Error of % + digits?

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I just bought an amp clamp meter, and it states the error is "+/- 1.9% + 3 digits".  What does the "3 digits" part mean?

Re: Error of % + digits?

Answering my own question, I found this page, it means aswell as the percentage error, the last digit (eg the 2 in 147.2V) can vary by 3.:

https://www.baylor.edu/content/services/document.php/49697.pdf

Re: Error of % + digits?
On 6/18/2020 6:33 PM, Commander Kinsey wrote:

If your meter should read, say 1.875 A, the correct reading could
be anywhere from 1.872 to 1.878. This is a possible error in the
display presented to you in the analog-digital display conversion
process. The +/-1.9% possible error is about the measurement
taken including - but not only - any error made by the sensor.

To put it another way: If the actual current is 1.875 A,
inaccuracies in the sensor and associated circuits may process it
as somewhere between 1.875 A +/-1.9%. The analog-digital process
may introduce a further error of +/- 3 counts in the least
significant display digit. Therefore a current of 1.875 A may be
displayed as anywhere from 1.836 to 1.913 A.

Re: Error of % + digits?

Thanks, I wonder why all my other meters only list a % error.  Is it included within it somehow, or are they just lying, or do some meters not have this error?

Re: Error of % + digits?

Most that use a digital meter should know the last digit is not accurate
because of a rounding error.  Say it shows 1.5 volts.  It could be 1.45
to 1.55 or so and still show 1.5.  Some meters such as the one under
discussion is less accurate and can be 3 numbers high or low on the last
digit.  That is why on digital meters you should try to use a range that
shows as many digits as you can.

-My several hundred dollar Fluke meter shows DC volts to be .05 % +- 1
digit.

Re: Error of % + digits?

Yes, but I was surprised to see up to 7 digits out on this one, depending on the range.  I think DC amps is 3 or 5 dependant on range, and AC amps is 5 or 7.

The meters I have are not several hundred dollars, so are you saying they're only +/- 1 digit?  Is the error much higher on the one under discussion because it's a clamp meter?

Re: Error of % + digits?

The larger error is because of the price difference.  It costs more to
make a part that is .01 % than it does to make one that is 2 %.  The
.01% parts may just be the 2 % ones that are hand sorted to .01%.

I am sure that the clamp part does play some part in how accurate the
meter is.

Re: Error of % + digits?
On Fri, 19 Jun 2020 00:38:29 +0100, Ralph Mowery <rmowery28146@earthlink=
.net> wrote:

- 1

they're only +/- 1 digit?  Is the error much higher on the one under dis=
cussion because it's a clamp meter?

But what I'm surprised at is a A3%5 multimeter (not clamp) not giving a =
digits error.  Maybe precision on a simple voltmeter is cheap as chips n=
owadays?

Re: Error of % + digits?

You have to be careful how you throw precision and accurecy around.

A meter that shows 4 digits is more precice than one that shows only 3
digits, however the 4 digit one may only be 1% accurate and the 3 digit
one may be .5% accurate.

It is easy to get precision, but difficule to be accurate.  Think of it
as shooting a gun.  Precision may be how close the bullets land to each
other where ever they land on the target, but to be accurate the bullets
have to land on the center of the target.  Such as all the bullets could
land very close to each other, but not even hit the target.

As I mentioned, a good meter will not have a digits error outside the +-
one digit due to rounding.

Re: Error of % + digits?
On Fri, 19 Jun 2020 23:55:42 +0100, Ralph Mowery <rmowery28146@earthlink=
.net> wrote:

a digits error.  Maybe precision on a simple voltmeter is cheap as chip=
s nowadays?

t
t
h
ts
ld
+-

That didn't help.  I interchange the two.  I just want to know how close=
to the correct reading the readout is.  Adding another digit doesn't im=
prove anything if it's incorrect.  And shooting all the bullets in one p=
lace doesn't help if they all miss.

Re: Error of % + digits?
On 6/20/2020 4:29 AM, Commander Kinsey wrote:

Take pi as an example. It can be said that 3.14 is accurate as a
three-digit value, but 3.1416 is more precise because it has a
higher resolution.

OTOH, deriving it from 22/7 or 3.1429 has the same 5-digit
resolution and is just as precise as far as the number it
represents is concerned but is less accurate.

In this particular case, 3.1416 is both more precise and more
accurate than 3.14 but that's not always the case with measurements.

My mechanical slide caliper has a resolution of 0.001 inch. This
means that it can display measurements with a precision of 1 mil,
but that doesn't guarantee that a measurement taken with it will
be accurate to 1 mil. I may not always press the jaws snugly
enough and the scale may not be perfectly accurate.

Re: Error of % + digits?

ink.net> wrote:

..
ng a digits error.  Maybe precision on a simple voltmeter is cheap as ch=
ips nowadays?

3

git
it

ach
lets
ould
e +-

ose to the correct reading the readout is.  Adding another digit doesn't=
improve anything if it's incorrect.  And shooting all the bullets in on=
e place doesn't help if they all miss.

I'd need to contract OCD to understand that.  There's only one thing in =
question here, how close is the reading to the correct value.  You can't=
split that into two.  3.1416 is better than 3.14, and that's it.  All y=
ou can state with a reading is it's correct to within a certain percenta=
ge.

Re: Error of % + digits?

Try this.

A doctor does a very complicated operation on your left arm like a joint
replacement.  It all goes very well.  Very precise.

However he should have done the operation on the right arm that was
causing trouble.  Not accurate.

That is why a voltmeter can show 3 digits and be accurate to only the
last digit being in question by one number either way, but a 5 digit
volt meter can show many numbers, but if it is not calibrated corrctly
the 2nd digit to the 5 th digit  could be way off and the meter not
accurate at all.

Re: Error of % + digits?

Nope, because the first one is 100% useless.  I wouldn't call that precise at all, as he was out by half a metre.

Showing those extra two numbers is pointless if they're wrong.  All that matters is how many volts difference between the actual voltage and what is shown.

Re: Error of % + digits?
On 6/20/2020 9:28 PM, Commander Kinsey wrote:

You keep saying that it's only the accuracy that matters. That's
true to some - and only some - extent.

Now let's compare two different hypothetical meters, both 100%
accurate. Let's say that meter A has 3.5 digits (max count 1999)
and meter B is 4.5 digits (19999). Use them to measure a battery
cell of exactly 1.612345V.

Meter A will display 1.612V whereas meter B will show 1.6123V.
Meter B allows you to evaluate the result to a higher degree of
precision.

Further suppose that both meters are not perfectly accurate and
read 1% low. A will show 1.596V while B will read 1.5962V. B is
still more precise in showing you what it thinks the voltage is.
An order of magnitude more precise, in fact, even though both
meters are off by -1%.

That's how the term 'precision' is used in engineering. Perhaps
what's confusing you is the fact that the term is more loosely
applied in everyday language.

As to the +/- 3 count (or 1 or whatever) possible error, it's an
*uncertainty*, not a fixed inaccuracy, in digitizing an analog
quantity. It will take too long to explain in detail here. Let me
put it this way: If you measure the example voltage above
multiple times with a meter with +/-3 count uncertainty, you may
get a reading that varies from measurement to measurement by up
to 6 points in the last digit. That's not a percentage inaccuracy.

Re: Error of % + digits?

Sometimes it is precision.

I worked at a company making polyester from raw materials.  In a room
was a panel with about 10 temperature gauges.  At a certain time all
gauges were marked and a sample of the material was sent to the lab.  If
it came back good, then the object was to keep all the gauges on the
mark.  It did not matter how far off the gauges were from the actual
temperature.  No mater how well we calibrated the guages there were
several other factors that we had no control over. Such as the
thermocouples they were connected to.  The specifications were +- 3 deg
C. on the thermocouples from the factory.   If the  temperature varied
more than 1 deg C at 300 deg C it could mess up the material.

So the object was precision and not accuracy.

Re: Error of % + digits?
On 20/06/20 19:15, Ralph Mowery wrote:

I once worked for a company that made an instrument that
measured cable attenuation to 0.001dB +- 0.1dB. The customers
didn't care about the 0.1dB, since all they were interested
in was the /stability/ of the 0.001dB and the ability to
measure small changes.

Why? Because the instrument measured the attenuation change
as a function of temperature, and each temperature cycle
test took 7 days. Yes, it was a /large/ drum of undersea cable.

Re: Error of % + digits?

If the goal was keep the needle on their marks it does't have to mean
anything was precise. Maybe your guages had no faces, or read mA instead
of degrees, and bent needles. As long as your +/- 3 degree thermocouples
and controllers did not jump up and down + and then -3 degrees all the
time, you were good.

It's like the zener diode or voltage standard that came up in this thread.
Those have no precision. They may not even be accurate. They might be
consistent though. Accuracy and precision by themselves can be useless
where time or multiple readings are needed.

Re: Error of % + digits?
says...

One good example of what we  had is this.

In a vat of material is a test hole.  In that hole is a rod about 3/8
inch in diameter and a foot long.  At the end there are two
thermocouples and two RTDs.  The thermocouples wires go about 100 feet
to a PLC (similar to a computer) card that converts the milivolts to
digital that is then displayed on a compute screen.  The RTDs go about
10 feet  to a converter that converts the change in resistance to a 4 to
20 miliamp signal.  That goes to a card on the PLC and then to the
computer display.

While the computer will display to 3 decimal places at 300 deg C from
the lowest to the highest temperature shown on the display can be around
3 deg  differnet and all 3 be within the limits of the equipmnet.

At a certain time a sample is sent to the lab and one of the computer
displays is set as a standard and the object of the PLC is to keep the
actual temperature , whatever it actually is, to that 'standard'.  Not
too accurate as to temperature, but very precice.  The operators only
needed to keep that one computer display as close to that 'mark' as they
can if for some reason the PLC messes up and they have to adjust  the
control manual.

Re: Error of % + digits?

What's the control loop if the PLC dies? How do people control temperatures
manually? Is there a foot pedal to stomp on to switch the heaters on and
off?

There's a couple machines I fuss with that use platinum junction RTDs and
we have alarm limits set. If the machine drifts into an alarm state,
outside of a warmup period that's pretty much the end of the day and
everything stops until it can be fixed. The loops on these machines are
tuned to maintain and hold a set point of less than 1 degree F. The
displays are all wrong, show fake levels of precision, and read in C, but
are wrong by several degrees, even if you do the math. We gave up trying to
calibrate the displays against what the real temperature with the offset
features when the probes were last changed. It just isn't worth the time.
Those machines are not accurate, they're not precise (as measured with
their own instrumentation), but they will absolutely hold a stable
temperature if you can determine the set points yourself.