**Differential pressure transmitters used for liquid level applications measure ****hydrostatic pressure head. Liquid level and specific gravity of a liquid are ****factors in determining pressure head. This pressure is equal to the liquid ****height above the tap multiplied by the specific gravity of the liquid. Pressure ****head is independent of volume or vessel shape.**

**Open Vessels :- A pressure transmitter mounted near a tank bottom measures the pressure of ****the liquid above.****Make a connection to the high pressure side of the transmitter, and vent the ****low pressure side to the atmosphere. Pressure head equals the liquid’s****specific gravity multiplied by the liquid height above the tap. ****Zero range suppression is required if the transmitter lies below the zero point ****of the desired level range. Figure shows a liquid level measurement ****example.**

**Closed Vessels :- Pressure above a liquid affects the pressure measured at the bottom of a ****closed vessel. The liquid specific gravity multiplied by the liquid height plus ****the vessel pressure equals the pressure at the bottom of the vessel. ****To measure true level, the vessel pressure must be subtracted from the ****vessel bottom pressure. To do this, make a pressure tap at the top of the ****vessel and connect this to the low side of the transmitter. Vessel pressure is ****then equally applied to both the high and low sides of the transmitter. The ****resulting differential pressure is proportional to liquid height multiplied by the ****liquid specific gravity.**

**Dry Leg Condition**

**Low-side transmitter piping will remain empty if gas above the liquid does not ****condense. This is a dry leg condition. Range determination calculations are ****the same as those described for bottom-mounted transmitters in open ****vessels, as shown in Figure**

**Liquid Level****Measurement Example:-**

**Let X equal the vertical distance between the minimum and maximum ****measurable levels (500 in.).**

**Let Y equal the vertical distance between the transmitter datum line and the****minimum measurable level (100 in.).**

**Let SG equal the specific gravity of the fluid (0.9).**

**Let h equal the maximum head pressure to be measured in inches of water.**

**Let e equal head pressure produced by Y expressed in inches of water.**

**Let Range equal e to e + h.**

**Then h = (X)(SG)**

**= 500 x 0.9**

**= 450 inH2O**

**e = (Y)(SG)**

**= 100 x 0.9**

**= 90 inH2O**

**Range = 90 to 540 inH2O**

**Wet Leg Condition :-**

**Condensation of the gas above the liquid slowly causes the low side of the ****transmitter piping to fill with liquid. The pipe is purposely filled with a ****convenient reference fluid to eliminate this potential error. This is a wet leg ****condition. ****The reference fluid will exert a head pressure on the low side of the ****transmitter. Zero elevation of the range must then be made. See Figure**

**Let X equal the vertical distance between the minimum and maximum ****measurable levels (500 in.).**

**Let Y equal the vertical distance between the transmitter datum line and the ****minimum measurable level (50 in.).**

**Let z equal the vertical distance between the top of the liquid in the wet leg ****and the transmitter datum line (600 in.).**

**Let SG1 equal the specific gravity of the fluid (1.0).**

**Let SG2 equal the specific gravity of the fluid in the wet leg (1.1).**

**Let h equal the maximum head pressure to be measured in inches of water.**

**Let e equal the head pressure produced by Y expressed in inches of water.**

**Let s equal head pressure produced by z expressed in inches of water.**

**Let Range equal e – s to h + e – s.**

**Then h = (X)(SG1)**

**= 500 x 1.0**

**= 500 in H2O**

**e = (Y)(SG1)**

**= 50 x 1.0**

**= 50 inH2O**

**s = (z)(SG2)**

**= 600 x 1.1**

**= 660 inH20**

**Range = e – s to h + e – s.**

**= 50 – 660 to 500 + 50 – 660**

**= –610 to –110 inH20**