Bushcraft thermodynamics, no that’s not an oxymoron. From now until the end of time all woodsman, bushcrafters & self reliance enthusiasts will continue to have a healthy debate on what they believe is the number one priority in outdoor survival, short term & long. Fire? Shelter? Water? The opinions & reasons are as numerous as the different types of bushcraft & survival scenarios you may face. Whether you’re in the woods intentionally (bushcraft) or unintentionally (survival) I think we can all agree that without control of your body core temperature even the biggest fire will only keep you warm for so long until you need to venture away from the warmth to get more fuel. You can have the most expensive, high tech, trendy tent from the fancy outfitter but without a proper sleeping bag in cold climates that tent is now your crypt. If you fall through the ice you will have all the water you can possibly drink, at least for the next 5 minutes until you’re a popsicle.
Maintaining your body core temperature by understanding basic thermodynamics will make your experiences more enjoyable & straight up save your life. Using everyday examples I’d like to explain the basics in thermodynamics & how they apply to everyday life & the outdoor scenario. My profession deals directly with thermodynamics & basically moving heat energy from one location to another. How you accomplish this is through conduction, convection & radiation.
Conduction – The second law of thermodynamics states that heat travels from warm to cold. By itself, it is not possible to travel the other direction. Think of the hot cup of coffee at your desk & how over the next couple hours it will be the same temperature as the room it is located in. The hot coffee is giving up its heat energy to the surrounding air in the room until the temperatures are equal. On it’s own your cup of coffee can’t take the heat from the room & warm itself. An electric stove top is another good example of how conduction is applied. Think of conduction as playing tag. It’s about touching & moving from next to next to next. How does this apply to bushcraft? The handle on the pot for your soup gets warm as the heat moves through the metal to the handle. The fire heats the metal, the metal takes the heat & puts it into your soup, the soup takes the heat & puts it into the surrounding air. Have you even seen someone boil water with a hot rock? If not then I strongly suggest you do so by visiting YouTube & watch the second law of thermodynamics in all its glory. It will also illustrate one other fact in that the greater difference in temperatures between the two objects the faster the heat transfer.
Convection – This is probably the easiest of the concepts to understand because I’m sure everyone has heard that hot air rises & cold air sinks. Cold air is heavy, more dense, the atoms are closer together & the reverse is hot air is lighter, less dense & the atoms are further apart. Convection is about transportation & heat energy is moved via a fluid & if you didn’t know, air is a fluid because it’s a means of transportation. There are two types of convection, forced & natural. Natural is everywhere & is going on at all times & my favorite example is the lava lamp. The light bulb warms the “lava” along with the water in the bottle. The lava and the water are very close to the same density. The cold lava is more dense & sinks and the heat source warms it & makes it less dense or more importantly less than the surrounding water, it expands & rises. As it rises it cools & becomes more dense & starts to sink again. Other examples are a hot air balloon & base board heaters. Forced convection is best illustrated by a furnace in a home. Your furnace fan takes the colder air in your house & forces it across the hot heat exchanger. That heat travels in the fluid (air) through your ducts & forced into your living space where the heat energy is distributed. In bushcraft, convection is understood by placing your pot of soup over the fire. Know anybody that puts the pot of soup next to the fire? Put your hand over the fire then put it next to the fire & you will experience convection first hand. As you are boiling water in your bottle the warmer water rises & the colder water sinks; imagine constant circles on a molecular level. I prefer to sleep under a tarp shelter & in colder climates your shelter is critical to keep the heat trapped inside your shelter otherwise it will rise away from you. Don’t forget that the wind is always against you & will act as forced convection & steal your heat away from you, by the fire & by your body.
Radiation – Plain & simple, the Sun. Lucky for us, thermal radiation from the Sun travels through the vacuum of space & makes contact with the Earth. In a very similar way to the Sun & Earth, when you sit next to the fire the radiant heat moves from the fire to your body. In bushcraft we have the power to manipulate the radiant heat energy, that’s right, we have the power to control the Sun & it’s called reflecting. The space blanket is designed specifically for this reason & the proper application of a space blanket will determine your comfort level. Sunlight also has the option to be manipulated in the form of starting a fire with a Fresnel lens by concentrating the energy to a single point creating heat for combustion. Dave Canterbury from the Pathfinder School has a great YouTube video on how to accomplish this & he does a great job in explaining the process & after his video you will have all the knowledge necessary to do this yourself.
Being in this field for almost ten years now I’ve obtained this knowledge from many different sources over this time. I would mainly like to credit my instructors at Rochester Technical College. Also, organizations that relate to thermodynamics such as ASHRAE, RETA & IIAR have wonderful resources & they have provided me with a great deal of knowledge.
This image is courtesy of www.spectrose.com & author Alex Robin
This is an archive of: http://aroundthecabin.com/bushcraft-thermodynamics/