5 ms·

So if a human burns ~2000 calories per day and has 1.5-2 m^2 of skin, that means that a reasonable upper bound on (mean) thermal flux density of human skin is a

So if a human burns ~2000 calories per day and has 1.5-2 m^2 of skin, that means that a reasonable upper bound on (mean) thermal flux density of human skin is around 65 W/m^2. So if a watch makes contact with an area of, say, 2 square inches, that comes out to... 85 mW of available thermal energy? Am I missing something?

theophrastus 10y ago
To a first approximation that looks fine[1]. To a second approximation one might consider that heat doesn't exit any endotherm in remotely equal surface fashion[2]. And if anything the wrists appear as local minima. [1] https://en.wikipedia.org/wiki/Spherical_cow https://en.wikipedia.org/wiki/Spherical_cow [2] http://i.stack.imgur.com/CsnO5.png http://i.stack.imgur.com/CsnO5.png
- Retric 10y ago
  Air and clothing vs a metal plate in skin contact makes that slightly less important. The real probably is you probably get 2% efficiency due to minimal temperature differences involved, and people take their watches off.
rasz_pl 10y ago
you can divide that by ~100, unless you manage to keep temperature difference at 100'
nickff 10y ago
Not a direct response to you, but if you calculate Carnot thermal efficiency with a skin temp of 37 C (likely too high), and ambient temperature of 20 C (probably about right), you get 5.5%.[1] Using this result (combined with yours), the watch has to run on less than 5 mW of electrical energy. [1] http://calistry.org/calculate/thermalEfficiencyCalculator http://calistry.org/calculate/thermalEfficiencyCalculator P.S. It would also be interesting to see how much waste heat is removed in breathing.
- nom 10y ago
  5mW should be enough for a simple e-ink watch. CPU and display have to stay idle most of the time though and you can't communicate a lot over radio. It should be possible, even with that little power if you can accumulate the energy in a capacitor. However it's not an easy task and poses a real engineering challenge, that's for sure.
  - nickff 10y ago
    Many of the Cortex M0s are very good at conserving power, and have a number of low-power and sleep modes, which make running at these levels quite easy. On the other hand, while Bluetooth 4.0 is low-power friendly, cost and size limits inherent to the watch will probably force the team to do a custom antenna and FCC certification, which is expensive, slow, and painful. I would say the electronics are not too complicated or difficult, but the energy harvesting and design will be tough, and the whole project will be quite arduous. It would be very interesting to see their development plan, and whether they are going for an iterative/modular design, or all-in.
    - nom 10y ago
      I agree, the CPU is probably the smallest problem. I'm curious with how little power you can get away with using a BLE radio. Especially if you constrain yourself to some kind of polling mode where the watch reaches out to the smartphone every minute or 5 minuetes or so.
david-given 10y ago
A basic low-power microprocessor (like the MSP430) will run at 1MIPS at about 0.5mA. (Less when sleeping.) At 3V, that's 1.5mW. A Sharp epaper display, like on the Pebble, consumes basically nil when not changing, although I can't find data on energy use when updating. A traditional STN LCD uses about 1mW. So I reckon it's plausible, just --- thermoelectric generators have traditionally been really inefficient, and I don't know whether getting 2.5mW of electricity out of 85mW of heat is possible. Particularly if you're like me and wear the watch quite loosely. Edit: Datasheet: http://www.ti.com/lit/ds/slase54a/slase54a.pdf http://www.ti.com/lit/ds/slase54a/slase54a.pdf That's an FRAM model with non-volatile RAM, so it can power down completely without losing memory. It claims it'll sleep with the clock on at 450nA. (That's nano, not milli.) It also claims it'll run with the processor on at 120 µA/MHz, so the 1 MIPS figure I quoted above must be about 4MHz. I've never run one below that but I gather they'll go down to about 500kHz. Enabling peripherals costs more, of course, but you can leave them powered off until you want to use them. So, yeah. Totally plausible.
- skykooler 10y ago
  Most smartwatches are also paired with phones. So there's extra power usage for bluetooth, too.
- rasz_pl 10y ago
  There is no epaper in pebble. You can get ~25 microwatts per cm2 per K temp difference (according to Linear). Enough for the Sharp memory display and simple clock circuit.
- fisherjeff 10y ago
  Yeah, it's the screen I'm curious about. The renders on their Indiegogo page make the screen look pretty awesome. FWIW, the original Pebble is rated at 2.8 mW and the Pebble Time is 4.6 mW.
- nom 10y ago
  There are CPUs out there with sleep currents well below 0.1 mA, for example the Microchip nanoWatt XLP. The product page advertises 9 nA in a best case sceneario.

theophrastus 10y ago

To a first approximation that looks fine[1]. To a second approximation one might consider that heat doesn't exit any endotherm in remotely equal surface fashion[2]. And if anything the wrists appear as local minima. [1] https://en.wikipedia.org/wiki/Spherical_cow https://en.wikipedia.org/wiki/Spherical_cow [2] http://i.stack.imgur.com/CsnO5.png http://i.stack.imgur.com/CsnO5.png

Retric 10y ago
Air and clothing vs a metal plate in skin contact makes that slightly less important. The real probably is you probably get 2% efficiency due to minimal temperature differences involved, and people take their watches off.

rasz_pl 10y ago

you can divide that by ~100, unless you manage to keep temperature difference at 100'

nickff 10y ago

Not a direct response to you, but if you calculate Carnot thermal efficiency with a skin temp of 37 C (likely too high), and ambient temperature of 20 C (probably about right), you get 5.5%.[1] Using this result (combined with yours), the watch has to run on less than 5 mW of electrical energy. [1] http://calistry.org/calculate/thermalEfficiencyCalculator http://calistry.org/calculate/thermalEfficiencyCalculator P.S. It would also be interesting to see how much waste heat is removed in breathing.

nom 10y ago
5mW should be enough for a simple e-ink watch. CPU and display have to stay idle most of the time though and you can't communicate a lot over radio. It should be possible, even with that little power if you can accumulate the energy in a capacitor. However it's not an easy task and poses a real engineering challenge, that's for sure.
- nickff 10y ago
  Many of the Cortex M0s are very good at conserving power, and have a number of low-power and sleep modes, which make running at these levels quite easy. On the other hand, while Bluetooth 4.0 is low-power friendly, cost and size limits inherent to the watch will probably force the team to do a custom antenna and FCC certification, which is expensive, slow, and painful. I would say the electronics are not too complicated or difficult, but the energy harvesting and design will be tough, and the whole project will be quite arduous. It would be very interesting to see their development plan, and whether they are going for an iterative/modular design, or all-in.
  - nom 10y ago
    I agree, the CPU is probably the smallest problem. I'm curious with how little power you can get away with using a BLE radio. Especially if you constrain yourself to some kind of polling mode where the watch reaches out to the smartphone every minute or 5 minuetes or so.

david-given 10y ago

A basic low-power microprocessor (like the MSP430) will run at 1MIPS at about 0.5mA. (Less when sleeping.) At 3V, that's 1.5mW. A Sharp epaper display, like on the Pebble, consumes basically nil when not changing, although I can't find data on energy use when updating. A traditional STN LCD uses about 1mW. So I reckon it's plausible, just --- thermoelectric generators have traditionally been really inefficient, and I don't know whether getting 2.5mW of electricity out of 85mW of heat is possible. Particularly if you're like me and wear the watch quite loosely. Edit: Datasheet: http://www.ti.com/lit/ds/slase54a/slase54a.pdf http://www.ti.com/lit/ds/slase54a/slase54a.pdf That's an FRAM model with non-volatile RAM, so it can power down completely without losing memory. It claims it'll sleep with the clock on at 450nA. (That's nano, not milli.) It also claims it'll run with the processor on at 120 µA/MHz, so the 1 MIPS figure I quoted above must be about 4MHz. I've never run one below that but I gather they'll go down to about 500kHz. Enabling peripherals costs more, of course, but you can leave them powered off until you want to use them. So, yeah. Totally plausible.

skykooler 10y ago
Most smartwatches are also paired with phones. So there's extra power usage for bluetooth, too.
rasz_pl 10y ago
There is no epaper in pebble. You can get ~25 microwatts per cm2 per K temp difference (according to Linear). Enough for the Sharp memory display and simple clock circuit.
fisherjeff 10y ago
Yeah, it's the screen I'm curious about. The renders on their Indiegogo page make the screen look pretty awesome. FWIW, the original Pebble is rated at 2.8 mW and the Pebble Time is 4.6 mW.
nom 10y ago
There are CPUs out there with sleep currents well below 0.1 mA, for example the Microchip nanoWatt XLP. The product page advertises 9 nA in a best case sceneario.