View Full Version : pH probe not calibrating right

01/30/2012, 12:41 PM
I have had my probe for about 9 months now and I have noticed the readings were not right. It started reading in the 5's.

I bought some calibration fluid and when trying to calibrate to 7 the numbers on the apex only read like 516 or so.

I tried calibrating to 10 and the numbers only went as high as like 831 on the Apex.

After calibration it read my pH as 7.52. I used a salifert pH test kit and it was reading my pH at 8.0

What am I doing? The first time I calibrated I had no problems. I stuck the probe in the 7 and the numbers read in the 700's same went with 10. The numbers went to like 1010.

Any help would be appreciated. I cleaned the probe bulb with a soft toothbrush as it says to in the unofficial user guide (very gently) but it looked pretty clean.

01/30/2012, 02:14 PM
If the wires of the pH probe are crossed with any other electrical wires, the readings may be off due to electrical interference IME. Hopefully that's the case. Otherwise your pH probe is old and needs replacement.

01/30/2012, 02:41 PM
I double checked that and Iit's not crossed... anyone else?

01/30/2012, 04:24 PM
Test kits are not so accurate to compare as you are with a calibrated probe.

The main verification comes during calibration; assuming the calibration fluid is good and accurate you should be able to place the probe inthe 2 fluids AFTER calibration completes and you should reading VERY CLOSE to the fluid's value. If this test works then the calibration is good and the probe should read accurate.

You can 'test' with some home products to verify the probe is reading correctly if you still have doubts; search the web (or this forum); there are many tables of home product PH. This type of testing should be a spot test as I suspect there is a range +/- for home products but it should give you some data points to validate against.

01/30/2012, 05:16 PM
Also, don't infer anything from the values that are shown during the actual cal process. They are nearly meaningless.

Test in both the 7.0 and 10.0 solutions aferwards to see how good the cal was.

That being said, I've always had trouble calibrating and always have to tweak my intercept and slope manually.

01/30/2012, 08:30 PM
You can try soaking it in vinegar for a couple of hours, then re-calibrate. That may help.

Make sure you don't have 'probe expansion' turned on. It's one of those old 'display module only' settings - in the System menu. Probably don't but just be sure.

If this is the full Apex, make sure you don't have it plugged into that second pH/ORP port and it's configured for ORP. That would mess you up also.

9 months is not that old for a Pinpoint/Neptune probe. If it's another bulk brand, then it might just be failing.