Why is my Salinity Fluctuating?

[RussC]

I am my own worst enemy! Having all these numbers and controls at my fingertips is as much my problem as it is my solution.

But I do hope to better understand and am reaching out to RC for your input. Why is my salinity fluctuating and seeming to actually increase?

Since 8/31 my salinity has slowly increased. Its minor but it gets me wondering and asking questions. If nothing else, to better understand more about this hobby. Since 8/31 its gone from 35.4 to 36. There was even a spike one day of 36.3. There is nothing but live rock and substrate in the tank. The tank is brand new, born on 8/29/17. My goal is to keep it between 34-36. My top off is with RO/DI water.

One thing that occurred to me could be undisolved salt. When I mixed my salt I did it in the tank since there was no life. Maybe there is some undissolved salt that is taking its sweet time dissolving?

McGyvr is probably going to let me have it. He has reassured me things are okay and to not be worrying so much. But like I said, I am my own worst enemy.

Thoughts?

 

[JustinM]

I would say it's undissolved salt as you said. I know when I mix my water up in my brute, it takes awhile for it to stabilize. Also temperature will affect the salinity as well.

Was your water brought up to temp or was it right from the RO/DI?

 

[der_wille_zur_macht]

Undissolved salt would only last hours at most, not days.

How are you measuring salinity? This could just be drift in your measurement, not drift in the actual value. Also, it will fluctuate some as you evaporate and top off.

 

[der_wille_zur_macht]

I would say it's undissolved salt as you said. I know when I mix my water up in my brute, it takes awhile for it to stabilize. Also temperature will affect the salinity as well.

Was your water brought up to temp or was it right from the RO/DI?

To be completely accurate, temperature affects density, not salinity. Some instruments have a built in conversation that assumes a specific temperature and/or some means of temperature compensation. But you are correct, depending on the measurement technique and if or how you are converting it, temperature can cause a difference in the final result.

 

[RussC]

When the water and salt were originally mixed, it was room temperature. I was not operating the heater at that time.

As for measuring, I monitor it through the Neptune probe as well as a refractometer. So if temp affects density, how will that affect measurements?

My refractometer says 20 degrees celsius and measures both PPT and specific gravity. And its matching up with my probe reading. Close enough, anyway.

Should I be looking for something regarding the reading based on the temperature at the time of the reading? Like an offset or some adjustment of some kind? For example, my temp is 76.7 and my salinity is bouncing between 35.8 and 35.9.

 

[RussC]

If I'm reading the charts I have correctly it looks like the warmer the tank gets, the denser the salinity in PPT it gets. So when the tank cools, I should see a small reduction in salinity that correlates with that drop in temp. Sound right? Still, though, I wouldn't expect to see an overall increase over time even if that increase is only .5. I would expect to see a decrease. Thus my questions...

Does the APEX system have a built in conversion? I don't recall reading that. Could have overlooked it, though.

 

[Maggie321]

Evaporation and not topping off as much as evaporation.

Sent from my SM-N920V using Tapatalk

 

[Xxero]

I know that you said there is only rock and substrate in the tank, but are you dosing Magnesium yet? I see that you have a BRS doser for Cal & Alk, so it makes me wonder.

Significant doses of Magnesium can sometimes raise your Specific Gravity.

 

[mako61]

My salinity reading varies also. For me it varies almost directly by temp. In apex you can overlay the temp graph on the salinity graph and see the correlation.

 

[der_wille_zur_macht]

If I'm reading the charts I have correctly it looks like the warmer the tank gets, the denser the salinity in PPT it gets. So when the tank cools, I should see a small reduction in salinity that correlates with that drop in temp. Sound right? Still, though, I wouldn't expect to see an overall increase over time even if that increase is only .5. I would expect to see a decrease. Thus my questions...

Does the APEX system have a built in conversion? I don't recall reading that. Could have overlooked it, though.

Density of water (in liquid form) decreases as temperature increases. So for a given salinity value, the equivalent density will be higher at lower temperatures. To think of it from the opposite perspective, if you're seeing a specific density measurement (say, 1.026) and trying to convert to salinity, the equivalent salinity will be lower at lower temperatures.

You mentioned you are using an Apex. I don't know if it has built in correction. I wouldn't be surprised if it was just hard coded to a specific temperature, since it's pretty darned safe to assume a reef tank is about 80 degrees F, and the Apex can't assume it has access to a temperature reading on the water sample. I've never used an Apex, but I've used other electronic salinity instruments and have built and designed a few myself. Most (all?) electronic salinity probes are actually measuring conductivity, and then converting to salinity. For a given conductivity measurement, the salinity will be higher at lower temperatures. It's the opposite as converting from density to salinity, since conductivity and density are inversely related.

For the record, we are way down the geek rabbit hole at this point. You don't really need to know or care about any of this unless you find it interesting. Your readings are totally fine and most people would consider them well within the margin of error. Depending on how you're topping off (all at once vs spread out) you could be getting variations due to salinity rising and falling through the evap/topoff cycle, which is normal and not a big deal.

It might be worth calibrating your Apex probe (I'm assuming it has a calibration function) if you haven't. I don't know what their recommended procedures are but any time I see something out of whack, my first reaction is to doubt the measurement procedure vs the actual value. By double checking or calibrating the measurement, you are eliminating possible causes of the variation.

 

[der_wille_zur_macht]

My salinity reading varies also. For me it varies almost directly by temp. In apex you can overlay the temp graph on the salinity graph and see the correlation.

Well that makes it pretty clear that they're not accounting for temperature, which is what I would expect.

 

[der_wille_zur_macht]

This is clearly not the issue here, but one other factor that comes up in these "why is my salinity slowly going off track" threads is the impact of skimmers. If you have a skimmer, you are essentially removing a little bit of saltwater every time you empty the collection cup. Over time, that saltwater will be replaced with (fresh water) topoff, unless you are specifically accounting for it. This can cause tanks to slowly drop in salinity. Usually people play catch up during a water change, probably without even realizing it.

 

[mcgyvr]

I am my own worst enemy! Having all these numbers and controls at my fingertips is as much my problem as it is my solution.

Thoughts?

enough said right there ^^^^ :lol2:

 

[Ron Reefman]

I'm going to join in with mcgyvr again.

The fluctuation in salinity is WAY too small to be concerned about. Evaporation and the replacement with new fresh water could be the cause. My tanks vary more than that and I see absolutely no effect on anything in the tank.

RELAX MAN!

 

[mako61]

I absolutely do not worry about small changes in salinity. I just noticed it changes with temp, more interesting than anything else. I just watch for a big change in case my ATO sticks on, so I set my alarm point accordingly.

As far as skimmer causing creep I dont think it matters unless you have a very small tank. I add 2 part which will I think raises salinity a bit over time. I would think reasonable water changes should cure both these issues.

 

[FullBoreReefer]

Since 8/31? As in a new probe on 8/31? A whole new apex on 8/31? Or just the salinity rise since 8/31?

 

[RussC]

enough said right there ^^^^ :lol2:

I knew I could count on McGyvrs support!:rollface:

 

[ca1ore]

This could just be drift in your measurement.

This would be my guess.