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Science Forum Index » Mechanics Forum » Microwave oven efficiency
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| Gordon |
 Posted: Mon Feb 18, 2008 1:35 pm |
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Guest
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How efficient is a microwave oven? Does the efficiency decline as the oven
ages?
Our 15-year old kitchen microwave appears to heat things slower than it did
when new, so I did an experiment to see how long it would take to heat 2
cups of water from near freezing to boiling. It took about six minutes,
but, by my estimate, should have been accomplished in under one minute:
(2 cups)*(2.366e-4 m^3/cup)*(1000 kg/m^3)*(1000 g/kg)*
(4.186 J/g-degC)*(100 degC)*(1 sec/5400 J)*(1/0.64)=57 sec.
The microwave is either 6500W or 5400W, depending on the voltage supplied,
so I assumed the lower. 4.186 is the specific heat of water. 0.64 is
Wikipedia's estimate of a typical oven efficiency (no citation given). The
time would have to be increased to account for the heating of the Pyrex
glass container, but glass has a specific heat about 20% that of water, so
little additional energy is needed to heat the glass. Am I missing
something?
Gordon |
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| N:dlzc D:aol T:com (dlzc) |
| Posted: Mon Feb 18, 2008 2:19 pm |
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Guest
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Dear Gordon:
"Gordon" <gordo432xRemove@comcast.net> wrote in message
news:ItydnVdL4qsjXCTanZ2dnUVZ_vShnZ2d@comcast.com...
Quote: How efficient is a microwave oven?
~ 60%.
Quote: Does the efficiency decline as the oven ages?
Good question.
Quote: Our 15-year old kitchen microwave appears to heat things
slower than it did when new, so I did an experiment to see
how long it would take to heat 2 cups of water from near
freezing to boiling. It took about six minutes,
Seems about right with my new one.
Quote: but, by my estimate, should have been accomplished in
under one minute: (2 cups)*(2.366e-4 m^3/cup)*(1000
kg/m^3)*(1000 g/kg)*
(4.186 J/g-degC)*(100 degC)*(1 sec/5400 J)*(1/0.64)=57 sec.
You have built the 5400 into your formula...
Quote: The microwave is either 6500W or 5400W,
Wahoo! You have three phase microwave, receiving 400-480vac 3
phase power, consuming the power of a 7.5hp motor? Better look
again.
David A. Smith |
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| Gordon |
| Posted: Mon Feb 18, 2008 6:55 pm |
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N:dlzc D:aol T:com (dlzc) wrote:
Quote: Dear Gordon:
"Gordon" <gordo432xRemove@comcast.net> wrote in message
news:ItydnVdL4qsjXCTanZ2dnUVZ_vShnZ2d@comcast.com...
How efficient is a microwave oven?
~ 60%.
Does the efficiency decline as the oven ages?
Good question.
Our 15-year old kitchen microwave appears to heat things
slower than it did when new, so I did an experiment to see
how long it would take to heat 2 cups of water from near
freezing to boiling. It took about six minutes,
Seems about right with my new one.
but, by my estimate, should have been accomplished in
under one minute: (2 cups)*(2.366e-4 m^3/cup)*(1000
kg/m^3)*(1000 g/kg)*
(4.186 J/g-degC)*(100 degC)*(1 sec/5400 J)*(1/0.64)=57 sec.
You have built the 5400 into your formula...
The microwave is either 6500W or 5400W,
Wahoo! You have three phase microwave, receiving 400-480vac 3
phase power, consuming the power of a 7.5hp motor? Better look
again.
David A. Smith
You are correct. This microwave oven is part of a combination conventional
oven/microwave cooking center. The name plate is located inside the
microwave but no doubt applies to the entire appliance, which is nominally a
6.5kW appliance. I eventually found in a manual that the "microwave power
output" is listed as 800 watts (IEC-705 Procedure), which already has the
efficiency factored in. Replacing the 5400*0.64 above with 800 changes the
result to 4.1 min. If that time is increased by another 20% to account for
heating the glass container, the estimated time is now about five minutes,
not so far from my measured time of about six minutes.
Gordon |
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| N:dlzc D:aol T:com (dlzc) |
| Posted: Mon Feb 18, 2008 7:39 pm |
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Guest
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Dear Gordon:
"Gordon" <gordo432xRemove@comcast.net> wrote in message
news:u-Odnd4BG6lZkSfanZ2dnUVZ_rCtnZ2d@comcast.com...
....
Quote: You are correct. This microwave oven is part of a
combination conventional oven/microwave cooking
center. The name plate is located inside the microwave but no
doubt applies to the entire
appliance, which is nominally a 6.5kW appliance.
I eventually found in a manual that the "microwave
power output" is listed as 800 watts (IEC-705
Procedure), which already has the efficiency
factored in. Replacing the 5400*0.64 above with
800 changes the result to 4.1 min. If that time is
increased by another 20% to account for heating
the glass container, the estimated time is now
about five minutes, not so far from my measured
time of about six minutes.
Klystron tubes go out of fashion, but they really don't
"degrade". There is a "window" that allows the microwaves to
enter the cooking chamber. Sometimes this gets occluded with
debris, and this can reflect some of the energy back into the
kystron. So a good thorough cleaning (with cautions) might speed
things up.
David A. Smith |
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| Brian Whatcott |
| Posted: Mon Feb 18, 2008 9:53 pm |
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Guest
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On Mon, 18 Feb 2008 12:35:11 -0500, "Gordon"
<gordo432xRemove@comcast.net> wrote:
Quote: How efficient is a microwave oven? Does the efficiency decline as the oven
ages?
Our 15-year old kitchen microwave appears to heat things slower than it did
when new, so I did an experiment to see how long it would take to heat 2
cups of water from near freezing to boiling. It took about six minutes,
but, by my estimate, should have been accomplished in under one minute:
(2 cups)*(2.366e-4 m^3/cup)*(1000 kg/m^3)*(1000 g/kg)*
(4.186 J/g-degC)*(100 degC)*(1 sec/5400 J)*(1/0.64)=57 sec.
The microwave is either 6500W or 5400W, depending on the voltage supplied,
so I assumed the lower. 4.186 is the specific heat of water. 0.64 is
Wikipedia's estimate of a typical oven efficiency (no citation given). The
time would have to be increased to account for the heating of the Pyrex
glass container, but glass has a specific heat about 20% that of water, so
little additional energy is needed to heat the glass. Am I missing
something?
Gordon
I have a suspicion that the heating rate of my domestic unit is not
linear with target mass. Two cups seem to take more than twice as long
as one cup. But I have not tested this objectively - perhaps you
might though?
Brian W |
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| Brian Whatcott |
| Posted: Mon Feb 18, 2008 10:03 pm |
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Guest
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On Mon, 18 Feb 2008 16:39:52 -0700, "N:dlzc D:aol T:com \(dlzc\)"
<dlzc1@cox.net> wrote:
Quote: Dear Gordon:
"Gordon" <gordo432xRemove@comcast.net> wrote in message
news:u-Odnd4BG6lZkSfanZ2dnUVZ_rCtnZ2d@comcast.com...
...
You are correct. This microwave oven is part of a
combination conventional oven/microwave cooking
center. The name plate is located inside the microwave but no
doubt applies to the entire
appliance, which is nominally a 6.5kW appliance.
I eventually found in a manual that the "microwave
power output" is listed as 800 watts (IEC-705
Procedure), which already has the efficiency
factored in. Replacing the 5400*0.64 above with
800 changes the result to 4.1 min. If that time is
increased by another 20% to account for heating
the glass container, the estimated time is now
about five minutes, not so far from my measured
time of about six minutes.
Klystron tubes go out of fashion, but they really don't
"degrade". There is a "window" that allows the microwaves to
enter the cooking chamber. Sometimes this gets occluded with
debris, and this can reflect some of the energy back into the
kystron. So a good thorough cleaning (with cautions) might speed
things up.
David A. Smith
The microwave generator found in domestic microwaves is
(I thought?) a magnetron. As its name implies, the magnetron
features a high magnetic field across a copper block with a central
hole for a cathode and peripheral holes for tuned cavities in the
anode(s). The field spins the electron cloud in a rotary spoke pattern
round the cathode, exciting the cavities in turn. RF is taken out
though a tap in one of the cavities to a short waveguide section
to a 'mode stirrer'..
Brian Whatcott Altus OK |
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| Gordon |
| Posted: Mon Feb 18, 2008 10:56 pm |
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Guest
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Brian Whatcott wrote:
Quote: On Mon, 18 Feb 2008 12:35:11 -0500, "Gordon"
gordo432xRemove@comcast.net> wrote:
How efficient is a microwave oven? Does the efficiency decline as
the oven ages?
Our 15-year old kitchen microwave appears to heat things slower than
it did when new, so I did an experiment to see how long it would
take to heat 2 cups of water from near freezing to boiling. It took
about six minutes, but, by my estimate, should have been
accomplished in under one minute: (2 cups)*(2.366e-4 m^3/cup)*(1000
kg/m^3)*(1000 g/kg)* (4.186 J/g-degC)*(100 degC)*(1 sec/5400
J)*(1/0.64)=57 sec.
The microwave is either 6500W or 5400W, depending on the voltage
supplied, so I assumed the lower. 4.186 is the specific heat of
water. 0.64 is Wikipedia's estimate of a typical oven efficiency
(no citation given). The time would have to be increased to account
for the heating of the Pyrex glass container, but glass has a
specific heat about 20% that of water, so little additional energy
is needed to heat the glass. Am I missing something?
Gordon
I have a suspicion that the heating rate of my domestic unit is not
linear with target mass. Two cups seem to take more than twice as long
as one cup. But I have not tested this objectively - perhaps you
might though?
Brian W
Apparently the current standard for testing the output power of microwave
ovens is the international IEC-705 test procedure.
http://www.gallawa.com/microtech/output.html
This procedure does what I did with the heating of water. The multiplier of
70 appearing on this web page can be verified as follows:
Output Power=(4.186 watt-sec/g-degC)*(1000 g)/(63 sec.)*T=66.4T,
where 4.186 is the specific heat of water, a joule is a watt-sec, and T is
the temperature rise. The standard uses the multiplier 70 instead of 66.4.
There is no explanation in the cited article, but perhaps the difference is
intended to account for some heating of the water container, which certainly
occurs.
Brian's suspicion about a dependency on mass is discussed here and referred
to as "well-known"
http://www.patentstorm.us/patents/5883801-description.html
See the two paragraphs containing "IEC 705".
Gordon |
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| N:dlzc D:aol T:com (dlzc) |
| Posted: Mon Feb 18, 2008 11:15 pm |
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Guest
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Dear Brian Whatcott:
"Brian Whatcott" <betwys1@sbcglobal.net> wrote in message
news:nrdkr3pjg7th97onmdpuh7a1pnnrqu1men@4ax.com...
Quote: On Mon, 18 Feb 2008 16:39:52 -0700, "N:dlzc D:aol T:com
\(dlzc\)"
dlzc1@cox.net> wrote:
Dear Gordon:
"Gordon" <gordo432xRemove@comcast.net> wrote in message
news:u-Odnd4BG6lZkSfanZ2dnUVZ_rCtnZ2d@comcast.com...
...
You are correct. This microwave oven is part of a
combination conventional oven/microwave cooking
center. The name plate is located inside the
microwave but no doubt applies to the entire
appliance, which is nominally a 6.5kW appliance.
I eventually found in a manual that the "microwave
power output" is listed as 800 watts (IEC-705
Procedure), which already has the efficiency
factored in. Replacing the 5400*0.64 above with
800 changes the result to 4.1 min. If that time is
increased by another 20% to account for heating
the glass container, the estimated time is now
about five minutes, not so far from my measured
time of about six minutes.
Klystron tubes go out of fashion, but they really
don't "degrade".
....
The microwave generator found in domestic
microwaves is (I thought?) a magnetron.
Correct. A klystron tube receives RF from a magnetron... in
radar applications. Thanks for the catch.
David A. Smith |
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