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<div class="moz-cite-prefix">Dear all,<br>
<br>
I appreciate and understand Crispin's comments. First a
question, and then a reply.<br>
<br>
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.<br>
<br>
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.<br>
<br>
As Crispin points out from Prof. Lloyd's work, even the position
of the temperature probe makes a difference. <br>
<pre class="moz-signature" cols="72">Paul S. Anderson, PhD aka "Dr TLUD"i
Email: <a class="moz-txt-link-abbreviated" href="mailto:psanders@ilstu.edu">psanders@ilstu.edu</a> Skype: paultlud Phone: +1-309-452-7072
Website: <a class="moz-txt-link-abbreviated" href="http://www.drtlud.com">www.drtlud.com</a></pre>
On 1/6/2013 6:45 PM, Crispin Pemberton-Pigott wrote:<br>
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<p class="MsoNormal"><span style="color:#1F497D">Dear Tom<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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’.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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 <i>at</i> the boiling point.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">When it comes
to temperature, measure with care!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Regards<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Crispin<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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