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is_razi
Joined: 06 Apr 2002
Posts: 163
Helped: 6
Location: Tabriz
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 18 Sep 2004 18:48 pleaes guide me to build a liquids conductivity meter. |
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hi,
i want to build a liquid conductivity meter.
the principle is to flow a current through the and mesure the voltage developed across the sensor.
please guide me about this purpose.
thanks very much.
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DoctorX
Joined: 02 Sep 2004
Posts: 73
Location: USA
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| 22 Sep 2004 16:13 Re: pleaes guide me to build a liquids conductivity meter. |
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| is_razi wrote: |
hi,
i want to build a liquid conductivity meter.
the principle is to flow a current through the and mesure the voltage developed across the sensor.
please guide me about this purpose.
thanks very much. |
You may want to use an opamp circuit, detecting the resistance across the liquid. Better yet, use a differential opamp circuit, with the reference input coming from a controlled (depth) liquid. Post back if need further info.
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flatulent
Joined: 19 Jul 2002
Posts: 4856
Helped: 292
Location: Middle Earth
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| 22 Sep 2004 16:32 Re: pleaes guide me to build a liquids conductivity meter. |
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One thing to do is use AC for the measurement drive. This will prevent the chemicals in the liquid from plating onto the electrodes as they would with DC. If you want direct readout in conductance, use the inverting op amp circuit with a fixed drive. Put your sample of liquid an electrodes as the "resistor" from the drive to the inverting amp input. You will have to rectify the output of the amp to get a DC level proportional to the conductance.
Another thing to keep in mind is the shape of the liquid sample. You will either have to use a very close electrode spacing compared to the liquid sample size or make your own cowling around the electrodes. Calibrate the system with a liquid of known conductivity such as a NaCl solution.
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is_razi
Joined: 06 Apr 2002
Posts: 163
Helped: 6
Location: Tabriz
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| 24 Sep 2004 19:40 Re: pleaes guide me to build a liquids conductivity meter. |
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thanks for your responds.
but please introduce me a practical circuit.
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bomba
Joined: 09 Sep 2003
Posts: 136
Helped: 1
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| 03 Oct 2004 16:18 Re: pleaes guide me to build a liquids conductivity meter. |
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I made a project similar with it, but with extra conditions the current going through the solution couldn't be higher than 20nA, the wave form must to be sine and biphasic (very low offset). First of all I thought to implement a four wire instrument inject 20nA current sine wave and measure the voltage. I tested a lot of OPAmp current source. Later I thought to use a linealized bridge, I implemented a modification and work well, but I still have problem with the noise susceptibility. I post them.
I still thinking that is not a good configuration to do that, I would like to known wich is the best configuration to do that? And what kind of method I must to select for measure impedances between .5 to 100MOhms?.
Thanks to all, and best regards
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is_razi
Joined: 06 Apr 2002
Posts: 163
Helped: 6
Location: Tabriz
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| 07 Oct 2004 7:24 Re: pleaes guide me to build a liquids conductivity meter. |
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| bomba wrote: |
I made a project similar with it, but with extra conditions the current going through the solution couldn't be higher than 20nA, the wave form must to be sine and biphasic (very low offset). First of all I thought to implement a four wire instrument inject 20nA current sine wave and measure the voltage. I tested a lot of OPAmp current source. Later I thought to use a linealized bridge, I implemented a modification and work well, but I still have problem with the noise susceptibility. I post them.
I still thinking that is not a good configuration to do that, I would like to known wich is the best configuration to do that? And what kind of method I must to select for measure impedances between .5 to 100MOhms?.
Thanks to all, and best regards |
i have seen several industrial conductivity .they inject sine current to have a voltage with predefined amplitude across the sensor.
to solve noise problem you must use instrumentation amplifier such as AD620 immediately after sensor block.
hope to be useful
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