In any process industry, level measurement is very important for safety and process purpose.

Level can be measured by two methods.

**Direct method****Indirect method**

Here we discuss about indirect method and which is using DP transmitter for level measurement.

A Differential pressure transmitter is a common and well understood technology for liquid level measurement. If the tank is closed or pressurized, a DP measurement must be made to compensate for the vessel pressure.

**DP Transmitter Level Measurement for Closed Tank**

In closed tank DP level measurement, LP leg is connected to the top of tank.

There are two methods in closed tank DP Level Measurement.

- Dry leg method: Dry leg method is used in normal close tank where vapor is not condensate and temperature of process is equal to atmospheric.
- Wet leg method: Wet leg method is used where vapor has a tendency to make the condensate and temperature of process is high or low from the atmospheric .

For DP Transmitter Configuration, we have to find out Zero Level & Span Level. Accordingly we have to configure Lower Range Value (LRV) and Upper Range Value (URV) using HART communicator.

**DP Transmitter Level Measurement for Closed Tank – Dry Leg Method**

Simply when LP side of the DP transmitter is filled with any gas/air then we call it as Dry Leg & we apply Dry Leg Method for calculations.

**At zero level (LRV) = pressure acting on HP leg – Pressure acting on LP leg**

=H2 x specific gravity – 0

= 200 x 0.9 – 0

= 180 mmwc

**At 100 % level (URV) = pressure acting on HP leg – Pressure acting on LP leg**

= (H2+H1) x specific gravity – 0

= (200 + 500) x 0.9 – 0

= 630 mmwc

**Range = URV – LRV =** 630 – 180 mmwc = 450 mmwc

So, we have to set Lower Range Value (LRV) = 180 mmwc and Upper Range Value (URV) = 630 mmwc in the DP Transmitter using HART communicator.

**DP Transmitter Level Measurement for Closed Tank – Wet Leg Method**

Simply when LP side of the DP transmitter is filled with liquid then we call it as Wet Leg & we apply Wet Leg Method for calculations.

**Y = H1 +H2 =** 500 + 200 = 700 mm

**At zero level (LRV) = pressure acting on HP leg – Pressure acting on LP leg**

= H2 x SG1 – Y x SG2

= 200 x 0.9 – 700 x 1.2

= 180 – 840

= – 660 mmwc

**At 100 % level (URV) = pressure acting on HP leg – Pressure acting on LP leg**

= (H2+H1) x SG1 – Y x SG2

= (200 + 500) x 0.9 – 700 x 1.2

= 630 – 840

= -210 mmwc

**Range = URV – LRV =** -210 – (-840) = 630 mmwc

So, we have to set Lower Range Value (LRV) = -660 mmwc and Upper Range Value (URV) = -210 mmwc in the DP Transmitter using HART communicator.

Dear Sir

I have question, why you neglect the LP side When you do a calculation on a dry leg mode? could you explaine that to me please?

Best regards

Ahmed

when we consider a dry leg, it means that LP side is subject to only Vapor / gas pressure, in the other hand the vapor/gas pressure is applied to both sides (LP and HP). so we don’t consider it (ie. the vapor/gas pressure) altogether.

Horziontal close tank calibration procedure ( DP type transmitter)

capillery type LT