This post is about temperature measurement in Ultralight tent stoves.
Introduction
I consider my tiny tent stoves to be ‘heat radiators’ rather than tent air heaters, although they do both. Consequently, estimating the correct temperature of the stove is critical understanding the radiant heat optimisation. It is the subject of this post. My companion posts consider stove surface emissivity to improve radiant heat output and also on its complex effects on temperature measurement. Another post is about the effect of stove temperature on radiant heat output.
Most of my tent stoves burn 400g or less of damp wood sticks per hour. Consequently, I am not concerned about the efficiency of fuel use per se.
Importance of stove surface temperature
Radiant heat increases as Temperature4. This comes from the Stefan–Boltzmann law. It describes the power radiated from a black body in terms of its temperature in degrees K.
This means that higher stove temperatures will cause enormous increases in the amount of radiant heat that will be provided to campers by their tent stove.
This means that radiation increases according to the absolute temperature T4 (degrees K multiplied by itself four times). If I have the maths correct, this means that compared with a stove at 300C the same stove at 400C would radiate twice as much. It would be three times as much at 500C and 5.3 times at 600C. “These are big and important differences that are largely invisible to us if we cannot reliably measure stove temperature.”
To manage and optimise this important heating factor, I need to have crudely accurate temperature measurements that I currently do not have. “As the saying goes, if you can’t measure it, you can’t manage it.”
What is the true temperature of my stoves? Or does it matter?
The short answer is that it probably does not matter so long as it is ‘bloody hot’ and does your cooking and keeps you warm. All my stoves fit this description.
However, it would be nice to have some more objective measure of temperatures for safety and objectively understanding a stove heating power to optimize in-tent comfort with the absolute minimum of weight, volume and complexity for ultralight winter backpacking trekkers.
A crude but effective ~100C ‘digital’ temperature measurement
For years I have used a simple temperature ‘measurement’ to do a safety check on my flue pipe temperature where it passes close to the nylon tent canopy where its exits. I use a ‘dab of saliva’ on my fingertip. I repeatedly test down the pipe until I find the zone on the flue pipe that spits back. I assume that this is at ~100C if it can make steam.
An ode to a primitive digital thermometer that you can really trust:
Digital thermometers on both hands did sit,
Alas, for only two temperatures, were they fit,
For 100 C to display or absolutely 373 K,
Both reached when steam from fingertip did spit.
Caution: On a more serious note, this test is only for ultra-thin SS or titanium flue pipes, nothing thicker. The unique physics of this situation is critical to risk management. When the steam point is cautiously found by moving down the pipe, a slight ‘zzzt’ sound can be heard and ‘bubbly feeling’ can be felt. However, it does not hurt or feel particularly hot.
I think this is because the thin metal foil is a very bad heat conductor and has negligible thermal mass. This means that only a tiny amount of spit turns to steam that you can hear and feel. It simply collapses in the remaining spit and does no harm. “It has a kick with no heft!”
Colour/temperature estimation
Using eyeballing to estimate temperature has been a time-honoured method used by Blacksmiths. An example of this colour/temperature relationship in the chart below from is from Reddit Inc
We don’t need to simply trust in tradition as Mick Wests informative YouTube video analysis validates that the colour of a very hot object, that is perceived by the human eye gives a reasonably accurate estimation of temperatures.
Limitations to colour/temperature estimation
Colour is a rather subjective sense and no person can know what another person is seeing.
I am a good example, as I am partially red/green colour-blind (as many blokes are). My colour discernment is not so good on pigmented red surfaces of mixed colours. However, I do quite clearly see purely transmitted light in the visible red wavelength range (e.g. LED traffic light arrays are just wonderful), particularly when observed in relative darkness where the background or surface colour is largely invisible to me and probably anyone else.
All this means that if anything, I should perceive less red colour than other normal people and this may cause me to underestimate temperature a little.
“This may diminish my stove boasting power, but it should still provide a robust estimate of temperature and as I discuss later, it will be much more accurate than cheap infrared thermometers.”
Recording, sharing and comparing colours by photograpy?
Theoretically, I could photograph my hot stove and ask a kind and non-colour-blind person to calibrate the colour/temperature. However, such a grand plan may be very unreliable unless I have the appropriate camera with the right features and the skill to make the appropriate settings. This is demonstrated in Mick Wests informative YouTube video analysis when he used an iPhone as well as a sophisticated Canon camera.
I make similar observations about colour distortion when using my Samsung Galaxy S4 Android phone camera. However, in contrast to Mick’s, my still photos appear to ‘inflate’ the red colouration (As Mick’s did) of my hot stoves, when compared with what I eyeball. The red area is more extensive and at least one colour band (100C) higher. On the other hand, the video from the same camera deflates the redness (the opposite to Mick’s observation).”All this means that a mobile phone camera just will not cut-it.”
This little Instagram video of experimental pine tree branches burning demonstrates the vast difference between the still photo (in the introductory photo) and video recording. My recollection of the colour is somewhere in between the two.
Temperature measurement by infrared thermometer
Infrared thermometers make an attractive cheap tool for stove makers to estimate surface temperature. I am certain that my infrared thermometers grossly underestimate stove temperature. They for example I ‘measure’ 300C when a surface is visibly red to a colour blind person (~700C by colour estimation as discussed above).
This temperature estimation is at its worst with a shiny new stove. The low infrared surface emissivity of such a shiny stainless steel surface is likely to cause to this underestimation. This is because the cheap IR thermometer cannot compensate for low emissivity.
As an example, “in the ‘good-old-days” when we enjoyed a leisurely pot of real tea, we used to have shiny stainless steel teapots to retain the heat in the tea so that your subsequent cuppa was still hot.
The natural loss of lustre of my stoves, with ravages of hours of the burning, improves their emissivity a little. However, it is not nearly enough to make IR temperature measurements correct.
High emissivity stove coatings
For some years, I coated my tiny tent stoves with my ugly DIY refractory render (or paint). When applied to the stove surface they should greatly increase the stove emissivity. It may not make it into a perfect black body. “And it will still be ugly!”
I discontinued this refractory coating for multiple reasons (or excuses). It tedious to do, easy to get wrong, makes the stoves look ugly, and makes them glow a much duller red when ‘showing-off’ my glowing stoves to visitors at campsites.
“Yes, I can hear you saying; that’s just laziness and vanity.” That could be true! If vanity, it would be shameful if it caused me to miss one of the most powerful and lightest stove improvement that I could ever have made!
With the benefit of my current analysis and hindsight, I can now see that continued use of the coating would make temperature sensing with IR thermometers much better.
More importantly, the render should greatly improve the amount of radiant heat emanating from the tiny stove in a tiny tent.
“That’s a story for another post (add link when ready). Making the render darker with better mineral additives to approximate a black body might be another challenge and another post (Add link when ready.”
Reassessment of my stove temperatures
All my dome stoves have small to extensive patches of quite visible red surface colour showing to my deficient eyeballs. This red spreads over most of the fire dome in my hottest stoves in their peak operational mode. “If the stoves did not go red, I would have been very disappointed and would have consigned them to the shelf of ‘failures & lessons & challenges’.”
This red colour, which I perceive, according to the above colour/temperature charts is ‘redder’ than ‘faint-red or dark-red’ colours and closer to ‘cherry red’. It approaches ‘orange-red’ in my highest performing dome stoves with their aggressive inverted J-burners. On this basis, it means that even the least of my dome stoves must in part be reaching 500C. The superstars are probably considerably higher.
An ode to non-colour-blind folk:
What is this vague sense that many call red,
The range of colours is beautiful, it’s said,
Imaginations figments or real colour pigments,
Wavelengths and frequencies are just sensed in the head.
conclusion
I will rely on the crude estimation of stove temperature by eyeballing combined with the use of colour/temperature charts.
This analysis also means that I will need to revise substantially upwards my past reports of stove temperatures that were based on my IR thermometer. However, this does not change the comfort that they provide. but adding a more emissive coating will.
Oddly, if a very effective emissive coating (e=~1) were applied to my stoves, it might just bring the stove surface down to a temperature that can be measured within the top range (550C) of many cheap IR thermometers such as mine.
Tim
Other related posts:
