and apply statistics.

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    747 Freight Pilot bicycleflyer's Avatar
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    Any measuring instrument, whether it be a micrometer, scales, or a air pressure gauge need to be calibrated from time to time. While I do own a precision 1" block to check my micrometer and a precision weight to check my scale, I do wonder what is used to check an air pressure gauge. I know they are calibrated with something, I'm just curious how.

    Anyone here read or listen to "Click-n-Clack" of Car talk radio? I remember one article they did a few years ago in answer to a question about which air pressure gauge type was more accurate. The went out and bought a bunch of different air pressure gauges and compared them to their shop's "calibrated" gauge. While hardly scientific, their results were interesting. For example, they found the most accurate gauge right out of the box was the digital models. Even the cheapest of the Chinese imports was accurate. They found that the worst gauges were those little slider types, you know, the ones that 90% of us have in our glove compartments. Those models could be off by as much as 20 psi.

    It would be interesting if someone had the resources and time to do a bicycle version of that test. But it would have to be someone who has a recently calibrated gauge, and not just something they "know" is accurate.
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    Quote Originally Posted by bicycleflyer View Post
    Any measuring instrument, whether it be a micrometer, scales, or a air pressure gauge need to be calibrated from time to time. While I do own a precision 1" block to check my micrometer and a precision weight to check my scale, I do wonder what is used to check an air pressure gauge. I know they are calibrated with something, I'm just curious how.
    I manufactured bicycle pressure gauges for a number of years until cheap imports made it impractical.

    Gauges are calibrated against known pressure. In our place we had an air line kept at constant pressure and having a check gauge plumbed in-line. The line also had a port for checking with a "master gauge" which was built to a higher accuracy spec. and kept off line except for our weekly check. Once a year the master gauge was sent out for checking which was done using a "dead weight" device which basically was an inline pneumatic cylinder supporting a known weight, and so could be known to maintain a very accurate pressure.

    In short, air gauges, like all measuring instruments, and calibrated against masters, which in turn are calibrated against better masters, and at some point can trace their calibration to the masters, or weights kept at the National institute of standards.
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  11. #11
    747 Freight Pilot bicycleflyer's Avatar
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    Quote Originally Posted by FBinNY View Post
    I manufactured bicycle pressure gauges for a number of years until cheap imports made it impractical.

    Gauges are calibrated against known pressure. In our place we had an air line kept at constant pressure and having a check gauge plumbed in-line. The line also had a port for checking with a "master gauge" which was built to a higher accuracy spec. and kept off line except for our weekly check. Once a year the master gauge was sent out for checking which was done using a "dead weight" device which basically was an inline pneumatic cylinder supporting a known weight, and so could be known to maintain a very accurate pressure.

    In short, air gauges, like all measuring instruments, and calibrated against masters, which in turn are calibrated against better masters, and at some point can trace their calibration to the masters, or weights kept at the National institute of standards.
    FBinNY, Very Informative...and thank you for the reply. I may have to build myself one of those inline cylinders.

    You still have that equipment? You may be the guy to do the testing I mentioned above.
    Flying an airplane is really very simple...Push the stick forward, the house gets big. Pull the stick back, the house gets small. Keep holding the stick back, the house gets big again.

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    I still have the master gauge that I use for calibration, but haven't checked it in years. I never owned a dead weight unit, I sent my calibration gauge out for inspection/calibration annually to a company that did.

    BTW- they're gone now, all the manufacturing long since shipped to the orient.
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    Senior Member himespau's Avatar
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    A man with one watch knows what time it is. A man with two is never sure. I'd imagine the same thing goes for pressure gauges.
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    i have just tested a dozen or so guages that are in my workshop, a quick test on a 26 x 2 tyre, not conclusive but it is possible to see how much they vary, a more informative test would be on a larger tyre, like a truck tyre, and use any of the available guages as a master guage (i prefer the one on the compressor) and use this one to set the pressure prior to each test.

    it is also important how well the guage seals on the valve at first attempt, with some of them by the time the guage was read, so much pressure had been lost that the tyre needed inflating again.

    2 digital guages varied by about 5 lbs although this was at 70 psi where the range of one was 2-60 and the other was unmarked. one pencil type was out 30+psi.

    i suppose, theoretically one could make up a test unit from something like the cylinder and piston from a foot pump, and if the bore equated to 3 sq" it would support a weight of 150 lbs at 50 psi?

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    Senior Member himespau's Avatar
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    Most gauges designed for cars don't go up to bike tire range. Did you refill the tire after each testing to replace the air let out in the previous test?
    Punctuation is important. It's the difference between "I helped my uncle, Jack, off a horse" and "I helped my uncle Jack off a horse"


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    no i didn't. like i said it was a quick test, and will do it more thoroughly when i get time, but generally after testing a few the pressure dropped by a couple of pound or so each time, when it varied a great deal, say from 58 to 38 then i tried the previous one again which would now be somewhere around 55. so that a couple of them did stand out as being inaccurate, whilst the others (having been compared several times) were pretty much within expected tolerances.

    like i say, i will do the test more thoroughly when i have time,

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