4.3.5 Calibration Requirements
The location of the high side seal near the bottom of the tank
and the low seal near the top of the tank provides increasing
transmitter output for increasing liquid level. The actual
differential pressure applied to the transmitter is up to
maximum value only when the process is at its minimum level
due to the elevation difference between the seals. As the tank
fills, the process liquid tends to cancel the initial seal system
head pressure and the measured differential pressure is
therefore at the minimum value when the level reaches its
maximum value.
Note also that the maximum differential is applied from the low
side because the low seal has the higher elevation.
To accommodate the head pressure conditions imposed by a
liquid level installation, the transmitter zero must be adjusted to
elevate the output. This cancels the effect of the low side head
pressure and provides zero percent output for the minimum
level condition. For example, the range values for a level
installation might be -120 to -5 inches of water. This indicates
that the level change (span) is 115 inches of water; the
differential at minimum level is -120 and at maximum level is -5
inches of water.
The minus sign indicates positive differential pressures applied
to the low side of the transmitter.
Determination of the span and range values for a specific
installation must take into account the level change, elevation
difference between the seals, height of the process liquid
above the high seal datum line at minimum level, and the
specific gravity of the process liquid and the fill liquid in the seal
system.
4.4 Interface level measurement
A differential pressure transmitter with two remote seals can be
used to measure interface level. The high side seal element
must be located near the bottom of the tank. The high seal
datum line must be at or below the lowest interface elevation.
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 highest
interface elevation. The minimum level of liquid in the tank must
be above the low seal element under all interface conditions.
The transmitter can be located between the seals, above both
seals or below both seals when the tank is open or operating
under positive pressure. The preferred location for 266DLH the
transmitter is directly mounted on the bottom connection. This
location enables the user to eliminate the capillary and
consequently allows a more accurate measurement. If the
operating pressure range includes pressures below
atmospheric, the transmitter must be located below the high
side seal (refer to the vacuum service information in Liquid
Level Measurement Section).
Figure 11: Interface level measurement
Note.
Total liquid level must always be above the top transmitter
tap. Both the densities must be predetermined and should remain
constant for accurate measurement
4.5 Density measurement
A differential pressure transmitter with two remote seals can be
used to measure liquid density or specific gravity.
The high side seal element must be located near the bottom of
the tank, and the low side seal must be located near the top of
the tank. The minimum level of liquid in the tank must be above
the low seal element under all density conditions. The
transmitter can be located between the seals, above both seals
or below both seals when the tank is open or operating under
positive pressure. 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. If the
operating pressure range includes pressures below
atmospheric, the transmitter must be located below the high
side seal (refer to the vacuum service information in Liquid
Level Measurement Section).
Figure 12: Density or specific gravity measurement
Note.
For this kind of measurement, it is essential to minimizes the
required capillary length as well as grant a uniform distribution of
the ambient temperature.
12 OI/S26-EN Rev. B
| 2600T Series Pressure transmitters
