The seal protects the instrument from the process. In exchange, it adds temperature error and response time.
A diaphragm seal isolates the sensing element from the process fluid. Between the two sits a fill fluid that transmits the pressure. It is the standard solution when the process cannot touch the instrument.
When a seal is justified
- Corrosive fluid that would attack the sensing element.
- Fluid that crystallises, polymerises or solidifies and would block the impulse line.
- High temperature, where the instrument must be kept away from the process.
- Sanitary applications, where the connection must be flush and free of product entrapment.
- Highly viscous fluid that would not transmit pressure through a conventional impulse line.
The price you pay
Temperature error. The fill fluid expands. A change in ambient or process temperature shifts the instrument's zero. The larger the diaphragm and the greater the fluid volume, the larger the effect — and with long capillaries the effect is significant.
Response time. The capillary introduces lag. In a fast control loop, that can destabilise the loop.
Minimum span. A seal with a long capillary has a minimum span below which temperature error dominates the measurement. The manufacturer specifies that limit — respect it.
How to reduce the impact
Use the largest diaphragm diameter the connection allows: the more flexible the membrane, the less force required and the smaller the error. Keep the capillary as short as possible. In level measurement with a dual seal, run both capillaries at the same length and exposed to the same temperature, so the thermal effects cancel out.
