Which to believe, pH probe or test kits?

[IsaaX]

Hello, I've had an AC Jr with the lab grade pH probe for a couple months now. Yesterday, out of curiousity I decided to compare the pH probe readings with two pH test kits (Salifert and RedSea). My test kits came up with 8.2 while my pH probe read 7.8. These readings concern me because I do not know which one is correct. I recalibrated the probe twice and came up with the same results. I also recalibrated with the probe line away from any power cords and also tested the tank water in a cup away from the tank. I have temp compensation turned off. I did not change any of the pH offsets in AquaNotes. My probe is floated on a foam block with 3" of the probe submerged in the sump.

Is the probe possibly going bad or defective?

 

[IsaaX]

I forgot to add that I do not have a computer connected to the AC jr.

 

[bobt2]

i had the same thing with a different brand of probe. called their tech support and they assured me the meter was a lot more accurate than the test kits.

 

[IsaaX]

Thanks for the reply. Curt do you have any opinions?

 

[dhoch]

Have you tried testing some know sample with the test kit to see how it performs?

Dave

 

[bobt2]

opinions? no. i deceided to trust the meter. i have 3 tanks and all are doing great. so until i see something wrong the meters the standard i use

 

[clsanchez77]

I had the same issue and went with pH probe. I made my decision from buying a bunch of calibration fluids, callibrating the probe and consistently reading 7.0 and 10.0 calibration fluids from different batches.

Chris

 

[Aquaduck]

So all these pH test kits are giving false high readings?

 

[bobt2]

yes, from what i understand!

 

[Slickdonkey]

Interesting, I have the same problem. I have the lab-grade probe with my AC3 pro that I've been running for 4 months. My pH always reads low, i.e. 7.8-8.0. My test kit (Salifert) reads 8.3 regardless of the time of day, so I don't trust it.

I also have a Pinpoint pH monitor. Last night I calibrated it and my AC3. Both monitors read the calibration solutions (7 and 10) within +/- 0.02 units. However, when I put them back in the tank the AC3 Pro reads 8.0 and the Pinpoint reads 8.25!

Not sure which to believe. The Pinpoint probe is over a years old, maybe it's time for replacement. But I just can't be sure without some more tests. Any suggestions are appreciated.

 

[Aquaduck]

Slick, can you swap the probes between the pinpoint and the AC3? See if they still read the same. If they do then I would think there is a bug in the calibration routine that adds an exagerated negative offset on all the controllers. edit: recalibrate after you swap probes too.

 

[Slickdonkey]

Duck, that's a great idea, didn't even think of it. Will try that tonight!

 

[IsaaX]

My buddy has a ac jr and a pin point monitor. He used the same probe on both units and the ac jr always had a lower pH reading than the pin point.

 

[Slickdonkey]

That's interesting... I just can't figure out why it reads the calibration solutions so accurately, but gets thrown off in the 8.0 region.

 

[IsaaX]

I don't either. It makes me very confused.

 

[agave60]

I have the same problem, my AC Jr with lab grade probe reads .07 less than my pinpoint. I have tried swapping probes, and the results stay the same (I recalibrated when swapping probes). I'm going to do the borax test with each and report back.

 

[pfish]

This is very interesting. I had the same thing. I have an ac jr and a Milwaukee ph controller. I used pinpoint probes for both units. I calibrated numerous times and switched probes between the units and again calibrated numerous time and they both read the calibration solution accurately but the ac jr always read .07 to .1 lower than the Milwaukee. So from reading this it appears the ac jr is consistently lower.

 

[pfish]

For clarification.
Read the tank water lower than the Milwaukee.

 

[Slickdonkey]

Well I hope that's the case, it would go a long way in explaining my low pH readings. Really wish I knew what was going on though.

 

[IsaaX]

I just went into AquaNotes and looked at the pH Probe configuration. One of the settings is "OffSet" and shows a value of -0.05. Anyone else have this?