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

On 04/23/2012 05:31 PM, Nathan McCorkle wrote:
> I want to watch RH for my incubators, this seems like the best option due to resistance to damage at 100% RH :
> http://www.sparkfun.com/products/10167

OK, that's a $10, $8 sensor. We had a $2.80 bill of materials going for this incubator so far...

You might want to use that if 92% RH was optimum and you needed that. But...
it's a little out of line price wise, so....

With an incubator that stirs air for even temperatures, it will have some flow of air you
can use to advantage to measure RH by definition. The definition is pretty much a differential
temperature between wet and dry with plenty of air flow over the wet especially since
max evaporation, ( as in swinging a wet bulb thermometer around and around for a while),
has some relation to temperature drop in various humidities, and you can create a look up table of
values to do the translation. So, in an incubator, you rig a diode voltage vs. temperature sensor
($0.25 for the first one, $0.11 for the second), for wet and dry using a 1N4007 diode
for each, (and ones from the same manufacturing batch), measured by and ADC channel
all coming from the 46 cent MSP430G2230IDR microcontroller I mentioned earlier, and referencing the same
precision voltage accuracy standard MSP430G2230 has built in, and you get the same result, accurate
rel. humidity, as you get with a $10 sensor.

The $2.80 and climbing bill of materials incubator version may lack some kind
of convenience feature the sensor has, but for incubation purposes, it will hold RH
and temperature rock steady if you have some kind of movable door to let air flow across
a wick into some distilled water, or not, by degrees, in a negative feedback control loop
via the code and the above mentioned measuring sensors and ADCs.

The bill of materials is climbing higher now since we have added more performance to
the requirements, but the system is set up for heaters, and controlling one more is easy,
so we can consider Ni-Ti springs as a wet/dry door mover and be climbing the performance
hill for cheap! All that's left is ways to make 3D shapes in water proof materials
inexpensively. Then you can make the swirl chamber for air flow, and wet dry areas
with a wick and H2O reservoir, and a snazzy outside look in 3D. How much would
that 3D enclosure cost?

For 3D enclosures, I've been thinking the well proven rapid prototype method of
ink jetted water/glue holding together starch could be good for some low volume
first articles -- you can harden starch with cyano-acrylate, (crazy glue). Or 3D
print the negative shape to get a mold, harden with cyano-acrylate, coat with something
for durability and mold release, and use it as a mold till it wears out and print another...

A good material for an incubator would be HDPE. For some shapes with low accuracy requirements,
your incremental cost of producing slow low pressure molded HDPE part of about 250 grams
could be (scrap price sm vol $2/kg) x.25kg + molding effort/costs $0.4 = $0.9.

That boosts the kit BOM total to maybe $4. (with a little mfrg infrastructure put in place)

John

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