Accurate and reliable level and pressure measurement are critical to safe and efficient industrial process control. In harsh industrial environments, these measurements often rely on diaphragm seal systems that separate the instrument from the process media while still allowing the process condition to be measured.
Capillary-based diaphragm seals have traditionally filled this role. However, they can also introduce accuracy and maintenance challenges tied to temperature effects, response lag, mechanical vulnerability, and installation variability.
Digital diaphragm seals address many of these vulnerabilities by digitizing the process signal at the diaphragm and transmitting data electronically. This article compares digital diaphragm seals and capillary-based diaphragm seals for level and pressure measurement applications.
The main difference is how the measurement signal is transferred. Capillary-based diaphragm seals use a fluid-filled capillary tube to mechanically transfer pressure from the diaphragm to a remote transmitter. Digital diaphragm seals digitize the process signal at the diaphragm itself and transmit data electronically.
Because capillary systems rely on a fluid-filled tube, their performance can be affected by temperature changes, capillary length, routing, vibration, and physical damage. Digital diaphragm seals reduce many of these variables by removing the long fluid-filled capillary from the measurement path.
| Factor | Capillary-Based Diaphragm Seals | Digital Diaphragm Seals |
|---|---|---|
| Signal transfer | Pressure is transferred through a fluid-filled capillary tube. | The process signal is digitized at the diaphragm and transmitted electronically. |
| Temperature effects | Fill fluid can expand and contract as temperatures change, which may cause measurement drift. | Digital systems are not affected by temperature changes in the same way capillary fluid is. |
| Response time | Long capillary runs can physically delay the pressure signal. | Without a long capillary tube, measurement response is faster and more reliable. |
| Mechanical vulnerability | Capillary tubing can be affected by kinks, vibration-induced fatigue, and incidental damage. | Fewer mechanical parts can reduce failure points and maintenance needs. |
| Installation | Length, routing, bends, and heat exposure can affect performance. | Without fragile tubing to route and protect, installation becomes more predictable. |
Capillaries are often the hidden weak point in level and pressure measurement.
Capillary-based diaphragm seals are designed to protect pressure instruments such as gauges, transmitters, and switches from harsh process media. They work by separating the instrument from the process fluid with a flexible diaphragm, while a fluid-filled capillary tube transfers pressure to the sensor.
Core failure and performance risks associated with capillaries include:
The fill fluid expands and contracts as temperatures change. Outdoor conditions can cause the reading to drift even when the process stays the same, leading operators to think the process is changing when it is not.
Long capillary tubes physically delay the pressure signal. The longer the run, the more lag. This slows measurement, hides real process changes, and makes operators chase “ghost” fluctuations.
Capillary tubing is prone to kinks, vibration-induced fatigue, and incidental damage during maintenance. Each of these can change the fluid volume or stiffness, distorting readings without other obvious external signs.
The length, routing, bends, and heat exposure of capillary tubing all affect performance. Even installations that appear identical can include small variations that lead to different measurement results.
For operators and maintenance teams, these issues do not always look like capillary problems. Instead, they show up as symptoms such as:
As these inconsistencies accumulate, control loops become increasingly difficult to tune. Yield and quality suffer, and maintenance teams spend more time validating instruments than supporting operations.
Compared to traditional capillary systems, digital diaphragm seal systems are more accurate, faster, and easier to maintain.
Rather than relying on mechanical transfer of pressure through a fluid-filled tube, digital diaphragm seals digitize the process signal at the diaphragm itself and transmit data electronically. This offers several advantages.
Digital systems are not affected by temperature changes like capillary fluid is. This means more reliable pressure readings.
There is no long capillary tube filled with fluid, so the measurement responds more quickly and reliably to pressure changes.
Traditional capillaries can leak, kink, or degrade over time. Digital diaphragm seals have fewer mechanical parts, reducing failure and downtime.
Digital diaphragm seals work well in variable pressures, vibrations, and long distances where capillary systems struggle.
Digital signals can be transmitted over long distances without loss, unlike capillary systems that require careful routing. Without fragile tubing to route and protect, instrumentation installation becomes more predictable.
Learn more from ABB, our manufacturer partner. Read ABB’s report on Digital Diaphragm Seal systems.
Digital diaphragm technology can be used in many industries, but it is especially useful where accuracy and reliability matter the most.
Replacing legacy capillary-based systems with digital diaphragm solutions does not have to be disruptive.
Because modern digital diaphragm transmitters often support standard communication protocols such as HART, they can be dropped into existing architectures without extensive redesign.
What is HART Protocol?
The HART Protocol, or Highway Addressable Remote Transducer Protocol, is a widely used, open standard communication protocol in industrial automation that superimposes digital signals onto traditional 4-20mA analog loops, allowing for two-way communication without needing new wires.
On the shop floor, this translates into faster commissioning, less troubleshooting, and ultimately more reliable operations.
ABB’s Digital Diaphragm Seal systems deliver more stable level and pressure measurements, reduced maintenance, and improved long-term reliability compared to traditional capillary systems.
MCE has expanded its ABB instrumentation support to give customers better access to instrumentation, fast support, delivery reliability, and a stronger local team. Learn more about MCE’s ABB instrumentation support.
The ABB HART 266 Digital Diaphragm Seal pressure transmitters are designed to address the same challenges as conventional remote seal systems while also eliminating many of the performance compromises and high costs traditionally associated with them.
The 266 Digital Diaphragm Seal pressure transmitter is an ideal solution for reliable level and pressure measurement of filling quantities in containers, silos, and tank systems where ambient temperature effects and fast response times are critical.
The DDS simplifies measurement by integrating two direct-mount gauge sensors, one on the high side and one on the low side, that operate independently.
Key features:
Help your engineers and operators focus on the process, not the problem.
Talk to an MCE instrumentation and process control expert today.
Capillary-based diaphragm seals have done the job for a long time, but they also introduce potential issues from temperature effects, slow response, and installation variables. By digitizing the signal closer to the process, ABB digital diaphragm seals remove uncertainty.
MCE collaborates with ABB to help engineers, managers, and maintenance teams optimize performance with quality equipment, expert application support, and responsive service.
MCE offers:
Need help evaluating digital diaphragm seals or ABB instrumentation? MCE can help review your level and pressure measurement needs and connect you with instrumentation and process control support.