Basic level measurement is done using open tank level. Its calculation is relatively easy and is shown below.
Zero based direct mount application
The level transmitter is positioned in the same level position as the HP tapping point when it is connected.
Pressure on H.P. side of transmitter P.HP = Patm + (SG*h)
Pressure on LP side of the transmitter P.LP= Patm(open to atmospheric pressure)
The transmitter’s differential pressure is calculated as follows:
DP = P.HP – P.LP
DP = Patm + (SG*h) – Patm
DP = SG*h
LRV Calculation
In order to obtain LRV, the pressure on the HP side and LP side of the tank must be empty. When the tank is empty, there will be no pressure on either the HP side or the LP side. since there will be no liquid height.
So h = 0
Then DP = Patm+(SG*h) – Patm
PH.P = Patm + (SG*0)
=Patm
PL.P = Patm
LRV = PH.P. – PL.P
= Patm – Patm
LRV=0
Estimating the URV
When the tank is completely full of liquid (h), determine the transmitter’s differential pressure (DP).
PH.P = Patm +(SG*h)
PL.P = Patm
URV = P.HP – P.LP
= Patm+(S.G*h) – Patm
URV = S.G*h
Range of the transmitter = 0 to (S.G*h)
DP calculator
Click here for more Instrumentation Calculators
 for the zero based direct mounting of transmitter in open tank level measurements){kind=link}