Big pH deviation in 2 weeks after calibration

Hello

I'm using a Profilux4 unit in a 540L freshwater aquarium. pH probe is used to control CO2 injection during the time lamps are switched on (I call it "day" here for short).
CO2 valve is completely disabled when lights are off ("night"), and controlled by pH probe when lights are on.

System works with no issue after calibration but two weeks later deviation in pH reading is so high that the CO2 valve remains closed most of the time.
I have replaced the pH probe, same behaviour, that is as follows:

- Calibration with 4.01 and 7.01 fluids => everything OK
pH value in Profilux matches pH value read by a Hanna HI98127, and also matches the aprox. value read by a Sera ph test (chemical test)
Values in aquarium:
at night (lights off, aeration on): 7.2
at day (lights on, aeration off, CO2 controlled by pH): desired pH value (6.5), after several hours of non-stop CO2 injection

- After two weeks:
pH value in Profilux does not match value read by Hanna HI98127. The reading of the Hanna HI98127 seems consistent with chemical pH tests and also with the CO2 concentration in water and Alcalinity.
pH value read by Profilux does not seem consistent with the measured CO2 concentration and Alkalinity.
Values in aquarium:
at night: Hanna reads 7.2, Profilux reads 6.85
at day: Profilux reads the desired value (6.5) but injection only last for two hours maximum. Hanna reads 6.7.

The deviation is not sudden. A small error can be yet seen first day after calibration. Deviation increase with the time.

Seems that the readings from the Profilux pH probe are more acidic (lower) than the reality, so CO2 injection is stopped before reaching the desired concentration.

Is this normal?
Should I be using a different probe, for freshwater aquarium?
Is this a calibration-related issue or is it caused more likely by biofilm in the probe? I clean the probe each time I recalibrate.

Regards

Pablo Gil

Hello Pablo,

You can check the calibration outcome and accuracy of the pH probe by letting it sit in pH4 fluid for a few minutes. See what value is displayed and compare it to the pH4 value fluid. Do the same test for pH7 fluid. If the numbers are within 0.1 of the fluid value, the probe is providing the correct numbers with tolerance. The tolerances of the Hanna device should also be considered instead of the fixed value it is showing.

Hello Vinny

Just after calibration of the GHL probe, both devices (GHL probe and Hanna HI98127) read exactly 4.01 and 7.01 +/- 0.1
Even, the Hanna device achieves that without being calibrated so often. The difference of course is that when not in use the Hanna device is not in the water; it's closed with a few drops of storing solution inside the cap.

As the days go by, readings from both devices are more and more different. After 2-3 weeks GHL readings are so low that it cannot perform a good job controlling CO2 injection: it stops injection before achieving desired CO2 concentration. At least, the error is always on the same direction (lower) so it's safe for the fish, no risk of overdosing CO2.

On last calibration, I recall putting the GHL probe in tap water (pH 7.8 here in Valencia, it's hard water) before cleaning and calibration, and it read 7.3. Hanna read the correct value.

Next calibration I will perform a 4.01 and 7.01 test before calibration just to report which values it's reading. I will do 3 times: before cleaning, after cleaning, and after calibration.
I will report the values here.

Regards

PH drifts are often the result of electricity in the water. Often pumps, heating, etc. are the cause. Also a faulty electrical grounding of the tank can be the cause.
Regards Pit

Hello Pit, thank you for feedback

After reading your message I performed the following test: I filled a 1-liter pot with water from the aquarium and I left it in a table near the aquarium. Then I placed the pH probe in the pot. Same water, no other electrical device in it. I left it for 15 minutes, the measured pH value was the same.

In the same pot (and in the aquarium) Hanna meter is reading 0.4 more (6.5 Hanna / 6.1 GHL). The drop checker colour is consistent with the Hanna checker (green with a bit of yellow). CO2 concentration at KH 3.5 is 33 ppm at pH 6.5 and 83 ppm at pH 6.1. The drop checker colour and observed fish behaviour (completely normal) are consistent with 33 ppm CO2. When I have performed direct CO2 concentration tests (chemical) at the same hour, the values have been around 40 ppm.

Before returning the pH probe to the aquarium I did another test. I cleaned the probe with Hanna Cleaning solution and then I placed the probe again in the pot. After 5 minutes it was reading again 6.1.

After searching documentation about pH drifting, I have seen that possible causes are: electrical potential in the water, clogging at the ceramic junction, and also low conductivity sampled media. My water is 185 ppm TDS (370 uS) it is soft freshwater very different from seawater, I wonder if it can be the cause. In the documentation also says that with low conductivity media you get wrong values but you get perfect readings if you place the probe in calibration fluids, this happens to me as well.

I wonder if I should be using a different pH probe. I will post the feedback requested by GHL at next calibration (maybe next weekend) and let’s see.