Anyone have a readTemperature calibration table for the RFM69?

[WhiteHare]

I did it on one prior occasion, when calibrating the temperature measurements given by a waterproof soil moisture sensor whose wireless signals (the raw values) I was able to intercept and decode.  I started it in a container packed with ice water, then put that into a cooler that had thick insulating foam, and then placed that in the attic, where temperatures would gradually rise through the day and slowly bleed into the cooler and raise the water temperature.  To get the full temperature range was a two day process, as the higher temperatures weren't reached until the second day.  So, it's doable, but there is an undeniable hassle factor, as you alluded to.  If one can't generalize the results from one radio to all the others--i.e. every radio has to be calibrated this way--it wouldn't be much fun.

The best temperature sensor I'm aware of at the moment is accurate to 0.1C--I've read that it's sensitive enough to detect the body heat from a hand even without touching it, provided you're within a few inches of it: http://www.mouser.com/ds/2/418/TSYS01_datasheet_rev0_3_20120113-710739.pdf  Digikey doesn't carry it, but Mouser does at $6.80 each.  It only takes 9 milliseconds to do a conversion while drawing 12.5uA, so although it's overkill in accuracy for most things, it's a tempting choice in it's own right, and it would obviously remove any need to do calibrations.  Also, I wouldn't have to worry about degraded performance over time, because of the extreme accuracy it would be starting with.

[joelucid]

That sounds like a reasonable procedure.

If one can't generalize the results from one radio to all the others--i.e. every radio has to be calibrated this way--it wouldn't be much fun.

You'd have to create a batch and measure them all at once for it to make sense - at least once you're in 'production'.

I'm thinking a 328p + rfm69cw + am1805. rtc switches total power off during sleep, 328p switches power to radio on demand. Frequent temp measurements to get precise slope measurements - say once every 10s. Only power radio on when there's a change from extrapolated slope.

Power consumption:

22nA - RTC
31nA - crystal wakeup radio
15nA - temp measurement Radio
18nA - temp measurement 328p
------
86nA - total

Now we also need to send updates. Say about once every 20 minutes (that should work from my experience to get 0.1C max error in rooms). That's about 83nA at 20mA transmit current at 5ms transmit time. Call it 100nA to account for radio startup costs.

For a total of 186nA or 1.63mAh per year. With self discharge of 1% per year a 220mAh CR2032 would last you 86 years. So there you have it: at least a lifetime T Mote.

[joelucid]

Oh I just realized that you have the radio startup costs for each temp measurement. That's clearly too expensive for this setup. So then maybe focusing on the RC oscillator would be more appropriate for ultra low power setups.

[WhiteHare]

Can a battery really last longer than 10 or 20 years?  For something ultra long-lived, I'm thinking energy harvesting might be the way to go, and for that, if you want to keep the small form factor, this chip seems pretty impressive:  http://cds.linear.com/docs/en/datasheet/31081fb.pdf  It can harvest from as little as 20 millivolts and eventually boost it up to 2.5v (well, for those of us still using winbond flash memory).  The atmega itself could run off the integrated 2.2v LDO.  If your electronics were encapsulated in epoxy, and using a capacitor for energy storage, maybe the system would run practically forever?

[joelucid]

Can a battery really last longer than 10 or 20 years?

Well - many of these lithium batteries have stated shelf lives of 10-15 years. An that's to ~90% of original capacity. Then why not use them for a couple of decades afterwards.

Practically speaking though: I have no idea 

[WhiteHare]

Even though the DS18b20 is a comparative power hog, and it can take up to 750ms (!) to take a temperature measurement, what I do like about it is the packaging: 3 long legs let you suspend it in the air, where it can be more attuned to rapid changes without the thermal mass of the main board (especially if encased in epoxy) creating latency and an averaging of fluctuating temperatures.  So, even though it's stated accuracy is +- 0.5C, in a real sense it may  offer a more accurate reading of current temperatures than higher accuracy surface mounted temp chips.

So, it would seem there just isn't a single, obviously "best" solution to incorporate into a general purpose platform.  For that reason, I think I'm trending toward a hybrid, which will avail itself to different trade-offs to fit different circumstances.  The MCP1640 boost converter is pretty cheap ($0.57 on Digikey), and so a  DS18b20 could be powered at 3.3v on-demand at any time, even if running from just a couple AA batteries.  Seems a bit unwieldy though, unless other sensors might (probably?) also need 3.3v  and so could leverage it.