Capillary String : A very small string, usually run along the outside of the tubing and banded to the tubing. Commonly used for hydraulic control of safety valves and sliding sleeves. May also transmit bottom hole gauge data
George E. King Engineering, 5000 Oilfield Terms: A Glossary of Petroleum Engineering Terms, Abbreviations and Acronyms, US 2010. http://gekengineering.com/Downloads/Free_Downloads /Glossary_of_Petroleum_Engineering_Terms_25_August _2010.pdf, (Oil - Amer) {593}