To the layman, specific gravity may seem like a misnomer. After all, isn’t there just one gravity? Specific gravity refers to the gravity of a specific thing, almost always a gas or liquid. Or more accurately:
“Specific gravity is the ratio of the density of a substance to the density (mass of the same unit volume) of a reference substance. The reference substance is nearly always water for liquids or air for gases.”
Specific gravity, then, in the case of our float tanks, is how dense solution is compared to regular, run of the mill water.
Water has a specific gravity of 1 (since it is being compared to itself). The ocean ranges from 1.023 to 1.035 and the dead sea which has been getting more saline over time is up to about 1.3 specific gravity, which is roughly where the salt reaches saturation. Even molten metals have a specific gravity: lead is 11.35 and gold is 19.3.
So, where should we keep the specific gravity of a float tank? A float tank will float someone starting around the 1.24 mark, and it may start to damage your pump and filtration system at or above a 1.3 specific gravity due to salt crystallization. The saturation point (for 93.5° F) is about 1.33. We’ve seen some float tanks with a specific gravity as low as 1.18, however within the range that works for floating people, we recommend being near the middle. We shoot to keep our specific gravity levels between 1.265 to 1.285 at all times.
Be warned, the water in your tank will gradually evaporate and as you operate your tank, the specific gravity will shift higher and higher as you go. Good for floating, but it does mean that your specific gravity will be fluctuating by minute degrees over time. Another good reason to keep it slightly lower.
At this point it may seem like you will never need to put new salt into your tank again, but alas entropy has other plans. Every floater brings out both water and salt with them (at Float On we’ve crunched the numbers, and came up with 1.1 pounds of salt per floater that we lose. Those darn floaters stealing our salt).
But how will you be measuring all of this? Enter: the hydrometer!
A hydrometer will float in a sample of your tank water, and depending on how high it floats in the water will give you a reading of how buoyant your water is. When purchasing a hydrometer, make sure it works in the 1.2 to 1.3 or more range, otherwise it will not be giving you usable data at all.
I recommend checking your specific gravity after about 25 floats in a given tank, unless the floats are happening over a long period of time, in which case checking it every week is a good practice.
Thank you all for reading, and as always, if you have even more in depth questions about this or anything else like it, just let us know.
And if you’d like more information on float water solution or how to treat it, head over to our Water Treatment Course (it’s absolutely free!)