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  1. #21
    gainer's Avatar
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    Quote Originally Posted by jacobj View Post
    Gainer's hypothesis isn't entirely flawed. I see one oversight though. The water heating scenario makes the incorrect assumption that microwave radiation heats similarly to convection--that is to say evenly. In fact, radiation tends to heat very unevenly, leaving hot-spots and cool-spots in the subject medium. Water tends to distribute heat very evenly through it's volume. When water is placed inside of a microwave and exposed to radiation, the exposure does not create lasting hot spots. Hot-spots ARE created, but they rapidly dissipate, especially on removal of the radiation.
    The disipation of hot spots is due to convection as well as the rotation of the turntable. Glycol, or for that matter any liquid, is subject to those forces. Some microwave ovens have temperature probes that allow heating to a set temperature. You also seem not to have studied about the difference between flashpoint, boiling point, autoignition and other characteristic temperatures.

    One does not live my 80 years without learning some precautions. Microwaves have timers for a reason. If you don't know how a substance will behave, try a short time, like 10 or 15 seconds and measure the temperature rise. As for using a hot plate, check the temperature of the plate. There must be a temperature gradient between the plate and the liquid you are heating, or heating will not take place.

    I doubt that anyone can prove that glycol vapor from a surface at the flashpoint (~210 F) will rise above the side of the container and fall onto a surface that is hotter than the one it came from, but if that is all that is required to ignite the glycol, then the hot plate is no better than the microwave, and may be worse since its surface is necessarily hotter than that of the microwave. A hot water bath heated by a submerged element might be the safest.
    Gadget Gainer

  2. #22

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    Quote Originally Posted by pesphoto View Post
    Peeps in the microwave are a thing to behold.



    reminds me of a guy who asks people to send him their OLD HARD peeps.
    he sets them up on the street in a little box and makes dioramas ...
    has a little hole in the box to look in, he calls it a PEEP SHOW ...
    some people are not amused ...

    but nuking-a-peep, THAT sounds like quite the peep show!

    -john

  3. #23
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    hehe..a peep show!

    They slowly expand to about 10 times their normal size and then...........

  4. #24
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    Convection in water is more rapid than PG due to viscosity. Therefore, I would believe that PG would develop hot spots more easily than water would. Also, since water has a very high specific heat, it is harder to drive it into boiling with a given amount of energy. IDK the factors for PG, but I can say that due to many physical constants associated with these two liquids, their behavior would be vastly different in all situations where heat is applied.

    PE

  5. #25
    gainer's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    Patrick;

    If the vapour comes in contact with an object at or above its flashpoint, it can ignite. So, your statement above that "the vapour must be at 700F or must come in contact with an object at that temperature" is in error as both Kirk and I have repeatedly pointed out to you in posts.


    PE
    Not by any definition of flashpoint I have ever seen, except yours. Not by any test of ignition I have ever seen, either. You have funny ideas about fluid dynamics. Heat a screwdriver to 250 or 300 F. You can do that in a deep fryer. Now hold it near the surface of a small amount of propylene glycol heated to 212 F. It will not ignite the glycol. Heck, plunge the screwdriver into the glycol. Time out while I go check the flashpoint of gasoline to see if I dare open my gastank.
    Gadget Gainer

  6. #26
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    Man, are we in deep do do. How did I ever fill my tank without blowing up the station! The flashpoint of gasoline is -40 F. If I have to wait till the temperature goes below -40 to gas up, I'll have to go to Antarctica, I suppose. Why don't you and Kirk go study up on a few things you obviously know nothing about. Partial pressure, solutions of things in air, convection currents, flashpoint, autoignition temperature, etc.
    Gadget Gainer

  7. #27
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    Besides smoking up the kitchen , my biggest concern might be damaging the microwave. Most that I've seen warn you not to run them empty, as it can damage the magnetron. It also seems possible (speculating here) that some organic compounds might be very low conductivity and could behave more like an empty oven -- my understanding is that the process is very dependent on the attributes of water molecules.

    DaveT

  8. #28
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    I really find this discussion hard to believe that it is taking place. Cooking gunpowder on the stove is also OK until something goes wrong. Fortunately it was only about a quarter pound.

    Since then I have chosen to do everything involving chemicals in the safest way possible. I would never introduce an unknown quantity. Extra short hair was enough to make me err on the side of caution for the rest of my life.

    If you want to play with flashpoint; try diethyl ether. That can give you some nasty surprises.
    Richard

    Why are there no speaker jacks on a stereo camera?

  9. #29

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    A clarification first: I misused the term "flammable liquid" when discussing propylene glycol. With a flashpoint above 200F, it is properly called a "combustible liquid".

    Quote Originally Posted by gainer View Post
    What, for instance, is the flashpoint of diesel fuel?
    Both the MSDS from Phillips and Cenex list the flashpoint of Diesel #2 as <125F. The autoignition temp is <500F.

    Quote Originally Posted by gainer View Post
    Is that the temperature required to ignite a fuel-air mixture in the cylinder by the heat of compression?
    The flashpoint of #2 Diesel is below the temp required for combustion in a Diesel cycle engine. And a Diesel engine has nothing to do with the safety concerns that are being voiced against your experiment here. Diesels operate at high compression ratios (16 to 25 or so) which cause high temperatures in the air/fuel mixture (about 700C or higher) and result in the self-ignition of the diesel fuel. We are not talking about Diesel engine here. We are talking about heating combustible liquids in a microwave oven and the potential hazards that arise from doing so.

    Quote Originally Posted by gainer View Post
    If there is no external ignition source, it will not ignite until it reaches that temperature.
    I completely agree. Can you guarantee that every person that reads your advise and trys this at home has a microwave that will not generate a spark? Or that the person using this technique is not smoking? A lot of people still smoke out there.

  10. #30

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    Quote Originally Posted by gainer View Post
    Furthermore, there IS such a thing as partial pressure of glycol vapor in the atmosphere.
    I know that very well. Here's a nice graph of the vapour pressure of progylene glycol: http://lyondell.com/lyondell/techlit/techlit/2518.pdf

    As you can see, the bottom line labeled 100% PG agrees with the MSDS listed value of 0.129 mm Hg at 77F. As we heat the propylene glycol up, by the time it reaches 100F, we are at about 0.3 mm Hg, at 150F we get 3 mm Hg, at about 185F it's 10 mm Hg, 210F, the flashpoint of propylene glycol, the vapour pressure is only about 21 mm Hg. Since you have a good understanding of vapour pressure, you most certainly know that this is not a very high vapor pressure. And note, we are no where near atmospheric pressure (about 760 mm Hg).

    Quote Originally Posted by gainer View Post
    The gradient is diferent in the closed cup test than in the open cup test. The situation in the microwave is that of the open cup tester.
    A closed microwave is a rather large closed cup tester.

    Quote Originally Posted by gainer View Post
    If there is a spark somewhere in the microwave, what conditions will allow it to ignite the surface of the container of glycol?
    The vapor goes to the spark so the spark does not have to go to the liquid.

    Quote Originally Posted by gainer View Post
    The partial pressure of glycol vapor will have to be the same as it is at the surface of the glycol at its flashpoint.
    No, here's where you are missing the point. The vapor in the microwave just needs to build up as time goes by with the hot glycol sitting in it. if you have the liquid hotter than the flashpoint, it will be evaporating at sufficient rate to fill the container with a flammable mixture given enough time. As long as there is enough vapor buildup,you risk a fire.

    Quote Originally Posted by gainer View Post
    In order for that to happen, the glycol will have to be at a much higher temperature than the flashpoint.
    No, it just needs to be at the flashpoint temp.

    Also, keep in mind that propylene glycol is a much more viscous liquid than water. It will not react to convection currents during heating as quickly as water and it will not relieve internal hotspots as easily.

    Another consideration is that microwaves are tuned to emit radiation that absorbed my the O-H bond in water. That absorbed microwave energy is turned into vibrational energy that causes friction of the water molecules against the surrounding food matter that causes it to heat up you food. While glycols do have some O-H bonds, they will not "heat" as efficiently as water does, and I believe it will require you to use longer heating times which may cause the glycol to risk superheating.

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