Need help with an equation

[GothFishKeeper]

I usually am more than capable of doing this kind of thing mentally, but after the week I’ve had my brain just isn’t working like that right now. So hopefully my fellow nerds on here can help me out:

I keep my salinity at 1.026, but I don’t test that as often as I do the rest of my parameters. I just did though and I’m at 1.024. I’m about to do a 20% water change on this system with 100g of total water volume (including displacement), so what do I mix the new water at to bring it up to 1.026? If it were a 50% water change it would be 1.028, but I can’t get my brain to change that math to what’s needed for a fifth instead of half.

If someone could help me out here it would be amazing, I don’t usually drink nearly as much as I have this halloween week and my mind is just mush.

 

[Subsea]

I usually am more than capable of doing this kind of thing mentally, but after the week I’ve had my brain just isn’t working like that right now. So hopefully my fellow nerds on here can help me out:

I keep my salinity at 1.026, but I don’t test that as often as I do the rest of my parameters. I just did though and I’m at 1.024. I’m about to do a 20% water change on this system with 100g of total water volume (including displacement), so what do I mix the new water at to bring it up to 1.026? If it were a 50% water change it would be 1.028, but I can’t get my brain to change that math to what’s needed for a fifth instead of half.

If someone could help me out here it would be amazing, I don’t usually drink nearly as much as I have this halloween week and my mind is just mush.

I can’t help you with the equation, but I can bump it for you.

Also, consider posting on reef chemistry forum.

 

[mmorrison55]

https://www.hamzasreef.com/Contents/Calculators/DirectSaltCalculator.php
That will tell you exactly how much to add assuming you know how much water you have.

 

[mmorrison55]

Or this one may be the one you need.

https://www.hamzasreef.com/Contents/Calculators/ResultingSalinity.php

 

[mmorrison55]

If you are changing 20 gallons and your tank has exactly 100 gallons of water is 1.024 and you want it to be 1.026 after the 20 gallon change, your water needs to be 45 ppt (1.034 sg).

But 99% sure you don’t have 100 gallons exactly, so I’d do 20 gallons @ 40 ppt (1.032), or even less) and check. As most likely if you changed for the 100 gallon #, that you may be over your desired goal.

 

[painter1982]

Mix the new 20 gallons of saltwater to 1.034

 

[BlueTide]

It's a weighted average formula.

80% * 1.024 = 0.8192

1.026 (desired salinity) - 0.8192 = 0.2068

0.2068 / 20% (amount of water to be changed) = 1.034

But 99% sure you don’t have 100 gallons exactly, so I’d do 20 gallons @ 40 ppt (1.032), or even less) and check. As most likely if you changed for the 100 gallon #, that you may be over your desired goal.

Agree here. Another alternative if you don't have sensitive livestock like SPS in the tank is to mix at 1.034 and just top off with RO water if you end up with too high of a salinity.

Practically speaking, if you're off in terms of the percentage of water you are changing by say 5% (i.e. you are really changing 25% instead of 20%) and you mixed the fresh batch at 1.034 that would translate to a SG difference of only 0.005, resulting in a salinity of 1.0265.

 

[Garf]

I usually am more than capable of doing this kind of thing mentally, but after the week I’ve had my brain just isn’t working like that right now. So hopefully my fellow nerds on here can help me out:

I keep my salinity at 1.026, but I don’t test that as often as I do the rest of my parameters. I just did though and I’m at 1.024. I’m about to do a 20% water change on this system with 100g of total water volume (including displacement), so what do I mix the new water at to bring it up to 1.026? If it were a 50% water change it would be 1.028, but I can’t get my brain to change that math to what’s needed for a fifth instead of half.

If someone could help me out here it would be amazing, I don’t usually drink nearly as much as I have this halloween week and my mind is just mush.

How did the salinity become low? If you didn't intentionally add freshwater, or make previous waterchanges too thin, I'd question your salinity testing method. There's a Randy Holmes DIY salinity calibration solution link in my sig if necessary.