[Stoves] Thermometers for WBT

[Crispin Pemberton-Pigott]

Dear Tom

 

Something further on the subject, Tom, is the work by Prof Philip Lloyd in
Cape Town on water boiling tests (meaning the boiling of water, not WBT's).

 

He noticed that as the temperature of water goes up at a steady rate, you
can project the time at which the pot should boil, but the targeted time is
always missed - the temperature levels off just as the moment of boiling
arrives. 

 

I believe that the cause is the investment of energy in the water at 100
degrees without reaching the point of evaporation which is 100 C + 2257
Joules per gram.

 

It only take 244 Joules to raise water from 0 to 100 C. That means water
which is in the pot and heated to 100 and 'halfway' to becoming steam holds
quite a bit of energy that is not being measured by the thermometer.
Consider the error involved:

 

Water at 90 C has a total enthalpy of about 1520 Joules/g. Between 90 and
100 + half way to becoming steam is another 1170 Joules. But only 42 of them
show up as 'a temperature increase'.

 

Thus as the boiling point is reached, a lot of energy disappears into the
mass of the water and does not change the temperature much. This is easily
seen on a temperature:time plot.

 

For this reason if you wanted to determine, for example, the heat transfer
efficiency of a stove design working with a certain fuel, pot size and
firepower, it should be done without crossing the boiling point.  Either it
should be measured when the water is below perhaps 70 C (as per SeTAR and
Indian methods) or when it has a fully developed rolling boil (as
recommended by Piet Visser). Trying to determine the heat transfer
efficiency or a proxy of it while crossing the boiling point pretty much
guarantees a large error because it is impossible to tell how much energy
has entered the water at the boiling point.

 

While this observation has been called 'speculative physics' that fact
remains that using a thermometer/thermocouple to get stove metrics is not
quite as simple as it first appears.

 

Using a clock at the same time one can get Time to Boil, with a scale: Fuel
to Boil, ditto for different pot sizes, but not get an accurate measure of
thermal efficiency, heat transfer rate or true energy content of the pot.
One of the proofs of this error is the calculation of the apparent energy
efficiency during the heating of a pot water and the same calculation to a
boiling pot. The latter efficiency is always a lower number. If you
calculate the difference, it is the heat invested in the water at 100 C
without boiling it away.

 

When it comes to temperature, measure with care!

 

Regards

Crispin

 

 

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