what is difference between upf &lpf watmeter?

Answer 1

An LPF (low power factor) wattmeter typically is used to measure no-load losses of inductive devices like motors and transformers. See ref. 1.
Information about UPF (unity power factor) wattmeters was hard to find. From ref. 2:
"How is LPF wattmeter different from UPF wattmeter? Some special features are incorporated in UPF wattmeter to measure LPF.
(i)Low value of resistance in pressure coil
(ii)compensation for pressure coil current
(iii)compensation for inductance of pressure coil
(iv)small control torque"

Answer 2

well one has a u and one has an l.