Neptune Apex Salinity Probe Drift After Cleaning

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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.

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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.
 
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Tobin VP

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Why would this be a reasonable timeframe?

Neptune confirmed their probe should measure "instantaneous" conductivity. The probe is in my display section just prior to my overflow and gets a large amount of flow.

As another example my Hanna Marine Conductivity Probe routinely goes from RODI rinse to tank and reads within 10 seconds. The only reason it takes 10s is that Hanna designed it to wait for temperature to stabilize and it takes about 10s for that to happen for accurate temp correction. But otherwise the conductivity reading is almost immediate.

The reason that I am asking this question is to understand the mechanics. If it is true that a biofilm grows and stabilizes on it in 3+ days then it would suggest that these probes that stay in tank should be left at least this long prior to calibration; and furthermore that any shifts in the tank biome that could lead to changes in algal build up on the probe (or even a small snail) would suggest that recalibration could be in order.

And if this turns out to be true then it really begs an additional question: Does the act of calibrating the probe (using a ref solution) possibly kill the film and then it sheds off once back in the tank and then has to reestablish itself yet again? This would lead to additional fluctuations in these intermitten periods and possible different biofilms each time.
 
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Neptune responded to my inquiry on this and they indicated that it is most likely due to micro bubbles being trapped at the sensor. This turned out to be the cause of the behavior above.

My probe holder requires me to place the probe into the water in a downward direction so in retrospect this explanation seemed plausible. after some testing, I was able to reproduce the low reading upon insertion into the tank water - followed by an approximately 1 ppt / day increase to the actual value. As soon as I tapped on the probe and swirled it within the tank water the correct value was read.

It turned out that a downward insertion of the probe into the water allowed micro bubbles to collect on the sensor and it took several days to clear those based on where my probe was located within the flow.

Lesson learned: swirl and tap conductivity sensors when placing into water !!!

(Asside: Turns out the Hanna portable Marine Conductivity probe instructs to do this same thing prior to measurements.)
 

Randy Holmes-Farley

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Neptune responded to my inquiry on this and they indicated that it is most likely due to micro bubbles being trapped at the sensor. This turned out to be the cause of the behavior above.

My probe holder requires me to place the probe into the water in a downward direction so in retrospect this explanation seemed plausible. after some testing, I was able to reproduce the low reading upon insertion into the tank water - followed by an approximately 1 ppt / day increase to the actual value. As soon as I tapped on the probe and swirled it within the tank water the correct value was read.

It turned out that a downward insertion of the probe into the water allowed micro bubbles to collect on the sensor and it took several days to clear those based on where my probe was located within the flow.

Lesson learned: swirl and tap conductivity sensors when placing into water !!!

(Asside: Turns out the Hanna portable Marine Conductivity probe instructs to do this same thing prior to measurements.)

Thanks for the update! :)
 
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