The Reed R5820 is a handheld loop calibrator built for sourcing and measuring 4-20mA signals when servicing two-wire transmitters and loop-powered isolators. It reads out on a large 5-digit LCD, offers an internal 24V loop power supply, and supports step or ramp output modes for linearity checks, with sourcing accuracy around 0.02-0.05%. It runs on 2 AA batteries for roughly 20 hours of field use.