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ADC-24 w reference electrode

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ADC-24 w reference electrode

Postby Avus » Wed Nov 23, 2011 2:55 pm

Hello,

I have a few questions regarding measurements using the ADC-24 data logger (with terminal board). Being an IT specialist I'm almost completely unfamiliar with this kind of operations, none the less I was tasked to make a software for the following device. We picked the ADC-24 as it appeared to be versatile enough for our purposes.

We have a sensor array consisting of one reference elctrode (calomel electrode) and eight measure electrodes. For testing purposes we connected the reference to "AG" and three of the measuring electrodes to channels 1, 3 and 5. The values we got were completely different than the ones we get from a multimeter.
So we made a "standard test" with an AA battery, plugging the negative to "AG" and the positive to channel 1 and, of course, the reading of approximately 1500mV was correct. The same test with the same result was with the multimeter, so we can assume that it's working fine.
How comes the measured values differ?

Thanks in advance.
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Re: ADC-24 w reference electrode

Postby PeterF » Thu Nov 24, 2011 2:42 pm

Hi,
Can you help me by telling me the approximate output impedance of the electrodes you use? If it is anywhere near the input impedance of the ADC-24, then readings could easily be distorted. How do you connect the ground terminals of the reference electrode and measuring electrodes? What voltage levels are you expecting and what are you measuring, both on the ADC-24 and the multimeter
Regards,
PeterF..
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Re: ADC-24 w reference electrode

Postby Avus » Thu Nov 24, 2011 3:19 pm

First off, thanks for looking into this.

Now to your questions:
    depending on the electrode the impedance is between 100 KOhm and 10MOhm
    the grounds of all the electrodes are not used (connected)
    the readings are as follows (all in mV):
    ADC-24
    ch1: -85
    ch3: 20
    ch5: 15

    Multimeter
    ch1: 330
    ch3: 200
    ch5: 30

HTH
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Re: ADC-24 w reference electrode

Postby PeterF » Mon Nov 28, 2011 10:45 am

Hi,
To read pH, you require a specialist logger with a very high input impedance. The ADC-24 is just 1 Megohm in single ended mode (2 Megohm in differential mode). Our only logger suitable for the measurement of pH is the DrDaq pH measuring kit:-
http://www.drdaq.com/USB-DrDAQ-Data-Log ... ?source=HP
http://www.drdaq.com/datalogger.html
Normally, a pH probe will have a BNC connector and that will connect directly with the pH input of the DrDaq, connecting both the signal and ground at the same time, (two connections are always necessary).
The only other alternative, using the ADC-24, is to make your own high impedance buffer amplifiers and use a scaling file to get the correct readings:-
http://www.picotech.com/experiments/ph- ... index.html
(The above article refers to the ADC-16 logger but the principle is the same for the ADC-24).
Please contact us again if you have any other questions.
Regards,
PeterF.
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Re: ADC-24 w reference electrode

Postby ciacco85 » Mon Dec 19, 2011 11:30 am

Now it's all OK, we are using the INA 129 and it's working fine.
We are developing a matrix of 8 instrumental amplifier where all the inputs are sharing the same reference electrode.
Is there any any problem if the reference elctrode is connected to the AG pins of the DataLogger.
I'm worried about the high impedence of the sensor and data logger, but the low output impedence of the INA
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Re: ADC-24 w reference electrode

Postby Pico Stuart » Fri Dec 23, 2011 11:34 am

Hi there Avis,

Thankyou for your inquiry with regard to the ADC-24. Looking through your application, I suspect its to do with impedeance matching and you will need to buffer the input.

The ADC-24 is just 1 Megohm in single ended mode (2 Megohm in differential mode).

The only other alternative, using the ADC-24, is to make your own high impedance buffer amplifiers and use a scaling file to get the correct readings:-

http://www.picotech.com/experiments/ph-measurements/index.html


(The above article refers to the ADC-16 logger but the principle is the same for the ADC-24).
Please contact us again if you have any other questions.


The following manuals will also be of help

http://www.picotech.com/document/brochures.html?id=44


You are more than welcome to contact me at support at picotech.com

Kindest regards,

Stuart
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