These adapters are used to make vacuum connections between hose and national pipe thread (NPT) fittings. Always use a pipe thread sealant when assembling tapered threads.  The hose should be made of soft flexible plastic or a type of rubber tubing. 

Common Dimensions:

Hose Barb: 1/8 inch ID Rubber Hose

NPT: 1/8 inch MALE National Pipe Thread