REDOX/ORP Schematic

Archiver

Hello Everyone,

I would like to read ORP/Redox values using an normal probe and
amplifier circuitry.

Does anyone have or know a link were I can find a schematic for an
ORP (REDOX) amplifier circuit,I know the cicuitry closely resembles
PH (e.g. Tracy Allen`s)cicuitry but I do not exactly know the
differences.

Thanks in advance,

Djago

Archiver

ORP sensors don't require much of anything as far as signal conditioning is
concerned. pH sensors require a buffer amp because they can only put out a
miniscule amount of current - whatever can make it through the glass bulb
surrounding the platinum electrode. Your typical multimeter has about 50k
ohms input inpedance which, to a pH sensor, is like a short circuit which
causes a voltage drop. pH sensor voltage also swings from positive (low pH)
to negative (high pH) which is something most ADCs aren't equipped to
handle. Tracy's circuit uses an LM358 to add 1 volt to the signal so it
always stays positive.

I have a multimeter with a 10Megaohm input impedance and even it draws the
voltage down. You need something in the neighborhood of 1 Gigaohm input
impedance to actually read the pH sensor output voltage. Even many general
purpose opamps can't read them. I was using the CA3160 but thanks to Tracy
(yet again!), I now use the LMC6482. Easier to buy and a much lower offset
error.

ORP sensors, on the other hand, put out lots of current. You can read them
directly with a dime-store voltmeter without any trouble. I hooked mine
directly to a TLC2543 ADC and it worked just dandy.

Having said all that, if you're using Tracy's pH amplifier circuit, it makes
sense to use an opamp on the ORP side. The pH signal is roughly 0-1.8V and
the ORP signal isn't likely to go over 900mV (if its for a swimming pool),
2X gain would give you better resolution with the ADC. This would put your
full scale pretty close to the same as the pH sensor. If you're range of
interest is lower than that, raise the gain.

The opamp just needs to be set up as a non-inverting voltage follower with
gain. Hook the ORP positive lead to the "+" input of an op amp. Feed the
output back to the "-" input though a 10K resistor. Then put another 10K
resistor from the "-" input to ground. Use precise resistors and try to get
exactly the same values or the gain would be exactly 2x. You could also use
a 5k pot and an 8k resistor in place of the 10k to ground. That way you can
adjust it for exactly the gain you want.


>

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Original Message

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> From: djago76 [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=mDW9Out8T5a0419AHLL5iWhkH8AwJusi6MS8eyjCmm4iDOFDsuBwEFXfO5oPEfWhu4kqb08e648GKgxXmw]pnumberi@h...[/url
> Sent: October 4, 2003 2:12 PM
> To: basicstamps@yahoogroups.com
> Subject: [noparse][[/noparse]basicstamps] REDOX/ORP Schematic
>
>
> Hello Everyone,
>
> I would like to read ORP/Redox values using an normal probe and
> amplifier circuitry.
>
> Does anyone have or know a link were I can find a schematic for an
> ORP (REDOX) amplifier circuit,I know the cicuitry closely resembles
> PH (e.g. Tracy Allen`s)cicuitry but I do not exactly know the
> differences.
>
> Thanks in advance,
>
> Djago
>
>
>
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