First, pet "immune" is a misnomer for how these detectors work as well as tolerate the presence of a pet within the protected area.

Usually there are two types of pet "immune" detectors. The first type, a passive infrared "PIR", ignores animals up to a certain weight, which is generally more a function of mass of the animal, as well as size and location. These units are usually the cheapest to purchase, as well as the least tolerant to animals. Commonly they will ignore a single small animal, such as a single cat or small dog.

The second, a dual technology, or "active" is usually more tolerant and resistant to animal caused false alarms. They require, what are essentially two detectors, to be triggered in a manner determined by the manufacturer, to generate an alarm condition. These units can usually ignore a good-sized animal or multiple small animals without generating an alarm condition.