Post by thread:Looking for PH sensor [OT]

<x-flowed>At 10:33 PM 5/2/00 +0100, Philip wrote: >Just to pick up on this thread, with regard to continuous pH monitoring. >Take a look at any commercial swimming pool. We (they) use continuous pH - >Redox - Chlorine monitoring, all the time! > >The only problem I find in my 'pool' is that because I use a special brew of >Chlorine and Bromine in the water the Glass pH probes only have a life of >some 10 months. But then 100 UK pounds to replace a probe against 400,000 >swimmers per year isn't a bad ratio. Combination electrode only have a shelf life of 1 year. Even when not used their life is short. To get long life, you need a refillable, 2-electrode system. Another issue in swimming pools and aquariums is ORP ("oxidation-reduction potential"). This is the voltage from pH plus that of other ionic species. Similar problems, similar solutions. One way around the difficulty of pH measurements in pools and aquariums to measure conductivity instead. Changing pH will change conductivity. So long as salt concentration is constant, this property allows development of a pH control loop. Conductivity is simply measured by the resistance between two narrow (1-3 mm) stainless rods a calibrated distance (10-20 mm) apart. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: spam_OUTralTakeThisOuTlcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================ </x-flowed>

Perhaps a better approach to approximating pH would be to measure dissolved carbon dioxide. Evaporation is a big problem in vessels open to the atmosphere--as water evaporates, salt concentrates, and the conductivity increases independent of pH changes. A similar problem occurs in measuring soil moisture. However, dissolved carbon dioxide is generally present at pH levels between abut 5.6 and 8.5, which is fine for aquariums and pools. Because carbon dioxide dissociates to form carbonic acid, which is an acid, the higher the CO2, the lower the pH. This is independent of conducitivity. And a nice pic project is to build a continuous CO2 sensor for an aquarium. Search altavista for CO2 sensor & aquarium, that should pop it. alice {Quote hidden}> > Another issue in swimming pools and aquariums is ORP ("oxidation-reduction > potential"). This is the voltage from pH plus that of other ionic species. > Similar problems, similar solutions. > > One way around the difficulty of pH measurements in pools and aquariums to > measure conductivity instead. Changing pH will change conductivity. So long > as salt concentration is constant, this property allows development of a pH > control loop. Conductivity is simply measured by the resistance between two > narrow (1-3 mm) stainless rods a calibrated distance (10-20 mm) apart. > > ================================================================ > Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: .....ralKILLspam@spam@lcfltd.com > Least Cost Formulations, Ltd. URL: http://lcfltd.com/ > 824 Timberlake Drive Tel: 757-467-0954 > Virginia Beach, VA 23464-3239 Fax: 757-467-2947 > > "Vere scire est per causas scire" > ================================================================

On 3 May 00, at 0:08, Alice Campbell wrote: > Perhaps a better approach to approximating pH would be to > measure dissolved carbon dioxide. Evaporation is a big > problem in vessels open to the atmosphere--as water > evaporates, salt concentrates, and the conductivity increases > independent of pH changes. A similar problem occurs in > measuring soil moisture. Alice, I'm just curious: have you try to measure soil moisture using a microwave moisture - meter ? This problem will be avoided . Vasile ********************************************* Surducan Vasile, engineer mail: vasileKILLspaml30.itim-cj.ro URL: http://www.geocities.com/vsurducan *********************************************

Well, yes and no. There may be several ways to measure carbon dioxide concentration, but the very common approach is to use a pH probe separated from the measured solution by a gas permeable membrane. The CO2 diffuses from the sample to the inner solution and changes its pH. This change is then measured with the pH probe. Unfortunately this type of CO2 sensor suffers serious interferences from NH3 and volatile amines, which are both likely to appear in the fish tank water. Regarding the pH probe impedance - it depends widely on temperature (and the glass type and thickness as well). At room temperature some 100MOhms sound reasonable, at 212F it can drop downto 1MOhm, but at 32F it can reach 1GOhm. Josef {Quote hidden}>Perhaps a better approach to approximating pH would be to >measure dissolved carbon dioxide. Evaporation is a big >problem in vessels open to the atmosphere--as water >evaporates, salt concentrates, and the conductivity increases >independent of pH changes. A similar problem occurs in >measuring soil moisture. However, dissolved carbon dioxide >is generally present at pH levels between abut 5.6 and 8.5, >which is fine for aquariums and pools. Because carbon >dioxide dissociates to form carbonic acid, which is an acid, >the higher the CO2, the lower the pH. This is independent of >conducitivity. ======================================================================= Electronical devices for chemical laboratory, custom electronics design ----------------------------------------------------------------------- Snail Instruments Josef Hanzal, M. S. Vojanova 615 phone/fax: +420-311-24433 266 01 Beroun e-mail: .....snailKILLspam.....iol.cz Czech Republic URL: http://www.vitrum.cz/snail/ GPS: 49¡58'28" North, 14¡ 4'35" East =======================================================================

On 4 May 00, at 5:20, Snail Instruments wrote: > Well, yes and no. There may be several ways to measure carbon dioxide > concentration, but the very common approach is to use a pH probe separated > from the measured solution by a gas permeable membrane. The CO2 diffuses > from the sample to the inner solution and changes its pH. This change is > then measured with the pH probe. Unfortunately this type of CO2 sensor > suffers serious interferences from NH3 and volatile amines, which are both > likely to appear in the fish tank water. Hi, Josef, you don't feel sometimes that engineers needs to make things too complicated...I think that maybe it's unnecesary to measure pH or conductivity or something else, the problem is only to keep some parameters like temperature and water saltiness in a loving range for fish... Lives in aquarium is not an industrial process to be severe monitorized. The simplest solution is to cover the top of aquarium for minimize evaporation. Of course a little spectrophotometer ( to test if water must be changed) is vital for fish... (mm..) Vasile ********************************************* Surducan Vasile, engineer mail: EraseMEvasilespam_OUTTakeThisOuTl30.itim-cj.ro URL: http://www.geocities.com/vsurducan *********************************************

<x-flowed>If anyone wants a cheap mini-pH electrode, you can check out this auction on eBay: http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=325849281 ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ralspam_OUTlcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================ </x-flowed>

Vasile Surducan wrote: > Lives in aquarium is not an industrial process to be severe > monitorized. The simplest solution is to cover the top of aquarium for > minimize evaporation. Of course a little spectrophotometer ( to test if > water must be changed) is vital for fish... (mm..) > Vasile > ********************************************* > Surducan Vasile, engineer > mail: @spam@vasileKILLspaml30.itim-cj.ro > URL: http://www.geocities.com/vsurducan > ********************************************* problem is that the top needs to be open for better gas exchange.

On 8 May 00, at 9:37, Snail Instruments wrote: > > Unfortunately not very profitable: I'm working in a dying research > >institute ( like fish into aquarium...) > Similar situation was here in Czech - young people left and the old slowly > retired. But it seems to change recently, many of my coleagues from > university work at some kind of research institution, they have grants > assigned either from the state grant agency, some even from abroad. > V.Surducan reply: I was hoping to hear some good news and probably a profitable common project proposal... BTW, Snail Instruments it's your ( personaly ) firm ? > >and in my 'spare' time I made > >medical equipment for electro-therapy an any electronics who fall in my > >hands... > I see, something like the electrical bath, where controlled current is > applied to the body? (I hope the term is at least close ;-). Or sime kind > of magneto-therapy? These medical apparatus have to supply on pacient (with to or four electrodes ) more than 100 different tipes of currents (I do not know if the terms are corectly translated ) , some of these curents keep the name of inventors: Klotz, Faradic, Huffschmitd, some name are usual like: galvanic (DC), diadinamic, interferential, tens, russian, rectangle, triangle, trapezoidal, all can be modulated with other tipes and must supply a patient resistance between 0.2K to 7K and a current up to 100mA. You may see the picture of them (and my pH-meters too at: URL:http://www.geocities.com/vsurducan/aparate/aparate.htm I'm serious thinking to an industrial pH-meter with an electrode-unit (adapted only for metalic electrodes) and a display-unit conected with a RS485 (or other current mode transmition) with electrode unit, I've think that romanian market needs something like that... By, Vasile ********************************************* Surducan Vasile, engineer mail: KILLspamvasileKILLspaml30.itim-cj.ro URL: http://www.geocities.com/vsurducan *********************************************