With the advancement of Cement Silo Dust Collector filters, magnehelic gauges are typically not needed on modern collectors.  20+ years ago 5 & 7 HP blowers were required on silo collectors in order to get the air to cloth ratio needed for BAG filtration.  Today we utilize pleated cartridges that have up to 35% more filter area and do not require the optional blower.  Static silo dust collectors should not need a gauge because there is no vacuum or blower that is incorporated into the silo collectors design.  The only blower or vacuum typically on a concrete batch plant would be on your mixer truck dust collector which includes a magnehelic gauge. 
The purpose of the magnehelic gauge is to display how efficient (dirty) the filtration system is operating at.  If the dust collector has a blower, then you will connect the 1/4" tube from the dust collector to the gauge on the "-" input to display the negative displacement or vacuum efficiency through the filters.  The higher the number the greater the vacuum.  If you do not have a blower on the collector, then the 1/4" tube would be connected to the "+" input to display the positive displacement through the filters.
If required by your local ARB, you can buy the Minihellic gauge (pn 145480) and install it.  However it will only function when a cement bulk truck is offloading.  Typically the ARB will request the gauge to be mounted near grade level.