As plants are becoming more and more automated there is a natural desire to ensure that the cleaning mechanism on the bin vent filter is automatically initiated when a truck is filling the silo. Putting a limit switch on the fill line is an obvious way to tackle this problem. The problem is that these devices are notoriously fragile, in a notoriously rough and tumble application. Nevertheless, there have been some creative solutions provided over the years (attaching the cap to a chain on a limit switch is a particular favourite).
A great alternative to a mechanical limit switch is to measure the difference in pressure between the silo and atmosphere. If there is a measurable pressure then the silo must be being filled. A differential pressure switch or transmitter on the bin vent filter (a commonly available option) measures this pressure, the transmitter option has the additional benefit of allowing operators to trend the silo filling pressure and predict maintenance intervals on the bin vent filter cartridges/bags.