OT Air purifiers, how do they tell when to change the filter?

In my case, I'm talking about a $100 air filter/purifier. The
light turns yellow, then red, depending on how clogged the air
filter is. Are they using motor current to determine that?

I would like to use my own custom filters. If they used motor
current, that will let me determine whether my filters allow
enough airflow.

Thanks

 

I suspect it simply be a really long timer. I have a whirlpool AP450 I've
been tempted to open to examine. It has a light that goes on when it's
time to replace the $80 HEPA filter.

 

Commercial air handling system use differential pressure sensors or flow
measurement. Most home/consumer products use a timer. In fact on my home
thermostat you can set the timer reminder interval depending on the
filter type.

 

I got a bunch of used Holmes air cleaners I got on eBay. I wanted these
because it's easy to configure your own. I buy charcoal media cheap in
bulk. I use various polyester electrostatic media. It's got a thumwheel
knob on top, which looks strictly manual for month. I guess your supposed
to keep track. HEPA filters are high restriction when clean. Some more
costly multi filter cleaners change speed monitoring room pollution, so I
suppose some measure differential pressure to change filters.

Mine HP2400

Greg