Salinity drift

[friendlyAlien]

I have been experiencing a salinity drift from 35ppt to 33pt over about two weeks. I have about 90 gallons of water. And no, it isn't a leak (though I'm happy to consider this, too). The sources of additional RODI water are twofold:

- removal of salt water through the protein skimmer (350 ml two times a week)
- adding freshwater through reagent for automatic KH monitoring using the Hydros iV. The test requires about 60ml of diluted acid. Alternatively, I could dump the test water, but then loose about 120ml salt water.

What I am really curious about is this: I can't be the only one who sees a salinity drift with all the automated testers and heavy skimming. My question to all of you is:
How are you all dealing with it?

In my head, it is clear that I should be able to compute the loss based on water volume, so I could consider compensatory use of saltwater at about 40ppt. However, somehow, the numbers I came up with feel way too high (like 4 liters of 40ppt to raise tank water from 34 back to 35ppt).

Anyway, how are you all dealing with this besides having a MUCH bigger tank and with that water volume?

 

[taricha]

on 90 gallons (~340 liters) the salinity drop from 35 to 33 psu represents something in the ballpark of trading out roughly 5 gallons (20L) saltwater for RODI.

The things you listed don't get you anywhere in the ballpark of 5 gallons in 2 weeks...

- removal of salt water through the protein skimmer (350 ml two times a week)
- adding freshwater through reagent for automatic KH monitoring using the Hydros iV. The test requires about 60ml of diluted acid.

So either the salinity measurement is not accurate and it's not actually changed nearly that much, or you have some other source of saltwater loss.

 

[KStatefan]

You could just add back what you have calculated you are removing.

 

[Garf]

Drifting salinity measuring device seems a likely candidate. How are you calibrating that?

 

[friendlyAlien]

I measure salinity in three ways: with a salinity probe (ignore it), the Hannah digital refractometer, and the tropic marine hydrometer. I did all three for this write-up, but day to day, I just use the Hannah digital one (I am paranoid about salinity and lose too much to wrong readings). I also validated the hydrometer with Randy's calibration mixture.

Back to the fun ... is it really 5 gallons? Even if it is 4 (removing sand and rocks) it is too much, to your point. I wonder what else dilutes. There is no leak, and no big saltcreep anywhere. The filter roller can't take much ... hmm ...

 

[Randy Holmes-Farley Verified Community Expert]

Back to the fun ... is it really 5 gallons? Even if it is 4 (removing sand and rocks) it is too much, to your point. I wonder what else dilutes. There is no leak, and no big saltcreep anywhere. The filter roller can't take much ... hmm ...

It is unquestionable that a drop of salinity from 35 to 33 ppt in 90 gallons must involve the replacement of (35-33)/35 x 90 = 5.1 gallons of tank water by fresh water.

If that did not happen in your tnak, then the only explanation is measurement drift/error. Note that most devices have substantial error bars. the Hanna only claims +/- 1 ppt for each measurement, which makes a 2 ppt change hard to reliably detect.

https://www.hannainst.com/marine-sa...O9HRQgOz_h0gJNb6Hvjs7TGjcoMoszyXuLaraCxF1R8Gx

 

[bobnicaragua]

If the overflow was empty and now it’s full of water. That would account for a few gallons.

 

[Joe31415]

The filter roller can't take much

Check the water level before and after the float triggers. Just quickly running some numbers, a 20g tank holds about 1.5 gallons of water per inch of depth. Assuming your sump is bigger than 20g, that might be your issue, or at least part of it.

Also, if you have an AWC, make sure it's doing what it's supposed to be doing. For example, if it's not pumping in as much water as it's supposed to, the remainder is going to be replaced with water from the ATO.

 

[friendlyAlien]

I'm doing more testing for another week and will report on why I lost so much water or whether it is the measurement's inaccuracy. You all are right; that is way more than it should be, given the simple Formula (thank you, Randy!) for the Salinity change.

I am still curious about the original question. Even if I just lose less, like half a liter, there is drift with all the things we are doing (automatic testing, skimmer, etc.). What do you all do to compensate for that?
I get that the bigger the system is, the less it matters. But what does everyone else do? (besides water changes).

 

[Garf]

I'm doing more testing for another week and will report on why I lost so much water or whether it is the measurement's inaccuracy. You all are right; that is way more than it should be, given the simple Formula (thank you, Randy!) for the Salinity change.

I am still curious about the original question. Even if I just lose less, like half a liter, there is drift with all the things we are doing (automatic testing, skimmer, etc.). What do you all do to compensate for that?
I get that the bigger the system is, the less it matters. But what does everyone else do? (besides water changes).

I don't skim and don't auto water test, lol.