Friday, 3 August 2007

Robert Reports.......Hooray

Well, it's here.....Roberts first post. After this weekend I hope to get him sorted so that he posts directly to the blog without having to send it to me as text to post but at least it's a start.

So!! I asked for it and you've got it:

Ah, bringing science into the Tudor kitchen!

We thought we would measure stuff and see what happens.
How hot are the charcoal stoves? Well, pretty hot, we typically run them at 600C. So we thought, as we cook in something as conductive as bronze, we must have some powerful cooking equipment...
So what we did is see how long it took to boil a gallon of water. It took twenty-one minutes.
OK then; now what?
Well, the energy needed to raise the temperature of 4.45kg of water from 19.5 to 100 degrees C is 1,527 kJ. Calculated by multiplying the mass and the temperature rise by 4.18, the heat capacity of water.
Kilo Joules may not be the most familiar of units, not as famous as kilo watts, a unit of power. For example, on the bottom of an electric kettle is marked the power in kW, mine says 2.2.
As kW are defined as kJ per second, and the gallon took l260 seconds to boil, the power transmitted to the water was 1.2 kW, disappointingly rather less than the kettle.
Anyway to check, my kettle took 192 seconds to boil two pints of water, or 1.9 kW. And it took 0.12 kWh of electricity to do that, in power terms that's 2.25 kW, so I suppose my kettle is operating at 84% efficiency.
It's not really comparing like with like, is it?
So I put three pints of water, in an uncovered saucepan, on the large ring on my gas stove, it took seven minutes to boil, which comes to about 1.38 kW, again a bit more than the charcoal stove. Incidentally, the calorific value of the gas used was the equivalent of 4 kW, about 34% efficiency. The small ring was only 0.6 kW, but 40% efficient.
But, I hear you say, the water on the charcoal stove was boiled in a heavy old bronze cauldron, that must have taken a lot of heating. Well, I weighed the cauldron, it was 7.7 kg, a lot heavier than the 4.45 kg of water inside. But as bronze has a heat capacity of only 0.435, it would only take 267 kJ to raise the temperature of the cauldron to 100 degrees, or 17% of the energy the gallon of water took.
Of course we tried to increase the efficiency by using a bigger cauldron, with more surface area exposed to the heat, this time the water took twelve and a half minutes to boil, about 2kW of power.
So what? After all this, we have found that the charcoal stove can boil water faster than my gas stove, but not quite as fast as a kettle.
One thing we do know is that the heat from our stove is comparable to ones we have at home, which is useful to know when experimenting with Tudor cookery.
Basic stuff, but it's a start.


Jorge said...

What I like about the project is having someone like Rob along who can do the maths stuff.

Helen said...

Hi Jorge

What you don't know is that, as the newbie, had Robert not the expertise (or if he'd been absent!) you'd have been on a VERY swift learning curve!!

(I'm as wicked as the rest of them, aren't I?)

Have you got your head round all the results yet, by the way?