Paul ([EMAIL PROTECTED]) commented on my earlier posting with: "Does your pH meter have a temperature compensation function ?"
Paul, You should ask Olga Caussade that question. She's the one with the pH meter. Personally, I doubt that a temperature compensation button would be much use unless a really good-condition top-notch model was being used. Having 1 or 2 decimal places on the read-out screen tends to seduce the user into thinking that the meter is accurate to that number of figures. This is seldom the case; on most cheap pH meters a reading of (eg) 4.78 could actually be a true pH of anything from 4 to 6 or (in some cases) even pH 3 to pH 7. My college uses a large number of pH meters of different designs, quality and price. When introducing my senior Chem students to pH measurements, I hand out a different meter to each student and get them to measure the pH of the same solution. The range of results is quite boggling .... and these are meters that are properly calibrated and reasonably well-maintained. If you wanted to accurately measure the very small pH variation in an aqueous solution caused by altering the temperature by 10 degrees Centigrade, you would need a high-quality meter that has a twin-point calibration routine, and this would have to be calibrated with freshly-made buffer solutions. If your pH probe is the glass type containing a salt solution, then the probe would need to be in good nick, filled with solution of the proper concentration, and certainly no salt-encrustation on the surface. How many orchid-growing amateurs maintain their pH meters to this level ? My bet is that the average orchid grower seldom maintains or calibrates their meter properly, if at all. Hell, most of the cheaper models cannot be calibrated to an external standard, can they ? My advice: treat pH-meter readings with scepticism, unless you are REALLY sure of your equipment. Peter O'Byrne Singapore BTW, Nick was right, I was wrong. During the daytime photosynthesis would exceed respiration, so plants would be net consumers of carbon dioxide. _______________________________________________ the OrchidGuide Digest (OGD) [EMAIL PROTECTED] http://lists.orchidguide.com/mailman/listinfo/orchids
