PH probe problem?

[dabob79]

I'm getting inconsistencies in my PH readings, and I can't figure out if it's my ph probe or not. To start off, I have the full Apex with lab grade ph probe. A few months ago when I first set it up I calibrated the probe following the Apex manual instructions, and it seemed right. I tested with a Salifert ph test to verify and and matched what my ph probe was reading. Fast forward a few months and I decided to use my Salifert test again to verify my ph probe again, because my ph was reading lower than normal. PH probe was reading 8.1, Salifert test read 8.3. So, I figured my probe needs re calibrated so I go out and buy some calibration solution packets and re calibrate. Immediately after re calibrating I notice it's reading even lower that it was before, now down to 8.0. So, I re calibrate again, and again the ph reading comes out a little lower. Testing this morning with my Salifert test and it says 8.1, but Apex probe is reading 7.64. I know there's no way my ph is that low. Alkalinity is at 9, windows have been open the last couple days and I run a co2 scrubber on my skimmer air intake. Am I doing something wrong? Is my probe going bad?

[jonmos75]

The way I verify if the pH probe needs to be calibrated is I stick it in a pH solution packet (7.01) and see what the probe reads if it is +-.2 then I will recalibrate the pH probe

[kurt_n]

You say you set it up a couple months ago... is the probe only a couple months old? Was it new? Probes should last at least a year - if not more - assuming it was stored properly before you bought it. There should've been moisture in the cap when you removed it when it was new.

Assuming the probe is good, after you calibrate it's good practice to immediately return the probe to the 10.0 and 7.0 solution packets. They should read 10.0 and 7.0 dead on (give or take .01 or so). If not, something went wrong with the calibration.

Another thing I like to do is rinse the probe in RODI water and shake off any excess water before putting them in the calibration solutions. That way, you're not contaminating the calibration solution and causing a shift in the pH of the solution.

If your probes read correctly in the calibration solution after calibration, then I'd believe what the Apex says when you immediately put it back in the tank. If the pH seems right when you put it in, but then starts to drift strangely, it could be the probe going bad or some type of electrical interference with the probe cabling. In my experience, a consistent "drift" either up or down implies bad probe... but erratic readings up and down without any rhyme or reason implies interference or perhaps a bad circuit internal to the Apex.

[dabob79]

Yes the probe and the Apex were brand new. I did leave it in the 10.0 solution after I calibrated it and it read 10.0 dead on, but in my tank it's only reading 7.6 when two different test are showing 8.04 (that's as of 10 pm this evening). There's no way my ph is that low. It's not drifting either, it's a solid reading. I just don't get it

[jonmos75]

If you have not done so I would open a support ticket (https://www.neptunesystems.com/support/contact-support/) with Neptune

[slief]

Sounds to me like you aren't letting the proble settle long enough for each solution during calibration. I usually let it settle for 7 to 10 minutes for each step. Another thing that is very important is to make sure the calibration solution is the same temp as the tank water during calibration. Otherwise, the temperature compensation will not be accurate. I usually add some tank water to a cup, place the calibration solution packet in the cup with the tank water and place that cup in a spot in my sump where it stays put and insures that the temp of the solution stays the same as the tank. The other alternative is to place the temp probe in the calibration solution with the pH probe. Also, don't open the calibration solution packets all the way. Just open them enough to get the probe into them. If you open them all the way, the exposure to the o2 in the air will change the pH pf the calibration solutions. be sure to rinse the probe in tap water and shake it dry very well before placing it in the calibration solution. Be sure to swirl it in the solution packets a couple times to make sure there isn't air trapped in the tip. Lastly, I use a soft tooth brush to clean the probe tip before calibrating.

[kurt_n]

Yes the probe and the Apex were brand new. I did leave it in the 10.0 solution after I calibrated it and it read 10.0 dead on, but in my tank it's only reading 7.6 when two different test are showing 8.04 (that's as of 10 pm this evening). There's no way my ph is that low. It's not drifting either, it's a solid reading. I just don't get it

Did you put it back in the 7.0 solution also? If it doesn't read dead on in BOTH solutions, the calibration was faulty.

I wouldn't trust any titration test over a properly calibrated probe. While I hear what you're saying about the variance between tests, I wouldn't assume the 8.04 is correct either.