A d/p transmitter is connected to the tank by a pressure tapping tube.

The liquid is tapped for the high pressure side, and the open air is tapped for the low pressure side.

The following relationship exists:

P=ρ1g ( H+h1)

P = the pressure

ρ1=density of the liquid

H=distance between the surface and the minimum liquid level

h1=distance between the minimum liquid level and the pressure detector.

**Liquid-level measurement: Open tank:**

Example:

A pressure transmitter connected at a position 10 cm below the bottom of a tank sends 13.57 mA to a computer. The transmitter was calibrated for a range of 0-200kPa to produce 4-20 mA. If the liquid has a specific gravity of 1.26, calculate the level of the liquid in the tank.

P=hρwgRD

RD=1.26

ρw=1000 kg/m2 g=9.81 m/s2

Reading 13.57 mA :(200 kPa *13.57mA/16mA)-50 kPa=119.62 kPa

h=P/ ρwgRD=119.62kPa/(1000 kg/m2 *9.81 m/s2*1.26)=967.75cm

hactual =h-10 cm =957.75 cm