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Old 01-16-2009, 06:06 AM   #11 (permalink)
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Quote:
Originally Posted by wyatt View Post
The definition of accuracy is that you get the same thing several times in a row, the definition of precision is the average of the readings is correct... you either want something both precise and accurate, or at least accurate so you can correct for it...
You got that backwards. Precision is when your readings are grouped around a small point. Accuracy is when your readings are near the true value.

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Old 01-16-2009, 11:16 AM   #12 (permalink)
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Quote:
Originally Posted by instarx View Post
You got that backwards. Precision is when your readings are grouped around a small point. Accuracy is when your readings are near the true value.
I'd actually take the correction one step farther; I define it in my Physics class around how we actually use measurements in the real world:

Precision is the exactness of a measurement:

--28.50 psi indicates a measurement made to the nearest 1/100th of a psi.

--31 psi indicates a measurement to the nearest 1 psi.

The difference between these numbers is the amount of precision implied in the measurement.

Accuracy is the closeness to the actual correct value.

--If the actual pressure in a tire is exactly 29.994 psi, the gauge reading 31 was more accurate, although it implied less precision.


The classic example of shots at a target and having grouped results isn't my favorite; I would consider getting the same results from a tool when measuring multiple times "consistent" rather than "precise".

The distinction with precision comes when you combine measurements in multiple calculations. If you know your tire loses 1 psi per month, that's a value precise to the nearest 1 psi. If you measured it as 28.943 psi last month, and you use your -1 psi/month figure, you can't say that your tire is now at 27.943 psi. You say it's at either 28 or 30 psi, depending on how exactly you use significant figures. In this case, I'd say 28 psi.

I recently heard about a traffic accident reconstruction expert from my wife (she was watching the trial) and the "expert" went through all his figures and said he determined with certainty that the vehicle had to be traveling 56.554 mph. I wish I was on the jury or on the opposing side of court; I'd have raised the BS flag on that one.
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Old 01-16-2009, 11:44 AM   #13 (permalink)
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I'd also like to add a pet peve of mine: the apparent (and totally bogus) precision people assign to digital gauges of all types. In the picture of the pressure gauge above you might think the tire pressure was 28.5 psi. In fact the gauge only has half a digit of precision, meaning the pressure is anywhere between 28.25 and 28.75 psi. (I have one of these and the gauge only reads 27.5, 28.0, 28.5...etc.).

This raises and interesting possibility: two tires measured at 28 psi with this gauge may be much further apart in actual pressure than two tires measured with this same gauge at 28 and 28.5 (which would happen if the actual pressures were 28.2 and 28.3 psi).

Having said that, it is only an academic problem. In my opinion it doesn't matter if tire pressures vary by half a psi. Precision for precision's sake is wasted effort.

For example, for the past 20 years there has been a strict standard for the velocity of the air jets that blow dust from the clothing of people entering Class III and IV clean rooms. I don't remember the exact value any more, but it is something like 85 feet per second. Millions of dollars have been spent making sure that the air-wash in clean rooms around the world have this velocity. Entire systems have been torn out and replaced to achieve this velocity. Clean rooms have been de-certified because they only had 80 fps wash air. What most people do not know is that the two researchers who established this "best" value arrived at it by holding a rag out a car window and noting the speed where it flapped the most.

Last edited by instarx; 01-16-2009 at 03:08 PM..
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Old 01-16-2009, 12:11 PM   #14 (permalink)
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adam728 -

This whole issue is why I asked Santa for this for X-Mas :

Pegasus - Intercomp 2.5" Glow in Dark Tire Pressure Gauge, 0-60 psi


It ain't cheap ($40), and it only goes to 60 PSI, but I am crossing my fingers that the website isn't selling me junk. And get this, it's MADE IN THE USA!!!!!!!!!!

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Old 01-16-2009, 01:05 PM   #15 (permalink)
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Quote:
Originally Posted by instarx View Post
You got that backwards. Precision is when your readings are grouped around a small point. Accuracy is when your readings are near the true value.
You are completely correct... Here I thought I was screwing up the definition a little bit with my wording, go figure I would get it completely backwards...

Quote:
Originally Posted by instarx View Post
In the picture of the pressure gauge above you might think the tire pressure was 28.5 psi. In fact the gauge only has half a digit of precision, meaning the pressure is anywhere between 28.25 and 28.75 psi. (I have one of these and the gauge only reads 27.5, 28.0, 28.5...etc.).
That's what I learned, if it shows something like 28.5, you can only trust it to the nearest half. If it goes to 28.75, to the nearest quarter... unless you know it to take more refined measurements, you can't assume that it does. I know my electric shaving razor goes in increments of 20%... I may have 20% left, and it may last for one shave, or perhaps two or three... hard to say... I agree, kind of annoying, but I expected nothing less.
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Old 01-16-2009, 01:24 PM   #16 (permalink)
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A few years ago I got a digital tire gauge from cabelas. Can't remember the brand though. It worked great...until one day. I was filling up the tires on my truck to 75psi. Well the last tire was just taking forever to fill up (according to the gauge). In addition it seemed as though my air compressor was not pumping up high enough pressure to actually get any into the tire. I stopped and looked at the tank on my compressor and it was at 135psi I got another gauge (mechanical slide type) and it about blew apart from the pressure in the tire. The tire was at 135psi!!!!!!!!!!!! Stupid digital gauge just about got me killed It went right into the garbage!
Took me about seven years to actually buy another digital gauge. I bought a goodyear one with an LED light in the tip. I'm always on the look out for a "slow filling" tire situation though.
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Old 01-16-2009, 05:11 PM   #17 (permalink)
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Quote:
Originally Posted by instarx View Post
...What most people do not know is that the two researchers who established this "best" value arrived at it by holding a rag out a car window and noting the speed where it flapped the most.
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Old 01-17-2009, 07:57 AM   #18 (permalink)
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A couple years ago I bought a nice brass dial gauge by Accu-Gage.
I got it locally for about $15 but it's also available from Accu-Gage's web site.


Accu-Gage site

I selected the gauge because they actually stated a specification of accuracy:
"A 60 psi tire gauge is accurate to +/- 1.2 psi from 15 to 45 psi and is calibrated to ± 1 psi at 30 psi. " They make it in other psi versions also.

Quoting from their site"
"All Accu-Gage® 1-1/2", 2" and 2-1/2" dial tire gauges are ANSI Commerical Grade B gauges, which is the best quality gauge typically used for tire pressure applications.
These tire gauges use a fully geared, solid brass precision movement with bronze bourdon tube. Unlike piston-plunger-type gauges, the bourdon tube movement is not affected by changes in temperature, humidity, altitude or air stream contaminants.
The mechanical accuracy rating is ± 2% from 25% to 75% of scale and ± 3% below 25% and above 75%. "
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Old 01-26-2009, 12:56 PM   #19 (permalink)
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Quote:
Originally Posted by gascort View Post
--28.50 psi indicates a measurement made to the nearest 1/100th of a psi.
Is this correct? It has been a long time, but I think that number only has three significant figures. Therefore it would be to the nearest 1/10 psi.

I may be wrong - it has been a long time I've had to worry about significant figures in anything other than my waistline.
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Old 01-26-2009, 10:09 PM   #20 (permalink)
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Re: 28.50 psi:
Quote:
Originally Posted by instarx View Post
Is this correct? It has been a long time, but I think that number only has three significant figures. Therefore it would be to the nearest 1/10 psi.
Typically, zeroes between nonzero digits or zeroes to the right of the decimal are considered significant in my experience/usage.

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