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How to measure the power of...
a Microwave Oven

(Scientific experiments can be easy!)


In this scientific experiment you can find out what the power of a microwave oven is, and that's even without looking at the label on the back which tells you the serial number as well! Besides, the actual power of the microwave oven might be slightly different from that specified.

Take a large plastic measuring jug of water with a scale on it which tells you how much water it contains. (1-3 litres is about right)

Give it a stir, and measure the temperature of the water with a reasonably good thermometer.

Next, put the jug of water in the microwave oven and microwave it on full power for a precise time. (about a minute per litre is about right)

Note: Choose an amount of water and a cooking time such that the water is not going to get too hot!

After the microwave oven has stopped, wait for a few seconds, then take the jug of water out, give it a stir, and measure the temperature again.

Subtract the initial temperature from the final temperature, so you have a figure for the temperature rise (in degrees Celsius).

The volume of water needs to be in cubic centimetres (cc), so if you've got it in litres, multiply by 1000.

You now have the volume of water measured in CC, the cooking time in seconds, and the temperature rise in degrees C.

Then, to find out the power, use this formula:

power = [volume of water] x [temperature rise] x 4.1868
[cooking time]

OK, there it is. The answer (in watts) should be in the many hundreds.

Incidentally, the 4.1868 is because that's how many joules there are in a calorie. One calorie is the energy required to heat 1cc of water by 1 degree Celsius. (There are a thousand calories in a Kcal as seen on food labels). As power is measured in watts, and a watt is a joule per second, it all fits together and makes sense. More about this explained at the page of Specific Heat of Water

Here's a test example experiment to test a microwave oven, and to make sure I've got the formula the right way around!:

Volume of water 1.5 litres = 1500cc. Initial temperature 19 degrees C. Microwave on full power for 2 minutes (120 seconds). Final temperature 30 degrees C. Temperature rise = 30-19=11 degrees C.

Therefore, power = 1500 times 11 times 4.1868 divided by 120 = 576 watts.

That's about right. Admittedly it's less than the 650 watts which that particular microwave oven is supposed to be, but it's not outside reasonable experimental results. It's quite an old microwave oven and it's not been cleaned for a long while, so I'd guess the muck and grease on the inside accounts for the extra power absorption.

Good luck with testing your microwave oven!

Notes:

* The reason why a lightweight plastic container is recommended is so the container doesn't take a lot of heat away from the water. If you used a weighty pot basin it would affect the results.

* I recommend cleaning the microwave oven, but I don't necessarily go to the trouble of doing that myself!

* A mercury thermometer of the type that used to be used in hospitals is best, but you have to be careful when stirring the water so you don't break the thermometer.

* The reason for stirring the water is to even out the heat, as microwave ovens don't always heat things evenly.

* When choosing a water volume and cooking time, it's best to avoid boiling the water, melting the container, etc. The aim is not to cook the water, but to get a measurable tepid water difference. So, if anything, err on the side of having too much water being heated up, rather than not enough.

* You can use old imperial measures instead, and convert them to metric to fit into the formula. A British pint is about 568.3cc , a US pint is about 473.2cc. A degree Fahrenheit is five ninths of a degree Celsius*, so if your thermometer is in F, divide the "temperature rise" figure by 1.8

Extra notes:

The method is not just for fun or to prove you can do proper scientific experiments in your kitchen. I have seen a microwave oven shop where they tested microwave ovens by such an experiment to see if the measured power matched up to that on the specification and therefore show the machine was working properly. If the oven wasn't heating the water at its rated power, it was probably off-tune and might need the magnetron replacing.


Also see: How a microwave oven works, science, how to convert a television into an oscilloscope and the Specific Heat of Water

* A degree Fahrenheit is five ninths of a degree Celsius. Yes, it's true. (Apologies for a previous mistake on this page). Boiling point of water is 212 Fahrenheit and 100 Celsius. Freezing point of water is 32 Fahrenheit and 0 Celsius. 212-32 F = 100-0 C. So a difference of 180 F is a difference of 100 C. Or, to put it another way, 1.8 degrees F is 1 degree C.