A sewer camera traces a line by being pushed along it with a sonde in its head; a flushable sonde traces a line by floating along it. It is a small sealed cylinder holding a battery and a 512 Hz transmitter, light enough to be carried by the water in a drain, and once flushed it travels down the pipe while an operator follows the signal across the surface with a receiver. For finding where a line goes, with no camera, no rod and no cleanout, it is the simplest tool there is. The cost is that it shows nothing of the pipe’s condition and that it does not always come back.
Quick answer: a flushable sonde is a battery sonde in a float, dropped into a toilet, a cleanout or a floor drain and carried by running water down the line. The receiver follows the moving signal and the route is marked as it goes, or the sonde is allowed to stop at a blockage or a belly and located there. It travels as far as the flow carries it, which may be to the main, where it is lost, or to a stop, where it marks the problem. It gives no picture and no distance count. It is retrieved at a downstream access point, by a drain cable with a retrieval head, or not at all, which is why cheap ones exist.
What it is
A sealed capsule, typically an inch or two in diameter and a few inches long, with a replaceable or sealed battery and a 512 Hz transmitter coil, sometimes with a float collar so it rides high in the flow. Some are rigid and some are flexible to pass bends. Battery life is hours to days. They are sold as drain sondes, float sondes or flushable transmitters, and they are found with the same receiver used for a camera sonde. What a 512 Hz sonde is and sewer camera sondes cover the camera-mounted version.
How it is used
Switch it on, confirm the receiver hears it, and drop it into the entry: a toilet bowl before flushing, a cleanout, a floor drain. Run water behind it from the entry and from upstream fixtures. Walk the receiver along the expected route, finding the peak and following it as it moves; mark the ground every few feet. Where the signal stops moving, the sonde has stopped, at a belly, a blockage, a change of grade, or the main. Locate it precisely there and mark it. Marking a dig covers the locate itself.
When it beats a camera
When the question is only where the line goes and there is no camera to hand, no cleanout for a rod, or a line too long or too convoluted for a push camera. When the pipe is a floor drain or a trap arm that a camera head will not pass but a small float will. When a quick route check is wanted before deciding whether a camera inspection is worth paying for. A septic line from the house to the tank, a long rural lateral, a storm drain to an unknown outfall and a floor drain to an unknown destination are the common cases. Tracing a lateral with a camera is the alternative where a camera is available.
When it does not
It gives no picture, so nothing about the pipe’s condition is learned; a line can be traced perfectly and still be about to collapse. It gives no distance count, so the stop point is known on the surface but not by footage from the entry. It needs flow to move, so a dry line or a line with little fall strands it near the entry. It stops at the first belly or blockage, which is useful information, but it will not pass to inspect beyond. And where the line reaches the main, the sonde goes with it. What a camera cannot detect is a longer list for the camera, but the sonde detects nothing at all.
How far it travels
As far as the water carries it and the pipe allows: tens of feet in a flat drain with little water, hundreds in a well-flushed lateral with good fall. Running water steadily behind it helps; a flat section or a belly stops it. In a line with a house trap or a sharp cast iron bend it may lodge at the fitting, which is itself a finding.
Retrieval
At a downstream cleanout, a manhole or a septic tank inlet, the sonde can be caught as it arrives, with a net or by hand at a tank inlet. In a line without downstream access, a drain cable with a retrieval head pushed from the entry can snag it, with the receiver guiding the cable to it. A sonde that reaches the public main is gone and the utility should be told, since a transmitter in a main is a small foreign object but a foreign object nonetheless. Inexpensive sondes are treated as consumable. Retrieving a lost camera head covers the same methods for a more expensive object.
Battery and signal
A float sonde’s battery is small and its coil is small, so its signal is weaker than a camera sonde’s and its depth range shorter. In deep or conductive ground it may be hard to hear. Check it against the receiver before dropping it in, and do not drop in a sonde whose battery is doubtful. Weak sonde signals and soil effects cover the limits.
Frequency
Flushable sondes are made at 512 Hz for sewer work and the receiver is set to sonde mode at that frequency. Some are made at other frequencies for other trades; a sewer operator buys 512 Hz to match the receiver they have. The 512 Hz standard and frequency matching cover compatibility.
Compared with a sonde on a cable
A sonde fixed to a drain cable or a push rod goes where it is pushed, stops where the operator stops it, comes back when pulled, and gives a distance from the cable markings; it needs a cleanout or a pulled toilet and some effort. A flushable sonde goes where the water goes, stops where the pipe stops it, may not come back, and gives no distance; it needs only an opening and running water. Each has its day. A sonde on a jetter nozzle is a third arrangement.
Who uses them
Plumbers finding a septic tank or a line’s route before quoting; home inspectors confirming where a drain goes; landscapers and pool builders checking a yard before digging; storm drain crews finding an outfall. Anyone who owns a receiver and wants a quick route without unpacking the camera. Sewer line locators and the locator finder cover the receivers.
Common questions
What is a flushable sonde?
A small battery-powered 512 Hz transmitter in a float that is dropped into a drain and carried by the water so its route can be followed from the surface with a receiver.
Can a flushable sonde replace a sewer camera inspection?
For finding the route, yes. It shows nothing about the pipe’s condition, so it does not replace the camera for inspection.
How do you get a flushable sonde back?
Catch it at a downstream cleanout, manhole or tank, or snag it with a retrieval head on a drain cable. One that reaches the main is lost; tell the utility.
Related guides
See septic line inspection, pre-purchase inspection, and locators and sondes.