Post by thread:Conductivity measurement

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_OUTvulcanTakeThisOuTihug.co.nz --------------------------

> 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.

Check pH probe Corning #476540. It generates an output in mV related to pH Jean Gavin Jackson wrote: {Quote hidden}> > 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 > -------------------------- > .....vulcanKILLspam@spam@ihug.co.nz > --------------------------

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.

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}> 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.

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.simpeltronicsKILLspamtip.nl -----Oorspronkelijk bericht----- Van: Gavin Jackson <.....vulcanKILLspam.....IHUG.CO.NZ> Aan: EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU <PICLISTspam_OUTMITVMA.MIT.EDU> Datum: zaterdag 14 februari 1998 0:25 Onderwerp: Conductivity measurement {Quote hidden}>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@vulcanKILLspamihug.co.nz >-------------------------- > >

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 KILLspamjemKILLspamiastate.edu Center for Nondestructive Evaluation Electrical Engineering student x-ray tech/modeller/programmer

Is your idee something like this? http://www.hydro-gardens.com/newitems.htm#GROW-MACS Leo -----Oorspronkelijk bericht----- Van: Gavin Jackson <RemoveMEvulcanTakeThisOuTIHUG.CO.NZ> Aan: spamBeGonePICLISTspamBeGoneMITVMA.MIT.EDU <TakeThisOuTPICLISTEraseMEspam_OUTMITVMA.MIT.EDU> Datum: zaterdag 14 februari 1998 0:25 Onderwerp: Conductivity measurement {Quote hidden}>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 >-------------------------- >RemoveMEvulcanTakeThisOuTihug.co.nz >-------------------------- > >

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

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 insadEraseME.....itacom.com.py

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 ******

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:RemoveMEinfoEraseMEEraseMEomega.com Josef RemoveMEeuroclassspam_OUTKILLspampha.pvtnet.cz