1/8 the trachea needs an outside diameter of the trachea
1/8 for 3.2 mm OD, you choose the trachea, then should look at the trachea is the diameter, not the outer diameter, generally as long as less than 3.2 millimeters in diameter, the tube OK. But don't be too small to avoid getting in.
1/8, you can transfer 4, 6, 8, and 10.SMC pipe connections, please refer to "SMC pneumatic service network""
The size of the pipe connection is not very large, or is not necessarily the size of the trachea,
