[TriEmbed] Coffee roaster

[Brian]

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
>>
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