pH probe reads 7.3 but that can't be right....help?

[Admonition]

I've been having an issue with my pH probe and I can't seem to determine if its a probe malfunction or not. My tank has been running normally at 7.7-7.9 for about six months now (due to high C02 in the apartment as proven by Randy's test, otherwise it would be higher). That being said, a few weeks ago my pH probe has been giving me very unusual readings of 7.3 low to 7.5 high- obviously not good. I quickly tested my pH with my backup API kit and it gave me a reading of 7.8, which is normal. An inspection of the corals also showed that my SPS all had normal looking polyp extension and nothing looked stressed out. I subsequently re-calibrated my probe and then tested it back in the solution packets and it gave an almost perfect reading; however, once placed back in the tank it went right back to a 7.5 pH reading.

 

I then decided to pick up a few other test kits to confirm it was the probe and not my API kit. In the week's time it took for the kits to get to me, the low probe readings continued but still my corals and fish seemed unaffected.

 

This leads me to yesterday and today when I used the test kits. It should be noted that due to the insanely cold weather here in florida, we had our windows open over the last few days. This caused the probe to start reading 7.95 and 7.68 high/low due to the circulation of fresh oxygen into the house. As for the details of everything, my probe is a 1.5 year old Neptune Systems lab grade pH probe attached to my Apex, which is up to date on its software. The kits I used were:

 

API (expiration 10/17)

Salifert (expiration 7/18)

Elos (expiration 2/17)

Milwaukee 10.01 solution (expiration 1/20)

Milwaukee 7.01 solution (expiration 10/19)

 

Yesterday, 02/07/16, I tested my water around 3pm with the above kits twice and received following readings:

 

API: 8.2 and 8.2

Elos: 8.3 and 8.3

Salifert: 8.0 and 8.0

Probe: 7.90

 

The above probe was also tested against the 7.01/10.01 Milwaukee solutions (which were floated in the tank water for twenty minutes) and gave a reading of 6.98 and 10.00.

 

Today, 02/08/16, I tested my water around 10am with the above kits twice and received the following readings:

 

API: 7.9 and 7.9 (it looked between the 7.8 and 8.0 colors)

Elos: 8.1 and 8.1

Salifert: 8.0 and 8.0

Probe: 7.68

 

The above probe was also tested against the 7.01/10.01 Milwaukee solutions (which were floated in the tank water for twenty minutes) and gave a reading of 7.00 and 10.00.

Looking at the data from yesterday I'd say the numbers were split between the API/Elos and Salifert/Probe readings. The data from today leans more toward the three kits averaging 8.0 with the probe far off at 7.68. To me it would seem that the probe may have reached its two year life expectancy early and I'll have to purchase a new probe. The only thing that I can't understand is why the probe would give me such drastically unusual readings all of a sudden but still measure out almost perfect in the Milwaukee calibration solutions. Any ideas or should I just purchase a new probe?

[markalot]

Is this on an Apex? And you did the calibration with the two solutions and waited for the probe to settle each time before continuing?

 

If this is an Apex probe did you make sure to keep the top of the probe out of the water?

 

Sometimes probes can be effected by electrical interference. Have you added anything or changed anything that might be causing issues?

[Admonition]

Is this on an Apex? And you did the calibration with the two solutions and waited for the probe to settle each time before continuing?

 

If this is an Apex probe did you make sure to keep the top of the probe out of the water?

 

Sometimes probes can be effected by electrical interference. Have you added anything or changed anything that might be causing issues?

Yes this is on a full Apex. And yes when I calibrated it I waited until the numbers to settle before moving on to the next. I also 'washed' the probe in RODI water between each solution to make sure the solutions didn't mix.

 

Yes, my probes are not fully submerged. They are only half submerged in water and subsequently held in place by Neptune Systems probe rack.

 

I thought it might be electrical interference as well but I haven't added or changed anything in the last six months, so I have no idea why it would be doing this at random now.

[markalot]

Yes this is on a full Apex. And yes when I calibrated it I waited until the numbers to settle before moving on to the next. I also 'washed' the probe in RODI water between each solution to make sure the solutions didn't mix.

 

Yes, my probes are not fully submerged. They are only half submerged in water and subsequently held in place by Neptune Systems probe rack.

 

I thought it might be electrical interference as well but I haven't added or changed anything in the last six months, so I have no idea why it would be doing this at random now.

 

I would say the probe is faulty, no other reason I can think of unless you are looking at the wrong PH port. You could try plugging it into the other Ph/ORP port and re-calibrating. I renamed my calibrated port PHc.