Re: Current measurement



As an old Professor kept reminding me when I was calibrating the lab
equipment, everything is relative. ;-)

You are right about the accuracy. The question is "how good is good
enough?". I think we probably both came up with analog meters where
the third place was interpolated. 5% was pretty good in a world of
20% resistors. Today's 3 1/2 digit multimeters often show a higher
level of resolution than their accuracy can back up. For a lot of
troubleshooting I would say the roll your own shunt and a cheap meter
are adequate. They are on par with what I used when I was young.

OTOH I have a couple of certified B&K's and certifiable Simpson and
Fluke's around. I just know my propensity for dropping a hammer on
the expensive stuff. ;-) If I was really worried I would calibrate a
cheap one. That 5 V standard is interesting though I doubt I will buy
one. In my world $30 here and $30 there starts looking like real
money. Especially when I only use it on rare occasions.

mark

On Fri, 24 Apr 2009 21:30:39 -0400, Neon John <no@xxxxxxxxx> wrote:

$3 meters... Copper "shunt"... Might as well just make up numbers.

A copper shunt looks like a good idea until you look at copper's
temperature coefficient of resistance. It is so high that copper is
used to make inexpensive RTDs (resistance temperature devices). When
the copper gets a little warm from the current flowing through it, it
changes value enough to make the measurement worthless.

Not worthless, just not directly as usable. Most folks might think
I'm quibbling, and I am. I think we both could figure out the actual
current by running the numbers to account for the difference in
resistance over the temp change. You could probably write a program
to do it. I probably would not get lost in whether my battery was
charging at 18 or 18.545 A and just be happy it was charging.



If one insists on ad-hoc'ing it, iron wire is much better. Both
higher resistance per foot and much lower tempco.

There's really no need to ad-hoc it when real shunts are so cheap.
Deltec http://www.deltecco.com/catalog.html has that market pretty
much sewed up. It's not hard to find a type MKA in the sub-$30 range.
In fact, here's the first google hit. $24.

http://store.solar-electric.com/mka-100-100.html

A 500 amp type MKB is also handy to have around for checking things
like starters and it costs just a little more, $27 from the same
source. Used ones go for half that.

I keep a 100 amp and a 500 amp shunt in my tool box all the time. I
also have a DC clamp-on meter or two but when I really need the right
answer, it's hard to beat a real factory-made shunt.

If one is to use the cheap ChiCom made meters, I highly recommend
getting one of these little gadgets:

http://www.voltagestandard.com/Home_Page.php

I have one and can say that it is right in there with my Fluke
calibration standard that cost many thousands of dollars new. This
gadget will let you know whether your meter is creating fiction or
not. Every sub-10 meter that I've ever tested has been guilty of
that, especially as the battery runs down.

I would go so far as to suggest that one take that gadget and a
partially discharged 9 volt battery to the store and actually test
candidate meters.

(not picking on you, Elliot. Picking on that other "advice")

John

On Fri, 24 Apr 2009 17:14:44 -0500, Elliot Richmond
<xmrichmond@xxxxxxxxxxxxxxxx> wrote:

On 24 Apr 2009 16:51:01 -0500, nothermark <nothermark@xxxxxxxx> wrote:


I agree, I just don't like the price. ;-)

Yeah, mine was free to me, costly to whomever lost it.


The point I am trying to make is that one of those $3 Harbor Freight
meters can be made to do a lot of usefull stuff if one wants to mess
around. For DC current measurements all one really needs is a known
resistance in the line.

I agree with this. However, it is going to be hard to get a big shunt
to a specific resistance. One needs a big shunt to work with high
currents.

I guess what one could do is take a meter of, say, 4 awg cable and
solder heavy brass lugs on each end. Then you could use a good
ohmmeter with a low ohms setting to measure the resistance.,Then you
could measure the voltage drop across the lugs when current is
flowing, and calculate the resistance. Remember Vultures always fly
over Indians and Rabbits, so I = V/R

The problem is, 4 awg has a resistance of 0.000831 ohms/m. At 100
amps, the voltage drop over a meter would be only 0.0831 V. But a good
digital multimeter should measure that okay.

Smaller wire means less current and more resistance, But also a more
inaccurate measure of the current when the shunt is not in the
circuit.



Elliot Richmond
Itinerant astronomy teacher
.

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