Re: [DIYbio] Isothermal microcalorimetry for bacterial activity.

On 04/16/2015 04:30 AM, Jeff Backstrom wrote:
> In other words, back to the ol' Wheatstone bridge.
>

A bridge with a thermistor in one leg could be very sensitive...but...

That just moves the place you need high accuracy from the volt reference
to the resistor temp coefficients of the bridge resistors... Something has to be accurate over
the temperature range of interest. Comparing your thermistor voltage to a good
voltage reference with a high res. A2D converter can be as good, and has the possibility
of correction factors built into a look up table for the A2D conversion numbers.
With a wheatstone bridge, more components need high accuracy, low drift.
With a wheatstone bridge, ambient temperature affects your readings and there's no
correcting that or linearity of the A2D in code.

When you're done, you probably want to
put the temperature back to near a standard starting point, so you want cooling
that has a heat flow path that is small when turned off. Thermoelectrics are not like that --
they have a big heat flow path on or off, and need a big heatsink to ambient to get rid of
all their excess heat of operating. So a heat exchanger connected by small diameter
PE tubing, with a valve near the bioreactor wall would do well. A microliter sized TE
cooled and TE monitored chamber would be much easier to do than a "pint sized" one that runs
continuously and uses its own characterized power flow as the heat flow sensor. Just from scale reasons.

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