The Fisher Labs SONDETW6 SR-81 Pipe Signal Transmitter is a self-contained heavy-duty pipe signal transmitter, specially designed for tracing the unknown path of a pipe in conjunction with the Fisher TW6. The SONDE-TW6 can also be used to find blocks within pipes, such as finding the location of a storm pipe blockage - like a tree-root penetration. This model is inserted into pipe or ductwork using a push-rod and located using a Fisher M-Scope receiver. A peak signal reading is noticed at three different locations in-line with the signal transmitter's physical length. The strongest peak signal is recorded at the center of the SONDE-TW6, accompanied with two weaker signals set at equal distances on each side, and in-line with the sonde.