Apex salinity probe slowly increasing

Agrass1992

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I have seen many threads about the salinity probe, but not one regarding this specific phenomenon. My entire system is new, the probes have been in the tank for about 3 days. I calibrated the pH probe yesterday but have been holding off on the salinity probe until readings stabilize. When I first placed the probe it was very low, I was able to give it a gentle shake and clear out some air bubbles and it quickly raised to a more expected value around 35. Since then it has been gradually increasing. It sways with temperature but overall the trend is still increasing. Is this expected behavior for the probe? How long do people typically wait to calibrate the probe?

Screenshot_20201217-085007_APEX Fusion.jpg Screenshot_20201217-084959_APEX Fusion.jpg
 

Brett S

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I would wait at least a week or two. I don’t know this for sure, but it’s my belief that a film of bacteria or something builds up on the probe over time and that’s what causes this drifting. Once the bacteria or whatever it is has completely colonized on the probe then it stops drifting. It’s worth noting that when you do calibrate it you should be careful not to clean it with any chemicals or anything because that will remove the film and it will just drift again. Instead just rinse it in salt water.
 

Randy Holmes-Farley

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I'm not a fan of leaving a conductivity probe in a reef tank 24/7 because of fouling and other issues, especially less sophisticated 2 electrode probes that are more prone to that concern. That said, I do not see why it is increasing as I would expect that most things that could build up on it will decrease conductivity, not increase it.

I would also note that hobby controllers have a seemingly poor track record with measuring salinity. I'm not sure why. Might be due to electrical interferences, inappropriate temperature compensation, or other issues.
 

Attreui

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I have seen many threads about the salinity probe, but not one regarding this specific phenomenon. My entire system is new, the probes have been in the tank for about 3 days. I calibrated the pH probe yesterday but have been holding off on the salinity probe until readings stabilize. When I first placed the probe it was very low, I was able to give it a gentle shake and clear out some air bubbles and it quickly raised to a more expected value around 35. Since then it has been gradually increasing. It sways with temperature but overall the trend is still increasing. Is this expected behavior for the probe? How long do people typically wait to calibrate the probe?

Screenshot_20201217-085007_APEX Fusion.jpg Screenshot_20201217-084959_APEX Fusion.jpg

It looks like its fluctuating between 36 and 37? What is your SG from a refractometer? A couple things: Every now and then some of mine will just spike. Like it will go from 36 to 45. If I take the probe out and shake it off and hold it out it will drop to 0 and when I put it back in will go back to normal. That doesn't seem to be the issue here but it will happen. Second, a 1-2 ppt variation is very much within tolerance. ppt is so finicky compared to SG. 36ppt is 1.0271 and 37 is 1.0279. I would go ahead and calibrate it and then keep an eye on it comparing it to your refractometer. If it is showing off from that, you can manually adjust the offset to correct it.
 

Randy Holmes-Farley

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Second, a 1-2 ppt variation is very much within tolerance.

Really? Pretty bad specification, if true. Why do you think that? I cannot find an Apex spec on that.

A good conductivity meter (like the Orion 128 that I use) has a specification of +/- 0.5%, which is about +/- 0.2 ppt.
 

Attreui

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Really? Pretty bad specification, if true. Why do you think that? I cannot find an Apex spec on that.

A good conductivity meter (like the Orion 128 that I use) has a specification of +/- 0.5%, which is about +/- 0.2 ppt.
Sorry I misspoke. I shouldn't have said tolerance because that is a scientific term. I meant experience. I agree with you, they are not that good. Over the past 10 years I have had 6 PM2 which are all still running and that is the tolerance I have had with all of them. I never did figure out if it was issues with the probes they use or what, I do know that something changed and the newer probes are actually less accurate than the ones they made back then. I think the Apex salinity system is good for a quick glance to make sure your tank is "ok". It's good to make sure nothing happened in the stand and dumped several gallons of fresh water into the system. I have had great luck with it holding true once I messed with the offset and got it to read correctly. I still use a refractometer to check against.
 

Randy Holmes-Farley

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Sorry I misspoke. I shouldn't have said tolerance because that is a scientific term. I meant experience. I agree with you, they are not that good. Over the past 10 years I have had 6 PM2 which are all still running and that is the tolerance I have had with all of them. I never did figure out if it was issues with the probes they use or what, I do know that something changed and the newer probes are actually less accurate than the ones they made back then. I think the Apex salinity system is good for a quick glance to make sure your tank is "ok". It's good to make sure nothing happened in the stand and dumped several gallons of fresh water into the system. I have had great luck with it holding true once I messed with the offset and got it to read correctly. I still use a refractometer to check against.

Thanks for the info. :)
 
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