[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re Bishop & Hurley pH controllers

>      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: