Greenlee 521a or 501 tracker 2

521 - The new and improved 521A Wire and Valve Locator builds on the success of the industry-standard Model 521. The new 521A has added features such as: loudspeaker listening, an extended locating wand for comfort, a slimmer case and a switch to easy to find "D" cell transmitter batteries. Like its predecessor, the 521A combines adjustable transmitter power with a sensitive locating receiver to allow location of buried wires and valves with speed and confidence.

501- The Model 501 is designed to locate the path and depth of buried cable, service wires, metallic pipe or conduit and locate the end of a cut cable.

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Likely the difference is the frequency.