;; 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)