I have a VFD fan that pushes air through a filter. Over time, downstream resistance gets higher due to the filter. Fan speed remains constant. We have looked at amperage draw and it has remained stable even with this added resistance. Does that mean that air flow remains constant? Kind of confused that we are getting constant amperage. I'm inclined to think that we should be seeing a decrease in amperage draw since we should be pushing less air to due added downstream resistance.