Re: [DIYbio] Re: Making my own incubator inexpensively

On 04/23/2012 03:28 PM, Cathal Garvey wrote:

Most other forms of temperature readout
that I've encountered are a bit batch-variable, and I wouldn't be
surprised if the same is true of something cool but hack-ey like your
onboard diode idea

It's the physics behind all the temp sensors, the differences are in the implementation details.
Why would you pay more for a method or mechanism if you can't get any better results
than with the low cost method?  You don't just always pay more, you shop for a low accuracy need for low dollars
and hi accuracy for hi dollars.  Precision, or fine grained resolution, is inexpensive,
and you can transfer the accuracy from another temp probe to your machine and get repeatable results
without a temp standard built in.

This particular MSP430, for example, MSP430G2230IDR, has a nicely engineered diode temp sensor
inside, that can be switched in to one of its ADC channels to read the temperature of the chip.
The diode will have a small batch to batch random variation, but will always repeat very closely
as temperature differences happen.  There is a temperature compensated volt reference for the
ADC so volts measured are truly accurate.  Temperature repeatability of .01 deg C is probably
possible, although that's not accuracy, and would only agree that closely on long equilibration
soak times after a temperature change.

All you need is to put the chip in the stirred air you want to heat and you're accurate
to a very reasonable precision, and you could get better accuracy by calibration.
Repeatability and precision are easy to get with silicon microcontrollers.
That one I mention costs $46/qty 100, or 46 cents each and has four channels of 10 bit analog converter.

Is 0.1 degree C inherent reference standard accuracy necessary for incubation?  No...
repeatability is nice, an easy cal feature is nice...  Your code can have a mode where
when you press a calibrate button, it changes its look up tables to use what your external
fancy temp sensor reference standard says.  The code could take input as up/down buttons
to adjust the temperature lookup table while controlling temp at 35 deg C.  You would put the tip
of your hi res. platinum temp probe in the incubator for the calibration for a few minutes for each
up/down command until stable at 35.0 deg C.  Then it will repeat, and be a transfer standard
for the cal thermometer for many months or years with as good accuracy and precision
as the cal thermometer even though it does not have an internal temp standard that
accurate.  It does need a very stable, repeatable volt standard inside or on board, and
to make no changes to the chips used to keep its cal.

John

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