Liquid Level Measurement - ABB S26 Series Operating Instruction

4.3 Liquid level measurement

A differential pressure transmitter with either one or two remote
seals can be used for liquid level measurement on open tanks,
closed tanks operating at pressures above atmospheric and
closed tanks operating under vacuum.
Liquid level measurements are based on the height of a
process liquid column with respect to predefined measurement
reference points on the transmitter. The transmitter has three
datum lines as follows:
— Primary Datum Line: located at the center of the seal
system connection to the transmitter body.
— High Seal Datum Line: located at the center of the high
side seal diaphragm.
— Low Seal Datum line: located at the center of the low side
seal diaphragm.
4.3.1 Open Tank Installation
A transmitter with one remote seal can be used for open tank
level measurement.
The seal is on the high side of the transmitter, and the seal
element must be located near the bottom of the tank so that
the high seal datum line is at or below the minimum level. The
transmitter can be located either above or below the seal
element. The low side of the transmitter must be vented to
atmosphere.
Figure 3: Level measurement of a liquid in an open tank
10 OI/S26-EN Rev. B | 2600T Series Pressure transmitters
4.3.2 Closed Tank Installation – Pressure Service –
Double Seal
When a transmitter with two remote seals is used to measure
level in a pressurized tank, the high pressure side seal element
must be located near the bottom of the tank.
The high seal datum line must be at or below the minimum
level. The low side seal must be located near the top of the
tank, and the low seal datum line must be at or above the
maximum level.
The transmitter can be located between the seals, above both
seals or below both seals on a pressure service installation.
The preferred location is mid way between the seal elements.
This location minimizes the required capillary length, and
usually provides the most uniform distribution of ambient
temperatures across the capillary length.
Figure 4: Level measurement of a liquid in a closed tank - vacuum or pressure
service
Note. This installation requires a double-seal transmitter. The
transmitter must be installed below the minimum level.
Figure 5: Level measurement of a liquid in a closed tank - pressure service only
Note. This installation requires a double-seal transmitter. The
transmitter must be located between the taps of the tank.