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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US'>Dear Marc<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US'>Firs I congratulate you on trying to make the engineering aspects of stove evaluation more accessible to the tinkerers that are genuinely trying to do something new and useful. Accessible science makes the world a better place.<o:p></o:p></span></p><div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>I've just published an online tool for simple water boiling tests:<o:p></o:p></p></div><div><p class=MsoNormal><a href="http://smallredtile.com/stove/wbt-buddy/">http://smallredtile.com/stove/wbt-buddy/</a><o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>Is it useful?<o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>It is because many people have no idea what to calculate when doing a simple test like bringing water to a boil (which is a useful task even if boiling is arbitrarily defined. The advantage is that if a person thinks a ‘rolling boil’ is needed each time, then they have a meaningful comparison.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>></span>What would make it more useful?<span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>It would be good to have some clear explanations about the implications of saying ‘overall thermal efficiency’ because as I read it, that is not the outcome. If all remaining fuel is considered to be ‘unburned raw fuel’ then the calculated result is not the overall thermal efficiency but the work done divided by the heat probably liberated by the fire, even if the fire does not burn the fuel completely and can’t do so alter on. The difference is quite small for stoves with a fine grate, and quite large for stoves with no grate, or which can’t burn charcoal produced. As the core problem with the mis-characterisation of stoves over the past 13 years has been this issue, it is worth trying to get the user of the site to put in numbers that will deliver a predictive result in the field.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>It is very likely that anyone who knows how much fuel mass they burned will also know the mass of the pot(s) and what they are made of. As demonstrated with the discussion about finned pots, the correct answer and comparison has to consider the heat absorbed by the pot. If you put a cell for pot mass and another for material (could be from a simple choice of stainless steel, aluminum and cast iron (called 1, 2 or 3) it would allow for a more accurate answer.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>Did I make any mistakes in the model?<o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><table class=MsoNormalTable border=0 cellspacing=0 cellpadding=0 width="100%" style='width:100.0%;border-collapse:collapse'><tr><td style='padding:.75pt .75pt .75pt .75pt'><p class=MsoNormal><span style='font-size:10.0pt'><o:p> </o:p></span></p></td></tr></table><p class=MsoNormal style='margin-bottom:7.5pt'><span style='font-size:10.5pt;font-family:"Helvetica","sans-serif";color:#333333'>“Overall Thermal Efficiency (OTE) is the ratio of energy absorbed by the pot of boiling water (E</span><sub><span style='font-size:8.0pt;font-family:"Helvetica","sans-serif";color:#333333'>water</span></sub><span style='font-size:10.5pt;font-family:"Helvetica","sans-serif";color:#333333'>) to the energy released by the fuel (E</span><sub><span style='font-size:8.0pt;font-family:"Helvetica","sans-serif";color:#333333'>fuel</span></sub><span style='font-size:10.5pt;font-family:"Helvetica","sans-serif";color:#333333'>)”<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>This definition is not correct. This is the fire-to-pot efficiency. The overall thermal efficiency (the exact term adopted by the Indonesian National Standards Committee at the last meeting) is the energy gained (net) by the pot divided by the energy available from the raw fuel consumed (as defined above) considering the material and mass of the pot as well as the wood type, moisture content and the usability of the fuel remaining.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>Is it possible to make the Water Boiling Test calculation simpler and easier to follow?<span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Definitely! But more to the point, how about using a burn cycle that is, overall, more representative of actual cooking. The Hot Start is particularly a problem because no one operates a stove like that. Prof Annegarn won’t let the students perform if because he says it is plainly unsafe.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>Can we make the steps of the calculation more visible? (to help newcomers, to help identify problems with the calculation)<o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I think that is clear enough. Most people not used to formulas have a problem with representational math so you could put it into words, or have the word formula pop up if the mouse hovers over it. That is pretty easy to code.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>I find that, even with an engineering background, the WBT Excel sheet is not easy to use. The equation for finding thermal efficiency is buried in one cell and looks like this:<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>=IF(W26=0,0,(4.186*SUM((W12-$N15)*(AA8-W8),(W13-$N16)*(AA9-W9),(W14-$N17)*(AA10-W10),(W15-$N18)*(AA11-W11))+2260*W27)/(W26*$E20))<o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>This can be made <i>much</i> easier to understand if the cells are Named. It also helps trouble-shoot it so that errors like having $E22 instead of $E20 (which was there for several years giving strongly misleading results) are easily spotted.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>A major problem with the WBT (well, the versions up to 4.2.1) is that it oscillates between using energy and fuel mass. This is rooted in the old method of using fuel mass to discuss energy – very much and EPA thing. Emissions per kg of fuel and all that. The whole ting should not use mass at all once the fuel is burned.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>Also, some of the steps of the calculation (moisture content, char energy content) aren't necessary for many use-cases of the WBT. For example, if I'm trying out a new design, all I want to know if the general range of efficiency: 10-20%, 40-50%, etc. <span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>That is OK but over time it is really hard to try to get fuel that is the same – seasonal changes are significant so it is important to let us put in the moisture content – an initial and final mass of a sample from the pile. Again, it only takes a scale to get that and the sheet presumes the user has a scale.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='color:#1F497D'>></span>I think there may be some benefit to emphasizing that the WBT is an <i>estimate</i>. Wasn't it originally designed as a quick-and-dirty test for the field?<o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Yes it was. As time passed, VITA settled the fuel remaining question in 1985 by recommending that anything left be ignored (as consumed). The University of Eindhoven earlier came to the same conclusion. Importantly, Nate Johnson found in his work in Mali that if he ignored char remaining (as if burned) the CoV was cut in half. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div><div><p class=MsoNormal><span style='color:#1F497D'>></span>So, I simplified the engineering model, tried to build a user friendly interface, and laid out the step of the calculation as clearly as I could. <o:p></o:p></p></div><div><p class=MsoNormal><span style='color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>It captures a lot of it.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I think you have to provide two outputs on the efficiency side: a best guess at the heat transfer efficiency and an overall thermal efficiency that represents fuel consumption.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Regards<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Crispin<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div></div></div></body></html>