Emerson Rosemount 3152, Reference Manual

Page: 55 / 84

Reference Manual
00809-0100-4835 Rev BE
February 2018
Rosemount 3150 Series
Operation 46
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, 4-20 mA dc signal based
on the following formulas:
𝑃𝑃 = 𝑘𝑘
1
�
𝐶𝐶
2
− 𝐶𝐶
1
𝐶𝐶
1
+ 𝐶𝐶
2
�
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.
𝑓𝑓𝑉𝑉
𝑝𝑝−𝑝𝑝
=
𝐼𝐼
𝑟𝑟𝑟𝑟𝑟𝑟
𝐶𝐶
1
+ 𝐶𝐶
2
Where:
I
ref
is the reference current.
V
p-p
is the peak to peak oscillation voltage.
f is the oscillation frequency.
𝐼𝐼
𝑑𝑑𝑑𝑑𝑟𝑟𝑟𝑟
= 𝑓𝑓𝑉𝑉
𝑝𝑝−𝑝𝑝
(
𝐶𝐶
2
− 𝐶𝐶
1
)
Where:
I
diff
is the difference in current between C
1
and
C
2
.
Therefore:
𝑃𝑃 = 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑥𝑥 𝐼𝐼
𝑑𝑑𝑑𝑑𝑟𝑟𝑟𝑟
= 𝐼𝐼
𝑟𝑟𝑟𝑟𝑟𝑟
�
𝐶𝐶
2
− 𝐶𝐶
1
𝐶𝐶
2
+ 𝐶𝐶
1
�