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Hurley CO2 Controller error

> I also recall, Jim mentioning an error on the prints and
> subsequently recommended changing one of the resistors of a given value to
> something else.  I've lost Jim's addendum and am unable to relocate it. Does
> anyone know if the plans on the Krib contain the change.
> http://www.thekrib.com/Plants/CO2/hurley-schemat.gif

	The updated version of Hurley's controller now posted at The Krib
should work fine, as far as I can see.  I have a copy of an earlier
posted version which has all the same resistor values, but one glaring
error: U6 is connected to the output of U5, not U2.  Maybe that's the
error you remember?
	In the circuit description,
Hurley also recommends using a better (higher input impedance) op amp at
U1.  I think this is a good idea.
	I'm certainly no guru, but I did work on similar circuitry at the local
university for years.  I'd be happy to discuss this, or any other
aquarium electronics, if you'd care to drop me a line.
Sherman Lovell, lovell at wa_freei.net