The transmitter mainly consists of a detection part and a signal conversion and amplification processing part.

The detection part consists of a detection membrane and two fixed arc-shaped plates on both sides. The detection membrane can move axially under the action of pressure difference, forming a movable capacitor plate, and together with the fixed arc-shaped plate, it forms two variable capacitors C1 and C2. The structure and electrical principle can be seen in Figure 6-11.

Before testing, the pressure in the high and low pressure chambers should be balanced, P1 =P2； According to the structural requirements, the fixed arc-shaped plates of the two variable capacitors and the detection membrane are symmetrical, with equal pole spacing, C1 =C2。

When the measured pressures P1 and P2 enter the high and low pressure chambers through the inlet tube, the center of the isolation diaphragm will shift due to P1>P2, compressing the electrolyte and reducing the volume on the high pressure side. When the electrolyte is an incompressible body, its volume change will cause the center of the detection membrane to shift towards the low-pressure side, which is equal to the displacement of the center of the isolation membrane. According to electrical engineering, when the distance between the two poles of a capacitor changes, its capacitance will also change, that is, from C1=C2 to C1 ≠ C2.

According to the electrical schematic, when no displacement occurs, I1=I2=0； ι1+ι2=ιc； After displacement occurs, due to the change in relative pole spacing, the accumulated charge on each electrode plate also changes, forming a charge displacement. At this time, I1 ≠ I2 is reflected, and a current difference will be generated between the two. If the magnitude of its value and its relationship with the pressure difference are detected, the flow rate can be calculated.

2. The relationship between transmission current and voltage difference

Assuming no displacement occurs, according to the definition of capacitance:

In the formula, K represents the proportionality constant;

ε - dielectric constant;

S - Absolute area of curved plate;

D0- Relative average distance between the curved plate and the movable electrode plate.

After the displacement Δ d occurs, according to the definition of capacitance:

From Figure 6-11, it can be seen that under the driving of a high-frequency power supply with an electromotive force of e and an angular frequency of ω, the charging and discharging current difference is:

By substituting the expressions for C1 and C2 into the above equation, we have:

From the derivation results, it can be concluded that the current difference is proportional to the center displacement of the movable electrode plate (detection diaphragm). Since this displacement is proportional to the measured pressure difference, the current difference is proportional to both the measured pressure difference and the flow rate.

3. Characteristics of capacitive differential pressure transmitter

The capacitive differential pressure transmitter is entirely composed of sealed measuring elements, which can eliminate the instantaneous impact and mechanical vibration caused by mechanical transmission. In addition, the high and low pressure measuring chambers are cast as a whole according to explosion-proof requirements, greatly suppressing the influence of external stress, torque, and static pressure on measurement accuracy.