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On Italian forums the question has been coming back for years: at what pressure does the flow rate in the catalogue apply [60]? And people often choose between two compressors by comparing the litres of intake air, without anyone mentioning delivered air [61].

Two different numbers

• Intake air (displacement): how much air enters the cylinder on each revolution, measured at the inlet. It is always the bigger number and the one most often printed on the box.
• Free air delivery (FAD): how much free air actually comes out at working pressure, the air the tool needs. It is lower, and it falls as the pressure rises.

A manufacturer says so too: Metabo’s compressed-air guide warns against confusing the effective delivery with intake air [19].

How big the gap is, according to those who declare both

Einhell and Parkside declare delivered air at several pressures, Metabo at 80% of maximum pressure. In the table at the bottom:

• the Einhell TC-AC 200/24/8 OF draws in 180 l/min and delivers 45 at 7 bar, a quarter;
• among the models that declare delivered air at 7 bar, delivery ranges from 25% to 48% of intake air.

An independent measurement, made with the tank fill-time method, found about 6.6 CFM against 10 declared on an imported compressor, with an uncertainty the author estimated at about ±30% [63]. It is a single test and does not concern the models in this table, but it indicates that even the declared figure should be taken as a maximum.

Manufacturers who declare only intake air

In the documents we have read, ABAC, Fiac, Fini, Nuair, Dari and Scheppach give only intake air [22][24][26][28][29][30]. This does not mean their compressors deliver less, but there is no figure to tell. The calculator shows them separately and makes no estimate. Before you buy one, ask the seller for the delivered air at the pressure you need. For the Scheppach HC52DC a manufacturer page archived in 2019 also gave the delivered air at 1 to 8 bar [71]: the calculator uses it with the archive date, as a statement made at the time and not as a confirmation of the model sold today.

An example

A Metabo DB 10 air drill has a declared consumption of 360 l/min at 6.2 bar [38]. Only three compressors on this site give that much according to the declared data, all three-phase Güde models on 400 V; none on 230 V: with a home compressor the drill can only work in intervals, on the air stored in the tank. Open the example in the calculator and see the guide on how long the tank lasts.