for access to the slide plate 28 fig2 shows more detail of a smaller version of a fixed plate 38 having an array of only 7 in the slide direction u00d7 3 openings 30 the construction details of this smaller version are fully applicable to the larger versions shown in fig1 four brackets 40 can be welded to the fixed plate 38 or punched bent into the fixed plate 38 for holding the edges 42 of the slide plate 44 to the fixed plate 38 four holes 46 in the middle slide direction ribbing 36 are provided for fasteners 48 50 shown in fig3 7 to further support the slide plate 44 a marking 52 this one provided as a short line with a marking u201c u201d to indicate u201c percent open u201d is also provided on the fixed plate 38 depending upon the velocity of the air flow the desired thickness of the slide plate 44 and or fixed plate 38 may be too thin to prevent flexing of the ribbings 36 if ribbings 36 flex into the openings 30 of the other plate the plates 38 44 can bind and prevent sliding back to a position where the ribbings 36 on the two plates 38 44 overlap to prevent the flexing ribbing causing binding situation separator columns.