On Tuesday, December 26, 2017 at 7:19:26 AM UTC-8, Andy wrote:
I bought an Envion Therapure TPP240 Permanent HEPA Type Air Purifier.

I sent them an email, but got no answer.

Anyone know how the sensor works that determines when the filter needs cleaning?

Thanks,
Andy

The flow of air trough it is how it works. Most owners manuals will give you a time factor for changing the filter. Depending on the time of year, your location and if you have pets will be a good guide. When you do change the filter check it out to see how dirty it is and that will tell if you waited too long.