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OT-Precison vs acccuracy

hanermo

Titanium
Joined
Sep 28, 2009
Location
barcelona, spain
Great thread.
Imho it conflates the 2, like always tends to happen.


Accuracy is the ability to make, or measure, a part or feature of a part to an established standard, like a perfect inch or meter or whatever.
A perfectly accurate meter is the exact same length as the one in the museum (arts y metiers iirc)I visited 3 years ago in france.

Precision implies accuracy but does not guarantee it.
Precision is making a needle valve for an ic engine, or a lens for a home telescope.
The lens is probably precise in its curvature to about 0.05 microns, or 0.0005 mm, or 0.0000025".

But the lens, or valve, is not necessarily accurate, because it does not have an established size, measurement, radius, or curve profile.
They can have excellent fits, and fantastic resolution, and amateurs routinely fit stuff to better than 1 micron, often better than 0.1 micron in telescopes etc.

Precision is usually badly used.
It should be == accuracy, and resolution used for machine tool stuff.
Resolution is the min. size you can move, or make a feature like a divot, whatever.
If you can make a cut of 0.001 mm your machine tool has a resolution of 1 micron.

This does not mean that your machine tool can cut a piece to 10.001 mm - not at all.
Or that it can make 3 pieces with 0.001 mm ledges, of size 10.001 mm, for example.

Resolution is a better word than precision.
Accuracy is a good word. It says anyone anywhere can measure the piece and come up with a result that others can repeat if needed.

Most / many here could make flat ledges in steel of 0.001 mm feature size.
But they mostly would only moderately match ledges cut by someone else.
They would have precision and resolution, but would not be accurate to 0.001 mm levels.

Many here could also work the ledges to be generally accurate to 0.001 mm.
This would be specialist work, and a customer would pay a lot of money for it.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY


Uniform distributions are also common in practice.

The weirdest distribution commonly seen in practice is Gaussian or uniform, but with the central ~tenth of the range punched out.

This most often happens when there is a tighter-tolerance grade sold for more money - it's done by cherry-picking the run of production.

A common pattern is that the run of production is Gaussian, and what is sold as the lower-precision grade is Gaussian with central region missing, and the higher-precision grade has a uniform distribution the same width as the hole in the Gaussian.

If there are more than two precision grades, uniform-with-hole distributions will appear.

That used to be carbon composition resistors - 20, 10, 5, and two percent tolerance. They would paint on a red, silver or gold last band - or no band at all.
 

CalG

Diamond
Joined
Dec 30, 2008
Location
Vt USA
I suggest we all apply the three "quality" terms to a round of golf. Linear measurement is not the only field of endeavor.

Precision
Accuracy
repeatability.
 

barbter

Diamond
Joined
Oct 27, 2007
Location
On Tour...
A perfectly accurate meter is the exact same length as the one in the museum (arts y metiers iirc)I visited 3 years ago in france.

I believe this hasn't been correct for quite some time as the metre was redefined relating to light in a vacuum/distance and time?
 

Lewie

Cast Iron
Joined
Jun 7, 2018
Location
Albuquerque NM
That used to be carbon composition resistors - 20, 10, 5, and two percent tolerance. They would paint on a red, silver or gold last band - or no band at all.

:-) As a retired Electronic technician, and later, engineer , That takes me back some 50/60 years. :-)
...lew...
 

crossthread

Titanium
Joined
Aug 5, 2004
Location
Richmond,VA,USA
This is the way the resister bands were taught to us in the military.
Bad
Boys
Rape
Our
Young
Girls
But
Violet
Gives
Willingly

Anybody remember that one from the 70's?
 

CITIZEN F16

Titanium
Joined
May 2, 2021
So where do repeatability and quality fall? If those thermometers were the coil spring type and the metal the springs were made out of had quality issues, that could cause the variance.
 

CITIZEN F16

Titanium
Joined
May 2, 2021
This is the way the resister bands were taught to us in the military.
Bad
Boys
Rape
Our
Young
Girls
But
Violet
Gives
Willingly

Anybody remember that one from the 70's?

I don't remember that one, but the classification rank for animals. Ken put coffee on fathers good sofa.
kingdom, phylum, class, order, family, genus, species.
 

Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
Look at what a Morse taper angle was supposed to mean. But he blew it so they have to be made accurately to a a bodged up standard. I will let you look it up but he was trying for soemthing like 5/8" per foot for all sizes. Each size is different taper angles, close but non are the 5/8" per foot.
Bill D.
 

Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
When they first surveyed Mount Everest they came out to exactly 29,000 feet with no inches. They threw in two extra feet so they would look like they had not rounded it off. They may have thrown in some inches as well?
We call them 1,2,3 blocks but they aren't.
Bill D
 








 
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