Emerson Process Management 2051 manual Rosemount, Reference Manual, 00809-0200-4101,Rev AA July

Rosemount 2051

Reference Manual

00809-0200-4101, Rev AA July 2008

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.

X

Y

Z

LT

H L

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 SG1 equal the specific gravity of the fluid (1.0).

Let SG2 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)(SG1)

=500 x 1.0

=500 in H2O

e = (Y)(SG1)

=50 x 1.0

=50 inH2O

s= (z)(SG2)

=600 x 1.1

=660 inH20

Range = e – s to h + e – s.

=50 – 660 to 500 + 50 – 660

=–610 to –110 inH20

ZERO ELEVATION 20

mA dc

4

-610 -110 0

inH2O

2-32