;; I realised that you can estimate this "performance" of a compressor
;; by how quickly it raised the pressure in the tank as it is
;; started-up.
;; 1litre=1000cc=1000cm^3
(setq litres2ftcub
(/
;; cubic mm per cubic foot
(let ((mm2inch 25.4)(in2ft 12))
(expt (* mm2inch in2ft) 3)) ;; 28316846.591999985
;; cubic mm per litre
(let* ((cmcub2l 1000)(mm2cm 10)(mmcub2l (* cmcub2l (expt mm2cm 3))))
mmcub2l) ;; 1000000
))
;; 28.316846591999987 l per ft^3
;; compressor took 15s to raise pressure from 90psi to 105psi as filling its 50litre tank
(* (/ 60 15) (/ 50 litres2ftcub)) ; 7.062933344297721cu ft per minute
;; so 7cfm at 90psi (actually as pumping to 105psi)