Thermography uses an infrared camera (thermal imager) to capture two-dimensional representations of the surface temperatures of parts of buildings. Often, those images reveal temperatures or temperature differences that indicate conditions contributing to the waste of heated or cooled air and, as a result, needlessly excessive energy costs.
There is no reason to ever put gauges on an air conditioning or refrigeration system after the initial installation unless a problem with the mechanical refrigeration circuit is suspected. The refrigerant charge can be accurately checked using a thermometer and manufacturer's charging chart.