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: ral@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" ================================================================