What is the difference between impulse tubing and capillary tube?

Comparing Functions in Industrial Instrumentation

Abstract: Clarify the differences between impulse tubing and capillary tubes. Understand their distinct roles in pressure measurement and how precision manufacturing impacts accuracy.

In the field of industrial instrumentation, the terms "pulse line" and "capillary tube" are frequently mentioned, yet their applications are entirely different. Understanding this distinction is crucial for engineers designing process control systems. A pulse line (or pulse tube) connects a process line directly to a measuring instrument, such as a pressure transmitter. Its primary function is to transmit the pressure of the process fluid to the sensor with no significant lag. It is typically a thick-walled tube designed to withstand the full system pressure.

On the other hand, a capillary tube is characterized by a very small inner diameter (usually less than 1 millimeter). In instrumentation applications, it is typically filled with a special fluid that transmits pressure to a remote sealed diaphragm, thereby protecting the instrument from high-temperature or corrosive process fluids. In other scenarios (such as refrigeration systems), it acts as a flow restrictor. The key distinction lies in the application: impulse tubes serve as direct pressure conduits, while capillary tubes typically function as protective transmission lines or throttling devices.

Both require precision manufacturing to ensure system accuracy. MTSCO offers comprehensive small-bore tubing solutions through its Siri Tubing facility. Siri Tubing’s advanced production lines ensure precise inner and outer diameter tolerances for impulse tubes and capillary tubes. Our products undergo 100% inspection, including PMI material verification and pressure testing, to meet the stringent requirements of the instrumentation industry. Through our integrated manufacturing model, we provide consistent quality and full traceability for all tubing needs.