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“My impact wrench won’t undo anything” is a question that has kept coming up on Italian forums for years. In one documented case the problem was solved, among other things, with a larger hose [59]; in another, a user estimates that the compressor supplied about half the air the wrench needed [62]. The hose is the part you can check most easily.

How much you lose

Pressure lost in a smooth hose, with 6.3 bar at the compressor outlet, without fittings:

Diameter matters much more than length: for the same air, going from 6 to 8 mm cuts the loss by a factor of about four, and from 8 to 10 mm by about three. A 360 l/min tool that needs 6.2 bar, such as the Metabo DB 10 drill [38], with 20 m of 8 mm hose would want almost 7 bar at the compressor, and at 7 bar many hobby compressors deliver less air.

Open an example in the calculator and try changing the diameter.

How we calculate it

• Loss: Darcy-Weisbach equation, with the Colebrook-White friction factor solved by iteration starting from the Swamee and Jain formula [58].
• Air at 20 °C: density 1.204 kg/m³ at atmospheric pressure, rising in proportion to the absolute pressure in the hose [55]; dynamic viscosity 18.13 µPa·s [56].
• Roughness: 0.007 mm, the upper value for smooth plastic hoses [57].
• Left out: quick couplings, coiled hoses and reducers add losses. A narrow quick coupling can lose more pressure than a few metres of hose.

Air hoses are usually labelled with the internal and the external diameter, for example 8 × 12 mm: the first number is the one that counts.