[Stoves] Simple change to the WBT Re: Thermometers for WBT

[Paul Anderson]

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:  psanders at ilstu.edu   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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