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Measuring microwave wattage

T

Terry Pinnell

Jan 1, 1970
0
My beans on toast were not quite as hot as usual this lunchtime. I
suspect that I may have damaged our microwave recently (placing a
ready meal in a metallic container into it by mistake for a few
seconds) so that it's no longer delivering its spec of 650 W.

The following test seems to confirm this, but I'd appreciate a check
of my calculation please.

I poured 200 g of tap water into a small container. It was a thin
plastic container weighing 24 g, so I've ignored its thermal mass.
After a few minutes (while I fetched my DMM and a couple of
temperature probes) I measured the water's steady starting temperature
as 19 C. I heated it for 50 s and its finishing temperature was 43 C.

So the microwave raised 200 g of water 24 degrees C in 50 seconds.

I had to look up the Calorie/Joule relationship, which is that 1 cal =
4.1868 joules.

So wattage = (200 * 24 * 4.187)/50 = 401 Watts

OK?

I've been intending to replace this old unit with a more modern 800 W
oven for a couple of years, so it looks like the time has come.
 
P

Phil Hobbs

Jan 1, 1970
0
Terry said:
My beans on toast were not quite as hot as usual this lunchtime.

So you eat a lot of microwaved beans on toast for lunch? *There's* the
problem. Switch to something more palatable, such as Spaghetti-Os or
Kraft "Macaroni & Cheese". Jim T. puts Merlot in his. ;-)

Cheers,

Phil Hobbs
 
T

Tim Hubberstey

Jan 1, 1970
0
Phil said:
So you eat a lot of microwaved beans on toast for lunch? *There's* the
problem. Switch to something more palatable, such as Spaghetti-Os or
Kraft "Macaroni & Cheese". Jim T. puts Merlot in his. ;-)

Hey! Give Terry a break, beans on toast is a lot healthier and tastier
than the usual English fare. :))
 
G

Guy Macvon

Jan 1, 1970
0
Terry Pinnell said:
My beans on toast were not quite as hot as usual this lunchtime. I
suspect that I may have damaged our microwave recently (placing a
ready meal in a metallic container into it by mistake for a few
seconds) so that it's no longer delivering its spec of 650 W.

The following test seems to confirm this, but I'd appreciate a check
of my calculation please.

I poured 200 g of tap water into a small container. It was a thin
plastic container weighing 24 g, so I've ignored its thermal mass.
After a few minutes (while I fetched my DMM and a couple of
temperature probes) I measured the water's steady starting temperature
as 19 C. I heated it for 50 s and its finishing temperature was 43 C.

So the microwave raised 200 g of water 24 degrees C in 50 seconds.

I had to look up the Calorie/Joule relationship, which is that 1 cal =
4.1868 joules.

So wattage = (200 * 24 * 4.187)/50 = 401 Watts

OK?

I've been intending to replace this old unit with a more modern 800 W
oven for a couple of years, so it looks like the time has come.

Added the experts group.....

The advice might be you need a frying pan, or two, and perhaps a saucpan.

(And some On!ons)

DNA
 
L

Lyle

Jan 1, 1970
0
Added the experts group.....

The advice might be you need a frying pan, or two, and perhaps a saucpan.

(And some On!ons)

DNA
No you need some weenies!
Lyle
 
J

John Larkin

Jan 1, 1970
0
My beans on toast were not quite as hot as usual this lunchtime. I
suspect that I may have damaged our microwave recently (placing a
ready meal in a metallic container into it by mistake for a few
seconds) so that it's no longer delivering its spec of 650 W.

The following test seems to confirm this, but I'd appreciate a check
of my calculation please.

I poured 200 g of tap water into a small container. It was a thin
plastic container weighing 24 g, so I've ignored its thermal mass.
After a few minutes (while I fetched my DMM and a couple of
temperature probes) I measured the water's steady starting temperature
as 19 C. I heated it for 50 s and its finishing temperature was 43 C.

So the microwave raised 200 g of water 24 degrees C in 50 seconds.

I had to look up the Calorie/Joule relationship, which is that 1 cal =
4.1868 joules.

So wattage = (200 * 24 * 4.187)/50 = 401 Watts

OK?

I've been intending to replace this old unit with a more modern 800 W
oven for a couple of years, so it looks like the time has come.


A small container like that probably isn't a perfect match for the
microwave field, so you might get more power into a bigger load, like
a chicken maybe. So maybe it's OK.

John
 
Z

Zak

Jan 1, 1970
0
Terry said:
So the microwave raised 200 g of water 24 degrees C in 50 seconds.

I had to look up the Calorie/Joule relationship, which is that 1 cal =
4.1868 joules.

So wattage = (200 * 24 * 4.187)/50 = 401 Watts

OK?

Microwaves are quite sensitive to line voltage changes. Be sure to
check. But they also wear out.


Thomas
 
K

Ken Taylor

Jan 1, 1970
0
Terry Pinnell said:
My beans on toast were not quite as hot as usual this lunchtime. I
suspect that I may have damaged our microwave recently (placing a
ready meal in a metallic container into it by mistake for a few
seconds) so that it's no longer delivering its spec of 650 W.

The following test seems to confirm this, but I'd appreciate a check
of my calculation please.

I poured 200 g of tap water into a small container. It was a thin
plastic container weighing 24 g, so I've ignored its thermal mass.
After a few minutes (while I fetched my DMM and a couple of
temperature probes) I measured the water's steady starting temperature
as 19 C. I heated it for 50 s and its finishing temperature was 43 C.

So the microwave raised 200 g of water 24 degrees C in 50 seconds.

I had to look up the Calorie/Joule relationship, which is that 1 cal =
4.1868 joules.

So wattage = (200 * 24 * 4.187)/50 = 401 Watts

OK?

I've been intending to replace this old unit with a more modern 800 W
oven for a couple of years, so it looks like the time has come.

Can you check another unit of same ilk to see if the result is the same?
Power delivered into the load may not be the 650W that's on the spec.
depending on efficiencies in the oven. Nice checking though! :)

Cheers.

Ken
 
I

Ian Stirling

Jan 1, 1970
0
Terry Pinnell said:
My beans on toast were not quite as hot as usual this lunchtime. I
suspect that I may have damaged our microwave recently (placing a
ready meal in a metallic container into it by mistake for a few
seconds) so that it's no longer delivering its spec of 650 W.

The following test seems to confirm this, but I'd appreciate a check
of my calculation please.

I poured 200 g of tap water into a small container. It was a thin
plastic container weighing 24 g, so I've ignored its thermal mass.
After a few minutes (while I fetched my DMM and a couple of
temperature probes) I measured the water's steady starting temperature
as 19 C. I heated it for 50 s and its finishing temperature was 43 C.
I've been intending to replace this old unit with a more modern 800 W
oven for a couple of years, so it looks like the time has come.

I would consider putting 2l of tap water into a large covered container,
and repeating, over maybe 500s.
(stir before measuring)
200g may be too little to well match.
 
J

JustMe

Jan 1, 1970
0
Terry Pinnell said:
My beans on toast were not quite as hot as usual this lunchtime. I
suspect that I may have damaged our microwave recently (placing a
ready meal in a metallic container into it by mistake for a few
seconds) so that it's no longer delivering its spec of 650 W.

The following test seems to confirm this, but I'd appreciate a check
of my calculation please.

I poured 200 g of tap water into a small container. It was a thin
plastic container weighing 24 g, so I've ignored its thermal mass.
After a few minutes (while I fetched my DMM and a couple of
temperature probes) I measured the water's steady starting temperature
as 19 C. I heated it for 50 s and its finishing temperature was 43 C.

So the microwave raised 200 g of water 24 degrees C in 50 seconds.

I had to look up the Calorie/Joule relationship, which is that 1 cal =
4.1868 joules.

So wattage = (200 * 24 * 4.187)/50 = 401 Watts

OK?

In the good old days, when watts were watts and not some mathematical
construct designed to sell microwave ovens and stereo amplifiers on the
"bigger is better" principle, the method to measure microwave oven output
went like this.

Use 1 litre of cool tap-water in a microwave safe plastic jug. Measure
temperature in Centigrade. Place off centre on turntable and run on high
for 2 minutes. Stir and re-measure temperature, then subtract initial
measurement. Multiply temperature gain in degrees Centigrade by 35 to obtain
a realistic power output.

Compare readings taken at mid- morning or late evening with readings taken
at peak electricity consumption times. Significant variations in power
output will indicate a mains power regulation problem. This is not an
uncommon situation, affecting many older ovens that use the
transformer/voltage doubler psu type. Switching type supplies are, of
course, designed to compensate for low input voltage.
 
G

GPG

Jan 1, 1970
0
I have used the same idea but 2L (icecream container) and *70
 
M

Mac

Jan 1, 1970
0
Microwaves are quite sensitive to line voltage changes. Be sure to
check. But they also wear out.
If you read the above sentence verbatim, and ignore context, it sounds
quite funny. I mean, how would a wave wear out? Do X-rays and short-waves
wear out, too? What about UV and vis radiation?

But of course I know what you mean.

--Mac
 
A

Alt Beer

Jan 1, 1970
0
John Larkin said:
A small container like that probably isn't a perfect match for the
microwave field, so you might get more power into a bigger load, like
a chicken maybe. So maybe it's OK.

John

What size chicken would give the best SWR :)
 
T

Terry Pinnell

Jan 1, 1970
0
In the good old days, when watts were watts and not some mathematical
construct designed to sell microwave ovens and stereo amplifiers on the
"bigger is better" principle, the method to measure microwave oven output
went like this.

Use 1 litre of cool tap-water in a microwave safe plastic jug. Measure
temperature in Centigrade. Place off centre on turntable and run on high
for 2 minutes. Stir and re-measure temperature, then subtract initial
measurement. Multiply temperature gain in degrees Centigrade by 35 to obtain
a realistic power output.

Thanks. That gave me a 13 C delta, so a significant improvement to 455
W. But still well down on its 650 W rating!
Compare readings taken at mid- morning or late evening with readings taken
at peak electricity consumption times. Significant variations in power
output will indicate a mains power regulation problem. This is not an
uncommon situation, affecting many older ovens that use the
transformer/voltage doubler psu type. Switching type supplies are, of
course, designed to compensate for low input voltage.

The above was at 08:25 UK time, a heavyish consumption period I
imagine, so I'll repeat mid-morning to see if I can nudge it up a bit
more.
 
R

rex

Jan 1, 1970
0
What size chicken would give the best SWR :)

Once you get above a Cornish hen, I would think the density or moisture
content is most important. Maybe how recently it drank prior to losing
its head, or what kind of diet it was raised on. Not aware of any
studies of chickens as microwave terminations (not to say there are
none.)

I did do an impromptu study of the effect of ~2 GHz microwaves on the
small brown ants that occasionally invade my house.

Left a plate with some sweet remnants on my counter. Came back to
discover several dozen ants on it. This didn't please me so I wanted to
do a mass assassination. Popped the plate into the microwave oven on
high for several minutes. No effect. Happily strolling around without
the slightest sign that I was blasting them with heavy energy. (Now even
crawling off the plate and through the chamber. Little bastards!)

Flash of insight. Microwave ovens probably don't work well on things
that are significantly smaller than 1/4 wavelength.

So if you want to fry ants in your microwave, be sure to add some water.

[Later, I found my favorite method to be a fast moving, low flame,
propane torch applied in situ. No chemical residue and little damage to
the surroundings with good technique.]
 
T

Terry Pinnell

Jan 1, 1970
0
Terry Pinnell said:
Thanks. That gave me a 13 C delta, so a significant improvement to 455
W. But still well down on its 650 W rating!


The above was at 08:25 UK time, a heavyish consumption period I
imagine, so I'll repeat mid-morning to see if I can nudge it up a bit
more.

I did so, and got a further improvement to 520 W. That's still 20%
down though, so I think a new microwave is on the cards.

BTW, assuming proportionality, that seems to imply a 7% increase in
local mains voltage within that couple of hours.
 
M

Mike Yarwood

Jan 1, 1970
0
<snip>

400 watts out of 650 available into one measly cup of water seems pretty
good to me. Probably generated a bit of steam too so you could weigh your
cup before and after to find out what that was and guesstimate what used to
be called latent heat of vapourisation or something. Better still nip round
to your next door neighbours and cross-check on her machine.
The advice might be you need a frying pan, or two, and perhaps a saucpan.

(And some On!ons)
Best of Luck - Mike61/2
 
J

JustMe

Jan 1, 1970
0
I did so, and got a further improvement to 520 W. That's still 20%
down though, so I think a new microwave is on the cards.

BTW, assuming proportionality, that seems to imply a 7% increase in
local mains voltage within that couple of hours.

--

I don't think there is anything very linear about the transition from
partial power to full power related to mains voltage. About as linear as a
zener knee if you get my drift.

In general, modern magnetrons do not fail gracefully as yours would seem to
indicate. They sometimes fail short circuit internally, or more commonly
where the feed-through filter capacitors are built into the base connector
assy. A blown short protector diode and line fuse is the usual result.

You may want to ponder a little more before you consign your old friend to
the municipal tip. If the magnetron has developed a partial internal fault
the power that is directed into it is transformed to heat instead of
microwaves. This can be detected as a sort of run-away effect in the air
blown through the cooling fins. The longer you run it under load, the less
output you get and the hotter the exhaust air stream. Pretty easy to spot.


Common causes of low output power, not related to mains supply, can be
intermittent spade connections to the magnetron base, (there are 10 to 20
amps at low voltage driving the filament) or overheating of the magnetron
caused by fluff blocked cooling air intakes to the cabinet. These are
usually on the bottom plate and rarely seen.

Most on this newsgroup will have some idea what a few kilovolts at half an
amp will do, and the potential for a sneak attack by the doubler capacitor.
It is usually permanent. Take great care.
 
J

John Larkin

Jan 1, 1970
0
[Later, I found my favorite method to be a fast moving, low flame,
propane torch applied in situ. No chemical residue and little damage to
the surroundings with good technique.]

Fourmi bruillé, old French country recipe.

John
 
J

Jim Thompson

Jan 1, 1970
0
[Later, I found my favorite method to be a fast moving, low flame,
propane torch applied in situ. No chemical residue and little damage to
the surroundings with good technique.]

Fourmi bruillé, old French country recipe.

John

The better cooking-ware stores around here carry miniature torches for
such occasions.

...Jim Thompson
 
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