Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 02/07/2008 00:58:13
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Evening/Morning/Afternoon
See Mrs Johnson of 33 Station Lane, Somwhereshire ?
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She's a fine looking filly isn't she ?..see her there preparing a hot meal for her husband who has been working all day. Mrs Johnson knows her place....and is happy, thankful and grateful for the modern convenience that she has utilised to offer her husband a nutritious meal. "Well Done Mrs Johnson"....and like all good wives you'll be expected to ' sort Mr Johnson out ' in his trouser department later won't you ?
But hark !!....I hear that although you can enjoy the marvelous features of this modern microwave oven...that in years to come it will lose some of it's power !!.....What are you going to do Mrs Johnson ?
I know....I'll ask the good people here while you put the kettle on and go and knit something !
So.....is this true ?...do microwave ovens lose their power/efficiency over then years ?....to what degree ?...when should Mrs Johnson replace her microwave oven ?...and why do they lose power ?
Please keep your answers in nice digestible portions....Mrs Johnson is a girly and knows her limits !
Thank You
Neil
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Haha Neil. You make me laugh.
I'd have to say that microwaves don't really lose all their power, but the might lose some of its cooking ablity. My grandpa has had a microwave for years and years and I think his works just fine, but he doesn't use it a million times a day like we use ours.
Our microwave completely breaks down and beyond all hope of fixing it every 2 years. Ha.
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A magnetron will lose output over time but I would think it would have to run for thousands of hours before it becomes noticeable but it is possible that the frequent on-off use may shorten its life considerable.
'Valves' (vacuum tubes), which includes klystrons, magnetrons. CRTs, Travelling Wave Tubes (used a lot in satellites) have finite lives mainly because the cathodes lose emission.
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Haha Neil. You make me laugh.
I'd have to say that microwaves don't really lose all their power, but the might lose some of its cooking ablity. My grandpa has had a microwave for years and years and I think his works just fine, but he doesn't use it a million times a day like we use ours.
Our microwave completely breaks down and beyond all hope of fixing it every 2 years. Ha.
LOL....glad to make ewe smile chum !!.....Gosh !!..ewe guys get serious mileage out of your microwave !!...which is nice !!
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A magnetron will lose output over time but I would think it would have to run for thousands of hours before it becomes noticeable but it is possible that the frequent on-off use may shorten its life considerable.
'Valves' (vacuum tubes), which includes klystrons, magnetrons. CRTs, Travelling Wave Tubes (used a lot in satellites) have finite lives mainly because the cathodes lose emission.
Thank Ewe Pumblechook !
So it is true !!....even if the diminishment (new word I just invented !)...is tiny...
Thank you again
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From a website...
7.3.7 Magnetron Life Expectancy
The typical life of a magnetron tube is approximately 2000 hours of operation. Some factors that can diminish the life of a magnetron are: 1) no-load operation, 2) operating with too much metal in the cooking cavity, 3) line voltage consistently too low or too high, 4) improper phasing, 5) improper installation or removal, 6) replacing associated high-voltage components with substitutes that are incompatible with the system design, 6) continual operation at the upper limit of its heat tolerance due to inadequate air circulation, 7) obstruction in the wave-guide, 8) failed stirrer operation.
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We use ours about 15 mins a day which would mean 2000 hours would be reached in 22 years but as they say there are a lot of factors which may reduce its life. The design and parameters of the oven will make a considerable difference. E.G. A highish power magnetron always underrun will last much longer than a smaller one run at its max ratings.
A restaurant might reach 2000 hours in only a year or so and industrial machines whether for cooking food or other processes will require frequent magnetron replacement.
I haven't worked on magnetrons but used to work on high power Klystrons which are a vaguely similar devices.
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THANK YOU Again Pumblechook !
2000 hours doesn't seem like many hours but , yes, as you say......when you consider that you rarely use it for more than a few minutes a day I can see now that the longevity is there......even though there are many ways to add to the shortening of it's life
Wooo !!KLYSTRONS (http://en.wikipedia.org/wiki/Klystron) look well meaty !!
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Haven't you read the posts??
Off course there is an 'inbetween'. Magnetrons will lose output over time although this may not be noticeable in domestic ovens which get relatively little use.
I worked on similar devices over many years and was responsible for recording the running hours and history of the tubes and ordering new tubes when they could no longer deliver the output and performance required. ... High power transmitting valves, travelling wave tubes and klystrons.
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I have had my present microwave for just over 13 years and it still works fine even though I have had to move it around my home.
The power is still as strong as the first day I received it as a gift. Although I don't use my microwave every day just when I don't have enough space left on the top of the cooker's hob.
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I have just tested our 800 Watts oven using a heating test and I get about 600 Watts. We have had it quite a few years. It seems to be using about 1100 Watts from the mains. It should be 1400 W.
The power test is heating 500 mL of water for 60 secs.
Power output (roughly..not an exact test) = temperature rise x 35. I get a 17 deg C rise
35 x 17 = 595 Watts.
We hadn't noticed a fall-off.
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I wonder if the thermal cycling etc could cause the resonant frequency to drift off in time? The resonance of water absorption is fairly broad, so it would have to drift off quite a lot.
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I gather the main mechanism is dielectric heating and not conduction, even in water. Substances with no water still warm up such as PVC. Microwave ovens work at 450, 900 (not strictly morowaves) (industrial processes, food or other stuff) as well as 2450 MHz. As long as the wavelength is short enough wrt to the size of the metal chamber of the oven any frequency will work..lower the better because lower frequencies will penetrate further into the food. A magnetron does drift but not much. The frequency is governed by the size of the cavities. 2450 is use I suspect because for lower frequncies the magnetron would be bigger..doesn't matter in a factory but does in a kitchen.
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Does chewing gum lose its flavour on the bedpost overnight?
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I have had my microwave since 1994 and it is still working as well
as the first day that I had it only it's always lost the odd minute on the microwave's clock.
It cooks and it's no matter to me about the clock. I'll use it until it comletely "dies" on me. (stops working)
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Do microwave ovens use more power when they get older? I can only see this as possible if something is monitoring the output, and increases the power to the magnetron as it ages and (possibly) becomes less efficient.
The reason I ask is because my 11 year old microwave is suddenly tripping my 15 amp circuit breaker. Nothing else is on this circuit.
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Power output (roughly..not an exact test) = temperature rise x 35.
Cool where did u get the formula from?
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Power output (roughly..not an exact test) = temperature rise x 35.
Cool where did u get the formula from?
I'm afraid that is a vast simplification. Firstly, is it degrees C or F? Then, the quantity to be heated must be relevant.
Thirdly, and most important, it's Energy not Power which produces temperature rise so you need Power times Time.
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It is times time. It is a 60 sec test.
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and how much food (mass) and C or F?
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noice neilep,, google gave me a autofill search straight to your question.
mine microwave oven used to heat a cup of water(2 minutes) to the extent that when i added instant coffee granules the water in the cup would boil, suffusing my kitchen with a mouth watering aroma!
now sadly i heat my cup of water for 5 minutes and it no longer boils when granules are added.
thus i decided to google your question as it is also now mine.
fyi mine microwave oven(800 watts) is approximately 7 years old, installed as per manufacturers instructions, but gets used regularly as i never use the conventional oven.
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mine microwave oven used to heat a cup of water(2 minutes) to the extent that when i added instant coffee granules the water in the cup would boil, suffusing my kitchen with a mouth watering aroma!
now sadly i heat my cup of water for 5 minutes and it no longer boils when granules are added.
Boiling when adding granules/powder is due to the water being superheated ... http://www.thenakedscientists.com/forum/index.php?topic=38875.msg353158#msg353158
If you want to try to replicate that you'll need a pristine cup (without scratches which are nucleation sites) and pure water which has no particles, (e.g. distilled water).
NB: superheating water is dangerous, it can boil-over instantly if the container is knocked.
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I have just tested our 800 Watts oven using a heating test and I get about 600 Watts. We have had it quite a few years. It seems to be using about 1100 Watts from the mains. It should be 1400 W.
The power test is heating 500 mL of water for 60 secs.
Power output (roughly..not an exact test) = temperature rise x 35. I get a 17 deg C rise
35 x 17 = 595 Watts.
We hadn't noticed a fall-off.
As a matter of interest how do you allow for the amount of energy used to heat the water container?
Or don't you need to?
Is it the volume of water or the mass of the whole thing being heated. Then there will be a certain heat loss due to the fan assisted ventilation taking heated air out of the system.
Takes me back to my days at work, trying to ensure that what we measured was what we wanted to measure, no bugger factors in play!
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Nope lol
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Over the years, I've had many microwaves. All of them eventually meet there end. There are two ways that happens.
- The suddenly stop working.
- The slowly drop in cooking power until they stop working.
It is really absurd to talk about magnatron life and then conclude microwaves don't change with time. Virtually all electronics, including microwaves use digital chips, capacitors, and diodes. All of these are effected by aging. Capacitor aging is the most predictable of these. As your microwave becomes further out of specs with the manufacturing tolerances, it will drop in performance. That happens with any electronics. Think about it, if they could put in a capacitor that would make it a higher power microwave, then they would have. So moving out of specs is only going to drop power.
Now, the effects of this aging is not predictable, but it is measurable. Generally one of the first things I try in a new microwave is cooking a bag of microwave popcorn. Often the new microwave is so powerful, that if you use the popcorn button and allow it to cook to the end you'll have burnt popcorn. But after a year or so, you'll notice the popcorn button works about right. Eventually, if your microwave doesn't suddenly fail, you'll notice you have to add extra time when cooking popcorn to get all the kernels popped.
Of course this is only a valid comparison if you stick to the same brand... You'll also notice it over time with how long you have to heat water for tea, and such. However, this is much more subjective and dependent on the cup being used, how full it is, where you placed it in the microwave, etc.
I have not notice the power loss in every microwave I've owned. But then I used to not be as observant, so it could be all microwaves lose power over time. Or it could be that only some lose power over time, and others drift beyond manufacture tolerance in other ways as they fail.
BTW. My most recent microwave that reduced power over time, I actually paid enough attention to understand why. You see a magnetron is designed to put out a constant amount of power, so to first order you would not expect a power drop over the life of the microwave. But I noticed when running the microwave on high power I would hear it do the same sound changes a microwave normally does on a lower power setting. The sound change is actually the microwave turning on and off the magnetron for short periods. So what was happening is as the microwave got older, it would run continuously for shorter and shorter periods. Perhaps a capacitor was arching, causing it to turn off the magnetron, or perhaps the heat sensors would detect to much heat being produced by the electronics and wait for the fan to cool things off. I don't really know. But the net effect is even though the magnetron could still output the same amount of power, it could not do it for long, so by the time I threw it away it could not heat my popcorn rapidly enough for it to remain hot and pop my popcorn at all.
The microwave I had after had inverter technology and spontaneously burst into flames after 9 months. Man, they were surprised to see the insides of that microwave when I exchanged it at Costco... Normally you would only expect that to happen by putting metal in the microwave, but I confirmed my wife used a microwave safe bowl, so who knows what happened. I'm just glad she hadn't left the microwave unattended... I exchanged it for the exact same model, because there have been no recalls, so I'm convinced it was some very strange fluke.
Bill
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I have had the opportunity to investigate the failure of commercial tubes for a very long time, and concluded that the Barium coated cathodes loose their power over time. In broad cast transmitter this mend a replacement after app. every 2000 hours. While catastrophic failure in microwave ovens is most often due to the high in-rush current, which will destroy the cathode. Modern microwave ovens, those build for example by Mitsubishi contain inverter circuits which contain a soft start-up heater circuit which should allow for longer service intervals.
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For some reason the inside surface of microwave is flaking off.
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No problem! The television (more food programs than the microwave) carries hundreds of advertisments for stuff that ladies put on their faces to stop their skin flaking off, or possibly to remove the stuff....anyway, one of them is bound to contain a really expensive molecule that defeats the seven signs of microwave ageing and brings admiring princes to your feet, or something.
Seriously, though, I've noticed localised paint blistering inside a domestic microwave, probably due to frequent exposure to steam, producing rust under minor scratches. A quick rub down with emery paper and a spray of radiator paint should cure it. Speaking of my mother-in-law....
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I have always understood that the loss of performance of vacuum tubes over time was due to the build up of resistance between the cathode and the emitting surface the special tubes used in the TAT cable used Platinum cathodes to overcome this effect and ran for 25 years without failure.
The Magnetrons in microwave ovens always run at full power variable cooking effect is achieved by switching them on for variable times.