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PICList Thread
'Conductivity measurement'
1998\02\13@174404 by Gavin Jackson

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Hi there

I have a supervised project that involves
converting a conventional greenhouse
into one for hydroponics. I have to
automate the entire thing and will
be using a 16C74 for the job.

Being hydroponics, I have to monitor
the solution for pH and conductivity.
I have been looking around for
any info on measuring these two
variables, but I have not found much.

I was wondering if there is anyone that
has ever had any experience with measuring
either of these two. The conductivity
is measured using two probes in the solution.
An AC signal is passed between the two
to prevent any build-up on one of the electrodes.
I presume the volt drop is then measured to
determine the conductivity which is measured
in micro-Siemens/cm.

pH is usually measured with a glass probe, but
I'm not sure what to expect at the other end of
the probe, as I have not come across any data
sheets on them.

Any help would be most appreciated.

Gavin
--------------------------
spam_OUTvulcanTakeThisOuTspamihug.co.nz
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1998\02\13@182019 by John Payson

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> I was wondering if there is anyone that
> has ever had any experience with measuring
> either of these two. The conductivity
> is measured using two probes in the solution.
> An AC signal is passed between the two
> to prevent any build-up on one of the electrodes.
> I presume the volt drop is then measured to
> determine the conductivity which is measured
> in micro-Siemens/cm.
>
> pH is usually measured with a glass probe, but
> I'm not sure what to expect at the other end of
> the probe, as I have not come across any data
> sheets on them.

From my impression, there are a number of ways of measuring pH; one very
simple method is to use two disimilar metal probes and measure the current
generated by the created cell.  I don't know what problems there are with
either probe aging or with solution contamination; you'd probably want to
have the probes open-circuit when you weren't actually taking measurements
or maybe even run a very small reverse current through them (to reduce the
effects of current leakage through the solution).  Perhaps others here know
more about such things.

1998\02\13@211413 by Jean Mercier

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Check pH probe Corning #476540.
It generates an output in mV related to pH

Jean


Gavin Jackson wrote:
{Quote hidden}

1998\02\14@061955 by paulb

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Jean Mercier wrote:

> Check pH probe Corning #476540.
> It generates an output in mV related to pH

 It does.  Note that it has an impedance in the GigOhm range, requiring
a FET input op-amp buffer/ scaler.  It is conventional to use a guard
rail connected to the inverting input of the op-amp to surround all PCB
connections to the non-inverting (probe) input.  The guard rail may also
be extended to the inner screen of double-screened co-ax to the probe,
but that's *real* precision stuff.

 Back on the "old days" this was real high-tech.  They used special
electrometer triodes( I think) in pH meters, possibly along with
transistors.  But of course FET op-amps are better!

 If you're going to do a good job, it is supposed to be temperature-
compensated too.  I don't know the rules for this.  Perhaps data sheets
for the above probe...

 Cheers,
       Paul B.

1998\02\14@065939 by paulb

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Gavin Jackson wrote:

> The conductivity is measured using two probes in the solution.  An AC
> signal is passed between the two to prevent any build-up on one of the
> electrodes.

 Hint:  Put a good (NOT electrolytic!) capacitor in series with the
conductivity probe.

 Cheers,
       Paul B.

1998\02\14@081932 by Jean Mercier

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Have a look at application note on Max125 from maxim-ic.com
An hardware approach is taken to compensate the temperature effect.
With a microcontroller the hardware is simplified and the compensation
is done via soft.

Jean

Paul B. Webster VK2BZC wrote:

{Quote hidden}

1998\02\14@093454 by Leo van Loon

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Hey man, this is chemistry, no electronics.
I am living in the middle of automated greenhouses and there is a lot more
to mesure and control as pH and conductivity. In the greenhouse nextdoor
there are two PC's controlling some tenth's of PLC's that do the jobs.

You have to learn first why and how these values are mesured before you try
put them in a PIC.

Leo van Loon
sbb.simpeltronicsspamKILLspamtip.nl

-----Oorspronkelijk bericht-----
Van: Gavin Jackson <.....vulcanKILLspamspam.....IHUG.CO.NZ>
Aan: EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU <PICLISTspamspam_OUTMITVMA.MIT.EDU>
Datum: zaterdag 14 februari 1998 0:25
Onderwerp: Conductivity measurement


{Quote hidden}

1998\02\14@095747 by James Merritt

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Hi Gavin,

I am just getting into PICs, when my engineering classes will allow me.
I have been on the STAMPS list for about 2 or 3 years and I remember a
thread talking about pH.

You might check http://www.parallaxinc.com and click on "Links" button
then scroll to the bottom of the screen where it says "Search past
posts".  As I recall from the thread, lots of good information was
there.

I hope this helps.
--
James E. Merritt N0SRB          Iowa State University
KILLspamjemKILLspamspamiastate.edu                 Center for Nondestructive Evaluation
Electrical Engineering student  x-ray tech/modeller/programmer

1998\02\14@120138 by Leo van Loon

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Is your idee something like this?

http://www.hydro-gardens.com/newitems.htm#GROW-MACS

Leo
-----Oorspronkelijk bericht-----
Van: Gavin Jackson <RemoveMEvulcanTakeThisOuTspamIHUG.CO.NZ>
Aan: spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU <TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU>
Datum: zaterdag 14 februari 1998 0:25
Onderwerp: Conductivity measurement


{Quote hidden}

1998\02\15@163545 by John Sanderson

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Hello Gavin & PIC.ers,

pH has a whole set of problems of its own.
I have a 88 page A5 book from Ingold of Switzerland which looks like
it covers your needs. :-

`Practice and Theory of pH Measurement'
An outline of pH measurement: Information and practical hints
1989
Ingold Messtechnik AG
CH-8902  Urdorf, Switzerland
Tel 01-736 2211
fax 01-736-2636

Maybe they're on the www too, can't say.

Best regards,   John
email from John Sanderson   /   JS Controls, Boksburg, RSA
Manufacturer & purveyor of laboratory force testing apparatus
and related products and services.
Tel/fax: Johannesburg 893 4154    Cellphone 082 453 4815

1998\02\15@205500 by Alberto Smulders

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Try to get the Cole Parmer 1997-1998 catalog; there's a lot of theory on pH
and conductivity measurement in it, including temperature corrections and
the like; they also have al kinds of pH electrodes (with and without
termistor, for lab use or for field use, etc. etc. - more than 100 different
types), although I think they are rather (very) expensive with their
products..... I've seen the same products elsewhere at 50% or less of their
listed prices (I think they must have the Government as their biggest
customers..... !!!)
I also found somewhere on the web schematics of an amplifier for pH
electrodes, but don't remember the URL. Try a search on Yahoo or Hotbot....

Albert Smulders
InSAD - Encarnaci—n, Paraguay
insadEraseMEspam.....itacom.com.py

1998\02\16@083629 by Mark A. Corio

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Another reference that I have not seen mentioned for PH or conductivity is
Omega. They have an entier hardcover catalog on only PH and conductivity
measurement products. They also have catalogs for temperature, humidity,
pressure/strain and others). They also advertise a web site (I have not looked
at it):

http://www.omega.com/

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel: 716-328-5850
Fax: 716-328-1144
http://www.frontiernet.net/~rmi/
****** Designing Electronics for Research and Industry ******

1998\02\16@090926 by Alberto Smulders

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See

  http://pitirre.lce.org/equipment/phmeter/phoper.html

for a very simple pH-probe amplifier circuit (1 x TL081....)

Albert A. Smulders
InSAD - Encarnaci—n, Paraguay
EraseMEinsadspamitacom.com.py

1998\02\16@123314 by Josef Hanzal

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Hi Gavin,

>I was wondering if there is anyone that
>has ever had any experience with measuring
>either of these two.
you are lucky, I work with both

>The conductivity
>is measured using two probes in the solution.
Right. Use inert material, like stainless steel or graphite, platinum used
in laboratory applications is probably unnecessary for you.

>An AC signal is passed between the two
>to prevent any build-up on one of the electrodes.
>I presume the volt drop is then measured to
AC signal correct, several hundred Hz up to units of kHz. Better work with
constant (and small, <1V effective) AC voltage and measure current, which is
proportional to conductivity.

>determine the conductivity which is measured
>in micro-Siemens/cm.
Yes. You have to calibrate your measuring cell constant using KCl solution
of known conductivity. Use destiled water to adjust zero.

>pH is usually measured with a glass probe, but
>I'm not sure what to expect at the other end of
>the probe, as I have not come across any data
>sheets on them.
Its output is voltage, most probes give zero voltage at about 7 pH and
change its output by about -58 mV/pH (negative slope, voltage decreases with
increasing pH value). Slope is temperature and probe dependent, also changes
with time. You may need adjustment pots (or adjust in software) for both
zero point and slope or zero point only, and calibrate using solution(s) of
known pH (buffer solutions). Last but not least, the output inpedance is
several hundred megaohms, so you need a FET OpAmp right after the probe.

>
>Any help would be most appreciated.
No problem. You may also try to call Omega company and request pH and
conductivity handbook. Besides all their wanderful product info with even
more wanderful price-tags you will get a short overview of pH and
conductivity measurement in the book.
Omega phone: (203)539-1660
fax: (203)539-7700
Toll-free: 1-800-TC-OMEGA
email:RemoveMEinfoEraseMEspamEraseMEomega.com

Josef
RemoveMEeuroclassspam_OUTspamKILLspampha.pvtnet.cz

1998\02\17@081912 by Tim Modra

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Has anyone experimented with double torroids for conductivity measurement ?
Would there be any advantage in single torroid and Linear Hall
effectsensor, or does anyone think that simply a magnet and hall effect
sensor could work ?

Tim Modra: RemoveMEtimTakeThisOuTspamspammodra.com.au

Modra Technology: http://www.modra.com.au
Custom Sensors and Embedded Control

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