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Discussion Starter · #1 ·
There was a good thread a few weeks ago about gauge accuracy and there was opinions on both sides with regards to digital and analog. Having worked with both analog and digital test equipment throughout my career I know that there is nothing inherently more accurate about digital.

However, here is the question. I have a better quality stick gauge and two good quality dial gauges. All read within one pound of each other.

I bought a new digital Craftsman gauge last week. I know it is actually one of the better Accu-Tire gauges because the readout is identical. My analog gauges measured 35,35, and 34 lbs. Surprisingly, the new digital measured 38 lbs! What's up with that?

I know that every major city has companies that specialize in calibrating test equipment, but is there an easier way to verify accuracy without going through that much trouble. I don't know now if the new gauge is bad or if all of my analog gauges are bad. Any thoughts?
 

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Gauge Accuracy

Hello Larry:

...Ohio's not that far from southern Ontario

...I had 2 dial gauges and 1 pen/stick gauge; one of the
dial gauges matched the stick gauge; took them both to
3 local mechanic shops and they all reasonably matched
...the other dial gauge was out by 3-4 lbs

...I only used both the dial and pen/stick gauge; well, the glass
dial recently blew off the gauge so now I only use the stick gauge
and I'm quite confident with that

...my air compressor, a "professional" tire inflator, was always
out by 3-4 lbs or so; contacted the mfg'r and he said there's
many variables; altitude above sea level; humidity; air temperature
etc; in other words, every reason as to why this air compressor
for $50 plus wasn't accurate

..anyways, if you have a stick and dial air gauge that match, feel
comfortable

...of course, there's always the possibility that your 1 dial gauge is
accurate and the other dial gauge and stick gauge are wrong!!
(just kiddiing)

regards.....Ken
 

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Gauges

Gauges of this price range fall into the 10% accuracy range. This is normal. To obtain better would require calibration against a standard. As for the stick gauge I never really understood how they work:)
 

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Larry, That is one the best, hardest questions one can ever ask anybody. How do you know for sure the accuracy of a temp gauge and a air pressure gauge? Think about it.

I have been in your shoes many times. I take them all and see which ones are the most consistant. That is the only way we can be sure. I do it by odds.
 

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--------------------------------------------------------------------------------

Motorcycle Consumer News did a comprehensive test on 23 different brands of tire pressure gauges and presented a complete report in their November 2005 Issue.
They were evaluated for accuracy, form/fit, readabiity and durability (drop test). Of the 23 brands tested, They reccommended an Auto-on Digital Tire Gauge whcih can be purchased for $9.99 from Griots Garage www.griotsgarage.com. Also reccomended were CyclePump EZAir Guage avaialble for $25. from cyclepump.com, NAPA Tire Pressure Gauge (pencil type) for $7.99 from NAPA Autoparts Stores and a Road Gear Guage for $24.90 from roadgear.com.
 

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I still don't understand how altitude can affect air pressure in a tire or the reading of a tire gauge.

The gauge uses a calibrated spring for a reference and the amount of compression that occurs from the air pressure against a piston. This would be the same even in a vacuum. (this is assuming that the tire doesn't expand, of course)

The pressure in the tire, a sealed container, doesn't change with altitude.

The only difference in a digital gauge is the readout display. The actual mechanism is still analogue as far as I know.

Of course, this is just the way I have it figured. I have been wrong once (or twice) before.
 

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Tire gauges

The last I read, can't remember where, the most accurate type is the bourdon tube gauge. I use an accu-gage with a short hose attached. The valve end goes on the valve cap with a retainer that stays on and the pump goes on the short, pressure relief valve attached to the gauge. The gauge is inline to the tire and there is a button to let air out if necessary. I found this is the best for me as I could never get any other gauge to fit the first time to the tire valve and always ended up letting quite a bit of air out of the tire before I got it positioned right to get a reading. And I was always afraid I was going to bend the right angle valve stem and break the thing. Hope this helps. Can't find a model # but you should be able to find a photo online somewhere.
 

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Discussion Starter · #10 ·
Ken that is good to know, but I'm really not trying to locate the most accurate gauge, just a way to find out which ones that I have are reasonably accurate.

Apprarently this question has stumpted this board. I think the suggestion made earlier about just getting together with a bunch of riders and comparing readings is about the most reasonable answer. I'm not looking for absolute perfection, just to be reasonably close. I don't want to be off by 3 pounds.
 

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riffraff said:
I still don't understand how altitude can affect air pressure in a tire or the reading of a tire gauge.
A pound is a measurement of mass relative to the force of gravity. The farther you get from Earth's center the lighter a pound becomes. Therefore, an 800 lb GoldWing at sea level weighs considerable less at 12,000 feet (remember you take your Wing to 58,000 feet [11 miles] miles from Earth and it’s almost weightless since there is less gravity). As far as tire air pressure goes, we measure that in PSI (pounds per square inch). Stated differently, it represents how many pounds of weight are sitting on each square inch of tire patch touching the pavement. At sea level, we can have 41lbs of pressure in the rear tire, and if the rear weight on that tire is 600 lbs then the tire patch size is 14.3 squared inches (or an oval patch covering the same area). At 12,000 feet the bike is lighter, therefore, the tire patch is smaller and if you measured the tire pressure it will be less also.
 

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Tire guage

If you can find some one with a guage accuracy traceable to the US bureau Standards. That is the final say-so on accuracy. Any airport repair staion may have a way to check one or make a party of it and bring several. Tom
 

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Discussion Starter · #13 ·
riffraff said:
I still don't understand how altitude can affect air pressure in a tire or the reading of a tire gauge.

The gauge uses a calibrated spring for a reference and the amount of compression that occurs from the air pressure against a piston. This would be the same even in a vacuum. (this is assuming that the tire doesn't expand, of course)

The pressure in the tire, a sealed container, doesn't change with altitude.

The only difference in a digital gauge is the readout display. The actual mechanism is still analogue as far as I know.

Of course, this is just the way I have it figured. I have been wrong once (or twice) before.
I hate when you guys do this to me. I can never resist researching this stuff when a question gets my curiosity.

I had to do a little research since my physics memory is a bit rusty, and then I had to smack myself because I realized I should have known the answer to this.

With most measurements, there is no such thing as an absolute zero. There are two ways to measure pressure, absolute pressure and gauge pressure. Scientists use absolute pressure, which measures pressure relaitive to a perfect vacuum. (0 psi.) The measurements we make use gauge pressure. In the case of a tire pressure gauge, the zero reference is the surrounding air pressure. What this means is that as altitude increases, your zero reference becomes less accurate and so does the gauge. Knowing that, if you measure absolute pressure of a tire, no the tire pressure does not change with altitude. Measured with gauge pressure, which is what we are concerned about, yes it does change.

Here is the real sticky part. To measure this difference you would have to be using a differential gauge that samples the surrounding atmospheric pressure at the same time. Since our gauges can't do this, the pressure we measure will be the same at all altitudes.

Fortunately, once you start measuring pressures that are several times greater than atmospheric, the amount of change is so slight that it is insignificant. That makes a debate about pressure change with altitude a moot point because the difference is far less than the accuracy of virtually all consumer grade gauges.

Fortunately, even though Rifraff's logical reasoning was incorrect, I think I was able to answer my original question after my research. Lacking enough information to know what elevation a particular gauge is calibrated at, coupled with the inherent innacuracy of consumer gauges, it is virtually impossible to attain the tire manufacturer's recommendations of staying within 10% of the recommended pressure. My only recourse is to monitor my tire wear and compensate based on what I know about how pressure affects wear. This could explain why so many people have tire cupping problems and so many don't. We are relying on our gauges and recommended pressures and not using common sense.
 

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Re: Tire guage

tom smith said:
If you can find some one with a guage accuracy traceable to the US bureau Standards. That is the final say-so on accuracy. Any airport repair staion may have a way to check one or make a party of it and bring several. Tom
You said "traceable" and "standards" in the same sentence, I'm drooling...

Actually there is no US Bureau Standards, but the sentiment is correct. It used to be called the National Bureau of Standards(NBS), but, in typical government fasion, it is now called the National Institute of Standards and Technology(NIST). I know this is all semantics, but precision is my business.

I could go into a long disertation about "Gage Pressure" vs "Absolute Pressure", but I don't think it will resolve this issue. There is a fundamental difference between analog and digital tire gages and that is the method they use to gage pressure. A previous poster is correct in saying that the bourdon tube variety is inherently more accurate at this price level. However, the digital transducer is more repeatable over time.

When discussing touring motorcycle tires, repeatability is more improtant than accuracy. Larry is right to say that the accuracy available in a cheep tire gage is not sufficient to give valuable results. We need to be dilligent and watch our tires and adjust not according to some arbitrary number, but what is actually working best for the individual. Also, +/-2psi really shouldn't make any difference to us. To a racer it will, but not for our concerns of longevity and tread wear.

My recommendation is to use one gage. This will give you consistant results and that is more important than anything else for our purpose.

Further, test your repeatability. Using the same gage, check and recheck the same tire a few times. If you get the same reading every time, then you have good technique. If you get different readings, either you have a bad gage or your technique needs work. Either way, work on it until you get consistant readings.
 

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guage pressures

to Rastoff
Can you forgive an retired aviation Tech for refering to Standards Bureau. I think I remember hearing about that change of name. I guess technologies should be part of goverment speak these days. Thanks for the correction. What's happening at Edwards these days. Tom
 

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Re: guage pressures

tom smith said:
What's happening at Edwards these days. Tom
Well, we're calibrating a lot of tire gages...

Don't sweat the NIST name thing. I wasn't trying to stomp on ya, it's just the anal side of me coming out. Shoot, the fact that you even knew about the bureau puts you ahead of 98% of the world.
 
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Here is the one I use. It is a bourdon type gage and is as accurate as it needs to be. Don't get too anal about tire pressure. A pound or two either way isn't going to amount to a hill of beans. We are not racing Nascar or Grand Prix vehicles or flying F16's.

This one is an AccuGage and is rugged and reliable, and very easy to use. I like the rigid section of the hose and the angled tire chuck. Many others seem to have good things to say about AccuGage products.

Ride safe.

 

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Discussion Starter · #18 ·
OTOH, many (most?) electronic pressure sensors, of the type used
in inexpensive 'digital' gauges, basically respond to absolute pressure.
That is not true. If it were, your tire gauge would read about 14.7 lbs of pressure without the gauge even being hooked up to the tire and would read about 50 psi with the tire fully inflated.

I used scientists as an example because most labs reference their pressure readings against a perfect vacuum, not atmospheric pressure, the way a tire reads.
 

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All I know is I usually lose a pound of air when I check the dam pressure by not getting the guage on perfectly so out comes my compressor - put air in, check, let air out, check - it is right on - to be sure I check again and I lost that pound again. Vicious cycle.

Ever go to Walmart or ACE hardware and look at all of the thermometers hanging on the wall for sale and notice how the temperature is all over the place. Especially those big dial type jobs.
 

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I am a Metrologist and I work at a nuclear power plant. I have been doing this job for 30 some odd years. So far I have been being quiet about this thread but I am being pulled in anyway. The reason that the temperature gages (thermometers) are all over the place has to do with the uncertainty of the measurement. The same applies to the measurement of the tire pressure. I will not get real long winded about this. Measure your tire pressure with the same gage and do it the same way every time. When you find the number that makes you feel the bike is riding the best, just keep hitting the same number. Always use the same gage. Even though 2 gages look exactly the same, they may not measure exactly the same. If you change the gage it might vary the desired indicated pressure by a pound or 2.
Metrology is the science of measurement.
PS: I calibrate pressure gages and thermometers for a living.
 
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