pH sensor malfunctioning (profilux 3)

Hi,

We got the profilux 3 from a colleague and hooked it to our saltwater system. The pH value was around 7,5/7,6, which is what we measured with our lab pH meter so we didn't calibrate.
Slowly over the time we saw an apparent descrease in pH in our system, we tough we had something going on with our fish, and after few weeks it reached 7,00.

It was so slow that we didn't think it could be due to the electrode. Finally I unplugged everything and checked with known solutions, the electrode was not reading good values, so I calibrated it using the 2 points method (4 and 7 solutions from our laboratory stock). It seemed to work for a day and then we had the opposite issue, the pH is apparently going up now to 8,1. I remeasured our known solutions and there are errors:
4 is read 4,20, 7 is read 7,20, 9 is read 9,30+ and 10 is 10,30+ (keeps of going up slowly, I have stopped measuring after 10 minutes).

What could it be due to?

The electrode needs some special cleaning? Or do we need to get a new one?

Thanks in advance,
Best regards,

Jonathan Roques

Hi Jonathan,

Do you know how old the probe is? It could have already reached the end of it's usable life. I would recommend calibrating with 7 and 9 calibration fluids because these are within the normal ranges for saltwater tanks.

Hi Vinny,
Thanks for the quick reply!
I will ask my colleague about the age of the probe and I am going to try calibration with 7 and 9 solution. I will let you know ASAP.
Cheers!
Jonathan

Hi Vinny,
Sorry for the delayed reply. I have asked my colleague and indeed the electrode was quite old so he ordered a new one and now I have installed it and calibrated it (7 and 9.00) and it seems to be fine, oscilating between 7,64 and 7,58 for the last 4 days.

I have another concern about the conductivity electrode now, it is pretty stable but I think it is quite high (43,2 mS/cm2), when I measure it with another device I have something close to 37 (salinity is 29,7 ppt, temperature 10,5 degrees). I did re-calibrate it 15 days ago, before that is was oscillating daily, alsways the same pattern, from around 42 in the morning and down to 37). I guess I should maybe consider replacing it as well?

Or is there a special calibration technique? When I was using the calibration liquid (50), I was in a quite cool room, the liquid temperature was stable at 18 degrees so I used this value to calibrate. Is there a special trick to calibrate or do you think the electrode should be changed?

Thanks again for your help!
Cheers!

Jonathan

Hi Jon,

Depending on the age of the Conductivity probe, it may be giving false readings due to age. We typically see users swapping these probes every year or two. It ultimately depends on when the probe itself no longer holds calibration as long as it did when it was new. Before you go swapping it for a new one, I would suggest you re-calibrate.

- Make sure the 50mS calibration fluid is the same temperature of the tank water. You can leave the whole bottle floating in the sump for a few hours before doing the calibration.
- Use the primary temperature probe as the temperature compensation.
- When you are asked to enter the temperature of the fluid, enter the correct temp.
- Take the bottle out of the tank only when you are prompted to dip the probe in the 50mS fluid. This will eliminate the chance for the fluid temp to drop in-between calibration steps.

If you find that the probe ends up deviating from a reasonable value in short time, it may be best to replace the probe.

Hi Vinny,

Thanks a lot, I will try out today and see if it works, otherwise I will get a new one from our reseller.
Have a nice week end!

Jonathan