If an electronic air cleaner filter is employed, a discharge counter counts the frequency of pops occurring across {HEAD ~ the electrodes} of {TAIL ~ that filter} 
If an electronic air cleaner filter is employed , a discharge counter counts the frequency of pops occurring across { HEAD ~ the electrodes } of { TAIL ~ that filter }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL