Capillary tube
A capillary tube: the thin, sealed, fluid-filled tube of a filled-system instrument. It is drawn as a line crossed by small x marks.
Easily confused with
How to read it
Kuphaldt’s Lessons In Industrial Instrumentation describes filled-bulb temperature sensors, where fluid sealed in a bulb expands with heat and a capillary tube connects the bulb to the transmitter or indicator. The DOE’s handbook lists the instrument capillary among the line types on engineering P&IDs.
Kuphaldt’s chart and the DOE’s both draw the capillary as a thin line crossed by small x marks.
Also searched as: capillary tube p&id symbol, capillary tube symbol.
This explains what a symbol represents on a diagram. Companies vary the symbols, and a diagram's own legend sheet takes precedence.
Look-alikes
A line with pairs of short slashes is a pneumatic signal. Kuphaldt’s chart also has an on/off pneumatic signal from older editions of ISA 5.1, drawn with the slashes crossed by a backslash; the 2009 edition dropped it.
Read next
- Next in P&ID symbolsGate valveA gate valve, which starts or stops the flow in a line. It is drawn as two triangles meeting point to point across the line.
- The same shape in European flagsFlag of GreeceGreece’s flag is nine horizontal stripes, five blue and four white, with a white cross on a blue square in the top corner by the pole. Law 851/1978 fixes the design but gives it no meaning; readings of the nine stripes, such as the nine syllables of “Freedom or Death”, are popular tradition.
More P&ID symbols
The same shape elsewhere
Browse symbols drawn with crosses or lines.
Sources
- Lessons In Industrial Instrumentation, Tony R. Kuphaldt, version 2.33, 2024 (CC BY 4.0), Tony R. Kuphaldt (ibiblio.org), 7.5.1 Line types; 21.2 Filled-bulb temperature sensors
- DOE Fundamentals Handbook: Engineering Symbology, Prints, and Drawings, Volume 1 of 2, DOE-HDBK-1016/1-93, January 1993, U.S. Department of Energy (University of Cambridge Engineering Department mirror), Module 2, objective 1.4
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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