Reference Manual
00809-0100-4835 Rev BD
July 2016
Rosemount 3150 Series
4-2
TRANSMITTER THEORY OF
OPERATION
The block diagram in Figure 4-1 illustrates the operation of the 3150
series pressure transmitter.
The 3150 series pressure transmitters have a variable capacitance
sensor (see Figure 4-2). Differential capacitance between the sensing
diaphragm and the capacitor plates is converted electronically to a 2
wire, 4-20mA dc signal based on the following formulas:
+
−
=
2
1
1
2
1
C
C
C
C
k
P
Where:
P
is the process pressure.
k
1
is a constant.
C
1
is the capacitance between the high-pressure
side and the sensing diaphragm.
C
2
is the capacitance between the low-pressure
side and the sensing diaphragm
2
1
C
C
I
fV
ref
p
p
+
=
−
Where:
I
ref
is the reference current.
V
p-p
is the peak to peak oscillation voltage.
f
is the oscillation frequency.
(
)
1
2
C
C
fV
I
p
p
diff
−
=
−
Where:
I
diff
is the difference in current between C
1
and C
2
.
Therefore:
P = constant x I
diff
= I
ref
1
2
1
2
C
C
C
C
+
−
Summary of Contents for 3152
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