against primary air through the grid

English to Italian Tech/Engineering Chemistry; Chem Sci/Eng CFB boiler (incinerator)

Each bed pressure tap is connected to a transmitter that directs a signal to the DCS indicating bed pressure. What the transmitter is actually measuring is back pressure against a constant purge air supply caused by gravitational pull upon fluidized bed material ***against primary air through the grid**.

Troppo contorta: quell'against finale non capisco come si collega con la parte prima!
rileva la contropressione in base alla bla bla dovuta alla bla bla esercitata sul bla bla...e poi?