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Reference Manual
00809-0200-4101, Rev AA
July 2008
Rosemount 2051
2-32
Wet Leg Condition
Condensation of the gas above the liquid slowly causes the low side of the
transmitter piping to fill with liquid. The pipe is purposely filled with a
convenient reference fluid to eliminate this potential error. This is a wet leg
condition.
The reference fluid will exert a head pressure on the low side of the
transmitter. Zero elevation of the range must then be made. See Figure 2-20
Figure 2-20. Wet Leg Example.
Let X equal the vertical distance between the minimum and maximum
measurable levels (500 in.).
Let Y equal the vertical distance between the transmitter datum line and the
minimum measurable level (50 in.).
Let z equal the vertical distance between the top of the liquid in the wet leg
and the transmitter datum line (600 in.).
Let SG
1
equal the specific gravity of the fluid (1.0).
Let SG
2
equal the specific gravity of the fluid in the wet leg (1.1).
Let h equal the maximum head pressure to be measured in inches of water.
Let e equal the head pressure produced by Y expressed in inches of water.
Let s equal head pressure produced by z expressed in inches of water.
Let Range equal e – s to h + e – s.
Then h = (X)(SG
1
)
= 500 x 1.0
= 500 in H
2
O
e=(Y)(SG
1
)
=50 x 1.0
=50 inH
2
O
s=(z)(SG
2
)
= 600 x 1.1
= 660 inH
2
0
Range = e – s to h + e – s.
= 50 – 660 to 500 + 50 – 660
= –610 to –110 inH
2
0
ZERO ELEVATION
LT
Y
H L
mA dc
20
0
4
-110-610
inH
2
O
X
Z