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'Sensing microwave oven temperature [OT]'
1998\04\04@062616 by Josef Hanzal

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Hi all,

I have just got a very weird request - to convert microwave oven into
termostated device. Do you thing there is a chance to sence the temperature
of some tissue sample inside the oven ? (and keeping budget under the price
of the oven ?) I just remember sparkling of the golden rim on a plate I used
once by mistake.

The microwave has several power settings (from 350W to 900W), does anybody
know how is the power reduction achieved ? Perhaps "PWM" ? Switching voltage
on the magnetrone ?

BTW, I also know more convenient ways to achieve constant temperature... TIA

Josef

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1998\04\04@073303 by Clyde Smith-Stubbs

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On Sat, Apr 04, 1998 at 01:27:29PM +0200, Josef Hanzal wrote:

> The microwave has several power settings (from 350W to 900W), does anybody
> know how is the power reduction achieved ? Perhaps "PWM" ? Switching voltage
> on the magnetrone ?

If my microwave oven is typical, it's purely done by cycling
the magnetron on and off, on the basis of several seconds on, several
seconds off. I suspect a magnetron would not take kindly to
varying voltage or pulsing at too rapid a rate.

IOW, the magnetron output is constant, but its average power is
determined by its duty cycle just like a hotplate.

Measuring temperature would be tricky, since it would depend on the
depth of the sensor in the heated sample (the microwave heating effect
is much shallower than you might think).

One thing you could do is detect a rise in humidity which would indicate
that at least some of the heated item had reached 100C (boiling point).
This is used by some ovens for "sensor-cook" settings.

--
Clyde Smith-Stubbs               |            HI-TECH Software
Email: .....clydeKILLspamspam@spam@htsoft.com          |          Phone            Fax
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HI-TECH C: compiling the real world.

1998\04\04@101114 by peter

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Josef Hanzal wrote:
>
> Hi all,
>
> I have just got a very weird request - to convert microwave oven into
> termostated device. Do you thing there is a chance to sence the temperature
> of some tissue sample inside the oven ? (and keeping budget under the price
> of the oven ?) I just remember sparkling of the golden rim on a plate I used
> once by mistake.

A standard oven meat probe will do
the type with a 1/4 jack plug.

Just make sure your 1/4 inch jack socket is earthed
and the probe does not make contact with (or close to) any metal
(quite dificult unless the chicken your cooking has
had hip replacement therapy)

As for power control, Clyde told you the answer
about 10 seconds for each cycle is about right
ie: 4 sec on, 6 sec off = 40% = 3600 watts (for 900w oven)

Peter Cousens
email: .....peterKILLspamspam.....cousens.her.forthnet.gr  phone: + 3081 324450, 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

1998\04\04@121429 by Mike Keitz

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On Sat, 4 Apr 1998 13:27:29 +0200 Josef Hanzal <EraseMEeuroclassspam_OUTspamTakeThisOuTPHA.PVTNET.CZ>
writes:
>Hi all,
>
>I have just got a very weird request - to convert microwave oven into
>termostated device. Do you thing there is a chance to sence the
>temperature
>of some tissue sample inside the oven ? (and keeping budget under the
>price
>of the oven ?) I just remember sparkling of the golden rim on a plate
>I used
>once by mistake.

Many of the higher-price ovens have a temperature probe which can be
plugged in inside the oven and inserted in the food.  I think the probe
is just a thermistor.  Both the thermistor and all the wiring need to be
shielded to prevent the microwaves from inducing a current that would
cause self-heating.  The control system can then be instructed to cook to
a certain temperature.  I never found it very useful in actual use, but
the kitchen really isn't my domain.

>
>The microwave has several power settings (from 350W to 900W), does
>anybody
>know how is the power reduction achieved ? Perhaps "PWM" ? Switching
>voltage
>on the magnetrone ?

Yes, the magnetron operates either full on or full off.  I think it is
hard to make a magnetron operate continuously at less than its rated
power.  So, the entire magnetron circuit is switched on and off at a rate
of about once every 20 seconds.  This slow rate is used because both the
heater and the plate of the tube are powered from the same transformer.
The start of each on cycle has to wait for the tube to heat up.  The long
"on" time, even at low power settings, can overheat small items of food.
I have an old Litton oven with two transformers.  The power control PWM
is much faster, on the order of 1 second.  It is much better than the new
ones when operated at low power.



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1998\04\04@134844 by Reginald Neale

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>Many of the higher-price ovens have a temperature probe which can be
>plugged in inside the oven and inserted in the food.  I think the probe
>is just a thermistor.  Both the thermistor and all the wiring need to be
>shielded to prevent the microwaves from inducing a current that would
>cause self-heating.  The control system can then be instructed to cook to
>a certain temperature.  I never found it very useful in actual use, but
>the kitchen really isn't my domain.
>
I always wondered how it was possible to use a metal-jacketed probe. Is it
some special kind of material?

Reg Neale

1998\04\04@175716 by Mike Keitz

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On Sat, 4 Apr 1998 13:50:15 -0500 Reginald Neale <nealespamspam_OUTSERVTECH.COM>
writes:

>I always wondered how it was possible to use a metal-jacketed probe.
>Is it
>some special kind of material?

It appears to be ordinary stainless steel.  Metal in a microwave oven
isn't necessarily bad.  A good electrical conductor won't absorb any
microwaves, thus it won't heat up.  Stainless steel wouldn't be my first
choice for probe metal because it isn't a good electrical conductor.
However if the metal conductor forms a loop a large current may be
induced in it.  If the conductor ends abruptly (seems to be the case with
a probe), a large voltage may be induced at the end of it.  I think that
if the probe assembly is an integral number of half-waves long, this
effect would be minimized (The wavelength of household oven microwaves is
about 12 cm).  The walls of the oven are of course metal, but they just
reflect the microwaves, they don't become hot.   The sensitive end of the
probe is going to be inside a large item of food that will absorb most of
the microwave energy as well as conduct most of the heat generated by any
waves absorbed by the probe.


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1998\04\06@010043 by tjaart

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Mike Keitz wrote:

> On Sat, 4 Apr 1998 13:50:15 -0500 Reginald Neale <@spam@nealeKILLspamspamSERVTECH.COM>
> writes:
>
> >I always wondered how it was possible to use a metal-jacketed probe.
> >Is it
> >some special kind of material?
>
> It appears to be ordinary stainless steel.  Metal in a microwave oven
> isn't necessarily bad.  A good electrical conductor won't absorb any
> microwaves, thus it won't heat up.  Stainless steel wouldn't be my first
> choice for probe metal because it isn't a good electrical conductor.
> However if the metal conductor forms a loop a large current may be
> induced in it.  If the conductor ends abruptly (seems to be the case with
> a probe), a large voltage may be induced at the end of it.

This the same principle you use when boiling an egg in your microwave.Cover an egg with tin foil,
and smooth the foil so no edges or points are
sticking out (if you don't, you'll have fire works). Put the egg in a cup
of water, and nuke it. The heat from the water actually cooks the egg
because it is in a faraday cage (tin foil).

Hey, how did we get to cooking lessons on the PIClist?

--
Friendly Regards

Tjaart van der Walt
KILLspamtjaartKILLspamspamwasp.co.za

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1998\04\06@064631 by Caisson

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> I am  sorry to post the letter a again because I did not receice the post
>on my pc,  I feel very strange.


> My  question is ,I got a pic, it's supposed to be  16c84,  but its  ink
> had been wiped.

> I  read it, the strange thing happened.

> I read nothing, if it were 16c84, then I'm supposed to get scrambled
code.

You allways read something, even when it is 0xFF or 0x00 ....

> but this time, in the program data all  00h, config is 09h.

As far as I know, the upper 6 bits will be ex-ored with the lower 6 bits.
(when in code-protect)
Because of the fact that (for the 16?84) the upper bits are all one's,
the real configuration should be 0x09 xor 0x3f => 0x16. This would be
Watch-dog ON and a High-speed oscillator.

> if  it were 16f84, then the program data  should be 00h, and the config
> should be 0fh.

Using the above trick : This config is actually 0x10, meaning  only the
Code-
protect bit is set.

> I tried for a new 16f84 under protect bit set, I read , the program data
> is 00h and the config is 0fh.

> what this chip could be?

It still could be either of them.

>  Is the trick for 16c84 no more available for the 16f84 ?

What trick are you talking about ?

Greetz,
 Rudy Wieser

1998\04\07@133022 by juha.tuomi

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----------
> From: Tjaart van der Walt <spamBeGonetjaartspamBeGonespamwasp.co.za>
> To: TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU
> Subject: Re: Sensing microwave oven temperature [OT]
> Date: 6. huhtikuuta 1998 7:59
>
> Mike Keitz wrote:
>
> > On Sat, 4 Apr 1998 13:50:15 -0500 Reginald Neale <RemoveMEnealespamTakeThisOuTSERVTECH.COM>
> > writes:
> >
> > >I always wondered how it was possible to use a metal-jacketed probe.
> > >Is it
> > >some special kind of material?
> >
> > It appears to be ordinary stainless steel.  Metal in a microwave oven
> > isn't necessarily bad.  A good electrical conductor won't absorb any
> > microwaves, thus it won't heat up.  Stainless steel wouldn't be my
first
> > choice for probe metal because it isn't a good electrical conductor.
> > However if the metal conductor forms a loop a large current may be
> > induced in it.  If the conductor ends abruptly (seems to be the case
with
> > a probe), a large voltage may be induced at the end of it.
>
> This the same principle you use when boiling an egg in your
microwave.Cover an egg with tin foil,
> and smooth the foil so no edges or points are
> sticking out (if you don't, you'll have fire works). Put the egg in a cup
> of water, and nuke it. The heat from the water actually cooks the egg
> because it is in a faraday cage (tin foil).
>
> Hey, how did we get to cooking lessons on the PIClist?
>
*****
Maybe someone has read too often the PIC COOKBOOK !

Greetings, Juha

*****
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