A nice simple one is to count parts over 10 seconds, store the data into a 6 word shift register (shifting down each 10 secs)
then reset the count, add the 6 registers & this gives you a 1 min rolling average every 10 secs. this is a good compromise as you refresh every 10 secs so after no parts for 1 min the parts start counting after 10 secs you have say 10 parts, 20 secs 20 parts, 30 secs 30 parts & so on if the parts are one every second then = 60 ppm but losing a part gives 59 ppm