salinity dropping, no auto-topoff, no skimmer

[fryman]

Yes I know salinity is the most basic measurement we do, so what kind of newb struggles with this? Please bear with me a min.

Not my first tank, I've had several reef tank over the years. Starting a new 50gal just set up a week ago. Got the Apex installed a few days ago, and after calibrating noticed salinity showed this sort of see-saw trend over time:

I thought it was odd but probably an artifact of the probe. The probe is located in the sump near the overflow so gets flow but not a ton of flow.

Now the salinity appears to be dropping? I don't have an auto topoff or skimmer setup on this tank yet. I checked this morning uaing a digital refractometer, which agreed with the Apex measurement. I'm not topping off with RO, because salinity is too low I thought I'd wait for evap to drive it back up. Instead it's dropping? How the heck?

Setup is brand new so not all equipment installed yet. I have pumps and heater, that's it. And now the Apex. Can't find a leak although even if there were I don't see how a leak drives down salinity unless I was topping off with ro.

I'm not adding RO. The heck is going on?

 

[jassermd]

No need to get concerned!
Clean your probe. This is VERY typical for the salinity probe. I have this happen all the time...
The conductivity probe is sensitive to micro airbubbles, debris, algae, etc. It doesn't measure salinity directly, but rather the conductivity of the solution.
Take the probe out, rinse it in RO, wipe it gently with your hands or a soft towel, rinse again, and put it back in your tank/sump.
If the problem persists, you can recalibrate.
Remember, salinity will for the most part, will only change through evaporation (more concentrated) or by adding RO (dilutes). If neither of those are the cause, check your equipment.

 

[zalick]

The ups and downs likely correspond with your temperature swings. Check the graphs and I bet they match.

as for the salinity dropping, @jassermd is right. I personally stopped bothering with recalibration since I had to do it too often. I no longer care about the absolute value it’s showing but rather that it stays in the current uncalibrated range.

 

[fryman]

I think you are both right. I should just wait and see I will clean the probe tonight it's just when I comfirmed a low readimg with refractometer I was perplexed. I knew salinity was a bit low but expected it to increase and it hadn't. I think it's just too soon and I need to wait longer. In my small tank salinity can change significantly with daily evaporation but this is a bigger volume.

The see-saw salinity does indeed correlate with temp. My temp swings about 1C and this is apparently enough to affect conductivity.

TY both

 

[Randy Holmes-Farley Verified Community Expert]

I think you are both right. I should just wait and see I will clean the probe tonight it's just when I comfirmed a low readimg with refractometer I was perplexed. I knew salinity was a bit low but expected it to increase and it hadn't. I think it's just too soon and I need to wait longer. In my small tank salinity can change significantly with daily evaporation but this is a bigger volume.

The see-saw salinity does indeed correlate with temp. My temp swings about 1C and this is apparently enough to affect conductivity.

TY both

The device ought to be correcting for temperature changes. maybe there is a setting for it that isn't checked off properly.

 

[fryman]

The device ought to be correcting for temperature changes. maybe there is a setting for it that isn't checked off properly.

Good call!

I found temp compensation in the advanced tab:

I set this "TC factor" to 2.2 per the help button, and then made 53 mS/cm conductivity solution using Randy's guide:

http://www.reefkeeping.com/issues/2004-06/rhf/

Basically 3.29% salt by weight in RO.

For anyone having this issue, using Manual Calibration is better than automatic. It's basically the same procedure just gives you the actual sensor reading (and no time limit). Per Neptune, reading in air should be 60-90 and then 200-900 in the 53 mS calibration solution. You need to wait for the reading to stay constant for at least a minute. It might stay constant for 30 seconds then suddenly change so be patient.

Randy, I don't know if people tell you this often enough but you are a scholar and a gentleman. I have read many of your articles and found them to be very helpful.

Thank You!