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tmc_31
11-03-2009, 04:32 PM
Hi all,

I hope you can enlighten me about my electronic measuring tools. I am curious about the 6th digit in the display as in 0.00005 . This is the digit that only displays 0 or 5. Does this digit roll over to 5 when the 10ths column reaches 1/2 a tenth or does it roll over when the tenths column reaches 2.5 tenths or something else less than 5/10ths? I am trying to determine how to interpret that last digit.

Thanks,

Tim

Walter
11-03-2009, 04:48 PM
It rolls at 1/2 a tenth, but in all honesty forget about that last digit. It's so small that...
A. It just doesn't matter.
B. The stated accuracy on the best of digital mics isn't close to that tight.

MTNGUN
11-03-2009, 04:51 PM
Good question. I don't know the how they are programmed.

But....... I read to 4 digits and ignore the 5th, even on a mic that displays to 0.00001". I'm not convinced that the 5th digit is reliable. Even the 4th digit is probably only reliable to 0.0002". I'm not saying you can't read finer, but for everyday measurements where I only use reasonable care, as opposed to extraordinary care, I have faith in 0.0002".

Likewise on HF calipers that display to 0.0005", I only read to 0.001" and it takes effort to even get it to repeat to 0.001".

tmc_31
11-03-2009, 08:21 PM
Thanks guys,

I too suspect that they roll on 5/10ths of the previous digit, just wasn't sure. I also suspect that even with the best mic s one should discard that last digit. I have a new 0-1" starret and a new 1-2" mitutoyo. They differ slightly, not that it matters with the tolerances that I use, it's just annoying. Dang, I guess that this is a "good" excuse to buy the set of starret grade B gauge blocks Iv'e been wanting.

There is no end to this, is there.:D
Tim

spope14
11-03-2009, 09:28 PM
Want some fun? measure something to that fifth digit then breathe on the part - measuring area- and watch it spin around.

This is just for show. If I really want to get to that point, I use a millimes indicator or some real tight mics made especially for this purpose - or a renishaw type of probe type gauge.

JMS6449
11-04-2009, 07:36 AM
.00001 is necessary when building things such as connector dies, mold and the like, where there are stacked parts. The sizes are checked with dial or electronic indicators against gage blocks. As the earlier poster said breathe on or handle the part or use an uninsulated/isolated device and you can measure .0001, but which .0001 of .001 is it.

Example:

24 laminations with a +/-.0001 = +/-.0024 difference = .0048 total

24 laminations with a +/-.00001 = +/- .00024 difference = .00048 total

The numbers have a importance with application.

Typical one off inventor/prototype work is make it fit so it works, then we can we worry about tolerance in the design of the production parts. This is not true if you are building a lets say a astronomical device. You must
understand the degree of accuracy needed.

Glenn Wegman
11-04-2009, 08:03 AM
Hi all,
I hope you can enlighten me about my electronic measuring tools. I am curious about the 6th digit in the display as in 0.00005 . This is the digit that only displays 0 or 5. Does this digit roll over to 5 when the 10ths column reaches 1/2 a tenth or does it roll over when the tenths column reaches 2.5 tenths or something else less than 5/10ths? I am trying to determine how to interpret that last digit.
Thanks,
Tim

In theory, it should roll over at "1/2 a tenth" as you put it. The important thing to consider ids the ACCURACY of the measuring instrument, not the RESOLUTION. Resolution is in basic terms the number of decimal places or graduations it displays. Accuracy is just that, how accurate thode numbers displayed are.

I was just looking at digital electronic micrometers recently in the MSC catalog in response to another similar question. Some of them listed the resolution as .00005", but the accuracy was .0001", so the fifth digit was pretty much useless. Fowler listed theirs as .00005" resolution, but did not list the accuracy, so I doubt it is even close. Mitutoyo listed some with .00005" resolution AND .00005" accuracy, so that means that .00005" is actually a usable feature. I have and use the Mitutoyo's with .00005" accuracy as that accuracy tells me that the fourth digit is pretty much what it says it is and not rounded off. (I do precision grinding and honing so tenths are important)

Same goes for digital calipers, they may display 5 decimal places or something rediculous for a caliper, but if you look into it the accuracy may only be to .001".

It's the accuracy of the digits displayed that counts, not just the number of digits displayed!

That's my take on it anyway.

tmc_31
11-04-2009, 10:23 AM
thanks Glen,

I do understand the difference between accuracy and resolution. I took the accuracy vs resolution into account when choosing what will be the "gold standard" mic s in my shop (Starrett 795XRL-1 and a Mitutoyo 293-331).

I guess, in the final analysis, one should just forget the last digit and and bank on the one preceding it (unless of course it is a Mit with an accuracy out to .00005 and a resolution to .00005:D ).

Tim