pH Probe Calibration Process / Accuracy

[shaywood]

Hello. Looking for some input.  I decided to replace an aging pH probe on my Apex using the following calibration process.  On my first attempt, I used a Milwaukie Double Junction probe and on my second attempt, I used a Pinpoint double junction probe.  

1. Floated 7 and 10 calibration fluid packs in sump to acclimate temperature
2. Removed new probe for transport fluid container and dried with a paper towel. 
3. Inserted probe into pH 7 pack and calibrated.
4. inserted probe into pH 10 pack and calibrated.

After successful calibration, I left the probe in the 10 solution and got a reading of 9.90 - 9.95.  Then placed in the 7 and got 6.82 - 6.85. Same results after a couple of attempts and the different brand probe. I would be happy with a .10 variance, but .20'ish seems a bit too much.

Is this normal? 

 

Shawn

[cjmdh]

The buffer solutions pH will drop after a short while when exposed to air due to CO2.  Depending how long between dunks that could be the variance.  

[Totroc3]

I understand your frustration. I just replaced a conductivity probe the same way and all goes well until I finish calibration and put the probe back in my sump. The probe goes from 35 or sometimes 35.5 to 38 even though my salinity is actually 33. I verified with 2 different calibrated Hanna salinity meters and 2 other meters. I then switched out my probe that was working perfectly in another of my displays and recalibrated it only to get the same result. I then put the second probe back in the original tank it worked in and it is at 37 instead of 34.7 where it was at. Neptune must have an issue after their last 2 updates. They are saying my Apex units have issues and want me to schedule support so they can get into the Apex. I have recalibrated about 30 plus times both through the task manager and manually. Grrr. I won’t dare recalibrate my pH probes now. Good luck Shaywood! Be glad the variance is small.