Sciencemadness Discussion Board

CS2 prep in microwave

halogenstruck - 7-4-2010 at 11:33

If phosphoric acid reduction by carbon can be done in microwave much lower than normal temperature of 1500`c just in 500,then

can we just mix S and C then put it in microwave and make CS2??i think C can only be heated up to 500'c in microwave.if thermodynamic allow the reaction in lower temperature then it will work.does thermodynamic allow H3PO4/C reaction in 500'c?

bbartlog - 7-4-2010 at 15:33

In the first place, we don't have any verification here that the phosphorus-at-500C process described in the patent actually works. As regards S and C, though, notice that S boils at ~440C. So even at 500C you'd need some sort of pressure vessel.

watson.fawkes - 7-4-2010 at 19:46

Quote: Originally posted by bbartlog  
In the first place, we don't have any verification here that the phosphorus-at-500C process described in the patent actually works. As regards S and C, though, notice that S boils at ~440C. So even at 500C you'd need some sort of pressure vessel.
You can use a "cold" finger to condense the sulfur vapor, where "cold" here means something around 150° C. You'd use a lower temperature condenser to liquefy the CS2 farther along in the vapor path. No need for high pressures at all in such a setup. As for building the cold finger, you can use oil as a coolant and a gear pump to circulate it. Perchance, I have a just such a pump (bought at surplus) in the shop that was specified for moving around used frying oil. If you make the vapor path out of metal and narrow enough, it will also serve as a wave stop for preventing microwave leakage.

I find it complete plausible that this might work at a lower temperature, by which I mean lower average temperature, because the surface of the carbon is where the bulk of the microwave absorption is going to be, making it rather reactive. It needs verification, though.

chief - 8-4-2010 at 00:18

There exist teflon-autoclaves for microwave-application, with maybe 100 to 200 ml capacity; overpressure-valves are typically for 30 Bar ... ; but these wouldnt hold that much temperature ..., are just for overheating water ...

halogenstruck - 13-4-2010 at 11:12

it seems the mixture does not absorb microwave,maybe it needs preheating or adding some good wave absorbing materials.maybe experimenting a mixture of Na2S5 and C is not a bad idea.
thx for the ideas :cold finger and teflon pressured reactor
they help the reaction to proceed

entropy51 - 13-4-2010 at 16:45

What a nice post

this could have been if halogenstruck had either looked up the temperature required to drive this reaction and calculated the microwave absorption of his putative reactants and showed that it was a likely possibility.

Or failing to be able to calculate a little thermo, that he had tried an experiment and had some meager data to report.

But no, we get the typical Science Madness brain fart, which smells of mercaptan and indole.

I wonder if the idea-prolific halogenstruck contemplated that the boiling point of CS2 is ridiculously low, and that the flash point is not much higher? Is it a good thing to toss half-baked ideas out there to an audience that may not know any better? Or is it criminal to post tripe that could actually get someone killed?

I've heard it said that the free-for-all of the net is a good thing, but is a little quality control too much to ask?

len1 - 13-4-2010 at 22:44

Of course you know reading the thread topic that there will be no practical because 90% of posters wont even get past first year theory. Not only that, but they have not the slightest inclination in their lazy body to try out what they have the energy to suggest - drugs makers excepted. The quality is absolutely abysmal.

Nicodem - 13-4-2010 at 23:25

I honestly think you two are exaggerating. First of all, I don't think anybody sane of mind and with a minimum of knowledge would heat a mixture of sulfur and carbon in a kitchen microwave oven knowing that sulfur boils at a temperature much lower that that required for the reaction and knowing that even if the reaction would have happened the CS2 formed would blow up the oven. If ignorant of even such obvious things, than nobody and no warning will help such a person.
About the laziness to do what they suggest... That is again an exaggeration and a pessimistic view. If people would really be interested in doing experiments they would go to a library, design an experiment properly and do it, rather than spend their time on internet forums. It is obvious that most (young) members are here mostly for other reasons (playing kewl, wasting free time, trying (not) to learn, investing their laziness into others work, exploiting others to do their literature searches...). However, some are honestly interested in chemistry and you should ask yourself what you can do to stimulate the interest of this minority instead of always complaining about the useless majority.
I also get depressed continuously seeing threads like this (or much worse) and even more depressed when I realize there is no way to enforce to the whole of this community even the very basic scientific method of using references, never mind the upgrades like proper experiment design ad analysis of the experimental data. But to me, one sign of progress in one single forum member compensates for the hundreds of lazy ass posters who regularly show up and often leave just as regularly seeing this is not a friendly place for kewls and the like.

len1 - 13-4-2010 at 23:42

Hey Ive done my best. Suggested a way to proceed .. got the usual tripe responses (check it its all on record)

Then did a whole pile of experiments to show how this forum can really work. People wanted this explained, this diagramised. I did all that.

Got lots of U2U some wanted to buy ready made stuff, some wanted to use it to make drugs, others were going to make something this weekend. Never did..

Thats enough for me .. You can try to interest this minority

Sandmeyer - 16-4-2010 at 17:18

Quote: Originally posted by Nicodem  
If people would really be interested in doing experiments they would go to a library, design an experiment properly and do it, rather than spend their time on internet forums.


Eh? Who goes to library before setting up an experiment? Luckily, I haven't been in library for at least a year - in fact, I see no reason in going there at all. All theoretical research can be done much more efficiently using the Internet. Just consider the vast amount of organic chemistry literature that is freely available on via Internet forums and other webpages.

halogenstruck - 16-4-2010 at 18:04

in roscoe "a treatise on chemistry Vol 1":
CS2 was made for the first time by heating FeS2 and C .
as FeS2 is a good conductor,it can get warm easily in Microwave,or even using Na2S5 as S source.

i know S boils much lower than 1000`c needed for reaction but normally reactions proceed in lower temperatures in microwave.
i know how to calculate delta Gibs, heat of reaction,... on NIST website and include physical status of reactants at given temperature and take into account Shomate Equations and...
but it`s wasting the time when i know already needed temperature and when experimenting just takes 3-5min to proceed and microwave also excite molecules,...

if you don`t mind ,instead of writing ... just mention calculations.then new users in forum can learn some thing useful.
it seems to me your are angry from another thread
https://www.sciencemadness.org/whisper/viewthread.php?tid=13...

mixing 1g S with charcoal does not make a big hazard for sure.chemistry is an experimental science.

in microscopic scale,in microwave,charcoal gets red and reach 500`c but S stays cold and get warms gradually by red C radiation. therefore continuously S gas passes over red C.

Ramiel - 16-4-2010 at 21:46

Len, you've distinguished yourself in this foray, and I understand your unhappiness as you percieve people haven't listened -- we are just lurking and absorbing every word!

I've read plenty of papers which included sections criticising the love-to-hate rival group. Such paragraphs boil down to "haha, your science sucks and you're shit"... scientists have a penchant for this perhaps, it's certainly part of the stereotype.

People are just trying to think differently about the problem. Some have little knowledge and some know better and say it anyway! You can't fault a person for trying something stupid!
At the end of the day, they are trying to help, to improve the quality of the board.

Please don't make posts which just bag out another scientist for suggesting an idea which seems stupid, and to you will always be a stupid idea, no matter how much work goes into improving it. Heavens may forfend there are things which even you do not know.

[Edited on 17-4-2010 by Ramiel]

Nicodem - 17-4-2010 at 02:16

Quote: Originally posted by Sandmeyer  
Eh? Who goes to library before setting up an experiment? Luckily, I haven't been in library for at least a year - in fact, I see no reason in going there at all. All theoretical research can be done much more efficiently using the Internet. Just consider the vast amount of organic chemistry literature that is freely available on via Internet forums and other webpages.

I'm relatively old and for me "going to a library" is synonymous for "doing a primary literature search" while not necessarily physically visit buildings with paper based books and journals. I do literature searches nearly every day, but physically visit a library maybe just once a month (the library of the institution I work for) and maybe just once or twice a year an external library, and even this only if the paper I look for is not available from publishers or illegal sources. I think that today you can do 99% primary literature search sitting comfortably in front of your computer provided that you have proper access passwords to the publisher's sites (or access to illegal archives). One would think that such an increasing ease in accessing literature would be followed by an increased interest in finding literature, yet sadly, I notice no such thing.
Quote: Originally posted by halogenstruck  

i know S boils much lower than 1000`c needed for reaction but normally reactions proceed in lower temperatures in microwave.

How about providing a reference for claims that go against the up to now thermodynamics theory? I could cite you a few papers that demonstrate exactly the opposite, that in homogeneous reactions (organic, at least) heating by microwaves or by convection has no effect whatsoever on the reaction rate as long as the reaction times and temperature graphs are equal. In heterogeneous reactions there is some differences because of differential heating of one phase over the other, but still, in such you can not say what the general temperature of the reaction actually was, because it depends on phase and phase boundaries. You can not say that the reaction of a heterogeneous mixture proceeds at 500°C under microwave heating, exactly because it is a heterogeneous mixture and as such it does not have a defined temperature under microwave irradiation unless under equilibrium conditions (which can not be achieved while the reaction proceeds).
Quote:
mixing 1g S with charcoal does not make a big hazard for sure.chemistry is an experimental science.

Sulfur vapours are not very dangerous from such an amount, but if CS2 would somehow form quantitatively, 1g sulfur and carbon can make just enough of it for the oven to explode in your face. It could not kill you, but it can do you harm for sure (if you are crazy enough to stand by the oven).
Yes, chemistry is an experimental science which means that it is based on designing and analysing experiments scientifically!

Edit: I had the "luck" of having the reaction vessel explode in our microwave reactor a couple of times already. The reactor cavity is shielded, but the blast is always terrible even though the reaction vessels used are less than 10ml size. It makes one big boom and the glass shrapnels go so violently that they can completely destroy the teflon container and penetrate into the aluminium alloy parts.

[Edited on 17/4/2010 by Nicodem]

len1 - 17-4-2010 at 04:52

I think you mixed me up. I did not attack the poster's science, entropy did that, and now that you mention it i agree with him. I accused the average SM poster of a) not following up anything he cares to post b) being lazy. That kind of puts them outside science altogether. They are not here for science.

They are here to show off, about something they dont much understand, never plan to do, and therefore dont care about. In front of someone whom they dont know, will never see, whom they dont care much about and who doesnt give a damn about them. If not the net I would never know there is a plethora of these wretched people in the world. They need help but society will never help them.

As far as CS2 is concerned, i have worked with it at temperature, it is not really that dangerous as its energy content is not much above that of the sulfur from which its made. it also seems to be a lot less flamable with sulfur vapour around. Also it can not accumulate at high temp mix with O2 and then explode since it autoignites immediately if it leaks out.

As far as microwaves are concerned I absolutely agree with nocodem

[Edited on 17-4-2010 by len1]

not_important - 17-4-2010 at 05:46

You could reach temperatures above 500 C by sandwiching the reaction tube between slabs of insulating board with grooves carved out for the tube. This is similar to the microwave kilns and metal melting setups that can reach 1000 C or so.

Obviously you can get away using borosilicate glass for the reaction tube in this case, silica or alumina tubing would be needed.

You also can't do this in a sealed tube, little CS2 would be formed unless it was removed from the reaction zone to prevent equilibrium conditions being reached. This means the tube would need to penetrate the microwave wall to attach to a condenser to capture the CS2. There'd be no pressure buildup, if the couplings were outside the cavity then there's much less opportunity for the buildup of CS2-air mixtures, leaks at temperatures above the ignition point become flames burning off the CS2.

Still would need some careful construction, and you don't poke holes in microwave oven walls lightly - proper design must be done to keep the microwaves inside.

If done with SiC instead of carbon you'd likely end up with CS2 (g) and SiS2 (s).

Damn, wish I had a spce for lab work and spare funds.

Field-expedient microwave radiation sensor

watson.fawkes - 18-4-2010 at 07:33

Quote: Originally posted by not_important  
Still would need some careful construction, and you don't poke holes in microwave oven walls lightly - proper design must be done to keep the microwaves inside.
One limitation of being able to do this on the cheap is a basic knowledge of electromagnetics, waveguides, etc. The other, though, which I've not seen discussed, is about the piece of test equipment that allows you to verify that your homemade reactor has radiation within acceptable limits. I had a conversation with a friend about this, and we didn't come up with anything that we were particularly satisfied with at the time. A direct field-strength meter is obvious, but (as I recall) my friend thought it would be hard to calibrate (or something like that); he's done microwave-frequency radio and antenna design work, so he's had practical experience with it. The other idea was discussed was a bolometer sensor. This is a piece of absorptive material that you stick a temperature sensor on. Adequate resolution on the cheap was the problem for this one.

So, any other ideas? Both of these ideas we talked about do work, but they don't work cheaply enough (as we could figure out) to compete with used commercial test equipment.

Since that conversation, I've had a third idea. You might be able to modulate the microwave generator, treat it as a carrier wave, and sync to that signal. Then you'd have an internal oscillator mirroring the modulation with an automatic gain control circuit whose fixed point is to match the amplitude of the received signal. (You might also need to phase-lock the internal oscillator.) Reading out the gain gives you the measurement. You'd only have to turn on the carrier modulation when testing, not necessarily during operation.

The WiZard is In - 18-4-2010 at 08:41

Quote: Originally posted by len1  


As far as CS2 is concerned, i have worked with it at temperature, it is not really that dangerous as its energy content is not much above that of the sulfur from which its made. it also seems to be a lot less flamable with sulfur vapour around. Also it can not accumulate at high temp mix with O2 and then explode since it autoignites immediately if it leaks out.
[Edited on 17-4-2010 by len1]



The Journal of the Society of Chemical Industry
No. 2.—Vol. VI. February 28, 1887.

XVII.-CHEMISTRY OF FOODS, SANITARY CHEMISTRY, DISINFECTANTS,
Etc.

(B) SANITARY CHEMISTRY.
Chronic Poisoning with Carbon Bisulphide. Jas. Ross, M.D., LL.D. Medical
Chronicle, 5 [4],257-269.

Two cases are reported. The first was that of a man aged twenty-four years,
unmarried, and who had never suffered from any serious illness up to date of the
seizure in question. He was admitted to the Manchester Infirmary 5th May, 1886.
He bad commenced to work, about eight months before, in an indiarubber
factory, and in this situation he was employed in the " curing-room," where he
inhaled the vapours given off from the materials used there, more or less of
these fumes consisting of carbon bisulpliide. After working for a few weeks here,
the man suffered from a burning sensation in the hands and face ; these were
also hot to the touch, and of a red colour. lie found, however, that on putting his
hands in cold water they immediately turned a livid colour, and became cold and
numb, as if dead, or they looked just as if frost-bitten ; in fact, he was compelled
to use warm water in washing. This numbness and weakness extending
gradually to his feet and legs, and being troubled with mental disturbances, he
was compelled to leave work for two or three weeks, during which time he rapidly
recovered. He now returned to his old work. His old symptoms soon returned,
but with redoubled force, and now he soon experienced the greatest difficulty in
walking, and could scarcely hold anything in his bands, which, besides being
feeble, trembled a good deal, more especially when the man attempted to grasp
anything. The senses of sight and hearing remained unaffected, but everything
seemed to smell of the vapours of the factory he had left, and his food either
seemed devoid of taste or to taste only of the loathsome vapours. The sight of
food was a burden ; be lost a stone in weight, and observed the wasting of his
arms and legs was out of proportion to that of the rest of his body. On leaving
work in the evening, lie often walked like a drunken person, and talked much
nonsense. The memory almost entirely failed ; at night he was restless, or, if
sleeping , was disturbed by horrid dreams. In the mornings he felt miserable and
depressed, and even found, on return to work, some relief from again inhaling
the gas—at first, at least. Sheer feebleness at length prevented this man from
crawling to his work, and he was laid up for four weeks. He next got employment
at a tarpaulin factory, but soon found himself unable to work, so feeble had his
hands become. Not only had there been a great loss of power of the brain
centres regulating memory, but those affecting important nervous functions had
also much suffered ; sexual appetite, for example, bad been lost. He contracted
colour-blindness, also, to some extent. The paralysis of principal muscular
centres was, of itself and alone, such as to reduce the patient to a condition of
pitiable weakness and helplessness.

The second case much resembled the first as regards the premonitory
symptoms of tingling, paralysis of muscles, trembling, livid condition of hands if
dipped in cold water, loss of taste for food, of memory, and in addition the sight
became affected. The man could not read, as the words all seemed, as he said,
" to run together." At length he became so feeble that he could scarcely walk,
and often fell down. Sexual appetite entirely failed ; shooting pains along nerve
courses; little sleep possible, and that disturbed by horrid dreams; violent
headaches, and a longing to get back to work, where the noxious fumes brought
temporary relief, and even an exaggerated feeling of pleasure. This soon gave
way to a feeling of intense apathy and wretchedness. These cases were not
exceptional ones, for this man was not affected, he declared, so much as some
of his fellow workmen. He stated that some of these, under the effects of the
gas, became very talkative, and often talked a great deal of nonsense. One man,
e.g., coming to work in the morning, declared he was in Liverpool the night
before, a statement that could not possibly have been true. Another workman,
apparently eager to escape from some imaginary danger, jumped through a
window, ran across an open court, and having crept under a joiner's bench, tried
to hide by covering himself with shavings One or two of the workmen had gone
quite mad, and had been sent to the lunatic asylum. The colour-vision of the
patient here considered was defective ; purple he called white, and could not
distinguish red from blue.

Dr. Ross states that these cases of paralysis from the inhalation of carbon
bisulphide show that such belongs to the group which is caused by various toxic
agents, such as alcohol, lead, arsenic, and certain animal poisons like that of
diphtheria. This paralysis, in fact, resembles alcoholic paralysis more nearly than
it does any other form of multiple neuritis. Sometimes symptoms are found
closely resembling those of delirium tremens, as in the case of the workman
jumping through the window to hide himself in a joiner's shop. The statements
and cases are well authenticated, and either alcohol nor lead played any part
whatever in the cases of these two patients, as causes or adjuncts in the
paralysis observed .

Dr. Ross concludes :—" If it be true, then, that we have in our midst certain
workshops in which the process of manufacture is so deleterious to the health of
the workmen that a certain proportion of them is reduced in a few months to the
pitiable condition of paralytic helplessness manifested by these two unfortunate
men, it is hardly necessary for me to maintain, in the face of all recent factory
legislation, that these workshops ought to be placed under the most stringent
regulations, and be subject to the frequent visits of a Government inspector. "-W. S.

Ephoton - 18-4-2010 at 09:03

safety first thankyou.

I wonder though what else can you tell us in time.

Lambda-Eyde - 18-4-2010 at 09:16

Quote: Originally posted by Nicodem  
I honestly think you two are exaggerating. First of all, I don't think anybody sane of mind and with a minimum of knowledge would heat a mixture of sulfur and carbon in a kitchen microwave oven knowing that sulfur boils at a temperature much lower that that required for the reaction and knowing that even if the reaction would have happened the CS2 formed would blow up the oven. If ignorant of even such obvious things, than nobody and no warning will help such a person.


"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former."
- Albert Einstein

densest - 18-4-2010 at 10:57

@watson.fawkes - googling on "microwave oven leakage" shows a number of radiation meters from $10 to $50. One of the more expensive ones purports to read down to 0.01 mw/cm2! I believe the FDA limit is something in the 1-2 mw/cm2 region. I'd probably want to use a video link to watch it for extended periods of time anyway ;)

The WiZard is In - 18-4-2010 at 13:43

Quote: Originally posted by Ephoton  
safety first thankyou.

I wonder though what else can you tell us in time.



Safety always. That said I found it humorous, which is why
it was posted. No — can't say that — it's — safety first.

jimmyboy - 18-4-2010 at 16:06

the combination of carbon and sulfur might be feasible in a microwave if you had a way to finely control the power output of the magnetron and did it in an oxygen free environment - a mixture of carbon and sulfur would easily reach the needed temperature but would then surpass it and break the chemical bonds as fast as it made them - like most things in chemistry the reaction is more complicated than adding A and B and heat - a tube furnace is still much more convenient for CS2 - if you get those two problems out of the way it might be a cheap convenient way to make it small scale - have you actually tried this in a used microwave already? results? make sure to use a ceramic container to withstand the heat

unome - 18-4-2010 at 21:17

Yes, chemistry is about experimentation - but we now know enough about the substance under discussion that we can design routes to it that won't kill us, unlike those on this list... (suprisingly doesn't mention Nobel who lost a brother)...

bbartlog - 19-4-2010 at 06:43

Quote:
and you don't poke holes in microwave oven walls lightly - proper design must be done to keep the microwaves inside


I'm not convinced of the extreme danger here (I suppose I did, in fact, do it lightly). I put a hole of ~25mm diameter in the left side of my microwave to allow for the possibility of piping out gases of via glass tubing. In normal (closed) operation I keep the opening covered with a metal plate and have a five gallon bucket of water sitting next to the covered hole. When running it open I keep the bucket of water as close as the setup allows and stand 2-3m away on the other (right) side of the microwave. I suppose if I wanted to watch closely through the front glass for minutes at a time while the thing was running open I would want additional protection.

watson.fawkes - 19-4-2010 at 07:08

Quote: Originally posted by densest  
@watson.fawkes - googling on "microwave oven leakage" shows a number of radiation meters from $10 to $50. One of the more expensive ones purports to read down to 0.01 mw/cm2! I believe the FDA limit is something in the 1-2 mw/cm2 region. I'd probably want to use a video link to watch it for extended periods of time anyway ;)
You are, indeed, illustrating exactly my point. I wouldn't trust a "leakage tester" either. The problem is that a leakage tester, if it's testing for leakage only, isn't a proper field strength meter. I would bet if you put one of those cheap meters into a full strength beam as it comes out of the magnetron that the detector would burn out immediately. Instant permanent false negative! The flip side of the high sensitivity you mention is that it's likely using a resonant antenna, so the voltages developed across it in a full-strength field are enormous. It requires special engineering effort to shunt away that behavior, whose costs are not justified if all you want is to check for leakage.

I spoke with my friend last night on this topic again, and he had a couple of great ideas. The first is to use a panel-mounted microwave attenuator at, say 60 db, mount in on the inside of the oven cavity somewhere, and take out the signal with a whip of microwave-capable coax cable. Presto, you've got a calibration signal (after, perhaps, a bit of conditioning) that you can feed into the detector where its antenna connects. This will allow you to detect burn-out. One of the cheap leakage testers could perhaps be modified to use this device. You still might burn out your detector, but at least you'll know.

His other idea was how to build a detection device. This idea is to use an intentionally non-resonant antenna. Designing with one of these makes the antenna signal insensitive to resonance effects and give you better assurances about the maximum power you're even going to see at the detector input. We're not doing long-distance communication here nor trying to an accurate low-level measurement; what's needed, at minimum, is a go/no-go tester with a threshold set at an acceptably low level. This kind of antenna eliminates needed to calibrate the antenna to find out its gain at the frequency of operation, lowering a barrier to entry by amateurs. Even with this device, you want to build a calibration source, just to ensure that it is indeed working.

The other thing that came out of that discussion is that the attenuator could also be used as the input to a simple field meter to observe how much of the beam energy was being absorbed. This could be used, for example, to ensure that a susceptor was adequate at the start of a process.

densest - 19-4-2010 at 09:15

@watson.fawkes - Agreed! Some of the meters look to be more useful & competently designed: decade attenuators & wide-band (500MHz?-5GHz?) response. Your point about testing with a reference signal is a good one. I don't think any of them would survive immersion in a full power field.

http://www.repairfaq.org/REPAIR/F_micfaq6.html has some notes on DIY leakage testers - old technology (neon bulb) and new technology (LED). The LED version could be made much more sensitive by using a low capacitance Schottky diode - probably a 100V one or even ($$) a SiC diode.

http://www.mdpi.com/1420-3049/6/10/831/pdf has a diagram of a modified oven used to synthesize phthalocyanine pigments.

There is a lot of material published over the last 15 years on using modified microwave ovens for chemistry. The most common way to shield holes in the oven is to extend a copper tube or mesh out from the oven at least 10cm with the conductive shield bonded to the inside of the oven. I don't have access to most of the journals. Somewhere in that pile of information must be a discussion of how to engineer such a shield.

One very interesting thread in the literature is whether non-heat-related effects occur. The consensus I saw was that careful evaluation of the products showed hot spots or areas in all cases. Still, one wonders about the high speed nitrations described some months ago on this web site - there doesn't seem to be any place where hot spots would occur, especially not enough to do bulk reactions.

watson.fawkes - 19-4-2010 at 13:00

Quote: Originally posted by densest  
I don't think any of them would survive immersion in a full power field.
[...]
One very interesting thread in the literature is whether non-heat-related effects occur.
That full power field, it's a bitch. And you really want a testing device that can withstand it. Suppose, by way of an example use, that you have an exit pipe in the wall of the oven; it's a common example. Assume that the cross-section has dimensions of less than λ/4 so that you're down below the waveguide cutoff. (Note to reader: if this makes no sense to you, don't modify your microwave; you don't know enough. Start by orienting yourself with the Wikipedia article on electromagnetic waveguides and then start studying.) You could just check the output leakage, but what you really want to do is to sample the attenuation in your pipe by sticking your probe down the tube at various depths and ensure it's attenuating correctly. You're in a field with a strength far above leakage, and it's good to have a device that can not be destroyed in the process. (It's OK if it saturates and doesn't give you a reading, but you don't want it to fail permanently with this kind of test.) If this seems unnatural, note that it's generic about any significant leakage that comes from a design or fabrication flaw. It's exactly when you have such a flaw that you want your test equipment to tell you.

Public Safety Announcement: Note that microwave ovens operate at a frequency designed to heat water and that your eyes are full of water. Please do not cook your eyeballs. See the repairfaq section on microwave ovens for non-standard applications.

As for the second point about non-heat-related effects, I would say that there aren't any, but that there are effects related to an out-of-equilibrium heat distribution, where some bonds are hotter than others and the chemical reaction time is faster than the heat relaxation time. This is partly semantics, since "heat" usually means equilibrium heat or steady-state heat flow. This situation is one where you have one bulk temperature and then, at least for a short time, a higher temperature for particular bonds. I'd say there are going to be some interesting pathway selection effects.

densest - 19-4-2010 at 16:39

Quote: Originally posted by watson.fawkes  
That full power field, it's a bitch. And you really want a testing device that can withstand it ... and it's good to have a device that can not be destroyed in the process.


A meter or so of RG-8 cable, a small tuned loop, a 50-ohm noninductive load with a series capacitor to allow DC continuity testing of the load, an electrically isolated thermocouple with RF choke(s) in the leads, and a temperature meter - total outlay near $0 if one has the thermocouple & meter, $30 or so for a cheap multimeter with thermocouple connections.

Ideally, some shielded twisted pair - maybe 2 pairs out of a high performance Category 6 shielded Ethernet cable in parallel, something magic from a new 10Gb copper Ethernet, DisplayPort, etc. would be better. Maybe some old FiberChannel copper twisted pair? But RG-8 coax with the 9.5AWG center conductor would work and withstand a lot of abuse.

A 1-5W incandescent bulb (auto brake light or turn signal) would be really tough to burn out and even vaguely the right impedance. One of the no-base wire lead ones would be good though it would require scraping the leads to bare metal to solder it. It is even an indicator of truly high power levels :P

Localized nonuniform energy distributions? Wouldn't a 2.5Ghz resonance be a molecular one or a group of molecules? Dunno...

watson.fawkes - 20-4-2010 at 14:37

Quote: Originally posted by densest  
A meter or so of RG-8 cable, a small tuned loop, a 50-ohm noninductive load [...]
If, by a "tuned loop", you mean one with a resonant frequency at 2.4 GHz, then that antenna won't fit down an output pipe that's supposed to be attenuating. The cross-sectional dimension of the pipe is, by design, smaller than (say) λ/4, but the resonant dimension is at λ/2. This is a bit of an over-simplification, but makes the essential point that such an antenna won't do everything you want. This device, however, would be reasonable for doing certain kinds of high-strength work, but you don't want to make it hand-held. Maybe on the end of a very long pole. A proverbial ten-foot pole.

I spoke with my friend again about this subject. He pointed out that the analogue of a proper field strength meter is a motorcycle helmet, that is, a modest buy-in cost of a fun and dangerous activity. Verbum sapienti satis est; a word to the wise is sufficient. All these low-end devices are useful, but at this point I'm of the opinion that they are secondary measurement devices, not primary ones.

A few other notes. The antenna in your meter may be polarized. Remember to rotate the antenna to check for field strengths at different polarizations.

Cracks in the conductive envelope of the oven will radiate (and the radiation will be polarized). The rule is that every possible dimension needs to be below an appropriate length limit. This is one of the reasons that waveguides are bolted together with lots of bolts around the perimeter. These bolts are acting as conductors to keep the radiation inside. For example, if you decide to make a demountable pipe with threads and teflon tape, it will leak radiation out, and quite a lot, because the pipe is now electrically isolated and acts as a passive re-radiator. So all joints should be hard-soldered (or brazed or welded), if permanent, or use lots of metal bolts at close spacing, if demountable.

The wavelength at 2.4 GHz is 12.5 cm, just a bit less than 5 inches. So it's reasonably to think about 1" ID pipe as an exhaust pipe, but it has to be long enough. Using 1/2" pipe would be better, because it attenuates faster. Fortunately, these dimensions are large enough that standard dimension glass tubes will fit inside them without any particular hassle.

Ensure that a gaseous output product is not a conductor at microwave frequencies. If it is, then it acts just like a coax cable in the output pipe and you'll need to do even more. Basically it means that the condenser has to go inside the shielded cavity, so that the output product becomes non-conductive before it leaves the conductive envelope.

The inverse square law is your friend. Even a kilowatt of power from a point source generates small fields at enough distance. So take your experimental apparatus out in a field with a long extension cord and approach it slowly, spiraling inward. Do not lean over it to test it from the top; that's not approaching it slowly from that direction. Instead, turn it on its side and do another round of field strength testing. Scan the field from your feet to your head while you're doing this.

densest - 20-4-2010 at 16:12

Agreed, again... do the engineering and then do a reality check! Finding out that corrosive gases chewed a hole in the shielding could ruin one's day, so check every time. The advice about reliable & continuous shielding is well taken.

My concept of the search loop was a small (1-2 cm) coil which is either self-resonant or resonant with a small added capacitor (a few pF) reminiscent of an old grid-dip meter. Yes it will be directional - one would have to rotate it or have several probes with different orientations. It will also not register in a standing wave null. RG-8 is quite stiff and could be used as a pole. It's about 1.1 cm thick so it would fit through a useful sized pipe.

Readings from anything amateur-built are of course a bit suspect :( but any reading beyond a very small one should be alarming! A simple probe which could detect (say) 10-100 mw/cm2 to check for gross problems (on a pole!) and a more sensitive meter to poke around would be a lot easier to make than a real lab instrument with wide input range.

The literature I've seen so far says that thermocouples give the widest dynamic range and reliability for the fields we would be interested in. I've seen a couple of "real" meters using thermocouple probes.

It would be instructive to take apart a few commercial field strength meters.