PH Sensor w/ ADC w/ stamp

Archiver

Hello,
This message was sent to the owner instead of the list itself, so I'm
posting it for Dominick.

Erik Wood
ewood@p...

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Parallax, Inc.

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Original Message

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From: DZappia [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=uc9B3Ncgggl6M1mUWlk-21NgaXjduKJsY6q7PPpGI2bvVqC0pX2eVx78ow1Mde2FQYPmV5X6fOcaWbgk]dzappia@y...[/url
Sent: Sunday, May 18, 2003 8:43 PM
To: basicstamps-owner@yahoogroups.com
Subject: Re: PH Sensor w/ ADC w/ stamp


Hi Dale,

I came across this message thread and thought you could help, if you
have the time. PH Sensors provide millivoltage to representing the
current pH of a solution ranging from negative to positive numbers
corresponding to 0 to 14 pH. I have no trouble reading the positive
values, but whenever the pH drops below 7, into the negative range, I
get nothing but 0 from my ADC0831 output. Can you help??

Thanx alot
Dominick Zappia
dzappia@o...

Here is the portion of the code that handles the sensor readings, if
it helps...

sensors:

debug cls

high CS ' be sure CS is high
low CS ' initiate the conversion
low CLK
pulsout CLK,210

shiftin AData, CLK, msbpost, [noparse][[/noparse]ADres\9] 'shift in the data

debug "Sensor Reading V: ", DEC ADres */ $139C DIG 3, ".", DEC3 ADres
*/ $139C, CR
debug "Sensor Reading V: ", DEC3 ADres */ $139C, CR
debug "Sensor Reading V: ", bin8 ADres

serout pin, baud, [noparse][[/noparse]bin ADres, CR]

goto mainloop

Archiver

Hi Dominick,

One solution is to provide a virtual ground for the pH
probe/amplifier. So, at pH=0 the output level would be 2.5 volts.
Then you could read both + and - pH values.

pH probe (+)

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|amplifier|----|ADC|---- stamp
pH probe (-)--;
|
+2.5 volts

instead of:

pH probe (+)

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|amplifier|----|ADC|---- stamp
pH probe (-)--;
|
common

the amplifier, ADC and Stamp are referenced to common in either case.

Other options are to change the amplifier, so that it provides the
offset, or change to a different ADC that allows both + and - signals.

-- best regards
Thomas Tracy Allen PhD
electronically monitored ecosystems
http://www.emesystems.com
mailto:tracy@e...

>

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Original Message

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>From: DZappia [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=0Ju-KMCJ7wQWk6EkSP2szgYYIyQUSTouJcOUal7n1ruJxzx2Sj1f1MiXaM7on7sZeElbctFm7e5Tt50]dzappia@y...[/url
>Sent: Sunday, May 18, 2003 8:43 PM
>To: basicstamps-owner@yahoogroups.com
>Subject: Re: PH Sensor w/ ADC w/ stamp
>
>
>Hi Dale,
>
>I came across this message thread and thought you could help, if you
>have the time. PH Sensors provide millivoltage to representing the
>current pH of a solution ranging from negative to positive numbers
>corresponding to 0 to 14 pH. I have no trouble reading the positive
>values, but whenever the pH drops below 7, into the negative range, I
>get nothing but 0 from my ADC0831 output. Can you help??
>
>Thanx alot
>Dominick Zappia
>dzappia@o...
>
>Here is the portion of the code that handles the sensor readings, if
>it helps...
>
>sensors:
>
>debug cls
>
>high CS ' be sure CS is high
>low CS ' initiate the conversion
>low CLK
>pulsout CLK,210
>
>shiftin AData, CLK, msbpost, [noparse][[/noparse]ADres\9] 'shift in the data
>
>debug "Sensor Reading V: ", DEC ADres */ $139C DIG 3, ".", DEC3 ADres
>*/ $139C, CR
>debug "Sensor Reading V: ", DEC3 ADres */ $139C, CR
>debug "Sensor Reading V: ", bin8 ADres
>
>serout pin, baud, [noparse][[/noparse]bin ADres, CR]
>
>goto mainloop