Locating Sondes

Follow Avatar Anthony Caplin
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Sonde overview

A sonde is a self-contained, battery-operated transmitter used for tracing the paths of pipes, ducts, sewers and drains and in the precise location of blockages or collapses. The sonde can be fitted to a flexible rod for insertion or pushing through pipes and the smaller diameter sondes can be used in conjunction with jetting machines, and blown through the duct.

Choosing a suitable sonde

Radiodetection offers a wide range of sondes to suit most applications: From the 6 mm (¼”) diameter S6 33 kHz Microsonde which, with a range of 2 m (6.6’), targets fiber-optic micro ducting or other small non-conductive pipes, to the 33 kHz Super Sonde, which with a depth range of 15 m (50’) targets deep sewer pipes.

Consult the precision locate accessory range brochure or web page on www.radiodetection.com for a full list of all available sondes and their technical specifications.

Check that the sonde has sufficient range for the application and is dimensionally small enough and sufficiently robust for the application. Ensure that the frequency of the sonde corresponds with the locator frequency; the locator will not locate the sonde unless the frequencies are the same. Ensure that the means of propelling the sonde is available together with the correct fittings and couplings.

Propelling a sonde

Sondes have a thread at one end for connecting to drain rods, or to other devices for inserting and propelling the sonde along a drain or duct. Sondes may be floated along drains at the end of a tether and floats are available for fitting to the sewer sonde and super sonde. Sondes can be strapped to high-pressure water jets or similar devices used for cleaning, maintaining and inspecting drains. Sondes used in underground drilling and boring operations are normally housed in the boring or drill head behind the boring or drill bit.


Insert a new battery into the sonde. A new battery or a freshly recharged battery should be used at the beginning of each day and preferably at the start of each new job.

Before inserting the sonde and propelling along the proposed route check that the sonde is working correctly and that the locator and sonde are at the same frequency. To do this, place the sonde on the ground at a distance from the locator that is equal to the rated depth of the sonde. Set the locator to Peak+ Guide mode, point the locator at the sonde with the antenna in line with the sonde (the opposite of using the locator to locate a line) and check that the bargraph reading exceeds 50% at maximum sensitivity.


Sonde deployment


Locating and tracing a sonde

Insert the sonde in the drain or duct access and locate it while it is still just in view at the drain or duct entrance. Hold the locator vertical directly over the sonde with the antenna in line with the sonde as shown below


RD5100S locator with 33 kHz standard sonde


Adjust the locator sensitivity so the bar graph reads between 60% and 80%.

The sonde radiates a Peak field from the center of its axis with a ghost signal at each end of the Peak. Move the locator a little way behind and then in front of the axis of the sonde to detect the ghost signals. Finding the two ghost signals positively confirms the location. Reduce the locator sensitivity to lose the ghost signals but still indicate a clear Peak response directly over the sonde. Locator sensitivity is now set for tracing the duct or drain unless the distance between sonde and locator changes.

Propel the sonde three paces along the drain or duct and stop.

(a) Place the locator over the supposed position of the sonde and move the locator backwards and forwards with the blade in line with the sonde and stop when the locator display indicates a clear peak response.

(b) Rotate the locator as if the blade were a pivot, stop when the display indicates a clear peak response.

(c) Move the locator from side to side until the display indicates a clear peak response.

Repeat (a), (b) and (c) in smaller increments with the locator blade resting on or near the ground.  The locator should now be directly above the sonde with the blade in line with the sonde.  Now mark the position.

Propel the sonde a further three to four paces along the drain or duct and pinpoint and mark.  Repeat this procedure along the route at similar intervals.  It should only be necessary to change the locator sensitivity while tracing the sonde if there is a change in the depth of the drain or duct, or the distance between the locator and sonde.

While locating the sonde it is useful to make use of the compass feature on the locator display to assist with the orientation of the locator with the sonde and for confirming direction of sonde.



Locating a sonde


Depth Measurements

All RD5100S locators use TruDepth™ to measure depth automatically when a good quality reading can be assured.


NOTE: TruDepth only indicates a locate depth when:

  • The locator is correctly oriented above the sonde.
  • The locate signal conditions assessed are good enough to ensure an accurate reading.

To help you orientate the locator correctly, you can use the locator’s compass feature on the screen.

Depth readings are displayed in the units of measurement according to the configuration of the RD5100S.

Depth readings are to the center of the sonde.

Note: Make sure that the compass on the LCD is in the East / West orientation as shown.


Depth readings

Depth range and accuracy vary depending on the makeup and type of target utility (for example pipe or sonde), its depth and external environmental factors like electromagnetic noise, ground conditions and interference.

Obtaining Depth Measurements

Pinpoint the sonde as previously described and rest the locator on the ground and in line with the sonde.  If using Peak+ guide mode, adjust sensitivity to provide bar graph response around 50%.  The locator will automatically display a depth measurement.


Verifying depth measurements

Check a suspect or critical depth reading by lifting the locator 50 mm (2”) above the ground and repeating the measurement. If the measured depth increases by the same amount it is a good indication that the depth reading is correct. When locating for a sonde, depth measurements should be precise to ±3% if conditions are suitable.


Verifying a depth measurement


Calculation method

If the locator does not provide a depth measurement or the sonde is too deep for the locator to display or calculate a depth, pinpoint the sonde. Move the locator in front of the sonde and still with the antenna in line with it, increase sensitivity to find the Peak of the ghost signal. Move the locator to behind the sonde ensuring that the locator blade is always in line with the sonde. Find the null positions A and B (see the illustration above - Verifying a depth measurement). Measure the distance between them and multiply by 0.7 to give an approximate depth measurement.