# Measuring temperature in a microwave oven

1. Sep 29, 2009

### redargon

I would like to put a cup of water in microwave and measure the temperature of the water as it rises with time in the microwave. Obviously electronic equipment will be fried in a microwave, but would it be possible to put a thermocouple in the water with its lead coming out of the microwave (with the door closed on it) and leading to the measuring device?

I read that it could be possible if the thermocouple is earthed to the inside wall of the microwave, is this true?

Any ideas for doing it a different way (within reasonable cost, ie. no thermal imaging devices. I already have a K type thermocouple and a handheld reader)?

2. Sep 29, 2009

### platipo

a thermocouple I guess will be metallic... that doesn't sound that smart, though I know little about microwave ovens.
btw, this led me to asking myself: would any measure you could get be of any physical significance? we usually call it thermodynamics, but the meaning of a measure outside equilibrium isn't that obvious.

3. Sep 29, 2009

### redargon

huh?

4. Sep 29, 2009

### vk6kro

You could probably have a metal tube attached to the wall of the oven, but eventually it has to leave the wall to get into the water and that's when you get sparking.

A safe way might be an air thermometer. You have a glass container with air in it and a fine glass tube leading through a small hole to the outside. This then pushes a manometer or other pressure gauge. Nothing to arc or melt.

You could get close by having the water in a polystyrene cup with a lid on it. Then stop the microwave every 15 seconds and poke a thermometer probe into a small hole in the lid and measure the temperature. Assume negligible heat loss or measure it and allow for it.

Then graph the temperature rise.

5. Sep 29, 2009

### redargon

now we're getting somewhere. nice ideas, thanks.

6. Sep 29, 2009

### vk6kro

The air thermometer probably could have a drop of Mercury in a glass tube as an indicator, so that it can move backwards and forwards without changing the pressure inside the bulb in the cup of water in the microwave.

These air thermometers give quite a large change in volume with temperature, especially over the range of room temperature to boiling water, so you might need up to a metre of tube for the Mercury to move in.

At least it can be calibrated outside of the microwave with hot water and a normal thermometer.

As an alternative to glass tubing, it might be worth trying some medical grade silicone tubing for its ability to cope with microwave radiation. It is expensive, though.