The 501 is a very cost-effective and versatile locator which features a Peak antenna in the Receiver and transmits a single ultra-high frequency signal (447.5 kHz) via Direct Connection, Inductive Clamp Coupling or an Inductive Antenna.

Inductive Clamps allow signals to be applied to cables without disconnecting them — this is ideal for working with pipes and cables that are in use; just ensure that they are safely insulated. Inductive Antennas broadly emit signals which can be coupled to a cable or pipe below ground — great when you know there’s something down there but don’t have access to the ends of the line.

The 501 is an excellent “first-timer” locator which is intuitive and versatile, for most situations and applications.

The IP54-rated 551 offers a high-powered Dual Frequency Transmitter which uses one low frequency (1.75 kHz) and one mid-range frequency (33 kHz). Lower frequencies are helpful not only for tracking deeper depths and longer distances, but to limit the amount of “crosstalk” or “signal bleed” onto other nearby lines. Dual Simultaneous Frequencies are also excellent for pinpointing faults as you can instantly compare them along a line. One frequency may respond to damage differently than the other. Like the 501, the 551 also includes a coupling clamp and a built-in induction antenna, but while the 501 operates only at 447.5 kHz, the 551’s Inductive Clamp injects 33 kHz into a line and the 551’s induction antenna emits 133 kHz.

The 551 Receiver detects any of the three active frequencies (1.75 kHz, 33 kHz or 133 kHz) with both Peak and Null antennas—Peak to make tracing simple and intuitive and Null for pinpointing and triangulating depth. The 551 also passively detects live lines carrying AC-current. The IntelliTrack digital noise rejection feature ensures successful locating around electrical interference.

If more effectively isolating single lines, tracking deeper and further, detecting and pinpointing damage, intuitively triangulating line depth, and passively sweeping for live electrical current is desirable, then the 551 is the better choice otherwise the 501 is simpler and more affordable.

The main differences between the 501 and the 521E involve the signal frequencies and orientation of the antennas in the receivers.

The 501 uses a very high-frequency signal at 447.5 kHz while the 521E uses a much lower 1.75 kHz.

The lower frequency of the 521E means there is less signal bleed so tracking a single wire in a bundle is easier (great for irrigation), but a lower frequency signal will struggle to travel to the end of a wire or cable if it is not buried (or grounded at the far end).

Meanwhile the higher frequency of the 501 is more susceptible to bleeding onto other nearby metal or wires, but it also allows for easier inductive transmission and puts out less current, therefore less risk of shock or damage as well as less power consumption by the transmitter.  The high frequency of the 501 will also work much better with fish tape / steel snake fed through a non-metallic pipe.

Furthermore, the 501 can also tend to track more deeply buried wires, cables and pipes when directly connected, but unlike the 521E, it does not require a direct connection; it can be used with the Inductive Clamp or Inductive Antenna.

The 501 uses a “peak” method of location, so the hotter the signal, the closer you are to the cable, while the  521E uses a “null” approach: the signal is hottest a few inches alongside the cable and cancels out when right above the cable.

The null method offers some benefits for the 521E:
– A hot spot in signal over an irrigation sprinkler valve box with a coil around the solenoid makes valve location very easy.

– In the hands of an experienced user, it can be used to locate ground faults.

Because the 501 uses the peak method of location, the signal does not change as much over a valve box and thus is not as helpful for locating them. If valve locating is your primary goal, the 521E is the better choice.

Tempo offers an industry-leading line-up of buried utility locators. Here's an overview to help you choose the right one:

	501 Tracker II – Trace wires and metallic pipes on active or dead systems without deactivating power.
	551™ Advanced Underground Locator – Dual-frequency locator with IntelliTrack™ for tracing buried utilities up to 20 ft deep.
	521E Wire and Valve Locator – Professional-grade tool for tracing buried wires, irrigation valves, and cable faults.
	508S-G Wire Finder – Compact transmitter and receiver for locating wires in walls and at shallow burial depths.
	BLL-200 Buried Line Locator – Locate buried lines up to 20 ft and measure depth up to 15 ft with intuitive indicators.