I conducted a 20% water change (on a 10 gal Fiji cube) and removed all of my Neptune Apex probes and cleaned/scrubbed them with a dilute citric acid bath. They then remained in RODI water for a couple of days while I did some other work on my tank. Upon putting them back into service the ORP, temp, and pH probes read correctly but the salinity probe read very low at around 31 ppt and then dropped under 30 ppt shortly after. At the time of probe placement I measured the salinity of the tank at 33.5 ppt using a Marin Tropic High Precision Hydrometer. I know that it is suggested to recalibrate probes after cleaning, but in this case since the other probes appeared to have not lost any calibration, and since it was drifting, I was curious to see what the steady state would become.
Three days later and my salinity probe now has reached a steady state at the correct value without any additional calibration. I confirmed the 33.5 ppt salinity reading initially at the time of the low reading and again now after steady state was achieved.
As can be seen the approach to steady sate is exponential and appears to be some kind of natural process.
I should note that a very similar situation occurred for me last December when after a water change (with no probe cleaning in this case) my salinity reading did the same thing to a lesser degree. I posted on it at the time here and in the end I discovered that I made an ATO sensor placement error and the increase in readings was accurate and do to evaporation. This is not the cause in this case as the ATO sensor remained in the same location and the salinity was measured before and after with a high precision hydrometer. There appears to be a break-in period of the conductivity probe after cleaning.
Previously @Randy Holmes-Farley had confirmed that saltwater mixing should occur very quickly and remain stable unless there are pockets of seawater not readily in flow (for example deep sand/gravel). I have 3/4" gravel that all gets very strong flow so this does not seem possible as the explanation. Moreover, the readings before and after exclude this as a possibility.
I also previously confirmed from Neptune that there is no time averaging going on with readings and that it should be providing "instantaneous" readings.
Also, temperature was maintained at 78-79 F during this entire period so it is unlikely to be an oddity with temperature correction in the reading.
Does anyone have more knowledge about conductivity probes that might be able to explain this phenomena? I'm thrilled that I did not have to spend another calibration packet on the probe at this time ... but I'm surprised at seeing this.
The only possible hypothesis I have right now is that when the probe was last calibrated it had a steady-state microbial/algal film on the sensor and that film plays a role in the conductivity reading. Then after starting with a clean probe it took about 3 days to rebuild (in this case) a similar steady-state film once back in the tank. It seems like fouling on the sensor could create some slight variations of conductivity ... but I am quite shocked to see that it would be this much.
Three days later and my salinity probe now has reached a steady state at the correct value without any additional calibration. I confirmed the 33.5 ppt salinity reading initially at the time of the low reading and again now after steady state was achieved.
As can be seen the approach to steady sate is exponential and appears to be some kind of natural process.
I should note that a very similar situation occurred for me last December when after a water change (with no probe cleaning in this case) my salinity reading did the same thing to a lesser degree. I posted on it at the time here and in the end I discovered that I made an ATO sensor placement error and the increase in readings was accurate and do to evaporation. This is not the cause in this case as the ATO sensor remained in the same location and the salinity was measured before and after with a high precision hydrometer. There appears to be a break-in period of the conductivity probe after cleaning.
Previously @Randy Holmes-Farley had confirmed that saltwater mixing should occur very quickly and remain stable unless there are pockets of seawater not readily in flow (for example deep sand/gravel). I have 3/4" gravel that all gets very strong flow so this does not seem possible as the explanation. Moreover, the readings before and after exclude this as a possibility.
I also previously confirmed from Neptune that there is no time averaging going on with readings and that it should be providing "instantaneous" readings.
Also, temperature was maintained at 78-79 F during this entire period so it is unlikely to be an oddity with temperature correction in the reading.
Does anyone have more knowledge about conductivity probes that might be able to explain this phenomena? I'm thrilled that I did not have to spend another calibration packet on the probe at this time ... but I'm surprised at seeing this.
The only possible hypothesis I have right now is that when the probe was last calibrated it had a steady-state microbial/algal film on the sensor and that film plays a role in the conductivity reading. Then after starting with a clean probe it took about 3 days to rebuild (in this case) a similar steady-state film once back in the tank. It seems like fouling on the sensor could create some slight variations of conductivity ... but I am quite shocked to see that it would be this much.
