An air hose connects the compressor to the tool and transfers air at high pressure. Strong and flexible air hose fittings are important to maintain an air tools performance from the constant flow of air and pressure. Measure the size of an air hose by the internal diameter. The bigger the hose I.D, the more air it can carry. Connectors, also known as couplings or splitters, are used to join a hose to a connector or tool. BSP threads are either BSPP or BSPT. Measure across the outside of the male thread and measure the inside diameter on a female thread.