Rosemount 1151GP - Liquid Level Measurement

Reference Manual
00809-0100-4360, Rev BA
August 2008
Rosemount 1151
2-18
Hazardous Locations
Certifications
The Rosemount 1151 was designed with an explosion-proof housing and
circuitry suitable for intrinsically safe and nonincendive operation. Factory
Mutual explosion-proof certification is standard for the Rosemount 1151
Transmitter. Individual transmitters are clearly marked with a tag indicating the
approvals they carry. Transmitters must be installed in accordance with all
applicable codes and standards to maintain these certified ratings. Refer to
“Hazardous Locations Certifications” on page B-2 for information on these
approvals.
Environmental
Requirements
Mount the transmitter in an environment that has minimal ambient
temperature change. The transmitter electronics temperature operating limits
are –40 to 185 °F (–40 to 85 °C). Refer to Section A: Reference Information
that lists the sensing element operating limits. Mount the transmitter so that it
is not susceptible to vibration and mechanical shock and does not have
external contact with corrosive materials.
LIQUID LEVEL
MEASUREMENT
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 2-10 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.