Level Measurement

Different Configurations of DP transmitter for level, density measurement

Differential Pressure Transmitter (DPT) are used to measure level in process tanks which compares the pressure measurement between pressure at the bottom of the tank with atmospheric pressure, or pressure at the empty space at the top of the same tank.

General Differential Pressure Transmitter Methods:

There is a variety of system arrangements for determining density from measurements of hydrostatic heads using differential pressure (DP) transmitters

DPT transmitter with a Wet Leg:

Applications with fluctuations in level or static pressure require compensation. Under these conditions, there are three basic density measurement arrangements.

A wet leg is installed between the lower and upper opening which is filled with a fluid that will sealed so that it won’t combine with the process fluid.

First, if a seal fluid that is always denser than the process fluid can be selected and will not be mixed with it. To ensure that the process liquid does not enter the purged wet leg, piping to the process vessel should include an appropriate barrier.

DP Transmitter with a Pressure Repeater:

Pressure repeater for the upper connection. This instrument reproduces any pressure that exists at the B connection in one form, from full vacuum to approximately 250 Pa positive pressure.

The repeater transmits to the low – pressure side of the DP transmitter the total pressure at elevation B. This subtracts the pressure at height B from the pressure at height A.

Special consideration should be given when using the repeater method for vacuum applications where the overall pressure on the repeater is lower than atmospheric.

DP Transmitter with Bubble Tubes:

This very simple system consists of two open – ended tubes, finished with V notches. They are immersed in the liquid with the V notches separated by a known fixed vertical distance H and purged at an appropriate pressure with a low but steady air flow (or inert gas).

The difference Tp in hydrostatic pressure at both points is measured by a DP transmitter connected between these tubes, with the higher – pressure side associated with the lower V notch.


Instrumentation Engineer

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