pH probe calibration

mdvistnes · 11/19/2008, 08:26 PM

Hello - I have a question on a lab grade pH probe calibration for a salt water reef tank using the AC Pro III. I recently ordered one from marine depot and tried with 3 different sets of new calibration fluid (7 and 10) to calibrate it, and it calibrates low. I'm replacing a 3 year old probe that consistently read pH of the tank at 8.1 to 8.2. A test kit currently confirms these values. When I calibrate the new probe my readings are 7.7 - 7.85 range no matter what I do. I put the temp probe in the same fluid to calibrate it as instructed. I called MD and they said send the probe back and they will ship a new one. So is there anything I should do differently? Does it matter if I decant the calibration fluid into a glass beaker to calibrate it or should I leave it in the foil packet? I let it settle until the level is stable. The temp correction on the AC III is set to on and pH is set to on when I calibrate it. Am I missing something? Thanks!

clp · 11/20/2008, 09:53 AM

Confirm that the calibration is accurate, by putting the controller on the run display, and then confirming that the 7.0 and 10.0 cal solutions read correctly. If so then the reading is likely accurate. The calibration solutions may be bad, but if they are single use packets then that is not likely.
Note that many color matching pH test kits are very inaccurate, and many of them are purposely skewed to read a higher pH than reality.

Curt

11/19/2008, 08:26 PM	#1
mdvistnes Registered Member Join Date: Apr 2005 Location: Palo Alto, CA Posts: 194	pH probe calibration Hello - I have a question on a lab grade pH probe calibration for a salt water reef tank using the AC Pro III. I recently ordered one from marine depot and tried with 3 different sets of new calibration fluid (7 and 10) to calibrate it, and it calibrates low. I'm replacing a 3 year old probe that consistently read pH of the tank at 8.1 to 8.2. A test kit currently confirms these values. When I calibrate the new probe my readings are 7.7 - 7.85 range no matter what I do. I put the temp probe in the same fluid to calibrate it as instructed. I called MD and they said send the probe back and they will ship a new one. So is there anything I should do differently? Does it matter if I decant the calibration fluid into a glass beaker to calibrate it or should I leave it in the foil packet? I let it settle until the level is stable. The temp correction on the AC III is set to on and pH is set to on when I calibrate it. Am I missing something? Thanks!

11/20/2008, 09:53 AM	#2
clp RC Sponsor Join Date: Aug 2001 Location: San Jose, CA Posts: 4,311	Confirm that the calibration is accurate, by putting the controller on the run display, and then confirming that the 7.0 and 10.0 cal solutions read correctly. If so then the reading is likely accurate. The calibration solutions may be bad, but if they are single use packets then that is not likely. Note that many color matching pH test kits are very inaccurate, and many of them are purposely skewed to read a higher pH than reality. Curt