GBM,

In looking around the web I came across the following sites, maybe this
will give you some additional pointers. As to prices, I think the off
the shelf device you are interested in will run you at least $200
dollars, if not more.

http://www.wholesalesolar.com/produc...etric2020.html
http://www.pkys.com/AmpHour.htm
http://www.solardepot.com/dpc_system_monitors.htm

Alternatively, a cheap inductance meter is about $50 and a data logger
can be built for under $100, but you'd need to be comfortable with
soldering, wiring, etc.

http://www.onsetcomp.com/Newsletters...ome_power.html
http://www.hobbyengineering.com/H1251.html

Hope this helps.

Cheers,

Robb

GBM wrote:
What is the best way of measuring the power drawn by a boat's
refrigeration unit?

Is there a way to do this without any expensive instruments?

I was thinking of installing an ammeter in the line to the compressor
to determine the current draw, but manufacturer does not recommend
this. Even with that, I would need an hour meter or some such device
to know how often and for how long the compressor runs.

I would like to test existing installation, then add some "temporary"
insulation to the inside of the box and then test again to see what
improvement I might get if I added permanent insulation.

GBM

Connect it to an isolated, fully-charged cheap auto battery of known
capacity, and see how long it takes to discharge the battery. I assume that
is what you want the data for in the first place. Not very accurate, but
hellish useful!

----- Original Message -----
From: "Dennis Pogson"

Connect it to an isolated, fully-charged cheap auto battery of known
capacity, and see how long it takes to discharge the battery. I assume
that
is what you want the data for in the first place. Not very accurate, but
hellish useful!


I do have a few 12v batteries ( I recently installed 6v,s) - using one of
them and monitoring the voltage could provide a rough measure of usage. If
the battery is (was?) say 100 Ah and the unit draws 50Ah/ day, I suppose I
would get a reasonable voltage drop if I let it run for say 12 hours
overnight?

GBM

I cannot see why an ammeter should not be in the power line. You only need
to measure the current once.

To measure the time it is on, assuming a 12v fridge, the cheapest way is to
get an "old" 12v analog (mechanical movement) car clock and wire it after
the thermostat, this will then give you the time the fridge runs. A car
scrapyard is the place to visit.

Alec


"GBM" wrote in message
...
What is the best way of measuring the power drawn by a boat's
refrigeration
unit?

Is there a way to do this without any expensive instruments?

I was thinking of installing an ammeter in the line to the compressor to
determine the current draw, but manufacturer does not recommend this.
Even
with that, I would need an hour meter or some such device to know how
often
and for how long the compressor runs.

I would like to test existing installation, then add some "temporary"
insulation to the inside of the box and then test again to see what
improvement I might get if I added permanent insulation.

GBM

Alec wrote:
I cannot see why an ammeter should not be in the power line. You only need
to measure the current once.

This is certainly not true for all refrigeration systems. It may be
that a Danfoss system with a simple evaporator has a constant draw,
but my larger DC holding plate system varies by 50% in draw, and 80%
in total load.


To measure the time it is on, assuming a 12v fridge, the cheapest way is to
get an "old" 12v analog (mechanical movement) car clock and wire it after
the thermostat, this will then give you the time the fridge runs. A car
scrapyard is the place to visit.

neat trick.

If it's 110V, just plug it into one of these:
http://www.thinkgeek.com/gadgets/electronic/7657/
They used to have these at Radio Shack, but they quit carrying them.
For $30, they're really worth it.

"Keith" wrote in message
oups.com...
If it's 110V, just plug it into one of these:
http://www.thinkgeek.com/gadgets/electronic/7657/
They used to have these at Radio Shack, but they quit carrying them.
For $30, they're really worth it.


The boat refrig (Waeco Coolmatic) is at present DC only, but I can
apparently get a converter called a Mobitronic MPS-35 that would allow me to
automatically switch to A/C at dock. This might be a useful addition anyway,
and with that the Kill a Watt would probably do what I am looking for as
well as check the quality of the A/C. may be worth a try?

GBM

You might spend more time fetching a car clock from the scrapyard than
if you just sat there with a watch and an ammeter. The mfgr
recommendation is probably to protect you from the surge at start up,
which might surprise you, if you expect say 10 amp draw, but have a one
second surge of 30 amps.

After noting a single cycle of start, run, stop, you then have the
onerous task of sitting beside the fridge all day with a beer or two,
counting, averaging the cycles per hour.

From a warm case of beer start, to keep score, or for all day, opening
the door for 20 seconds, closing it every hour, to work up a "typical"
duty cycle.

Alternatively, charge the battery check the specific gravity, let it
run for say 12 hours, then measure the sg of the battery, and
interpolate the number of amp hours from your battery specs.

How critical is all of this?

Your time is say, 10 bucks an hour. The test equipment to do this
properly starts out ridiculously expensive, then proceeds toward the
incredulous, depending on the degree of accuracy desired.

A laptop data port, some software, an interface, or multimeter with
data output would do for centibucks what you could accomplish by rule
of thumb for a little effort.

Actually, I wonder if a digital audio recorder like windat might record
DC voltages across a current shunt in the DC supply. You would need to
test and calibrate the data.

Terry K

"Terry K" wrote

After noting a single cycle of start, run, stop, you then have the
onerous task of sitting beside the fridge all day with a beer or two,
counting, averaging the cycles per hour.

From a warm case of beer start, to keep score, or for all day, opening
the door for 20 seconds, closing it every hour, to work up a "typical"
duty cycle.

Alternatively, charge the battery check the specific gravity, let it
run for say 12 hours, then measure the sg of the battery, and
interpolate the number of amp hours from your battery specs.

How critical is all of this?

It's not too critical - Just want to decide whether or not additional
(difficult to add) insulation is warranted.

I don't really need a typical duty cycle - just let the unit run with box
closed for say 12-24 hours, then add temporary insulation and let it run
again. If the external temperature changes this could upset the results. So
need to do test when weather is stable for a few days. I could measure
temperatures, acid SG and battery voltage at intervals.

I think this may give me an idea of whether or not to add the extra
insulation.

Thanks for suggestions!

GBM






Your time is say, 10 bucks an hour. The test equipment to do this
properly starts out ridiculously expensive, then proceeds toward the
incredulous, depending on the degree of accuracy desired.

A laptop data port, some software, an interface, or multimeter with
data output would do for centibucks what you could accomplish by rule
of thumb for a little effort.

Actually, I wonder if a digital audio recorder like windat might record
DC voltages across a current shunt in the DC supply. You would need to
test and calibrate the data.

Terry K

If the system is based on a Danfoss compressor with an electrical unit older
than about 12 months and doesn't have a "smart" controller like Fridoboat
SSC or Isotherm ASU it will run at a constant speed and use a constant
current so you only need total run time to calculate the relative power
usage. With the new Danfoss AEO electronics or a smart control the speed
varies and you do have to measure the current draw over time.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"GBM" wrote in message
...

"Terry K" wrote

After noting a single cycle of start, run, stop, you then have the
onerous task of sitting beside the fridge all day with a beer or two,
counting, averaging the cycles per hour.

From a warm case of beer start, to keep score, or for all day, opening
the door for 20 seconds, closing it every hour, to work up a "typical"
duty cycle.

Alternatively, charge the battery check the specific gravity, let it
run for say 12 hours, then measure the sg of the battery, and
interpolate the number of amp hours from your battery specs.

How critical is all of this?

It's not too critical - Just want to decide whether or not additional
(difficult to add) insulation is warranted.

I don't really need a typical duty cycle - just let the unit run with box
closed for say 12-24 hours, then add temporary insulation and let it run
again. If the external temperature changes this could upset the results.
So
need to do test when weather is stable for a few days. I could measure
temperatures, acid SG and battery voltage at intervals.

I think this may give me an idea of whether or not to add the extra
insulation.

Thanks for suggestions!

GBM






Your time is say, 10 bucks an hour. The test equipment to do this
properly starts out ridiculously expensive, then proceeds toward the
incredulous, depending on the degree of accuracy desired.

A laptop data port, some software, an interface, or multimeter with
data output would do for centibucks what you could accomplish by rule
of thumb for a little effort.

Actually, I wonder if a digital audio recorder like windat might record
DC voltages across a current shunt in the DC supply. You would need to
test and calibrate the data.

Terry K