Vacuum Gauge to Drafting Lift Readout
Reads the compound (vacuum) gauge during a draft: suction_head = vacuum_inHg x 1.13, against the altitude-corrected attainable ceiling (factor x (33.9 - elevation/1000)) with the margin left. 10 in Hg at sea level is 11.3 ft, only half the ~22.6 ft ceiling; 18 in Hg at 3,000 ft is 98% of the ceiling - the pump about to lose prime. Turns the gauge needle into feet and a cavitation warning. Pairs with draft-lift-max (the ceiling). A readout aid, not incident command.
Formula and source
suction_head_ft = vacuum_inhg x 1.13; theoretical_ceiling = 33.9 - site_elevation / 1000; attainable_ceiling = pump_factor x theoretical_ceiling; margin = attainable_ceiling - suction_head_ft.
IFSTA / NWCG fire-pump drafting practice vacuum-to-lift conversion, by name.
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