Black 3/16" ID (4.8mm) Nitrile / Buna N Fuel and Smoke Tubing, 5/16" OD (7.9mm), 1/16" (1.6mm) Wall thickness.  Buna N is more flexible than yellow Tygon and will not harden.

Sold per foot.  If you want 6 feet of Tubing order a quantity of 6 and it will be 1 piece that is 6 feet long.