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

If I made minimum wage in the U.S., $10 is just about 1.3 hours, its
well worth it to me to get rid of dealing with autoclaving shoelaces
and water storage containers to get rid of scum that builds... and
they're a source of scum/unwanted growth///possible routes for
contamination.

Maybe if you're talking developing world africa where people's time is
worth $10 every 1.3 weeks... but its just not worth it for me to
bother dealing with.

Not to mention the calibration, recalibration, curve fitting, etc...
with a sensor like this its done, and performs pretty well with a
simple buffer to extend the range of the sensor wires to any distance
a lab might see for incubator or environmental monitoring.

As for the question of how to fix humidity, I'm not sure if piezo
water misters can be autoclaved, but its a candidate choice for me.

On Mon, Apr 23, 2012 at 7:46 PM, Simon Quellen Field <sfield@scitoys.com> wrote:
> Why spend $8 on a sensor when you can just use a second temperature sensor?
> Use the one in the microcontroller for the air temperature, and use a $2
> LM34
> in a shoelace in front of the fan for the wet bulb temperature. The other
> end
> of the shoelace sits in the water bath that is keeping the temperature
> constant.
>
> What are you doing to change the humidity, once you decide that you don't
> like
> the sensor reading?
>
> And it just occurred to me that it might be silly to use an LM34 when
> another
> microprocessor is a third of the cost. The two processors can talk to one
> another
> via I2C or SIP. But since the first processor has spare ADC pins and a
> voltage
> reference, your wet bulb sensor might just be a 1N4004 diode at a penny
> apiece.
> You get 2 degree accuracy, which I'll bet is just fine for any incubator.
>
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>
>
> On Mon, Apr 23, 2012 at 3:31 PM, Nathan McCorkle <nmz787@gmail.com> 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
>>
>> On Apr 23, 2012 5:20 PM, "John Griessen" <john@industromatic.com> wrote:
>>>
>>> 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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--

Nathan McCorkle
Rochester Institute of Technology
College of Science, Biotechnology/Bioinformatics

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