Air Flow Indicator for Record Power AC400 Air Filter

I was never quite sure when the filters in my workshop dust scrubbing air filter needed to be changed. They're quite expensive, and sometimes hard to find, so I've tended to vacuum them out a few times and then, eventually and reluctantly, swap them out. If only there was something to indicate when the filters should be changed!

This is a very simple “meter”/indicator to give an idea of relative air flow during the life of a set of filters. I don't pretend that it's calibrated in any way, the needle is just pushed by the air against gravity, but I've taken some photos to illustrate how it can help…

The image below is my air flow indicator showing a reading of about 6½ notches (from the bottom notch - not a recognised metric or imperial unit!) with a fresh set of filters straight out of the packet, with the air filter set on medium speed.

The  image below is my air flow indicator showing a reading of about 2½ notches with some old filters that I doubted should still be in service (also with the air filter set on medium speed).

My guess is that I should change the filters to keep the indicator at or above 4 notches.

Looking at other air filter designs that seem to be clones of the Record Power AC400 (or vice versa!), this should also work with:

Scheppach HDA-14,
Axminster AW15 AFS,
Jet AFS400,
Fox W1830,
Felder Number 1 air filter LF 600
Rutlands R8560
WEN 3410

The basic design is intended to be mounted onto the top, central screw that fixes the wire mesh across the air outlet on the RP AC400, but you may need to improvise a mount for one or more of the above brands. The funny, segmental, boxy thing at the top is part of a damper mechanism that helps stop the needle oscillating in the turbulent air exiting the air filter. The indicator still oscillates, particularly for low air speed, but is much more stable than without the damper. The lid of the damper box is printed separately and glued on - it's best to glue this on, and to mount the needle, after fixing the air flow indicator to the air filter, otherwise access to the mounting screw is restricted.

The needle needs an M3 x 25mm cap head screw, which will self tap & should be tightened such that it does not impede the needle's movement.

I printed the scale and damper cover from Bambu Lab Basic PLA in Green, and the needle in Eryone PETG-CF in Black. No supports necessary.