Multimeters (My first post to sci.electronics.basics)

[P J King]

Hello:

I couldn't seem to find a valid FAQ to this newsgruop (unless the
Electronics Search FAQ was it, but it doesn't appear to say much), so
I apologize if this topic has been "beaten to death" in previous
posts.

I have about 15 multimeters at the school I teach at. There are 3
different brands (the names escape me at the moment), all of them are
digital. I set up two small lamps in series, and test the resistance
of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought
to be 600 ohms.

But when I test the connection across the two lamps, I get 560 ohms. I
thought this was a fault of the multimeter, but since I have 15 of
them, and 3 different brands, each one I used gave nearly the same
reading. BTW, I made sure that each time, the individual loads were
measured with the same multimeter as used to measure the total
resistance.

How is it that two loads hooked up in series give a reading that is
*less* than the sum of the individual loads? This makes absolutely no
sense. Calibration can't be the problem, since the same multimeter is
used for all 3 measurements. The error seems to be independent of the
brand of multimeter.

PJK

 

[John Popelish]

P J King wrote:

Hello:

I couldn't seem to find a valid FAQ to this newsgruop (unless the
Electronics Search FAQ was it, but it doesn't appear to say much), so
I apologize if this topic has been "beaten to death" in previous
posts.

I have about 15 multimeters at the school I teach at. There are 3
different brands (the names escape me at the moment), all of them are
digital. I set up two small lamps in series, and test the resistance
of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought
to be 600 ohms.

But when I test the connection across the two lamps, I get 560 ohms. I
thought this was a fault of the multimeter, but since I have 15 of
them, and 3 different brands, each one I used gave nearly the same
reading. BTW, I made sure that each time, the individual loads were
measured with the same multimeter as used to measure the total
resistance.

How is it that two loads hooked up in series give a reading that is
*less* than the sum of the individual loads? This makes absolutely no
sense. Calibration can't be the problem, since the same multimeter is
used for all 3 measurements. The error seems to be independent of the
brand of multimeter.

Try the same test with a pair of 220 or 330 ohm resistors. I suspect
that the meters pass enough current through the filaments to change
their temperature, raising their resistance above the room temperature
value. Testing two in series lowers that current so they heat less,
and each have lower resistance. Resistors should not have this high
temperature coefficient and should not show the effect.

 

[me]

P J King <xogblog@hotmail.com> wrote in
news:g1j3915ep8gtl2k12rrggncet5objd1gdn@4ax.com:

Hello:

I couldn't seem to find a valid FAQ to this newsgruop (unless the
Electronics Search FAQ was it, but it doesn't appear to say much), so
I apologize if this topic has been "beaten to death" in previous
posts.

I have about 15 multimeters at the school I teach at. There are 3
different brands (the names escape me at the moment), all of them are
digital. I set up two small lamps in series, and test the resistance
of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought
to be 600 ohms.

But when I test the connection across the two lamps, I get 560 ohms. I
thought this was a fault of the multimeter, but since I have 15 of
them, and 3 different brands, each one I used gave nearly the same
reading. BTW, I made sure that each time, the individual loads were
measured with the same multimeter as used to measure the total
resistance.

How is it that two loads hooked up in series give a reading that is
*less* than the sum of the individual loads? This makes absolutely no
sense. Calibration can't be the problem, since the same multimeter is
used for all 3 measurements. The error seems to be independent of the
brand of multimeter.

PJK

What is it that you teach?...

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[P J King]

On 23 May 2005 07:03:18 -0700, "Chris" <cfoley1064@yahoo.com> wrote:

Thanks for all your responses, they were a great help!

PJK

Welcome to s.e.b., P.J. - the water's fine here among the groundlings
and basic electrical/electronics questions are (usually) answered
without snark.

I'm assuming you have two very small incandescent bulbs. Incandescent
lamps have a non-linear resistance characteristic. That means their
resistance increases along with their temperature. In other words, the
measured resistance will decrease as current (power) used to measure
the resistor decreases.

Resistance is measured by putting current through a resistor, and
measuring voltage across it. Resistance is inferred through Ohm's Law
(R = V / I).

The higher resistance of the two bulbs in series may decrease the
measurement current from the meter. If that's the case, each bulb has
less power applied across it, and so it's a little cooler. This means
lower voltage across each bulb, therefore lower total voltage and lower
total inferred resistance. That may be why they don't "add up".

Don't depend on accurate resistance measurements of bulb filaments
unless you take accurate filament temperature into account. And you
should always keep in mind the effect of the instrument on the
measurement.

A friend of mine used to call our shop claw hammer his "Heisenberg
Uncertainty Principle Tube Tester" many years ago. Since he hated
wasting time swapping out marginal or possibly bad tubes with other
marginal or possibly bad tubes, he always tested them this way last
(inside a box in the shop garbage can). He said the advantage of this
test was, once you were done, you always had a definitive and
satisfying answer to whether the tube was good or not. And the
instrument definitely affected the measurement. ;-p

Good luck
Chris

 

[John Smith]

"P J King" <xogblog@hotmail.com> wrote in message
news:g1j3915ep8gtl2k12rrggncet5objd1gdn@4ax.com...

Hello:

I couldn't seem to find a valid FAQ to this newsgruop (unless the
Electronics Search FAQ was it, but it doesn't appear to say much), so
I apologize if this topic has been "beaten to death" in previous
posts.

I have about 15 multimeters at the school I teach at. There are 3
different brands (the names escape me at the moment), all of them are
digital. I set up two small lamps in series, and test the resistance
of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought
to be 600 ohms.

But when I test the connection across the two lamps, I get 560 ohms. I
thought this was a fault of the multimeter, but since I have 15 of
them, and 3 different brands, each one I used gave nearly the same
reading. BTW, I made sure that each time, the individual loads were
measured with the same multimeter as used to measure the total
resistance.

How is it that two loads hooked up in series give a reading that is
*less* than the sum of the individual loads? This makes absolutely no
sense. Calibration can't be the problem, since the same multimeter is
used for all 3 measurements. The error seems to be independent of the
brand of multimeter.

PJK

Of course everyone has told you why your readings are rubbish - the
resistance increases with temperatrure (which is why your house light bulbs
usually blow on or soon after switching them on).

No-one has suggested how to measure their resistance, but perhaps did infer
it: use Ohm's law. Measure the volt drop across the load, and the current
being passed through the load using a volt meter and a current meter. Then
calculate the resistance. Do this several times, measuring their temperature
too, and obtain the relationship for yourself. Good science project for the
kids I'd think.

 

[Chris]

P J King wrote:

Hello:

I couldn't seem to find a valid FAQ to this newsgruop (unless the
Electronics Search FAQ was it, but it doesn't appear to say much), so
I apologize if this topic has been "beaten to death" in previous
posts.

I have about 15 multimeters at the school I teach at. There are 3
different brands (the names escape me at the moment), all of them are
digital. I set up two small lamps in series, and test the resistance
of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought
to be 600 ohms.

But when I test the connection across the two lamps, I get 560 ohms.
I
thought this was a fault of the multimeter, but since I have 15 of
them, and 3 different brands, each one I used gave nearly the same
reading. BTW, I made sure that each time, the individual loads were
measured with the same multimeter as used to measure the total
resistance.

How is it that two loads hooked up in series give a reading that is
*less* than the sum of the individual loads? This makes absolutely no
sense. Calibration can't be the problem, since the same multimeter is
used for all 3 measurements. The error seems to be independent of the
brand of multimeter.

PJK

Welcome to s.e.b., P.J. - the water's fine here among the groundlings
and basic electrical/electronics questions are (usually) answered
without snark.

I'm assuming you have two very small incandescent bulbs. Incandescent
lamps have a non-linear resistance characteristic. That means their
resistance increases along with their temperature. In other words, the
measured resistance will decrease as current (power) used to measure
the resistor decreases.

Resistance is measured by putting current through a resistor, and
measuring voltage across it. Resistance is inferred through Ohm's Law
(R = V / I).

The higher resistance of the two bulbs in series may decrease the
measurement current from the meter. If that's the case, each bulb has
less power applied across it, and so it's a little cooler. This means
lower voltage across each bulb, therefore lower total voltage and lower
total inferred resistance. That may be why they don't "add up".

Don't depend on accurate resistance measurements of bulb filaments
unless you take accurate filament temperature into account. And you
should always keep in mind the effect of the instrument on the
measurement.

A friend of mine used to call our shop claw hammer his "Heisenberg
Uncertainty Principle Tube Tester" many years ago. Since he hated
wasting time swapping out marginal or possibly bad tubes with other
marginal or possibly bad tubes, he always tested them this way last
(inside a box in the shop garbage can). He said the advantage of this
test was, once you were done, you always had a definitive and
satisfying answer to whether the tube was good or not. And the
instrument definitely affected the measurement. ;-p

Good luck
Chris

 

[JeffM]

I set up two small lamps in series, and test the resistance of the lamps. >...The total ought to be 600 ohms...I get 560 ohms.
PJK (P J King)

I'm wondering if you had to change ranges (more juice applied)
to do the 2nd reading.
You see a similar effect to what Popelish described
with surge (inrush) current on cold heating elements
in stoves, etc. woth the current decreasing as they warm up.

I couldn't seem to find a valid FAQ to this newsgruop
(unless the Electronics Search FAQ was it...),

Ignore Jason. He trolls the groups once a month.

Probably the closest thing to a FAQ is a Mark Zenier post from 1996:
http://groups-beta.google.com/group/sci.electronics/msg/13651a897337a7a9?q=sci.electronics-*-*-*-*-discussions+superceded+is-now-a-bogus-group+unreliable

I apologize if this topic has been "beaten to death" in previous posts.

There has long been a Usenet archive.

Currently it is maintained by Google.
http://groups-beta.google.com/groups?q=ingroup:sci.electronics.*&scoring=d+-ingroup:economics

A caveat:
sci.electronics (no suffix) has not been a valid group for ~13 years.
http://groups-beta.google.com/group/news.groups/msg/3696b1eeee6a60ab?q=sci-electronics-0102+sci.electronics-*-y-*-*-1030+are-*-removed+flag-n+GROUP-DATE-SUPERSEDED-BY
Google still archives it and, because Google hasn't disallowed it,
folks still occasionally post there (largely from Google).


My first post to sci.electronics.basics)
:
The more, the merrier.