A 33g alpine hot water soft bottle- a sleeping bag warmer

This is about using a hot water soft bottle to provide sustained supplementary body warmth for those brave souls who dare to enjoy backcountry skiing (or snowshoe trekking). It would use heat from a tent stove to provide the boiling water. I find that the ski experience is more complete when the day finishes with the challenging pleasure of camping in a winter wonderland with only the aid of what you carry in your backpack and what nature abundantly provides.

Note: Successful hot water fill cycles 27 and counting.

A little thermal test

Before we get down to the nitty-gritty, let’s play a game and decide which of the following you would cuddle up to in your sleeping bag to provide the most supplementary heat on a freezing alpine night. (Leaving aesthetics out of the equation) We’ll pretend they all weigh 2kg and are heated to 100C.

Now, put them in order from the best to worst for storing heat to warm you.

Don’t worry about getting it wrong as you most certainly will if you don’t do preliminary research. I am a retired scientist and have been tinkering with this stuff for all my life and I did not have a clue.

Me

• Rock,
• Water,
• The human body (blended), and
• Dry Oakwood.

And now for the ultra-smarties, write down how much more heat the best material can hold when compared with the worst. I will give you help with multiple choices for selection and only one is approximately right when rounded to a whole number: I think we all will get a surprise. Here are your choices:

1, 2, 3, 4, 5, 6, 7, or 8 times more heat holding capacity/Kg/degree C

The classroom is out and the answers are at the bottom of the post, now back to the fun bit.

The alpine hot water soft bottle sleeping bag warmer

Background- a need for a sleeping bag warmer

I have had some failed attempts at making a little tent stove burn conveniently overnight to keep the frost out of my snow tent. My experiments with a little stove that achieved slow clean combustion of self-feeding long sticks failed to burn automatically for more than about 0.5h. I did not discover the Holy Grail of tent stoving. In desperation, the stove could provide heat all night if multiple people were prepared to take refuelling shifts.

I was inspired by the story of Shackelton’s ill-fated Antarctic trip where they survived by burning the fatty pelts of penguins (not often seals as we learned at school). They constructed the stove while their ship was being crushed by ice. “That would have put some pressure on their creativity with a sense of urgency.”

I experimented with burning drips of cooking oil in a tiny wood-burning stove by using the wood sticks as oil wicks. Here is a little video of the wooden oil wicks burning in a KISS tent stove that would normally burn sticks for fuel. The combustion of the wood sticks is greatly slowed by the protective coating of oil on the charcoal.

The next video shows the drips of oil and water moving down the fuel groove.

Next, I tried a burner with a DIY ceramic wick as shown in the photo below.

Both oil feeding methods worked (sustained fire and made gentle warming heat), but the combustion was dirty and inefficient and the Holy Grail had eluded me.

“I still am in awe of Shackelton and his men. However, I felt a bit better about my own experimental failure when I learned Shackelton and Hurley would sit up all night, chatting and feeding pairs of penguin skins into the burner every five minutes to keep the burner going. They did not find the Holy Grail either.”

Mothy The Elder

Shackleton’s Incredible Voyage

Heating the body and not the tent

I have designed my KISS tiny tent stoves to produce intense radiant heat to directly heat cooking pots and to directly warm campers’ bodies. This still heats the air in the tent, but the focus is on bodies rather than the air in the tent and both are welcome.

Experience told me that a similar approach of providing sustained supplementary warmth directly to my body, within my sleeping bag, might be a way of achieving ‘all-night-warmth’ and that it would be worth investigating.

This could be done by storing heat from the powerful tent stove while it is running and slowly releasing heat to me in my sleeping bag during the night. “Just let the tent freeze and defrost it with the stove in the morning.”

Choice of heat bank material

Without snow burying the ground, hot rocks from around a campfire can make an obvious sleeping bag warmer, providing they are not too hot. They can be wrapped in spare clothing for comfort and the slow release of heat during the night.

With no rocks available while camping on deep snow, water is the most obvious option and is somewhat safer because without a pressurised container it will not exceed 100C and somewhat less with lower alpine air pressure.

From my experience water is always more comfortable than rocks. If water has a good heat storage capacity this also will add to the benefits of using it as an overnight warmer in an alpine sleeping bag.

Going to bed with a rock or water may at first seem perverse. However, if you go to the effort of collecting water or melting snow to make liquid water, it seems pointless to let it freeze solid overnight. So a smart snow camper will keep their water in their sleeping bag. This is along with ski boot liners, gloves, avalanche beacons, batteries, gas canisters and cameras etc. So putting your water in your sleeping bag is no big deal. “It might as well be hot to keep me warm rather than be cold and steal my heat and my morning coffee will be ready sooner.”

“For the fun of it, I camped on the snow on the treeless top of Mt Hotham for a clear frosty winter night. I put my toothbrush in my metal mug of water after brushing my teeth and the brush was frozen in within a very short time before I had a chance to pack it away.”

Me

Choice of container

Vinegar bottle

Traditionally I have used 2*2L vinegar bottles for carrying water for daily use and meal preparation and rehydration at the end of the day. I insist on a pack design that has side pockets that accommodates such bottles. When empty they can be strapped on the outside. I also use a wine cask bladder in a DIY polyester bag with sewn tubes for forming carry handles with bush sticks.

The supermarket 2L vinegar bottles have excellent seals (much better than equivalent milk bottles). Their other virtues are a smooth tough roundish shape, a strong loop handle and they are made of plastic that is quite heat resistant when compared with other plastics such as PET. These bottles became my hot water bottle when I had an unlimited source of heat from an efficient wood-burning stove to heat the water.

Soft bottle

I never quite felt the need for hydration bladders or soft bottles. However, I was tempted to buy some 2L ones to see how they would perform as big soft alpine hot water bottles. They can be as flat as roadkill on a trucking route and are very compact and light (33g).

Testing a mega hot water soft bottle

Modest temperature

“I am a strange mixed beast, both conservative and radical, so I often like to cautiously test the limits where others would not go”. Consequently, I started conservatively by putting about 300ml of cold water in first and then topping up with boiling water. At first, I thought that the hot water soft bottle only comfortably fitted 1,800g or ml. I found it disconcerting pouring boiling water into something so thin and fragile looking. For my experiment, I wrapped it up in a small hand towel.

For the testing during early winter, I took the hot soft bottle to bed in a room with the heating deliberately turned off.

This was the practice of some early, crazy and wealthy Britains in their pre-season preparation for their conquest of the European Alps.

“Some even slept outdoors on their verandahs, but they had not quite yet dreamed up the idea of a tent, let alone a tent stove. It took a lady who dressed up as a man (To be allowed to climb.) while she chaperoned her sickly Nephew (Who needed Alpine air for his health). They discovered climbing and pioneered the use of a thing called a tent and it had a stove in it. Porters would bring up fuel and supplies when the weather permitted. This meant that they would just wait for perfect weather to climb to the nearby summits. Then they would come back to the safety and comfort of the high altitude tent and wait to summit other peaks when they chose. They broke the record for summits in a season. Meantime, the real men with pants were still doing ‘all day and night’ climbs. They hoped to return alive without suffering terrible frostbite. Some returned very slowly and were found a predictable number of years later in near perfect condition at the glacial melt face. The event was a source of amusement for many and boosted tourist numbers. Often issues of: ‘Did the rope break or was it cut?’ were often resolved at this time.”

My recollection of a story from: Killing Dragons: The Conquest of the Alps

Killing Dragons is a fantastic read and is on par with modern-day stories of Mt Everest expeditions. It dates back to when members of the ‘Royal Society of London for Improving Natural Knowledge’ would present serious ‘scientific’ papers on mountain dragons and what human behaviour would make them angry!

Back to the hot water soft bottle performance.

Instantly, I knew it was a very pleasant source of warmth and that it would last a long time. “Not too hot, just Goldilocks right. It would certainly be welcome in a snow camper’s sleeping bag.” The remaining question was just how long would this warmth last? I was delighted to find it was still providing warmth in the morning when it was time to get up.

I followed the same conservative routine for a couple more nights with the same success and importantly no disasters. It was time to test the soft bottle’s with the harshest heat test.

Maximum temperature

I put 1.8Kg of boiling water into the hot water soft bottle. It survived and was noticeably warmer but still pleasant as a bed warmer. This peek temperature was so high that it was not comfortable to have it in constant contact with my skin. A tiny separation made it very pleasant. Steady contact was pleasant after 2 to 3 hours.

With this hotter start, the soft bottle had noticeably more residual warmth in the morning after 8 hours. I would describe the temperature sensed by putting my finger onto the soft bottle surface as uncomfortably hot and I could not sustain contact with it for more than a few seconds. This was not a very scientific test but I have twice put the bottle into two other peoples beds (when they were having a sleep-in) and they quickly conclude that yes it was still warm and providing pleasant warming.

I later found that I could squeeze in the full 2 L of boiling water, and this has become my standard nightly test condition, as I probe to find if the poor thing has a breaking point. “So far so good (Up to 35 sleeps).”

Less water. I also did one test with only 1,000ml of boiling water in the 2L soft bottle to mimic the circumstances where water heating may be more limited than that provided with my tiny KISS tent stove. Yes, it worked well and had residual warmth in the morning, but it would not be my choice if I had the extra boiling water.

Discussion/conclusion

My experimental soft bottle has now survived 37 hot fillings and is still going strong. It will make an excellent alpine hot water bottle. To be practical, it will require a suitable lightweight insulating cover. Such a cover could also double as a food cooking cozy. Spare clothing could also be used to make a soft cover. I used my ski boot liners as part of this in the past when using a 2L vinegar bottle.

The use of spare ‘spare clothing’ has serious limitations while snow camping. “On very cold nights the term is an oxymoron! Most items are worn!” The few remaining items get jealously used to stuff my dry bag to make a pillow with nothing to spare.

Consequently, I consider that the cozy for the hot water soft bottle should be an ultralight and standalone device. It may benefit by having a variable slow heat-release surface. It could provide stronger heat for a shorter time or weaker heat for a longer time. However, you will just have to wait for post addendums for details of that device unless you beat me to it.  

Under harsh alpine conditions, I expect that the bottle will cool down more quickly than under my domestic test conditions. (Rather harsh, but not on the verandah.) I am waiting for the snow to do the real test.

My preliminary observations under field conditions, while ski camping, showed that the below-freezing nighttime temperatures caused the soft water bottle to cool down at twice the rate as in my cold bedroom. However, if required, it would be quite practical to light up the tent stove partway through the sleeping period to reheat the water.

While the already warm water is being re-heated I could make a hot drink, have a pee, punch back the accumulating snow around the tent and defrost the tent. I could even gauge the snowfall to embellish my dreaming about the great Telemark turns that await me when daylight comes, all with a fresh hot soft bottle and empty bladder, for comfort and liquid water for a breakfast cuppa.

The KISS stove has a rapid start-up and attainment of peak temperatures in about 3 minutes in my best tent stove. This would make it very suitable for this re-heating scenario if needed. I think this would almost pass the test of all-night warmth from a tent stove. Such a duty could even be shared if you have the pleasure of sharing a tent as I often do.

The heat capacity puzzle

When I investigated the relative heat capacities of rock, water and other materials I was astounded that water was king. It only came second to something like hydrogen. “Imagine taking a 3,360L hydrogen blimp to bed to get an equivalent heat store. Imagine the explosion risks. Makes a fart look pretty safe even with a tent stove burning.” I would have put my money on the rock (as most people do), but water can store nearly six times the quantity of heat than the same weight of rock at the same temperature.

In other words, the heat in a 2Kg hot water soft bottle would equal that of 11.3Kg of rocks at the same temperature! I think you could imagine that you would get a lot of comfort from that amount of hot rocks. However, it would be difficult to heat them while snow camping and would you want them in your sleeping bag?

This helps to explain the wonderfully persistent warming capacity of my 2.0kg of boiling water in a hot water soft bottle.

I don’t know how you ranked the potential body heating materials, but I got it the wrong way around. Even oak wood is better than rock. Our bodies are mainly water, so that was not a surprise that it was most similar to water.

Table of specific heats of materials and ratio of each when compared with the specific heat of sandstone.

Material	Joule/kg/K	The specific heat ratio of each material compared with sandstone (My calculation)
Hydrogen	14,300^^	19.3
Liquid water	4,182*	5.7
Human body	3,500^	4.7
Oak timber	2,400^^	3.2
Sandstone	740^^	1.0

Reference sources:

* https://en.wikipedia.org/wiki/Table_of_specific_heat_capacities ^ https://courses.lumenlearning.com/boundless-physics/chapter/specific-heat/ ^^ https://theengineeringmindset.com/specific-heat-capacity-of-materials/

The heating power of the soft hot bottle and the human body

Using a specific heat value of water (4,182 Joule/Kg/K), I calculated the heating power of 2kg of water at 100C as it cools to 50C over 8 hours is only about 5 Watts. This seemed to be quite a small value, to give such comfort when compared with the typical heating power of a resting human body of 100 Watts. Maybe a 5% improvement is all that it takes to feel more comfortable?

Perhaps the 5 Watts of the soft hot bottle is better compared to the 15 Watts low heating power setting of an electric blanket.

If you have got this far, you are probably into soft bottles and their multiple uses. A soft bottle hot shower is another such use.

Tim

Addendum 1- Bubble-pack cover

I have continued my testing and have reached 15 uses with no disasters. I also have made a dedicated cover for the soft bottle. I made this out of a discarded windscreen heat reflector that was made of bubble-pack (silver coating both sides). The material by the roll (US). Opportunity shops and $2 shops (AU). I found that my ‘blister tape’ from my first aid kit bonded strongly and made the cover easy to assemble. The bonding was excellent and the tab that holds the flap down could be carefully pulled undone without rippling the delicate bubble pack. Elastoplast adhesive tape (US) and Generic Priceline Pharmacy Tape (AU).

This bubble pack cover was noticeably warmer on the outside (Still pleasant, but not suitable for steady direct skin contact) when compared to the soft bottle with the cover made with a ‘hand towel’. The greater warmth would be appreciated while camping in the snow.

I have never heard anyone wake up after snow camping to declare that; They were too hot last night!

Mothy The Elder

As a result of the higher temperature, the device provided optimum warmth for an estimated 8-9 hours under moderate conditions. Under sub-zero conditions, two warming periods of 4 hours would also be acceptable. In either case, I consider this to be a very adequate amount of sleeping time, particularly if there is freshly fallen snow to make first ski tracks upon.

The bad news for the ‘ultralighties’ is that the cover adds another 34g, almost equal to the soft bottle weight (33g). There again, this weight could be discounted if the device was used as a cooking cosy and a sit-upon pad.

Addendum 2- Double bubble-pack cover

Tunable insulation of the cover. I made a liner for the cover by folding in two a scrap of bubble-pack (16g). The original cover was designed with a little spare space to accommodate this liner. The folded side of the liner was fed down into the cover and the soft bottle was place within the liner. The idea of the liner was to reduce the surface temperature of the cover when the soft bottle has been freshly filled with boiling water. It did initially reduce the surface temperature, as expected. It was more comfortable for direct skin contact. I think that it would have also extended the duration of very strong heating for several hours, but I was sleeping and unable to monitor the change.

Under extreme snow camping conditions, when extra warmth is so welcome, I imagine that the liner could be removed from the cover, partway through the sleep period, to provide greater heat transfer when the water temperature has dropped considerably. Similarly, the outer cover could eventually be removed to enjoy the direct heating of the soft bottle in the wee hours of the morning. This will provide very strong heat while you ponder that morning pee and a cup of hot coffee and that glowing hot tent stove.

The bubble-pack cover and liner should make a highly adjustable insulator for the hot soft bottle so that the heat release rate can be optimized for alpine campers sleep period. The 16g liner weight may be worthwhile, and if nothing else it will make the device a more comfortable substitute ‘sit-upon’.

Note: The tally of overnight tests has reached 35 without any sign of failure.

Addendum 3- A cover for softies during power outages and disasters

The hot soft bottle was inspired by totally voluntary adventuring in the winter wilds. However, our only planet is increasingly reacting to our profligate use of fossil fuels by making our weather more hostile to us humans. It IS causing major disruptions to our lifestyle and energy networks through bushfires, floods and unseasonal snow.

Having a small endless supply of heat through a tiny and efficient wood stove and using this for supplementary warmth while sleeping might be a prudent preparation for what is to come. Consequently, I have made a sewn soft cuddly towelling cover for the soft bottle. The double towelling layer performs similarly to the prototype hand towel cover and should make a non-adventurer very gruntled in a power supply emergency.

Addendum 4- A safety net for the risk-averse

From some comments that I have received, I am apparently a risk taker to do such experiments with boiling water in such a thin plastic container. That may be so, but I did my experiment under quite controlled conditions. I built up to using boiling water gradually. I also had experience with heat sealing of plastic films and knowledge of the nature of the failure of such seams. Slow leakage is the norm unless provoked by rather excessive pressure of supporting the weight of an adult person.

When I worked for a living, I had the experience of some visiting defence Boffins who came to look at some of my samples of liquid that was sealed in heat-sealed sachet from my machines. During my presentation to them, I playfully threw down some sachets and stood on them while presenting. After the presentation, we had discussions over a cup of coffee. Out of the corner of my eye, I noticed a rather large Boffin standing on a sachet. No, it would not burst. Next, he accelerated his body upwards to increase his loading on the sachet and it eventually burst. He was looking for the water on the carpet. I walked over, with a cup of coffee in hand, and smiled at his game and pointed to the water on the wall some 4m away.

Mothy The Elder

Less ignorance. I think most of the concerns are based on ignorance. Nevertheless, some people may think that the hot soft bottle idea is good, but they don’t understand the low risk of rupture that is involved. “The seals are exceedingly wide and of excellent quality. The plastic used is up to the task and the screw cap is secure and leak-free.” The doubters might like to implement a further precautionary measure of wrapping and sealing the soft bottle in a sealed plastic bag, before putting it inside the cover. “It will add to the insulation if nothing else.”

Lower temperature. Another cautious approach could be to only use 80C water. Two litres of that will still make a wonderful sleeping bag warmer. This is the temperature reached after about 2-3 hrs when using 100C water. It is still very warming and will have heat to spare when you get up to make your morning coffee.

Less water. The soft bottle could have less than its full capacity of water put in it. This may provide the cautious person with some extra confidence about the risk of rupture. However, the pressure inside will be approximately the same whether filled or half empty. In addition, the risk of rolling over onto the soft bottle and increasing the internal pressure is much greater when it is less full.

Where there is a will there is always a way, even for the ultra-cautious ‘worrywarts’.

Mothy The Elder

Barge-arse test. I am currently up to 50 overnight uses and a tiny leak has developed just as that target count was reached so, I will declare it a non-failure.

The leakage was not a cataclysmic failure that some expected just a tiny leak that was less than 1ml/h. Hardly bet-wetting stuff. Here is a photo of the water that leaked out over an hour of standing.

The ultimate test was to put my body weight on the filled soft bottle, as a simulated accidental rollover while sleeping, as shown in this video. Even with some Covid KG’s it did not burst. Maybe this will change some minds?

Unfortunately, there will be closed minds that are no longer capable of learning and will just keep repeating dogma. Most of them would probably not do backcountry skiing and snow camping because it entails an ‘unreasonable’ risk. Even our bodies rely on such complex, intricate processes. The delicate gut membrane that keeps faeces separated from our gut cavity (preventing peritonitis) is just one risky wonder. Just living our lives is an unreasonable risk! So let’s not let ignorance add to our concerns.

Mothy The Elder

Addendum 5- Alpine field testing of the hot soft bottle

During a recent ski trip, we camped out in some below-zero temperature conditions. It was so cold that all our spare water and my toothbrush froze solid overnight. As I speculated the hot soft bottle lost a lot more of its heat during the cold night when compared to my many tests in a cold winter bedroom (as described above). It ran out of useful heat after about 4 hours. On the bright side, this probably improves the soft bottles heating power from 5 to 7 watts per fill. Also, while shuffling around in the tent, I also did a real accidental sit-upon barge-arse test and can report there as no damage done to the soft bottle.

Addendum 6- A soft funnel for safe soft bottle filling

I now have a post on making a DIY soft funnel from cotton and silicone rubber. This simple device can make it safer and more efficient to fill the soft bottle with boiling water from wide camping pots. It is also is helpful when filling the bottle with water from a river.

Tim