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Comments on Jim Hurley's CO2 Controller


  1. ARO Needle Valves vs. Nupro
    by BOLLING37/ (Sun, 9 May 1999)
  2. Bishop CO2/pH controller
    by lovell/ (Sun, 07 Mar 1999)
  3. Re Bishop & Hurley pH controllers
    by lovell/ (Mon, 08 Mar 1999)
  4. Hurley PH Circuit
    by BOLLING37/ (Mon, 17 May 1999)

ARO Needle Valves vs. Nupro

Date: Sun, 9 May 1999
To: erik/

Hey Erik,


By the way, Hurley's schematic has a few typos in it, they caused me a few 
hours of grief.  You may be aware of these, maybe not.  They are easily 
noticed by the
more experienced hobbyists, but beginners may not see them immediately.
Typo 1:  The offset pot (Zero control) is to be fed with -12 v, not +12.
Typo 2:  The "CO2 ON" LED is backwards, and will not light as shown in the 
Ok, I know, they're not serious typos but I didn't notice them until I had 
the circuit and couldn't get it to work.  

             (In sunny, Macon GA, where my Water Lettuce is enjoying the 
humid air)

Bishop CO2/pH controller

by lovell/
Date: Sun, 07 Mar 1999

Regarding the Bishop pH controller at

>Has anyone else built this thing?
	I built a couple of versions of it, but couldn't get it to work well
enough to be useful.  It's a clever idea, but the probe is S-L-O-W to
respond to changes in the tank CO2 level -- so slow as to be useless, in
my experience.  If I were going to try it again (and I might -- it's a
clever idea), I'd try to speed it up by putting the probe IN the tank
("diving bell" style), and by increasing the surface area to volume
ratio, which I think it critical: if you have a 5 ml sample and 1 square
cm of surface area, you can wait all day for the sample color to change.
> I am unsure of the VCC
	It all runs on the regulated 5 volts.
> He also didn't mention how much reagent to add
	The usual 3 drops/5 mls seems like a good amount.
	My advice, for what it's worth, would be to go with the Hurley pH
control system ( with a $33
"Silver" probe from Pet Warehouse.
	If y'all techie types would care to continue this discussion off-list,
please drop me a line.

Re Bishop & Hurley pH controllers

by lovell/
Date: Mon, 08 Mar 1999

>      Gary... did mention in his article that (his pH probe) was slow to respond.  How much surface area do you think it needs?
	If I build another one, I'm going to shoot for one square cm of exposed
surface per cubic centimeter of sample water.  I think that would speed
it up a lot.  Also, by getting rid of two or three inches of pipe going
from the tank to the sensor, you cut down on a lot of delay.  So, it
looks like it might be a 3 or 4 centimeter long trough, suspended as a
diving bell, and somehow protected from light.  That's starting to get
tricky to make.  The idea is to shine an LED with a narrow "viewing
angle" (30 degrees) down the trough to the sensor.
>      If I build Jim Hurley's circuit, and use the 'Silver' probe from Pet Warehouse, how can I be sure this probe will work?  The probe Hurley used changed its output voltage 50 mv per 1 unit of ph.  (I think) Is this a standard value for ph probes?
	If you have a look at Omega's Tech Reference section
you'll find a good discussion of the basics of pH and probes. 
Electrochemistry is a fascinating and frightening world for me, but as I
understand it, all pH probes are like little batteries that put out
about -60mV/pH.  The problem is that this "battery" has EXTREMELY high
internal resistance, so you can't just read its output with a volt
meter.  Fortunately, in this age of techo-miracles, you can buy a TL082
op amp IC at Radio Shack for about a dollar.  If you hook up the probe
directly to the input of this (or just about any FET-input amp), you
have a very basic pH measuring device.  Hurley also multiplies the
voltage so it reads out in pH units, not millivolts, which is nice.
	I have been fooling with this stuff a lot lately (there are pH buffers
and wiring all over my desk here right now).  I'm going to post a bunch
of information about it at my web page in a week or so, complete with a
basic pH meter/controller circuit that can be built with less than $20
of Radio Shack parts (plus the cost of a volt meter and the $33 probe). 
I'll put up the URL here for people who are interested when it's ready. 
Tom Sasala has some pH measuring circuitry at his web page:

Hurley PH Circuit

Date: Mon, 17 May 1999
To: erik/

Hey Eric,

     I have a few more additions to the Hurley CO2 controller.  My probe and 
solutions arrived a few days ago, so I calibrated the controller and stuck 
the probe
in the tank.  PH was around 6.2.  The circuit seemed to be
working ok, that is until I tried to set the PH set point control low enough 
to trigger
the solenoid gas valve.  The comparator wouldn't trigger smoothly, and the 
started to vibrate and make a buzzing sound.  The hysteresis swing was around
.3 PH!!  I knew exactly what was happening: 60 hz ac line interference!!  I 
spoken to Sherman Lovell (an APD list member) about this before, and then I 
remembered that his ph controller circuit used a few caps in the first op amp 
stage to cut ac gain, limit ac line noise, and prevent false triggering of 
the relay.                A 10 uf, Non-polarized cap placed across pins 2 and 
6 of the first stage IC (U1) solved the problem.    Perfect!!
The relay triggered nicely without buzzing, and the hysteresis swing can now 
be adjusted to .05 or less. (If desired)  One other change that helped cut 
interference:  A .001 uf (1000 pf) polystyrene cap should be placed at pin 3 
(probe input) and the Ground.
A ceramic disc cap works well also, and is what I used.  
Both caps can be obtained locally from Radio Shack.  


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