Dear all,
I appreciate and understand Crispin's comments. First a question, and
then a reply.
Question: Does this make any difference? I think it does. I have
conducted enough WBT with the waiting for the final degree (local
boiling point) to be reached to know that there there can be even
minutes of waiting. And it is especially long if the fire is
low/small/weak.
Reply: Why not define the "End-of-test boiling point" to be either 1 or
2 degrees C less than the local boiling point? So when the test pot
water temperature reaches that number and remains steady (or increases)
for 10 seconds, the test is over. This would mean imprecision of
only a few joules instead of variations that could be 100 or more times
greater. Latent heat of 2257 Joules per gram is a significant amount
when compared with the energy to raise one degree when well below the
boiling point.
As Crispin points out from Prof. Lloyd's work, even the position of the
temperature probe makes a difference.
Paul S. Anderson, PhD aka "Dr TLUD"i
Email: [email protected] Skype: paultlud Phone: +1-309-452-7072
Website: www.drtlud.com
On 1/6/2013 6:45 PM, Crispin Pemberton-Pigott wrote:
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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