Top off amounts for correct specific gravity

[Bill B]

I have pondered this question over time and never have been able to get my mind around it:

When mixing a batch of salt water. I will usually come up with a specific gravity that is a little higher or lower than I am shooting for. For instance perhaps I am mixing 10 gallons of water with "2 plastic cups and a little" of salt and my reading is .28. Shoorly someone knows how to figure how much water to add to bring the batch to .25. (other than just add a little more water (or salt) and check again.) If this is actually complex, make that a big Rosanna Dana NEVERMIND; but I suspect it is just a proportion formula - then again I haven't figured it out.

Thanks

Bill

[10gReef]

what kind of salt are you using? I use redsea coral pro and find just weighing it out works pretty good. sure that doesnt help your current question but it might improve your next salt mix. best of luck.

[jestep]

To lower you just need to calculate what percentage of salt needs to be removed.

Reducing from 1.028 to 1.025 (37.1ppt - 33.2ppt) is roughly a 10% reduction in total salt, so in 10 gallons you would remove 1 gallon of salt water and replace it with RO water to get to 1.025. If you're just adding water you would need slightly more than 1 gallon because the overall volume is going up, but the ratio would still be the same, so roughly 1.1 gallons.

Best bet for initial mixing is to get a scale and through measured testing figure out how much needs to be added to get it right. It might still vary slightly based on how well the dry salt is distributed but measuring by weight with the exact same volume of water each time should be much more accurate than using a measuring cup.

[madsalt]

That is a tough question. Even when using the same salt mix I have found that my salt can vary from bucket to bucket, not by much but it can. After trial and error I found that if I mix 2 2/3 cups each time I am really close to where I want to be. I test each bucket so if I find one is a little low or high I adjust the next bucket slightly. So in short just make sure you use the same amount each time with the same level of water. If you are mixing large batches I can see where a specific formula might be necessary especially if you are not starting with an empty container.

[Planeden]

I just do my water change when the salinity in the tank matches the makeup water. I have marks on the sump that I can tell what is. So if I mix at 1.025 I change water when the tank matches and fill to the 1.025 line then top off water to 1.022. But with a scale I do get pretty close every time.

[Bill B]

Thanks for all your input. Your suggestions about how to get close from the start will help. Part of this real life situation was just an attempt to get the 'ol gray matter firing a little bit. @Planeden - I think you have some inferences and knowledge that I don't have, but it sounds like a unique approach. @jestep - thanks for reminding me about the ppt component of mixing salt water. That is definitely the aspect that I was missing to convert A to B. It's down right amusing how much time I have sporatically spent trying to figure out how to convert apples to diluted oranges.

Bill