Hand and Foot Warmer

Discussion in 'The Projects Forum' started by Major Tom MIB, Feb 14, 2011.

  1. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    I work outside and in the winter and it can get chilly, but it normally stays above 10 deg F during the daylight hours. The work is physical and I perspire making my gloves and work shoes damp. My problem is after I do a lot of physical work, I operate open air powered equipment or wait 'on-duty' for my next task. This period of relative idleness is when my hands and feet can get painfully cold. I have three sets of gloves and two sets of foot wear, but I can't always anticipate when they need to be changed (wind is a big factor), so I use hand and foot warmers to keep the 'chill' away.

    Chemical heat sources (slow and fast oxidizing) are ideal for 'all-day' infrequent usage (ski weekends), but for daily usage (at work) the inexpensive electric warmers with rechargeable batteries are significantly less expensive, but commercially available low cost electrically heated gloves, socks, and insoles lack heating capacity, temperature control, and fail after minimal use. This project will attempt provide a solution for these deficiencies while maintaining similar economics.

    I've managed to 'break' three sets of electric socks, two sets of electric insoles, and two sets of electric gloves, so I'm motivated to start this project :)

    ...

    I know the 2 watt insole warmers were not enough, so I'll experiment with something in the 10 watt range. I'm thinking a single cell as it would be nice to use those rechargeable, NiMH (1.2v) 11,000 mAh, D cells I bought for the electric socks, but I'll consider what happens when alkaline (1.5v) and NiZn (1.65v) are substituted. I'll use the heater wire from one of those personal heating pads as it appears to be fairly rugged--the one I have is about 12 feet long measuring about 5-6 ohms on each of its two strands. For now I'll forgo the temperature controller design as I'm not really sure how much power I actually need to control, but my guess for now is that I'll need a very low on resistance MOSFET of say 0.01-0.001 ohms that can carry 20+ amps since I'm looking at a heating wire resistance of just above 0.1 ohms. It'll have to be turned on when the temperature is below the 'users' setting.

    Experiment 1)

    Using Snow boots with removable insulated liners, wrap liner with entire length of heater wire, attach to 1.2/12/24v power source and measure/monitor surface temperature of heater wire (with foot in boot) at ambient air temperature of 20-25 deg C and repeat test at ~0 deg C (weather permitting or stick foot w/boot in bucket of ice water--that'll skew the test, but it's not unusual to be in slush and snow at those temperatures.) This will help test the environmental extremes from working outside and going into a building.
     
    Last edited: Feb 14, 2011
  2. wayneh

    Expert

    Sep 9, 2010
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    Proceed with caution. I don't fully understand everything you've written but it looks mildly dangerous. Not so much from a safety standpoint, but likely to produce hot spots and the occasional puff of smoke.

    One thing to do BEFORE an experiment is have a prediction to judge it against. For instance, a 1 ohm piece of wire supplied 1 amp of current should produce IR^2 watts, or 1 watt. That would be a problem in a short wire. The power produced will go up with the square of the current, so you should start low and go up slowly to establish the current and wattage that you like. Be sure to take enough measurements that you can calculate everything, voltage drop, current, wattage.

    Worry about the thermostat later - that part is easy. Getting the wire and power supply matched up is the bigger challenge. 20A? What are you going to use to supply THAT?
     
  3. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    Actually my first pair of commercial heated socks shorted out and started to burn while I was wearing them!

    Sound advise--That's primarily why I'm initially testing with the full 12 odd feet. Using an 8Ah SLA battery for simplicity the wire at ~ 12v drew ~3 amps. That was using both of the strands in parallel and heating the wire in open 'still' air from 23 deg C to ~78 deg C. Looks like a good test point to start experimentation.

    The thermostat is an interesting item as there needs to be 'too much' heat generated so that regulation is required :)

    20A MOSFET rating was to obtain very low 'on' resistance. By design I don't want the heating element to draw more than 1C from the NiMH battery.
     
  4. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    The first experiment was modified for simplicity. It became too complicated (time consuming) to apply a secured spiral wind of the heating wire around the boot liner. I actually just made and oval edge standing wind, well attempted to make it--the previous bends in the wire made for an ugly wind (see attached photo.)

    Testing outdoors with a foot in the boot also proved a little too time consuming since the boot's mass was heated by the prior indoor test. I decided just to expose the coil to the 4 deg C air (with ~5mph winds.)

    What did I observe?

    1) Indoor temp rise on exposed core was nearly identical to the outdoor rise--both about ~50 deg C.
    2) Indoor wire temp installed between liner and boot shell can exceed 80 deg C! I'm not sure at what temp the boots would melt, but I'd rather not buy another pair.

    Did I learn anything?

    1) I'll need to add an upper temperature limit shutoff to protect the material the heater wire might be installed within.
    2) A 36 watt heater in my boot would do a good job at keeping my foot warm.

    Thoughts on the design which occurred while testing...

    1) Need heat shielding material to avoid heating the boot exterior (just like some of the nice outdoor boots.)
    2) A heat mass/sink might prove helpful to provide an even heat distribution.
     
    Last edited: Feb 14, 2011
  5. wayneh

    Expert

    Sep 9, 2010
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    Whew! Glad to hear it.

    There are various strategies for the control. One is a thermostat based on a LM35 thermometer IC to sense temperature, a comparator to compare the temperature to a set point, and a MOSFET to switch the heater.

    But I also think you could just use PWM control of the heater. If it's too hot, just reduce the duty cycle. You'd still have the same MOSFET do the switching. It's not really a thermostat, but in use I think it would accomplish the same thing. It eliminates the need to put a sensor somewhere. And I'm sure there are other approaches also. Note that a logic-level MOSFET is going to need at least 5v to reach a full "on". A regular MOSFET needs about 12v.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Another new poster wanting something for nothing. :rolleyes:

    You're going to need more power in than you can expect out of a circuit.

    A NiMH battery pack won't likely be able to supply sufficient current. Perhaps Li-Po, but they have an annoying habit of bursting into flames. However, the flames from the burning battery might keep you warm if you got far away from it in time.
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Having had a similar idea for footwarmers, I first determined how many watts of heat I needed per foot (10.) Then, I decided what voltage I intended to use for power (12) and calculated the current (833ma.) Next, I checked the temperature of various diameters of nichrome wire at 833ma, looking for a temperature that would heat my feet but not burn. I settled on 100°F. As nearly as I could calculate, that meant AWG 26 wire. Finally, I determined what length of AWG 26 nichrome wire it would take to provide 14.4 Ω of resistance; the answer is about 5.5 feet at 2.6 Ω/foot.

    My idea was to buy a good pair of wool socks and have my wife sew the wire in a non shorting spiral pattern around the sock with the ends near the top of the sock to be connected to wires that led to a 12v battery pack. As you might expect, my wife didn't think much of the idea so it didn't proceed beyond that stage. But maybe you can sew? :D
     
  8. wayneh

    Expert

    Sep 9, 2010
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    Hah! Maybe no worse than rubbing pepper powder on your feet. :eek:

    But I know that battery-heated socks and gloves are actually in use, so I don't think the OP is too far off base in wanting to make some that actually work as people expect them to.

    OTOH, the REAL problem is moisture management. If you can get rid of the moisture, it's not so hard to stay warm. I can't help but think the OP may be happier in the long run using advanced fabrics and insulation technologies. They're stupid expensive but some at least are also for real, eg. Thinsulate and Gore-tex.

    I run all winter long, and it's stunning how wet you get even at 0°F. It's not bad until you stop, and then you don't have much time until you're in trouble, because you're sopping wet with little insulation.
     
  9. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    Yeah, I've read a few posts here--plenty of perpetual machine designs ;)

    Based on the commercial insole warmers I broke, not really. They used a flexible circuit with a resistor trace rated to run on 3v at 2 watts using two AA batteries. Their device worked 'okay' down to about freezing when my feet were dry and I used the higher voltage NiZN batteries. I replaced their battery pack with two NiMH 11,000 mAh cells and got my 5-6 hours of acceptable warmth on the warmer days.

    There's a 'wireless' insole insert with a Li-Po ~800mAh for about $100, but with my experience with the chemical and electrical warmers, I don't see it being effective below freezing for a whole day at work.
     
  10. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    That's a lot of uncontrolled heat waiting to make toast out of your feet when you walk indoors :) For $10, the commercial poly-blend socks with the under the toes only heating element modified to use and optional second battery in series would work quite fine, that is until your feet get wet :(
     
  11. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    Modern wicking fabrics do help, but with Gore-tex 'sealed' or rubberized boots where does the moisture go? Thick wool socks do help in keeping some distance from the bottom insole where water collects. The snow and water that always manage to enter my boots from above (and footwear in poor condition that allows even more water) always seems to make for damp cold feet.

    This would be an interesting thought--provide ventilation for the heated moisture to escape.

    I use to ride a bike home from work in the middle of the night all-year-round. The 4 deg F air temp, wind, and that pesky water in the footwear it made for a very unhappy ride.
     
    Last edited: Feb 15, 2011
  12. John P

    AAC Fanatic!

    Oct 14, 2008
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    I wonder if you should be looking at electrically conductive rubber compounds. I'd assume that compared with wire, they'd have less problem with fatigue as you moved around.

    I did some looking around and found this:
    http://www.sspinc.com/ssp_store/A_65_durometer_semi-conductive_silicone_compound____22.htm

    It has 7 ohm/cm resistivity and comes as either cured sheets, or "ready to mold". Maybe you could get the liquid form of it and embed something flexible but conductive as an electrode, like copper desoldering braid. You'd have to calculate the thickness of material and distance between electrodes in order to get the resistance you need for your desired power output. So if you had a 12 volt supply and you wanted 10 watts output, you'd need a 14 ohm resistance (V^2 / R = 10, 12*12/14 = 10).
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Definitely not an easy engineering challenge but it would be a great invention. Even summer hiking/climbing suffers from heat and moisture buildup. I wonder if the defense dept. has ever worked on ventilated boots? It seems like such an obvious thing to try.
     
  14. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    I improperly searched for 'resistive' and only found manufacturers in China with no links to distributors--thanks.

    In reverse order...Target supply voltage is 1.2v--single cell NiHM 11,000 mAh, so E^2 (1.2v) / P (10w) = R (0.144 ohm) if configured as a 'single' element. If they ever make a NiZn D cell, the same element would be 18.90625w while the alkaline hits 15.625 w. BTW: I only anticipate peak watt output to occur initially to warm the boot mass or the water that leaks into the boot. The power requirements to maintain a 'stable' temperature should be significantly less since the 'foot' is also a heat source and the boots are insulated. (It might be interesting to determine the volume of water contained in a 'flooded' boot so I could calculate the 'recovery' time to stabilize the temperature ;) ) So, no wading in water above the boot tops as you can expect the battery to last only minutes, not hours as warming water is an expensive endeavor and time consuming process at this relatively low power level.

    No sample sheets for the SSP1850C product, but there is for SSP502 (next highest resistance value, but several order of magnitudes lower 0.03 to 0.50.) But, I currently lack an understanding of the 'Resistive Volume' measured as ohms/cm :( since it's not cm^3 -- study time :(

    Having a heating element with a larger surface area would simplify the project and the flexibility of silicone sounds perfect! Not sure how to interpret the Tensile (psi) and Tear (ppi) for this material, but they sound 'good.'

    Given the size of the samples and the material anyone want to hazard a guess on the total resistance of these sheets (4" x 4" x 0.032 & 0.062" assuming the above suggested copper braid is attached by an electrically conductive adhesive (what sticks to silicone?) or by mechanical contact at opposite ends (and sides)? Might make it easier to spend my money on this material if I know it's a suitable alternative. Thanks.
     
    Last edited: Feb 15, 2011
  15. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    Current Gore-Tex based personal wear works well providing you keep 'it-on'--check out their site it's loaded with all the details. It's 'clowns' like myself, that refuse to keep 'bundled' and become exposed to wind and water that get into trouble. I like to 'work' with only a sweatshirt--I'll be sweating, but my hands and feet get cold. The additional heat source allows me to shed the bulky outerwear for mobility and stay comfortable. The higher power might allow me to operate in lower temperatures (wind chill & water included) and the rugged design considerations could help in keeping the units working when something like a nail penetrates the shoe or wood splinters rip the gloves.
     
  16. John P

    AAC Fanatic!

    Oct 14, 2008
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    You can think of the resistivity of a bulk material, given as ohm-cm, by imagining a 1cm cube with current passing from one face across the cube. Obviously, if you made the cube twice as long, you'd double the resistance. If you made the cube twice as wide and twice as high, you'd cut the resistance by a factor of 4. So if you double all 3 dimensions, you decrease the resistance by a factor of 2.

    If you had a strip of the material with conduction along the length, the resistance would be the basic resistivity, multiplied by the length, and divided by the cross-sectional area. It's really a very simple calculation.
     
  17. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    Easy enough using this Volume resistivity calculator.

    Yes, the SSP1529 compound appears an ideal solution to directly mold and cure an insole, but I asked them for a thick 12x12" sheet since molding and curing can get complicated.
     
    Last edited: Feb 15, 2011
  18. Wendy

    Moderator

    Mar 24, 2008
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    Ever study hot ice? Try searching the term on YouTube. Basically it is a common food preservative using kitchen chemistry. When triggered, it gets very hot, and slowly becomes a rock. The neat thing is it is reversible, a simple microwave will make it a clear liquid again, and it will stay somewhat stable that way.

    I bought some muscle warmers. My kids played with them to death, it was in medical bag (similar to breast implants and blood bags) with a metal crickets (another child hood toy). Click the cricket, the liquid started to solidify and got really hot. In four hours it was a white hot rock.

    My girl put it in the freezer (it gets cold enough it will also trigger) then dropped the frozen bag, which shattered. One of the joys of having kids.

    Basically it is a super saturated solution.
     
  19. wayneh

    Expert

    Sep 9, 2010
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    These were demonstrated in our local Sam's before Christmas. Very slick. The great thing about using a phase change is that it holds an exact temperature throughout the process, without any need for a control. Melting ice works if you want 0°C. I was curious what chemistry delivers the 45?°C these things deliver, but haven't bothered to do the research yet.
     
  20. Major Tom MIB

    Thread Starter Member

    Nov 28, 2010
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    This sounds like them: Supersaturated Solution .

    130 def F for 30 mins to 3 hrs in the $10-20 range (depending on size and quantity). I nice warm alternative to the disposable type.

    Bulk and high temperature is their drawback. The palms are very effective point to apply heat. I actually put the chemical packs there, but I not too sure how long a gel pack would last in that location while grasping heavy objects on a frequent basis. Toe warmers are more uncomfortable than insole warmers, but I think I saw some insoles with a depression to accommodate the packs.

    I must admit though, that I am biased toward an electric unit since the power output and temperature can be regulated.

    Anyway, I have lots of back up chemical heating pads and I choose not to get the reusable ones since I figured I'd forget to boil them and I choose not to get the fuel-cell type as I once had a bad experience with one.
     
    Last edited: Feb 16, 2011
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