Mixing reservoir salinity increase?

[Trey]

So this has happened the last two times I have made a batch of saltwater in my mixing bin using tropic Marin pro reef salt.
I employ auto water changes at a rate of 3 gallons per day on my Waterbox 180.5. I test sg in the display a couple time a week using a Milwaukee, as well as a standard atc refractometer. The last two times I made new batches of water in my mixing station, I heated the water to 78 to match my display, mixed the appropriate amount of salt until it it registered at 35 ppt (1.026-1.027), continued mixing for a couple of hours, then removed heat and turned off the pump. A week later (both times), I noticed the salinity in the DT had risen to 1.028ish. Upon checking the SW reservoir I had found, in both instances, that the salinity had risen to 1.030! Is there a simple explanation to this, and how might I prevent this in the future?

Full disclosure, I am using RODI, through (2) 7 stage units. The resulting mixed saltwater tested (in both instances) 35ppt after initial mixing and again 24ish hours later.
What could account for this raise in sg over an approximate week long period?

 

[sdreef]

What is the water temperature in the reservoir when you aren't heating it?

When water cools, the salinity increases related to increased density.

 

[Trey]

What is the water temperature in the reservoir when you aren't heating it?

When water cools, the salinity increases related to increased density.

The Milwaukee is an ATC instrument, and I heat the water I test (from the mixing reservoir) in a 1mm syringe suspended in the DT before testing to ensure accuracy.

 

[Randy Holmes-Farley]

What is the water temperature in the reservoir when you aren't heating it?

When water cools, the salinity increases related to increased density.

Salinity does not increase, but the density does. Different ways of measuring salinity may be impacted differently by the temp change (if at all).

 

[Randy Holmes-Farley]

So this has happened the last two times I have made a batch of saltwater in my mixing bin using tropic Marin pro reef salt.
I employ auto water changes at a rate of 3 gallons per day on my Waterbox 180.5. I test sg in the display a couple time a week using a Milwaukee, as well as a standard atc refractometer. The last two times I made new batches of water in my mixing station, I heated the water to 78 to match my display, mixed the appropriate amount of salt until it it registered at 35 ppt (1.026-1.027), continued mixing for a couple of hours, then removed heat and turned off the pump. A week later (both times), I noticed the salinity in the DT had risen to 1.028ish. Upon checking the SW reservoir I had found, in both instances, that the salinity had risen to 1.030! Is there a simple explanation to this, and how might I prevent this in the future?

Full disclosure, I am using RODI, through (2) 7 stage units. The resulting mixed saltwater tested (in both instances) 35ppt after initial mixing and again 24ish hours later.
What could account for this raise in sg over an approximate week long period?

If all the salt is dissolved and the solution is clear, the salinity is not increasing (except perhaps by evaporation of some of the water) and any effect you see is a testing issue.

If there is still undissolved salt, then that will increase salinity as it dissolves.

Precipitation of calcium carbonate over time will have a tiny effect to lower salinity.

 

[Trey]

If all the salt is dissolved and the solution is clear, the salinity is not increasing (except perhaps by evaporation of some of the water) and any effect you see is a testing issue.

If there is still undissolved salt, then that will increase salinity as it dissolves.

Precipitation of calcium carbonate over time will have a tiny effect to lower salinity.

Thanks for the reply Randy. I wonder what my issue is. I will have to watch more closely next time. Very odd to me that it has happened twice this way..

 

[Randy Holmes-Farley]

FWIW, the Milwaukee seawater refractometer has a very wide range (4 ppt) of acceptable tolerance, so it may just be random variation.

Milwaukee MA887 Digital Salinity Refractometer

The Milwaukee Instruments digital salinity refractometer is ideal for saltwater aquarium professionals and enthusiasts.

milwaukeeinstruments.com

 

[YankeeTankee]

Winter heating = dryness could have increased evaporation in your saltwater container.

 

[Trey]

Winter heating = dryness could have increased evaporation in your saltwater container.

I think my containers are pretty evaporate proof..