Ask Hackaday: The Many Uses Of Microwaves

When most think of a microwave, they think of that little magic box that you can heat food in really fast. An entire industry of frozen foods has sprung up from the invention of the household microwave oven, and it would be difficult to find a household without one. You might be surprised that microwave ovens, or reactors to be more accurate, can also be found in chemistry labs and industrial complexes throughout the world. They are used in organic synthesis – many equipped with devices to monitor the pressure and temperature while heating. Most people probably don’t know that most food production facilities use microwave-based moisture solids analyzers. And there’s even an industry that uses microwaves with acids to dissolve or digest samples quickly. In this article, we’re going to look beyond the typical magnetron / HV power supply / electronics and instead focus on some other peculiarities of microwave reactors than you might not know.

Single vs Multimode

The typical microwave oven in the millions of households across the world is known as multimode type. In these, the microwaves will take on typical wavelike behavior like we learned about in physics 101. They will develop constructive and destructive interference patterns, causing ‘hot spots’ in the cavity. A reader tipped us off to this example, where [Lenore] uses a popular Indian snack food to observe radiation distribution in a multimode microwave cavity. Because of this, you need some type of turntable to move the food around the cavity to help even out the cooking. You can avoid the use of a turn table with what is known as a mode stirrer. This is basically a metal ‘fan’ that helps to spread the microwaves throughout the cavity. They can often be found in industrial microwaves. Next time you’re in the 7-11, take a look in the top of the cavity, and you will likely see one.

Multimode microwaves also require an isolator to protect the magnetron from reflected energy. These work like a diode, and do not let any microwaves bounce back and hit the magnetron. It absorbs the reflected energy and turns it into heat. It’s important to note that all microwave energy must be absorbed in a multimode cavity. What is not absorbed by the food will be absorbed by the isolator. Eventually, all isolators will fail from the heat stress. Think about that next time you’re nuking a small amount of food with a thousand watts!

Single Mode microwaves are what you will find in chemistry and research labs. In these, the cavity is tuned to the frequency of the magnetron – 2.45GHz. This allows for a uniform microwave field. There is no interference, and therefore no hot or cold spots. The microwave field is completely homogenous. Because of this, there is no reflected energy, and no need for an isolator. These traits allow single mode microwaves to be much smaller than multimode, and usually of a much lower power as there is a 100% transfer of energy into the sample.  While most multimode microwaves are 1000+ watts, the typical single mode will be around 300 watts.

single vs multimode cavity

Power Measurement

Most microwave ovens only produce one power level. Power is measured and delivered by the amount of time the magnetron stays on. So if you were running something at 50% power for 1 minute, the magnetron would be on for a total of 30 seconds. You can measure the output power of any microwave by heating 1 liter of water at 100% power for 2 minutes. Multiply the difference in temperature by 35, and that is your power in watts.

There are other types of microwaves that control power by adjusting the current through the magnetron. This type of control is often utilized by moisture solids analyzers, where are more precise control is needed to keep samples from burning.

Have you used a microwave and an arduino for something other than cooking food? Let us know in the comments!

Thanks to [konnigito] for the tip!

73 thoughts on “Ask Hackaday: The Many Uses Of Microwaves

    1. You can also damage your gene depot by slamming the door shut before drying/nuking process of said parts is completed. You might want to look up “grape plasma” & “microwave” before you stick anything in there that contains a relvant percentage of H2O and that has a dipole topology … the effects would certainly be impressive – but also very painful. …

      1. The most shocking that I’ve witnessed is hotdogs. Cut them in quarters long ways, then into about 1cm pieces and drop them in a bowl. They will spark like mad, and “weld” themselves together. Unfortunately, it doesn’t get a good char flavor for adding to mac&cheese.

      1. I have never seen an isolator in a consumer-grade microwave oven, and I have seen the inside of quite a few.

        Mostly, I use my microwave to dry out silica gel.
        They make food taste funny and probably ruin it.

  1. I’ve also disassembled many microwaves (i.e. several dozen) and have seen a few kinds of magnetrons and stirring techniques including the metal “fan” method. The older ones have this, and the newer ones just use a turntable, as a general rule.

    I’ve never seen an isolator, but maybe I did, but just didn’t know what it was. All the ovens I took apart used a simple steel box channel wave guide with nothing in it. I’ve never seen anything that I could identify as being failed from heat stress that resembled the aforementioned isolator. Would like to know more on that.

    I have not yet seen the newer inverter-type ovens, still waiting for one to come my way. I hear that they lack the heavy transformer.

    1. Inverter microwaves work much like switching power supplies – they invert the incoming power to a much higher frequency, which lets you use a transformer a fraction of the size of the original. So they aren’t as great for harvesting parts if you are looking for a nice big transformer, but they will have a good amount of electronics to salvage.

      1. It could be that the inverter may be able to be repurposed just as well as the original transformer — now we can build inverter-welders from inverter-microwaves. Just do the same trick, reconfigure the high-voltage output coil with a few turns of high amperage wire. Inverters still use inductors in most cases. I can’t wait to get my hands on one.

      1. “What is not absorbed by the food will be absorbed by the isolator. Eventually, all isolators will fail from the heat stress”

        I guess if you’re a radio engineer or a user of industrial microwave devices, you might cook food on your antennas as a means of preventing the isolator to fail from the heat stress. Or not. It’s pretty obvious that the writer was referring to food-cooking microwave devices.

  2. “You can measure the output power of any microwave by heating 1 liter of water at 100% power for 2 minutes. Multiply the difference in temperature by 35, and that is your power in watts.”

    Are we talking about Kelvin or Fahrenheit .. ?

    1. I was wondering exactly the same question. When no units are given on a US website, when unspecified I’m never quite sure which unite they would use. But in science based websites I always assume kelvin.

    2. I don’t get how this could be Fahrenheit? The specific heat of water is 4.19 KJ / Kg.
      Watts is J/s. So Delta T would be 4180J / 120s = 34.91666… well I just answerd my own question. It *is* Celsius.

    1. Couldn’t be. That little thing would get glowing hot in short order if it were to absorb any significant portion of the output. I have noticed it getting hot, but not that hot, and cleaning the grease off it reduced the temperature. I think it’s just a cover. Googling microwave isolators, I found no mica, just some large ferrite devices; so perhaps these are only used in industrial microwaves, not in consumer.

    1. As usual, there are videos on YT. People heat a small spot on the side of a beer bottle using a torch, then quickly start the microwave. The spot melts and spreads until the entire bottle is melted.

  3. The “isolator” is complete BS. There’s no mention of them in “Practical Microwave Oven Repair” by Davidson. Just to be sure I took the magnetron out of an oven I have. It connects through a waveguide to the cooking chamber. If something between the magnetron and the chamber were effective at absorbing microwaves it would prevent the oven from heating food by absorbing the energy rather than letting it into the cooking chamber.

    I nominate this article for Fail of the Week and the author for unemployment. Don’t write on HaD about things you don’t know about.

    FWIW Mica will not fail from heat in any meaningful time frame. It’s used for furnace windows and similar applications. It would also not have a significant effect on RF energy other than dielectric losses which are small. In my oven they use a piece of plastic for the screen. It *is* important to keep food and moisture from accumulating on the magnetron tube to prevent arcing.

    1. **sigh**

      isolators are needed to stop reflected power from getting back to the magnetron in a multimode system. They direct the reflected energy (that is not absorbed by the food or whatever) into a dummy load. I would imagine the “made in china” microwaves you find at walmart and the goodwill do not have them. But any quality microwave* should have an isolator.

      * our definition of quality microwave might differ.

      http://www.microdenshi.co.jp/en/images/d/dummy.gif

      1. Picture from Micro Denshi Co., Ltd., a manufacturer of industrial microwave ovens. So your definition of quality strictly equals industrial. Where the cost of replacing a high-power magnetron manufactured in low volume, would warrant inclusion of a cheaper (but not cheap) sacrificial protection device.

        It was the association with consumer multimode ovens that was confusing. I haven’t seen an isolator in any of those. Not in my first, an early 80’s USA-made model, which lasted 25 years; technically it was still working when retired, but hearing the maggie sputter and arc made me wonder if it would die completely at an inopportune moment. Nor can I find evidence of an isolator in the original consumer model Amana Radarrange, never owned one of those, but they were built like a tank.

      2. That is a “circulator” as we would call it, with a water cooled load on its output (the arrows are showing water flow). You would use it or a directional coupler in reverse to capture reflected power and dump it as heat into the load. So yeah, it is an isolator. But I’ve never seen one in a counter top microwave oven unless the magnetron itself has something added to act as reflected power suppression. Then again, I work with microwave tubes themselves for a living, so ovens would be out of my expertise.

    2. An isolator would only absorb microwaves that are back-reflected and have a different polarization than the source. They are not used on multi-mode home microwave ovens.

    3. Isolators (which are actually circulators with built-in loads) are used mostly in single mode systems, despite what the article stated. Industrial multi-mode systems usually don’t use isolators. They essentially dump all the microwave power into a big box, where hopefully it couples to whatever you’re heating. The load seen by the generator is highly variable, but the multimode nature usually means that the power reflected back is also quite variable and minimal if there is a load in the applicator to absorb the power.

      Single mode systems are lot more sensitive to power being reflected back toward the magnetron, so an isolator (or circulator/load) are usually mandatory. We also use tuners in the system to minimize the power reflected back to the magnetron.

      1. This is odd. All of the single mode systems I’ve worked on have no isolator. All of the multimode systems I’ve worked on have isolators. I’ve replaced dozens of them. When they fail, you will have very low microwave power and the isolator will overheat and open the thermal sensor.

          1. **moisture solids analyzers, digesters, synthesizers and furnaces mainly

            **varies from new to 35 years old

            **almost every state in the country now.

        1. The systems you work on actually use klystrons that produce a single mode output with a single (known) polarization. That is why you can use an isolator in those systems. The waveguides used may be multi-mode, and therefore the energy delivered to the target is multi-mode.

          Cavity magnetrons used in home microwave ovens produce a different polarization and phase every time they start up, and therefore won’t work with an isolator.

  4. You say 2.45 GHz for the operating frequency of the microwave like it’s a set-in-stone fact, but there are plenty of microwaves that operate in the 900 MHz band as well.

    A common misconception is that ~2.4 GHz is used because it’s similar to the resonant frequency of the dipole of water, but this is just a coincidence. 2.4 GHz is used because it is an unlicensed band, just like 900 MHz is an unlicensed band. The longer wavelength of 900 MHz microwaves creates larger dead zones and is therefore more annoying in a home setting, but 900 MHz microwaves are used in commercial food settings all the time.

    1. There’s the slight issue that 900MHz RF is not in the microwave part of the spectrum, but 2.45GHz is. We don’t call them UHF ovens, do we?
      Anything that uses frequencies outside of the microwave part of the spectrum cannot technically be called a microwave oven.

      I am, however, quite interested in finding a magnetron that works at 900MHz. Can you show me any makes/models of RF ovens that operate in this range? I could make one heck of a ham radio amplifier out of one…

      1. Actually, no you can’t. Magnetrons are CW(ish) devices only. They don’t produce a very clean spectrum (think several MHz wide) and can’t be operated as a amplifier except through a frequency locking process. They are also slow to come to full power (several to tens of seconds depending on power) so modulating them is not really possible either. Smaller magnetrons are more responsive but my experience is with 100 kW units.

  5. I have dissassembled many microwaves, for all sorts of purposes. Mainly to get at the transformers, but the magnets are cool and the HV caps and diodes are useful as well. Epic fail was following a set of instructions to ‘grow a tiny diamond crystal by depositing graphite plasma’. Written by an evil genius, with almost all the science correct and enough references to ‘some of the guys doing this’ and ‘people on the forums reckon they get bigger crystals when’ and ‘NOTE: this is not a money-making scheme – the crystal isn’t even big enough to see with the naked eye, and is only useful for bragging/science demo purposes’ to make me believe the 10% of rubbish :P Long story short, don’t try to make diamonds in a microwave, especially if the article was published on April 1’st! First and hopefully last time I’ve been duped by a random on the ‘net :) good times

  6. As long as y’all are thinking microwaves anyone have a relatively inexpensive way to monitor the temperature of the target once it gets over 1000c? It would be really nice to alter the temperature profile after the susceptors have done their work and the ceramic target has started absorbing for example.

    1. Maybe some kind of IR (non-contact) Thermometer? You may need to add a small window to the door that allows the thermal IR wavelengths to pass. Something like Zinc Selenide may work.

    2. Over 1000c? The target should be visibly glowing by then. Probably can just use a standard silicon photo-diode and lens if you screen out ambient light. A bit more accurate would be to setup a red, green, blue LED as photo-detectors and look at the ratios between each color to infer what temperature of black-body spectrum they see.

  7. I use an old microwave for lots of things other then cooking:

    My personal favorite destroy solid state electronic devices. Put a populated circuit broad in and see what happens :)

    USB drives with the outer case removed and the USB connector off are great. The chips pop, exposing bits of the die and everything. (most large chips explode this way as well)

    Melting glass (had to after watching it done online).
    Making plasma from various things (NOTE: The microwave will not like you for this)
    Put a lit candle in and see what happens.
    Accidentally discovered not all cheap dishes are microwave safe, some off-brand brown ceramic bowls from target don’t heat food at all, the bowl gets VERY hot and the food is totally protected from microwaves. Painful lesson :(
    A bar of soap = crazy
    Super heating water is fun

    ANTS! :) – My friends house was over run with ants going after all the soda spills after a party. Deciding to entertain myself at the expense of a few ants and my friends radio cooker, I plopped a handful inside to see what would happen. Turns out ants are pretty cool, I took the spinning plate out of the system, put in ants, just lose and running around, waited for the expected POP!. What I discovered, the ants not caught immediately inside the hot zone were more then able to feel around and congregate into the dead zones.

    Here are a few more ideas : http://amasci.com/weird/microwave/voltage1.html

  8. Once when I was touring the non-secure area of Y-12, they showed us a startup that was planning to use powerful microwaves to smelt reactive metals in a vacuum. As I recall, the power there was about 10x-40x greater than a garden-variety microwave oven. They demonstrated the test apparatus to us smelting aluminum (I think).

  9. Here is a strange thing to do with your microwave at home, it’s not supposed to damage the oven and the event is very quick and over with in under 15 to 20 seconds. Take a glass microwave safe saucer (I used a Corelle brand saucer), a heavy walled drinking glass (I used a glass normally used for beer) and an table grape. Use a very sharp knife and quarter the grape, pat the freshly cut sides of the grape with a paper towel, just get the excess juice don’t over do it or squeeze the fruit to death, place the sections of grape on the saucer and place in the microwave, cover the fruit with the glass placed upside down on the saucer. Close the oven and set it for say 30 seconds at full power, In about 10 seconds or so the glass will pulse with what appears to be bursts of plasma! the display only lasts a few seconds and once it stalls the show is over and wont work again until the glass is wiped dry and a freshly prepared grape replaces the “used” one. I have a video of this stunt somewhere and if I locate it I will upload it or email it to anyone who would like to see it. The reaction is better with some grapes or types of grapes than others, some are very pronounced and active where others seem pathetic and barely work at all. I am unsure why this works with grapes or what exactly is going on but I have tried other fruits and veggies with none of them making even the smallest flash at all.
    Seems to work best with plump green seedless grapes, dry clean dishes are important once the glass gets fogged up or the plate gets any goo on it then the effect gets sluggish or doesn’t happen at all.
    Anyone have any idea of what the true reasons and scientific explanation for the “plasma” discharges from grapes might be, and why nothing else I have tested produces the same effect?
    Let me know what kind of results you get if you give it a go!
    I also dry rechargeable type desiccants in the microwave regenerates the stuff in under an hour for a pound or two instead of 12 to 14 hours in a conventional oven. You have to work a bit and alternate batches and place on fresh paper plates to get best results and not over heat the desiccant but 15 or 20 minutes for a regeneration of partially hydrated material beats hours especially when you need it in a hurry!

    1. This is usually done by taking a grape and cutting it almost in half, leaving it connected by a tiny bit of skin, and nuke. The two halves act as antenna, converting some of the RF to electricity. This flows through the skin bridge, until it burns out like a fuse, creating a tiny flame. Particulates in the flame continue to absorb microwaves, feeding additional energy into it, and making it much more intense for a brief period until the particulates are fully consumed.

      Anything capable of producing a flame seeded with microwave-absorbing particulates will produce this effect. For example, a piece of aluminum foil cut like a bowtie with a very small neck; which usually produces a better show, as the vaporized metal is very good at absorbing microwaves.

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