What is lpf and upf wattmeters?

LPF (Low Power Factor) and UPF (Unity Power Factor) wattmeters are instruments used to measure electrical power in AC circuits. LPF wattmeters are designed for circuits with low power factor, where the phase difference between voltage and current is significant, while UPF wattmeters are used for circuits operating at or near unity power factor, where the voltage and current are in phase. The choice between the two depends on the power factor of the system being measured, as each type provides accurate readings under its respective conditions.