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[Phys-L] Re: Digital Multi Meters (DMM)



Inspired by two private massages, and by what was posted yesterday by
JohnD (see below), I summarized my own thinking at

Http://blake.montclair.edu/~kowalskil/cf/258sampling.html

1) Is the Monte Carlo approach convincing? All comments and suggestions
will be appreciated. Thanks in advance.

2) A week from now I will be working in a private research lab trying
to replicate a French excess heat experiment (see unit #252). Two other
teams (one involving two students), will try to replicate the same
experiment. Our instrument will sample volts and amps at the rate of 2
kHz; the results will be sent to a computer file.

Ludwik Kowalski
Let the perfect not be the enemy of the good.


On Sep 19, 2005, at 3:04 PM, John Denker wrote:

ludwik kowalski wrote:
Digital Multi Meters (DMM) sample waveforms and display updated
avarages.

1) Does anybody know what the sampling frequencies are in cheap DMMs?

Long ago Rat Shack sold one that used a dual-slope integrator with
a one-second cycle time. Ewwwww.

2) What are these frequencies in more expensive DMM's?

I have a handheld Fluke (10+ years old) that is good to 100kHz,
true RMS.

I have (relatively) cheap National Instruments boards lying around the
lab that are good to 20MHz.

4) Any potential traps in using DMMs?

More than I know how to count.

Perhaps the biggest one is Dragon Breath: The instrument injects weird
noise out of its input into the device under test.
There are many types of Dragon Breath. One kind arises if the
nominally-floating (i.e. gauge invariant) probes aren't quite,
i.e. there is some nontrivial impedance (usually capacitive) to
the rack and/or the "ground" of the power line.

Then there's aliasing at high frequencies.

Then there's eyeball-aliasing, if you only look at it intermittently,
as opposed to logging *all* of the data.

Sensitivity.

Linearity.

Multiplicative calibration errors.

Additive DC offsets.

Non-infinite input impedance.

Thermocouple effects, if the thing you're measuring isn't at room
temperature.

etc. etc. etc.

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