Using Passive Infra-red (PIR) detectors in the presence of animals is always a compromise!
Visonic produce two general purpose wireless PIRs for use with the Powermax Wireless alarm system. one is labelled NEXT MCW and the other is labelled NEXT K9-85. The latter is promoted as being suitable for use where animals up to 85 lbs in weight are present.
In the hard-wired alarm world there are a lot of pet-friendly PIRs on the market. Some are unsophisticated and use simple optical cut-offs whereas others use complex algorithms to attempt to reliably distinguish between animals and human intruders.
Unfortunately the Visonic K9-85 falls into the former simple cut-off type. The internals of both the wireless Visonic offerings are identical, it is only the front lens that changes.
In the photo above the K9-85 PIR is on the left. The bottom half of the lens is opaque and effectively cuts off the received infra-red from the bottom half of the detector. Exactly what this has to do with the weight of the animal is presumably something to do with the typical height of an 85 lb animal.
The upshot of this is that the PIR detector must be fitted at the correct height. Also note that it will detect an animal climbing open stairs or say getting onto that comfy settee when you are out.
Climbing cats are also likely to cause problems. So use with caution.
Have you had good or bad experiences with these - then let us know.