Correcting a tank is not the same sum as mixing one
Mixing a brine from scratch is a ratio and you can do it on the back of the bag. Correcting one that has been in service is a mixture problem, because the salt already dissolved in the tank counts toward the answer and the water it is sitting in counts against it. The formula is the salt needed equals the target fraction times the total weight, minus the salt already there, all divided by one minus the target fraction — and that last division is the part people leave out. It is there because the salt you add joins the solution as well as the salt, so it moves both halves of the ratio.
On the tank in the form: 40 gallons at 9.6 lb a gallon is 384 lb of brine, twenty percent of which is 76.8 lb of salt. Going to twenty-three percent needs 14.96 lb of pure salt. Skip the division and you would get 11.52 lb and land at 22.3 percent instead — close enough to look right and wrong enough to matter over a season of corrections.
Twenty percent of what
The page prints every reading three ways because a brine percentage means nothing on its own. Salt at twenty percent of the whole solution is the same brine as salt at twenty-five percent of the water, and neither of those is 75.8 degrees on a salometer, which is what that brine reads if saturation on your chart sits at 26.4 percent.
All three describe one tank. A make sheet that says twenty percent and a salometer that says twenty are describing brines a long way apart. If you inherited a brine spec from anywhere, the first job is finding out which convention it was written in, and the second is writing that down next to it so the next person does not have to guess.
Saturation itself is a field on the form rather than a constant in the code, because saturation moves with temperature and the number that matters is the one on the chart that came with your own salometer, read at the temperature your brine actually sits at rather than at the temperature a table assumes.
Make-up water changes the answer
Wheels come out of a brine carrying salt and leaving room. Topping the tank back up with fresh water dilutes what is left, so if the water and the salt are going in on the same day the salt has to cover the water as well as the shortfall. Five gallons of make-up on this tank takes the correction from 14.96 lb to 27.42 lb — nearly double, for five gallons.
That is the number that surprises people, and it is the reason a tank that gets topped up without being corrected drifts down faster than anybody expects.
Reading the tank, not the top of it
Brine stratifies. A sample skimmed off a still tank reads the top few inches and nothing else, and a correction calculated from it is a correction to a layer. Stir the tank or draw from depth before the reading that goes into this form. The other honest limitation is time: a working tank moves between the day you measure and the day you next look, and the only way to find out how fast yours moves is to measure it more often than you think you need to.
Questions people ask
How much salt do I add to bring a brine tank back up?
Enter the gallons, the weight of a gallon you measured, the reading you took and the reading you want. On the tank in the form — 40 gallons at 9.6 lb a gallon, going from twenty to twenty-three percent of the solution — it is 14.96 lb of pure salt, or 15.11 lb weighed off a bag that is 99 percent salt. The formula divides by one minus the target fraction, because the salt you add joins the solution as well as the salt.
What is the difference between percent of solution and percent of water?
The first divides the salt by everything in the tank, the second divides it by the water alone, so the second is always the larger number for the same brine. Twenty percent of solution is 25 percent of water. They are two ways of writing one tank and a spec that does not say which one it means is not usable, which is why this page prints both for every reading it touches.
How do I convert salometer degrees to a percentage?
Degrees are a share of saturation, so 100 degrees means saturated and 50 means half of whatever saturation is. Converting needs to know what saturation is by weight, which is a field on this form rather than a constant, because it moves with temperature. With saturation at 26.4 percent of solution, 75.8 degrees is twenty percent of solution and 25 percent of water. Take the saturation figure off your own salometer chart at your brine temperature.
Why does topping up with water need so much extra salt?
Because the new water joins the solution and has to be brought up to strength along with everything else. On the tank in the form, adding five gallons of make-up water takes the salt correction from 14.96 lb to 27.42 lb. It is why a tank that gets topped up regularly but only corrected occasionally drifts down faster than the reading history suggests it should.
How strong should a cheese brine be?
This page does not say, and will not. Brine strength, temperature, acidity, calcium, how long a tank stays in service and whether it may be reused belong to your process, your make sheet and the authority over your dairy, and they differ by cheese and by jurisdiction. Put the figure somebody qualified gave you into the target field and the arithmetic will get you there; deciding what the figure is is not a job for a calculator.