Introduction
Remote diaphragm seal differential pressure transmitters are widely used for level measurement in closed vessels and separators.
The two diaphragm seals are connected to the transmitter through capillaries.
Selecting the correct capillary length is important because capillary design can affect measurement response and accuracy.
1. Measure the Actual Installation Distance
Do not select capillary length based only on the straight-line distance between the transmitter and the process connections.
Consider the actual routing path, including:
Vertical distance
Horizontal distance
Pipe and equipment obstacles
Cable tray or support locations
Maintenance access
Required bending allowance
The selected length should provide sufficient routing allowance without unnecessary excess.
2. Avoid Unnecessarily Long Capillaries
Longer capillaries may increase the influence of ambient temperature changes and can affect response time.
They may also make installation and protection more difficult.
Therefore, use the shortest practical length that satisfies the installation layout and manufacturer requirements.
3. Consider Elevation Differences
The elevation difference between the two seals and the transmitter is important.
The filling fluid inside each capillary can create hydrostatic effects.
For differential pressure measurement, unequal elevation or temperature conditions may introduce additional measurement errors.
The instrument configuration and calibration must account for the actual installation arrangement.
4. Consider Ambient Temperature
Capillary filling fluid is affected by temperature.
Different temperatures along the high- and low-pressure capillaries can create unequal effects.
Avoid placing one capillary near a heat source while the other is exposed to a significantly different environment, where possible.
5. Confirm the Filling Fluid
The filling fluid must be suitable for:
Process temperature
Ambient temperature
Minimum pressure conditions
Required response time
Chemical and safety requirements
Vacuum applications require particular attention to filling fluid vapor pressure.
6. What Information Should Be Given to the Supplier?
Provide:
Separator dimensions
High- and low-pressure tap elevations
Transmitter installation position
Required capillary routing distance
Process temperature
Ambient temperature range
Operating pressure and differential pressure range
Conclusion
Capillary length should be determined by the actual installation layout and measurement conditions, not by guesswork.
Correct routing, elevation compensation, filling fluid selection, and temperature management help improve the reliability of remote seal differential pressure measurement.
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