We all that how important to know the level in the tank especially when in an old installation where pressure gauge is installed below the tank to measure pressure than by a simple calculation we can know the level.
Nowadays pressure transmitter does everything and shows us directly level by doing some parameter adjustment but it is still important to know how exactly calculation is done.
Here the pressure is given by,
P = (S.G.) * H ----------(1)
P is pressure
S.G. is the specific gravity of a liquid
H is the height (Level) which we have to calculate
Let consider pressure gauge is showing us 20 psi(P).
Here our tank is open so two different pressure exists one is due to the liquid and other is due to the atmosphere. so,
P = P(liquid) + P(atm)
20 = P(liquid) + 14.69595
P(liquid) = 5.30405 psi
Now let convert it into mmH2o
P(liquid) = 5.30405 * 703 (1 psi = 703 mmH2o)
P(liquid) = 3728.74715 mmH2o
Now put all the value in the equation (1)
P = (S.G.) * H
3728.75 = 1 * H
H = 3728.75 mmH20