The REED R3500 is a compact, pen-style pH meter with a waterproof housing and a dual LCD that shows pH and temperature at the same time. It covers the full 0-14 pH range with automatic temperature compensation and a guided calibration routine, running on four replaceable LR44 batteries with auto shut-off to save power. The -NIST package bundles the pen, a protective cap, hard case, batteries, and a NIST calibration certificate, though it is a simpler non-logging meter aimed at quick spot checks rather than continuous monitoring.