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Hi! Welcome to the EEVblog.
I'm your host Dave Jones
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and this is episode number 26.
There was a comment on one of
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my earlier blogs from
someone called Walter.
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He had a question about multimeters and
the counts like a 4000 count multimeter
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What are counts and how that
relates to multimeter accuracy.
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I thought that's a pretty good question.
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It's something a lot of people
don't understand all that well.
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So I thought I'd clear it up.
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A 3½ digit meter can actually
display plus or minus 1999.
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The half actually means that the most
significant digit can only go to one.
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It can only be zero or one.
So a 3 ½ digit meter can go up to 1999.
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A 4½ digit meter can go to +/- 19999.
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They've started advertising 3¾ and
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three-quarter digits or
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three and two thirds digits and that's
where it gets a bit confusing is very
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real
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standard for that but Tom typically if
it if it's not like two-thirds of three
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quarters
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or something like that by reeling main
that he can go a bit faded with them
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once thought might be
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I'm 3 know I know I know I know it might
be to
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999 or even Fort 9900 something like
that
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not because it started becoming quite
confusing
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manufacturers started talking in terms
is our
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the number of care what's and I think
this is a much better way to do it
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so I i three and a half digit multimeter
at plus/minus 1990 is actually
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a 2000 count me to because it counts up
to
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plus/minus 2000 and likewise before and
after each meter is actually a twenty
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thousand county can use these tents
interchangeably
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now has this relight accuracy well
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it actually doesn't rely lauter nothing
to do with accuracy so let's talk about
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this
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now accuracy is a term when it comes to
motivate his people get
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roll all the time and mining I'm guilty
of sin at 23
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she laziness or and I have it it's wrong
to side
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that this may die has and accuracy
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a four-and-a-half digits or an accuracy
of 10,000 carrots
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that is completely wrong the
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camels the number I've I am
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digits is the resolution so it's
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you shoots I that this may not has a ten
thousand count
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resolution not a ten thousand count
accuracy
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and if you unite if you say the wrong
thing
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Mike use a market adults especially if
you're in a job interview or something
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amass a question I can make you look
like you don't know it
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talking about okay now let's take a look
at
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of three typical mode he made his and
hell
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the the number of digits for the
resolution
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fix I'm the usability of these made its
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now I've got a fluke 79 series 2
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he at now this is i on 4,000 kill
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or three and three-quarter digit made up
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I've got made me in 37 X sake and
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this one is a 10,000 count made a
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the flick 87.5% he is I to
four-and-a-half digit made up but I've
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got it in
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it's actually got a mud that he's
actually I'm 6,000 count low
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okay don't fade in the Sun Bowl teaching
two or three
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mayes and you can see they all read
fields love it and I've is this slight
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differences they
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but let's have a look what happens when
Roy chein the only jockey
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K now if I had a three and a half
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digit me to heal which I do it it
wouldn't actually dropped down to two
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decimal places
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because it would have been over the one
know and I know I'm limit
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but none of her reach that yet so let's
keep turning this up
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K&C which one
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actually dropped its range first
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and I'm sure you can guess which one its
gonna pay
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being a there it is the flu
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seven or eight series 2 because this is
a four thousand count major
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we r others I that 4,000 where
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basically over four so it's got it go up
to the next highest
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range so we lose one digit all
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resolution okay not accuracy now
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week a tennis up foreign
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that the flick a 75 here is a 6% sales
in Kansas in as a guest to six
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or just slightly II Bay its
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Chinese upper range also I so I this is
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up Dale these are giving the signs
resolution okay this is a much more
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expensive made up
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I'm at six thousand chaos I K but
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its but we've got the sign resolution
know this maiden name
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37 XIII 10,000 count you'll still get
him
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the extra digital resolution and this
won't change ranges I'm to a guy right
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up to a
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and to your casa no i know i know i know
i limit and being a race
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after just they're all back on an even
playing field but let's switch
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the fluke 87 52 for her digit night
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okay now we've got this extreme teacher
a resolution he
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not accuracy resolution now you for it
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that this ID 7542 hostage it may be that
is useful
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until once again we get up to
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game we get up to twenty adults and
bingo it's going back
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so you can save that for her digit made
up only has more resolution for twenty
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percent of its range
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ok the 10,000 count me to
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I said before that resolution the number
digits on the sly has nothing to do with
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accuracy n that's true
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I K accuracy of the made-up is
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based on the percentage of the actual
reading
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not a false Kyle so I made his accuracy
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will be specified as the percentage of
the Radian
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plus camps now I typical
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lie a good made up might have point and
accuracy of
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no 0.1 percent plus to
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camps now what that means is if you're
measuring
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one fault okay you have an it
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it will have and uncertainty all plus
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or minus one Miley vault
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.1 percent a one-vote is one million
baht
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but you have to add on two camps to that
plus or minus
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to Celts to get displayed value
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saw it it will be if it's actually
raining exactly one vote
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okay 1.00 0
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then it can actually be I one
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point 0 0
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the re or plus/minus
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for lockport no I'm 9
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7 so that's resolution and accuracy
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now let's talk about calibration
everyone knows
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that unite you should get you made a
calibrated
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was actually many what it means is that
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the meda has been checked it's been it
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doesn't calibration does not mean
adjustment will be made when you see in
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this away for calibration
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its they usually don't adjusted what
they would do
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is all measure it against a reference
stain did
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and I will give you 80 s report for
these
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I'm giving you the actual typically
giving you the actual fee got
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and you know and the a rock against that
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absolute reference all that transfer
stain
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and what you do with that calibration
information determines
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help this morning made is going to be
used
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within your company one applications
it's going to use for and how often you
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going to calibrate
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now just because a meter has been
calibrated yesterday
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does not mean it's going to be within
spec
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to die its I it's all about calibrations
about the history of this made at
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Racine's best traceability
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in a bit rice not the calibration of
this may DNA test article
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record of it overtime and then you build
up confidence
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in this may tap that it's not drifting
this is why some companies in some
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industries will actually I to unite you
have to actually use
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to the mall he made is too tight I'm
measurement just in case one of them
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has unite is is actually help and I will
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unite anyone serious about electronics
will actually have to multimeters I want
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rely on just
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one though have to you so that you can
cross check each other
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and you can actually use a good known
Mon made a as I
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as a bit of a transferred stain did to
not check
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and measure other instruments and if
you've got a punch him only made as you
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can all cross measure
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themselves and you get more confidence
that you made a is within
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specification usually you follow the
manufacturer's recommendation which my
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peace I every 12 months
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but when you first get used to it you
might wanna calibrated
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more often so that we build up more that
reference
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that historical reference data so you
can track it
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and one Chino this mater is not you have
to really stable it's not true 15 at all
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then you might actually widened here I
period of calibration
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12 months then United still has
interested or you might actually change
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it to you