Pet's weight in Pet Immunity PIR sensor

As I'm sure you realize, PIR's don't measure the pet's weight. The manufacturers use weight as a guideline for installers in selecting an appropriate pet resistant detector for the job. The detector looks for changes in the amount of IR (heat) energy striking various segments of a pyrolitic transducer. If the change is sufficient and the location of the change meets certain criteria an alarm is generated.

There are several approaches to making a PIR pet immune. The simplest is to look for simultaneous IR changes (movement) in upper and lower halves of the transducer. That would correspond to motion in the lower and upper part of the field of view respectively. If the detector only "sees" motion along the floor it's probably a pet walking on the floor. Likewise, if the motion is only detected up high it's possible the pet is sitting on top of a couch or whatever. If motion is detected high and low at the same time that's probably a person.

The above is the simplest type of pet resistant technology. Microprocessors in modern PIR detectors run complex algorithms to determine whether a given pattern of motion (actually, changes in received IR) merits a response. You're probably aware that there are "dual technology" detectors which couple PIR with microwave. Technically that isn't done so much for pet resistance as to prevent nuisance alarms from other stimuli.