On 10/24/18 7:34 PM, Pete Soper via TriEmbed wrote:
I can vouch for the max31850K chips for those thermocouples. They work
very well. But many handheld DMMS can handle type K thermocouples directly.
Useful knowledge if you're using a DMM to visually monitor temperature;
not so useful if you're trying to feed a temperature signal to a
microcontroller. :-D
Thermocouples are great for wide temperature ranges, but require
nontrivial signal processing to be useful to your run-of-the-mill
onboard ADC. I've never used the MAX31850 chips. Just from reading the
AdaFruit description of the 31850, I might suggest using the 31855
instead; SPI is slightly easier to handle than 1-Wire, and the AdaFruit
31855 breakout appears to have level-shifting built in:
https://www.adafruit.com/product/269
Before you go that far, though, I might suggest looking for a thermistor
with a suitable coefficient; K-type thermocouples are better-suited to
huge temperature ranges at a cost of accuracy; an NTC thermistor such as
this one from Littelfuse might be good for your application:
https://www.digikey.com/product-detail/en/littelfuse-inc/503JG1K/615-1143-ND/5230977
Its room-temperature resistance is 50k ohms, and drops to about 365 ohms
at 205 C. See note [1]. This along with a simple resistor-divider
network, or a Whetstone bridge if you want to get fancy, should allow
you to put your temperature of interest in the middle of the Arduino's
ADC range.
Cheers,
-B
[1] - NTC Thermistors (Negative Thermal Coefficient, resistance drops as
temperature increases) are often spec'd by a room-temperature resistance
(R25C) and a coefficient called beta. The equation for finding the
expected resistance at temperature t for an NTC thermistor:
Rt = R25C * e^(b * (1/t - 1/298))
Where:
R25C = room-temperature resistance from spec sheet
b = beta from spec sheet
t = temperature being measured IN KELVIN (C + 273)
Or, rearranged to find temperature given a measured resistance from the
thermistor:
t = (b * 298) / ((298 * ln(Rt/R25C)) + b)
...with the same symbols as the first equation. Remember that
temperatures in the equations are specified in Kelvin.
-Pete
On 10/24/18 7:23 PM, Craig Cook via TriEmbed wrote:
>As to why the designer chose that particular part, I can't guess! I can
only imagine that its operating temperature was in the right range for
the application (I suppose the coffee-roasting temperature doesn't
exceed 150 C?) and provided a useful signal when connected as described.
Huh. Thanks for the information. According to this instructable:
https://www.instructables.com/id/Build-a-Controllable-Coffee-Roaster-from-an-Air-Po/#step0
I need a sensor that works between 150 C - 210 C, like this:
https://www.adafruit.com/product/3245
Thanks
Craig
_______________________________________________
Triangle, NC Embedded Computing mailing list
To post message:[email protected]
List info:http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
TriEmbed web site:http://TriEmbed.org
To unsubscribe, click link and send a blank
message:mailto:[email protected]?subject=unsubscribe
_______________________________________________
Triangle, NC Embedded Computing mailing list
To post message: [email protected]
List info: http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
TriEmbed web site: http://TriEmbed.org
To unsubscribe, click link and send a blank message:
mailto:[email protected]?subject=unsubscribe
_______________________________________________
Triangle, NC Embedded Computing mailing list
To post message: [email protected]
List info: http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
TriEmbed web site: http://TriEmbed.org
To unsubscribe, click link and send a blank message:
mailto:[email protected]?subject=unsubscribe
