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Remote Seal Systems
May 2020
Figure 2-2: FFW Seal and Components
A. Process flange
B. Diaphragm
C. Gasket
D. Flushing ring (optional)
2.3
Seal system performance
2.3.1
Volume temperature effects (process temperature
effects)
Fill fluids expand or contract with temperature changes, creating a volume change that is
absorbed by the diaphragm seal and is seen as back pressure at the transmitter. This back
pressure creates a shift in the transmitter reading. For symmetrical or balanced systems,
this error is usually minimal due to the back pressure being equal on both sides. However,
head temperature effect is still present.
Note
Other factors that affect seal temperature effect include diaphragm thickness, seal type
and size, capillary length, and inner diameter.
Figure 2-3
transmitter. For smaller seal sizes, such as the 1½-in. size, the amount of back pressure on
the transmitter causes an additional 12.1 inH
size gives 1.7 inH
mmH
provides a more stable reading.
10
shows how diaphragm size can affect the measurement reading at the
O (43 mmH
2
O)error. Using a larger diaphragm can drastically improve performance and
2
O (307 mmH
2
O)and the largest 3-in. size shown only has 0.5 inH
2
Reference Manual
00809-0100-4002
A
B
C
D
O)error. Moving to the 2-in.
2
Rosemount DP Level
O (13
2
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