Section 2
2.1
OPERATION
The
LD303
Series Pressure Transmitters use capacitive sensors (capacitive cells) as pressure
sensing elements, as shown in Figure 2.1. This is exactly the same sensor as the LD301 series
uses, the sensor modules are therefore interchangeable.
Figure 2.1 - Capacitive Cell
Functional Description - Sensor
Where,
P
1
and P
2
are the pressures and P
1
≥
P
2
CH = Capacitance between the fixed plate on P
1
side and the sensing diaphragm.
CL = Capacitance between the fixed plate on the P
2
side and the sensing diaphragm.
d = Distance between CH and CL fixed plates.
∆
d = Sensing diaphragm's deflection due to the differential pressure
∆
P = P
1
- P
2
.
Knowing that the capacitance of a capacitor with flat, parallel plates may be expressed as a function
of plate area (A) and distance (d) between the plates:
d
A
C
×
≈
ε
Where,
ε
= Dielectric constant of the medium between the capacitor's plates.
CL
d
d
A
d
d
A
CH
≈
∆
−
×
∆
+
×
≈
)
2
(
and
)
2
(
ε
ε
However, should
CH
and
CL
be considered as capacitances of flat and parallel plates with identical
areas, then:
However, should the differential pressure (
∆
P) apply to the capacitive cell not deflect the sensing
diaphragm beyond d/4, it is possible to assume
∆
P as proportional to
∆
d, that is:
d
∆
∝
∆
P
By developing the expression (CL - CH)/(CL + CH), it follows that:
d
d
CH
CL
CH
CL
∆
=
+
−
2
Summary of Contents for LD303
Page 1: ...Profibus PA Pressure Transmitter L D 3 0 3 M E...
Page 8: ...LD303 Operation and Maintenance Instruction Manual VIII...
Page 22: ...LD303 Operation and Maintenance Instruction Manual 1 14...
Page 26: ...LD303 Operation and Maintenance Instruction Manual 2 4...
Page 74: ...LD303 Operation and Maintenance Instruction Manual 4 6 Figure 4 4 Exploded View...
Page 83: ...Maintenance Procedures 4 15 Figure 4 8 TPE Software Screen...
Page 102: ...LD303 Certifications Information A 4 CSA NEMKO and EXAM CEPEL NEPSI...
Page 104: ...LD303 Certifications Information A 6 CEPEL...
Page 105: ...Appendix A A 7 Control Drawing Factory Mutual FM...
Page 106: ...LD303 Certifications Information A 8...
Page 108: ...LD303 Operation and Maintenance Instruction Manual B 2...