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Q: Microwave Oven ( Answered,   5 Comments )
Subject: Microwave Oven
Category: Science > Technology
Asked by: ab41-ga
List Price: $10.00
Posted: 31 May 2006 19:27 PDT
Expires: 30 Jun 2006 19:27 PDT
Question ID: 734252
Is the microwave we use in our lab for moisture testing faulty? - it
is 6 months old and is used nearly 24/7 for routine moisture testing.
It runs for 5 -10 minutes every 1/2 hour but is already showing quite
variable heat output and is sometimes very low. The microwave is 1100
watt and a reputable domestic brand. Are we expecting too much from a
domestic oven? are there commercial types?
Subject: Re: Microwave Oven
Answered By: hedgie-ga on 26 Jun 2006 01:29 PDT
To measure moisture by weighing a sample before and after drying
is a reasonable method, but microwave owen is not a suitable 
device for trying the sample: There has to be a sufficient ventilation
and temperature has to be controled.

The ampint of water vapor will influence the heat output, and excess
temperature can cause 'dry destillation' of the hydrocarbon 
(caramelisation of the starch). Both can affect accuracy of results.

I would suggest you get an instrument intended for the purpose.
It will be faster and more reliable method.

Instrument such as

Also, look at search terms  (query types into a search engine)

moisture meter

If extra low cost, you may consider household appliance for drying food

Vaccum drying will always be more consistent that druing by elevated temperature

Here are examples of industrial fryers. leistungen/kammern.en.html

Clarification of Answer by hedgie-ga on 26 Jun 2006 01:31 PDT
Price comparison shows that food dehydrator will cost less than microwave:

It will also provide better results.
Subject: Re: Microwave Oven
From: nelson-ga on 31 May 2006 22:39 PDT
I doubt the microwave was designed to test moisture in a laboratory
setting.  I hope you aren't using this in a medical setting where
people's well-being is at stake.
Subject: Re: Microwave Oven
From: ab41-ga on 01 Jun 2006 15:00 PDT
Only solids levels in potatoes.
Subject: Re: Microwave Oven
From: brix24-ga on 01 Jun 2006 18:21 PDT
I take it that you are using the oven for drying. Liquid water is a
good absorber of microwaves. As the water is driven off, the
unabsorbed microwave radiation reflects back and forth, with some
eventually reflecting back to the magnetron and possibly/probably
causing damage.

You might improve your luck by getting an oven that will permit using
the oven when empty (that is, when there is nothing in the oven to
absorb the microwaves) (if you can find such an oven) or by pausing
before the potato is completely dry, then including another item with
moisture in it to absorb some of the "excess" microwaves, and finally
continue drying the potato (slices?) with that other item present. (I
would try something other than a glass of water since it is likely to
erupt explosively at unexpected times, including after the oven is
turned off and the door opened. In addition to that hazard, hot water
everywhere would obviously negate the attempt to determine solids. You
would still have to be careful about the second object, depending on
how hot it got.)

Panasonic manuals often have a section saying not to use them empty:
?2. DO NOT operate the oven empty. The microwave energy will reflect
continuously throughout the oven if food or water is not present to
absorb energy. This could damage the oven and result in the danger of
a fire.? (on the page numbered ?2,? or pg. 4 of the pdf doc)

(The page numbered ?5? also has directions for testing a container for
microwave use: ?Fill a 1-cup glass measure with water and place it in
the microwave oven along with the container to be tested; heat one
minute at HIGH.? (This last clause is in bold.)

The following implies that some ovens can be operated empty: ?Other
Tips for Microwave Oven Use: Some ovens should not be operated when
empty. Refer to the instruction manual for your oven.?
This is at the end of information dated March 8, 2000, at

Here are additional quotes on damaging microwave ovens:

?It may be possible to damage a microwave oven by leaving it cooking
on high power for a long time (particularly if there is no food in
it). But the amount and rate of damage (if any) depends on how well
the cooling fan works and whether the oven is protected in other ways
(some may even have temperature sensors that shut down the oven before
the components get too hot, or they may have a fuse protecting the
capacitor in the microwave source). As long as they don't get too hot
to damage anything, the metal parts should not wear out. The cooling
fan may wear out after a long time, not because of microwaves but
because of dust, grit, or loss of lubrication.?

Here is a quote indicating that damage can occur:
?What happens if you start the microwave oven with nothing inside?
The magnetron creates microwaves that travel into the cooking chamber
and should be absorbed there. If there is no food (or rather no
water-containing food), those microwaves will not be absorbed and will
eventually find their way back to the magnetron. Eventually the
magnetron will absorb as many microwaves as it emits. This situation
is hard on the magnetron, which works best when it has very little
radiation returning to it. That's why you should never run a microwave
empty for more than a second or two.?
This is item 326 in
This site has several similar quotes along the same lines, e.g., ?Can
I warm plates in my microwave oven? ? AC
Yes, but it's not a good idea. Depending on the type of plate, you can
either damage your microwave oven or damage the plate.

If a plate is "microwave safe," it will barely absorb the microwaves
and heat extremely slowly. In effect, the microwave oven will be
operating empty and the electromagnetic fields inside it will build up
to extremely high levels. Since the walls of the oven are mirrorlike
and the plate is almost perfectly transparent to microwaves, the
electromagnetic waves streaming out of the oven's magnetron tube
bounce around endlessly inside the oven's cooking chamber. The
resulting intense fields can produce various types of electric
breakdown along the walls of the cooking chamber and thereby damage
the surface with burns or arcs. Furthermore, the intense microwaves in
the cooking chamber will reflect back into the magnetron and can upset
its internal oscillations so that it doesn't function properly.
Although magnetrons are astonishingly robust and long-lived, they
don't appreciate having to reabsorb their own emitted microwaves. In
short, your plates will heat up slowly and you'll be aging your
microwave oven in the process. You could wet the plates before putting
them in the microwave oven to speed the heating and decrease the
wear-and-tear on the magnetron, but then you'd have to dry the plates
before use. ??

Here is a site that seems to indicate that no microwave oven should be
expected to escape damage if it is used empty for any length of time:
?Also, my experiments did not work with a cup of water inside the oven
along side of the grapes. The cup of water inside the oven is a
safeguard load for the microwave magnetron power tube. A microwave
oven should NEVER BE OPERATED with out some material to cook inside
the oven and all manufactures tell you to put a cup of water in the
oven if you want to test the oven. Heating just a few grapes with
nothing else in the oven could possibly damage the microwave tube if
the water in the grapes evaporates to [sic] quickly.?

Here?s a possibly different opinion:
?Damage to oven
There's a common belief that putting 'odd' things in a microwave may
damage the oven. I don't know where this belief comes from, since both
my own experience (no instance of magnetron damage yet, despite
numerous tests of very 'odd' objects), and my understanding of how
magnetrons and mw ovens work, suggest that such fears are baseless. In
general, I'd expect that damage to the magnetron might be possible in
cases of severe mw energy reflection back into the magnetron from the
oven cavity. However, surely one of the worst cases for power
reflection would be from a completely empty oven cavity. If a
magnetron can survive that (common) situation, then it should be able
to survive any other form of cavity load, right up to the almost total
absorption (heavy load) presented by (say) a large roast.?

Here is just one warning about hot water erupting explosively:
?Erupted Hot Water Phenomena in Microwave Ovens
The FDA has received reports of serious skin burns or scalding
injuries around people's hands and faces as a result of hot water
erupting out of a cup after it had been over-heated in a microwave
oven. Over-heating of water in a cup can result in superheated water
(past its boiling temperature) without appearing to boil.
This type of phenomena occurs if water is heated in a clean cup. If
foreign materials such as instant coffee or sugar are added before
heating, the risk is greatly reduced. If superheating has occurred, a
slight disturbance or movement such as picking up the cup, or pouring
in a spoon full of instant coffee, may result in a violent eruption
with the boiling water exploding out of the cup.
What Can Consumers Do to Avoid Super-Heated Water?
Users should follow the precautions and recommendations found in the
microwave oven instruction manuals, specifically the heating time.
Users should not use excessive amounts of time when heating water or
liquids in the microwave oven. Determine the best time setting to heat
the water just to the desired temperature and use that time setting
Subject: Re: Microwave Oven
From: ab41-ga on 05 Jun 2006 18:04 PDT
thanks for this comprehensive "comment". This makes sense. I will
investigate running the replacement microwave oven with something to
absorb excess microwave energy as the sample becomes dry.
Subject: Re: Microwave Oven
From: brix24-ga on 05 Jun 2006 20:54 PDT
Two other thoughts: mentions a sensor feature of some microwave ovens:

I don't know anything about the sensors, though. They may turn off the
oven too soon for your purposes.

Here is a quote from an individual on how one sensor works: "With the
sensor cooking, the food will not be overcooked and there will be no
uneven cooking. The sensor detects the humidity inside the oven that
is created when the moisture from the food is thrown off. Once enough
humidity is picked up, the oven stops cooking automatically so what
you have is food cooked perfectly."

Another part of the Consumer Reports site gives repair histories but I
think that this is available only to subscribers to the site. The repair histories don't indicate what the
repairs were for, but the histories might be a guide in choosing a
replacement.  (Your local library may have a copy of the appropriate
hard copy issue, but sometimes parts of these issues have been torn
out. You could call your local library to be sure before wasting a

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