Power Consumption Monitor

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Mitchell Buchman

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Mar 19, 1993, 2:27:01 PM3/19/93
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I am trying to find (hopefully) or build (if I have to) a meter to
measure AC Amps over to find the power consumption of specific appliances.
You can use a multimeter to measure Amps at one instant, but for a refrigerator
which turns on and off, you need to check it over time.

Any hints?

John De Armond

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Mar 20, 1993, 11:58:28 AM3/20/93
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meb...@afterlife.ncsc.mil (Mitchell Buchman) writes:

You can't just measure amps and volts to derive power because it does not
take into account power factor. By far the cheapest way to measure
total power is with the common utility-type power meter. Call your
local utility, ask to speak to someone in engineering and ask them for
a reference to a supplier. Some utilities will sell you a meter directly
if you convince them you're not going to use it to rip them off. Cost
is typically under $100.

I have just that kind of meter. It is mounted in a small round meter base
which in turn is mounted in a wooden box. The glass cover is removed
so access can be gained to the hands. Mine is the dial type rather than
the mechanical digital type. I can thus easily set the dials back to
zero by simply turning them.

These meters are incredibly accurate and hold their calibration over
time very well.

If you want to see the duty cycle of your refrigerator or other appliance,
Ampclamp makes a nice recording clamp-on ammeter that consists of
an amp-clamp feeding a small stripchart recorder. Retail is about $400
for the setup. I found one new but surplus and paid $50 for it.
Very good investment.

John
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Vance Morrison

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Mar 21, 1993, 11:36:45 AM3/21/93
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j...@dixie.com (John De Armond) writes:
>meb...@afterlife.ncsc.mil (Mitchell Buchman) writes:
>> I am trying to find (hopefully) or build (if I have to) a meter to
>>measure AC Amps over to find the power consumption of specific appliances.
>>You can use multimeter to measure Amps at one instant, but for a refrigerator

>>which turns on and off, you need to check it over time.

>You can't just measure amps and volts to derive power because it does not
>take into account power factor. By far the cheapest way to measure
>total power is with the common utility-type power meter. Call your
>local utility, ask to speak to someone in engineering and ask them for
>a reference to a supplier. Some utilities will sell you a meter directly
>if you convince them you're not going to use it to rip them off. Cost
>is typically under $100.

There was a very good tip I looked up in sci.electronics reciently that
is relevant. In the 1/7/93 Issue of EDN in the `Circuit Ideas' section
there is a very nifty circuit for measuring real average power. The
beauty of this circuit is its simplicity. One IC and a handful of resistors
is all that is needed (total cost under $5). Given the lengths I went
to to design a similiar circuit, I am really impressed with this one's
elegance and low cost.

The circuit is designed to output 200mv full scale (to be hooked up to
a digital volt meter) hand has two ranges 0-200W and 0-2000W (you could
easily add more ranges.

Now this does not really solve your problem as you wanted something that
will record this information over time, but it IS a big step in the
right direction (either have a PC or a strip chart recorder do the
logging).

For your original example of the refridgerator, you can certainly measure
the ON power consumption, and measure the duty cycle some other way
(do your homework in the kitchen and listen carefully for a couple hours?).

Note that if you do go the strip chart or PC data collection route, please
note that this circuit's output is NOT isolated from the AC lines. Thus
either your inputs to the recorder should be, or you should make sure
the inputs can take the common mode voltage.

I have not built one myself yet, but I have the only part I dont have
on order.

Vance

Mitchell Buchman

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Mar 22, 1993, 10:09:29 AM3/22/93
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A Few short comments to give a follow up status on my search for an AC power
consumption monitor:
- Checked with local power utility company (through 4 different divisions)
and they say that they can't help. I have a hard time believing
that and am not yet giving up.
- Found something that comes close. At Radio Shack, of all places, a
digital multimeter, with an RS-232 interface, software for a
PC and a very simple interface protocol. The catch is that the
AC circuit is not fused (poor design) and the duty cycle on the
meter is only 30 seconds (even poorer design). With that short a
duty cycle, why did they even bother to put in the PC interface.
- High cost, but it will do the job. I have found two devices that are
properly designed for power line monitoring, are fused, have
PC interfaces, and come with data analysis software. One comes in
at just under $500 and the other at $2,500. A good technical
solution, put not practical for this job.
The search continues.

William K. McFadden

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Mar 22, 1993, 12:34:05 PM3/22/93
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In article <C48zH...@cs.uiuc.edu> morr...@cs.uiuc.edu (Vance Morrison) writes:

>In the 1/7/93 Issue of EDN in the `Circuit Ideas' section there is a very

>nifty circuit for measuring real average power. One IC and a handful of


>resistors is all that is needed (total cost under $5).

I looked up this article and found that resistor R2 was unlabeled on the
schematic. In the March 4 issue, page 136, there is a correction that says
R2 is 1M, 1/4 W.
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E. Michael Smith

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Mar 22, 1993, 9:02:37 PM3/22/93
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In article <1993Mar22.1...@afterlife.ncsc.mil> meb...@afterlife.ncsc.mil writes:
> A Few short comments to give a follow up status on my search for an AC power
>consumption monitor:
> - Checked with local power utility company (through 4 different divisions)
> and they say that they can't help. I have a hard time believing
> that and am not yet giving up.
...

> at just under $500 and the other at $2,500. A good technical
> solution, put not practical for this job.
> The search continues.

Find your local building dismanteler (house demolisher) and buy a 'panel'
from them. Wire on a plug and socket. The panel is the thing on the
back of your house filled with breakers and a big glass thing that has
your watt-hr meter in it. This should be relatively cheap and easy.
Get a small panel if possible... This is where I got one once...

(I find it hard to believe that you can't get one of these new from
and electrical supply house ... maybe I'll check on my way home ...
You should be able to get one from the folks who sell Solar Power systems,
since even if you are 'off the grid' you might like to know how much
power you are using...)

--

E. Michael Smith e...@apple.COM

'Whatever you can do, or dream you can, begin it. Boldness has
genius, power and magic in it.' - Goethe

I am not responsible nor is anyone else. Everything is disclaimed.

Mitchell Buchman

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Mar 23, 1993, 11:41:07 AM3/23/93
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> (I find it hard to believe that you can't get one of these new from
> and electrical supply house ... maybe I'll check on my way home ...
> You should be able to get one from the folks who sell Solar Power systems,
> since even if you are 'off the grid' you might like to know how much
> power you are using...)

I have gone through the Real Goods catalog without any luck. The folks at
Real Goods are the ones to go to for living off the grid. They have lots
of monitors for battery systems, but nothing to measure current output over time.

Until I find a more graceful solution, I am now taking manual periodic
readings from the dials on the electric meter in my house. I am noting readings
about two or three times a day. This might help as a rough idea of consumption
patterns.

System Administrator

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Mar 23, 1993, 8:50:13 PM3/23/93
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Mitchell Buchman (meb...@afterlife.ncsc.mil) wrote:
:
: I am trying to find (hopefully) or build (if I have to) a meter to

Places like Princess Auto, Edmond Scientific, and so on carry hour
meters. Many of these are just an electric clock and a vibration
detector. Stick it on a motor, and whenever the motor turns, so does
the hour meter.

Something like a refrigerator is going to have a pretty constant demand
when running. So if you can determine the power when it is running, add
a small fudge factor for startup, and read the hour meter now and then.


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Anmar Caves

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Apr 2, 1993, 3:50:00 PM4/2/93
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> A Few short comments to give a follow up status on my search for an AC power
>consumption monitor:

The last few issues of Home Power Magazine has had ads for just that
device. Priced not too bad either, under $200 I think.
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