Differential Pressure Transmitter Sensing Element: Why a Capsule Structure?
In industrial measurement, the sensing element of differential pressure transmitters commonly adopts a capsule structure rather than a single diaphragm. This is not accidental but an inevitable choice based on measurement principles and reliability requirements. The capsule structure has significant advantages in sensitivity and overpressure resistance.
Comparison Between Capsule Structure and Single Diaphragm
- Capsule structure: Made by welding two diaphragms together, with silicone oil filled inside. It has high sensitivity and good linearity within the differential pressure range; when unidirectional overpressure occurs, the capsule is not easily damaged, and the performance impact is minimal.
- Single diaphragm: Easy to process and highly sensitive, but cannot withstand unidirectional overpressure, easily leading to permanent deformation or damage.
Therefore, the vast majority of differential pressure transmitters use a capsule structure to ensure long-term stable operation.
| Characteristic | Capsule Structure | Single Diaphragm |
|---|---|---|
| Sensitivity | High | High |
| Linearity | Good | Average |
| Unidirectional Overpressure Resistance | Strong | Weak |
| Damping Effect | Yes (silicone oil) | No |
| Output Stability | Stable | Poor |
The silicone oil filled in the capsule not only transmits pressure but also provides a damping effect, making the instrument output more stable. This is unmatched by a single diaphragm.
Taking E+H PMD55B as an Example: Measurement Principle of the Capsule Structure
Taking the E+H differential pressure transmitter PMD55B as an example, its measurement principle intuitively demonstrates the necessity of the capsule structure.
From the above figure, it can be intuitively felt that if there is only a single-sided diaphragm, the differential pressure cannot be measured. Because differential pressure measurement requires obtaining two pressure values simultaneously, the capsule structure can introduce the pressures from both sides into the same sensitive element, thereby accurately outputting the differential pressure.
The advantages of the capsule structure can also be seen from the application of differential pressure transmitters in flow measurement. Flow measurement depends on the stable acquisition of differential pressure, and the capsule structure can still maintain reliable performance under overpressure and pulsating conditions.
Summary
- The capsule structure has high sensitivity and good linearity within the differential pressure range.
- It withstands unidirectional overpressure, and silicone oil damping makes the output stable.
- A single diaphragm cannot meet the reliability and stability requirements of differential pressure measurement.
Therefore, in industrial differential pressure transmitters, the capsule structure has become the preferred sensing element.