Emerson Rosemount 2051 Reference Manual page 45

Reference Manual
00809-0300-4101, Rev BB
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 3-13. 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
Let SG
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
e = (Y)(SG
s = (z)(SG
Range = e – s to h + e – s.
–610
Hardware Installation
X
Y
equal the specific gravity of the fluid (1.0).
1
equal the specific gravity of the fluid in the wet leg (1.1).
2
)
1
= 500 1.0
= 500 in H O
2
)
1
= 50 1.0
= 50 inH O
2
)
2
= 600 1.1
= 660 inH 0
2
= 50 – 660 to 500 + 50 – 660
= –610 to –110 inH 0
2
Zero Elevation
inH O
2
Figure 3-13.
Z
LT
H L
H
i
Range
L
o
–110 0
Hardware Installation
November 2016
35