[Stoves] Effect of ambient temperature on stove testing at lowpower

Crispin Pemberton-Pigott crispinpigott at outlook.com
Wed Jun 1 13:42:46 MDT 2016

Dear Tony


>For this simmering or turndown portion of the test is there any published standards or test methods that could be considered "best practice" for stove testing that could be used regardless of the stove test environment??


I think you would get different answers from different people. The test should make something of a prediction: If you use this stove you will get X as a performance. Everyone expects that. There is little point in reporting that a stove can do something no one does or wants it to do.


So the concept of contextuality in test arises: The stoves performs how when used? So ‘use’ has to be defined and of course it varies from place to place. 


With respect to the ambient conditions, the performance is definitely affected because the fuel moisture takes less energy to heat, the water has ‘less distance to go’ and the heat losses from the pot vary with temperature (and wind, especially).


So if you want to know the low power performance of a stove, it should be checked while ‘doing something’ that represents use. Simmering a pot of soup is a pretty common task. The only thing you could quibble about is the length of time to do it. People often use 30 or 45 minutes.


The major issue is the measurements to make and what to calculate so you get a meaningful appraisal of what the stove will do. Fuel consumption is an obvious one: how much fuel is needed to replicate a task? We had suffered for years with a ‘Equivalent dry fuel consumption’ which is not the fuel required to complete a task, but the dry fuel equivalent of the energy that happened to be released from the missing mass of fuel. This means that leftovers are given an energy value and deducted from the total because it did not disappear. 


Correctly capturing the amount of fuel needed is useful and forms the basis of a comparison of fuel consumption.


The division of the fuel mass by the amount of water in the pot (whether initial or final) is far more suspect. That is not a ‘best practise’ because there is no relationship between the amount of water kept hot and the energy needed to do it. The relationship is entirely dominated by the colour and surface roughness of the pot, its surface area, ambient temperature, wind conditions and whether or not there is a lid on it. None of these are a ‘volume of water’. Thus dividing by a volume of water is a random act and hides the true performance.


That is why Pail A commented on not trying to adjust the stove design to achieve a ‘better result’ on an invalid metric.


>I am beginning to come to the opinion that the turndown is of low importance from a heat perspective as it is largely irrelevant for cooks if they have correct combination of pots and haybox or heat retention devices.


Turndown and appreciation of it is culturally dependent. In some cultures such as Central Java, turndown is very important and in fact turn down to a particular level is important because some foods are difficult to cook. Anything with coconut milk in it has to be done at low heat. 


Equally important is the cooking power on ‘high’. This can best be rated using the heat flux, which is Joules entering the pot per sq cm. This we found to be a very valuable number. If we know what people think is ‘good’ we can test in the lab whether or not a candidate stove reaches that level of performance. The same metric can be used to measure low cooking power. 


Another important variable that is ‘ambient dependent’ is the fuel moisture level. If the place is hot and humid, the fuel will be damp even if it is dried for months. In Jakarta it is often above 40°C.  Testing in a similar condition will give a predictive result and better decisions can be made about what stove will perform better or produce better quality air in the room. 


I don’t know if anyone remembers but there are a couple of stoves that incorporate a retained heat cooker inside them, sort of. Anyway they come as a kit. 


One is the Save 80 from Christian Koch which markets a wood stove with a specially shaped pot, and a retained heat cooker (RTC) of great quality that accepts only that pot. Looks sort of deliberate.


One of our list members had an enclosure that dropped over the whole pot and stove to use what little char might remain in the stove and it worked much better than any insulated container because it has a small heat source within it. 


Ideas abound…



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