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

GBM,

I'm curious why the manufacturer recommended against adding an ammeter
to the circuit. Generally these devices have very little measurable
effect on the current flow or voltage, so I'm puzzled about the
recommendation.

As to measuring current, there are several solutions but the problem is
that most of the "cheap, common" hand held devices will measure amperage
instantaneously, where as I believe what you are looking for is a graph
of the consumption over a very long period of time. In other words, I
think you really want the total amount of energy consumed by the system
over a given period of time, which in your case appears to be at least
one or more complete refrigeration compressor cycles.

To my very limited knowledge I'm aware of nothing "off the shelf" that
will do this, but just a very quick search on Goggle produced this link:

http://www.ferret.com.au/articles/16/0c006616.asp

which offers a TOPTRONIC T202 multimeter with a built in inductive
ammeter which can be then coupled with the CABAC TBM515 data logging
device. NOTE: I've no interest in either of these companies, nor have I
ever used either product. The above information is only an example of
how to capture the current draw over time. Any other product that can
measure current and log the data would work.

The advantage to this approach is that virtually no expertise is
required to capture the information you require, nor would you need to
modify the existing circuity in any way (the ammeter clips around the
wire and measures the current flow inductively), but you will need to
purchase two devices that you might otherwise not need. In my opinion,
I'd think both of these types of instruments would be useful for
debugging additional electrical issues in the future, but I tend towards
overkill in these matters.

Alternatively, one could build their own embedded system using a few ICs
(I would suggest checking out the PIC16 micro-processor line from
www.microchip.com) but this approach presumes some familiarity with
electronics.

Hope this helps,

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

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


What type is it? Danfoss or big DC compressor? AC? Holding plate?

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.

Why? This seems odd.

Even
with that, I would need an hour meter or some such device to know how often
and for how long the compressor runs.

That's just what I do - I have a "2 bank" amp-hour meter, actually a
Link 2000, which I use in the normal way for the first bank, but I
measure the fridge usage with the second bank.

This is not a cheap solution, but if you have AH concerns, maybe you
want at least a simple amp-hour meter that you could use in either mode.




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 writes:

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

(If this is DC power you can't use a clamp-on meter as some others
suggest.)

If you're trying to measure duty cycles, you can improvise a cheap logging
meter with an inexpensive multimeter from Radio Shack (etc) that has a PC
interface, and a laptop or other computer. You can even simplify by
omitting the computer and just pointing a video recorder at an ordinary
meter, and fast-forwarding through to find the on and off times. No need
for expensive logging instruments for a one-time procedure.

Adding insulation is not typically an improvement in anything of recent
construction. That's a fantasy factoid lingering from the 1970s energy
crisis, endlessly repeated by people who've never measured the realized
benefit.

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!

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.

Here is an idea that will cost about $150 and do a lot more than measure the
amps.

A Hobo 12-012 data logger with internal temp and RH sensors and 2 external
sensors.
http://www.onsetcomp.com/Products/Pr...a_loggers.html
Plug a temperature sensor in one of the external inputs and an Ampsense Hall
effect current sensor on the other. http://www.ampsense.com/

With the free basic Hobo software you can capture the data and load it into
a spreadsheet. That will give you the external temp and RH, internal box
temp and amp draw on a common timeline.

Besides adding insulation you can do a few other things. The Danfoss DC
compressors are most efficient when they run a slow as possible to maintain
box temperature. They actually use less amp hours when they run 90% of the
time. You can experiment with the thermostat circuit resistor to adjust
the speed.

--
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
...
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