- In this type of application, the differential pressure transmitter measures the pressure of the media in the tank, the pressure at the top of the tank, and the head pressure made by the fill fluid in the capillary.
- All smart pressure sensors on the market today can measure the pressure at the top of the capillary.
- But this fill fluid can be changed by temperature because it is a very small amount in a very long tube.
- As with all liquids, a change in temperature can cause a change in density (SG), which changes the head pressure measured by the sensor.
- As was already said, the differential pressure method can only be used if the density stays the same.
Temperature effects in differential pressure level measurements can arise from various factors:
- The capillaries on the high side are shorter than the ones on the low side.
- High-side and low-side experiencing different temperatures.
- Capillaries being too long, leading to temperature differentials.
A well-designed level system can correct these temperature effects:
- Ensure proper capillary length and symmetry for both high-side and low-side.
- Implement temperature compensation techniques to account for varying temperatures.
- Optimize capillary design to minimize temperature differentials and maintain accuracy.
Different Length Capillaries
- To account for the error caused by varying volumes of fill fluid in each capillary, industry uses different methods to address the imbalance between the high-side and low-side.
- Two effective methods are employed:
- Make both capillaries the same length to ensure they experience the same temperature and fill fluid volume, thus balancing the effect.
- Keep different capillary lengths but adjust the internal diameter of the capillaries to balance the temperature effect.
- Both methods are effective, with the first method being simpler but potentially costing more. The second method requires more engineering work upfront.
Differential Temperature
- Tank farms often experience differential temperatures between the high-side and low-side process connections on tanks due to their compact layout.
- The high-side components may be cast in shadow, while the low-side components are exposed to sunlight, leading to noticeable temperature differences.
- To address this, a reference capillary is added to the level measurement system, connecting the low-side to the high-side capillary.
- The reference capillary transmits the temperature of the low-side capillary to the high-side, achieving temperature equilibrium and eliminating the temperature imbalance.
Long Capillaries
- In applications with long capillaries, the temperature effect becomes more significant, potentially leading to inaccuracies in level measurements.
- Traditional methods to mitigate temperature effects may not be sufficient for extremely long capillaries.
- An alternative solution is to eliminate capillaries altogether and use wire-based systems.
- These wire-based systems consist of two pressure sensors, one at the high-pressure connection and another at the low-pressure side, communicating through electrical wiring.
- The high-pressure sensor uses information from the other sensor to generate the level output signal.
- Drawbacks of this system include higher cost (two transmitters instead of one) and potentially lower accuracy compared to a well-designed transmitter/capillary/diaphragm seal system for certain applications.
Conclusion
- Temperature can impact device performance, but proper design can minimize its effects.
- Differential pressure measurement for tank level is a cost-effective and reliable method.
- This approach offers accurate and swift level measurements to the controller.