Two instruments share one name
Before comparing anything, it is worth pinning down which device is on the table. Manufacturers sell an induction-based cover meter as a “rebar locator” or “rebar scanner”, and they also sell handheld radar units under names just as close to “scanner”. Two products described in nearly the same words work on entirely different physics and answer different questions.
The distinction that matters on site is the sensing principle, not the badge on the case. Ask whether the instrument works by electromagnetic induction or by radar, and the rest of this page follows from the answer.
How an induction cover meter works
The head generates an alternating magnetic field. Ferrous steel in that field distorts it, and the instrument converts the strength of the distortion into a position and a cover estimate. Because the physics only engages with ferrous metal, anything that comes back is metal — which is a genuine advantage: there are no ambiguous responses to interpret away.
The cover figure depends on bar diameter, which the operator supplies or the instrument assumes. Get that wrong and the depth is wrong by a predictable amount. Two bars close together read as one shallower bar, and a second mat under the first is generally invisible, because the field has already been captured by the steel above it.
How concrete GPR works
A high-frequency antenna emits a short pulse and records what reflects. Any boundary where the electrical properties change sends part of the pulse back: a steel bar, a plastic conduit, a post-tension duct, a void, the far face of the slab. Each discrete target crossed at right angles draws a hyperbola on the display, and a grid of passes assembles those into a plan.
That breadth is the whole argument for radar, and it comes with a cost the marketing rarely states plainly: a radar screen shows reflections, not labels. Distinguishing a 16-millimetre bar from a conduit of similar size is a judgement made from spacing, continuity, depth, and context, and it is only as good as the operator making it. The FHWA's bridge-deck protocols treat radar the same way — as a survey whose value rests on how the data is collected and interpreted, not on the instrument alone.
Cover meter and GPR compared
The row that decides most jobs is the third one. If anything other than ferrous bar might be in the way, induction is answering a narrower question than the one you are asking.
| Consideration | Cover meter (induction) | Concrete GPR |
|---|---|---|
| Sensing principle | Magnetic influence of ferrous steel | Reflection from any change in electrical properties |
| What it detects | Ferrous reinforcement only | Bar, conduit, tendon ducts, voids, and the back face of the element |
| Nonferrous targets | Not detected at all — PVC conduit and plastic tubing are invisible | Often detected, depending on size, depth, and contrast |
| Second mat and congestion | Poor; overlapping responses merge and cover drifts shallow | Better, though a dense top mat still shadows what lies below it |
| Slab thickness | Not available | Available where the back-face reflection is resolved |
| Output | A position and a numeric cover estimate | An image requiring operator interpretation |
| Speed on a clean single mat | Very fast, minimal set-up | Fast, but a proper grid and interpretation take longer |
| Depends on operator skill | Modestly — mainly correct bar-size entry and calibration | Heavily — the reading is the deliverable |
Which one to specify
Most providers carry both, so the practical question is not which instrument to buy but what to put in the scope. Describe the element and the risk, and let the provider tell you which method covers it — then check that their answer accounts for the things below.
Simple slab, single mat, shallow anchors
A cover meter is proportionate, quick, and gives a cover figure directly. Radar adds little on an element with nothing else in it.
Core drilling or a full-depth cut
Radar. A penetration through the section can meet a second mat, a conduit, or a duct that induction never reported.
Post-tensioned deck
Radar at minimum, briefed as post-tensioned, and often radiography. A cover meter cannot dependably separate a tendon duct from ordinary bar.
Conduits or radiant tubing suspected
Radar, because a plastic conduit or PEX loop produces no induction response whatsoever.
Condition assessment or slab thickness
Radar. Cover meters report cover on individual bars and nothing about the element as a whole.
Common questions
Frequently asked questions
Is a rebar scanner the same as GPR?
Not necessarily. The phrase is used for induction cover meters and for handheld concrete radar units. A cover meter senses ferrous steel magnetically and reports cover; GPR reflects a pulse off anything that contrasts with the concrete and produces an image. Confirm which principle the instrument uses before assuming what the scan will cover.
Which is more accurate for rebar depth?
On a clean single mat with the correct bar diameter entered, a cover meter's cover estimate is generally the more direct figure. In congestion, or where a second mat sits below the first, its estimate drifts toward the shallowest steel in range while radar can often still separate the layers. Both produce estimates rather than measurements.
Can a cover meter find post-tension cables?
Not dependably. The strand is ferrous, so something usually responds, but an induction instrument cannot separate a tendon duct from ordinary reinforcement, and tendons drape through the slab depth rather than holding constant cover. Where post-tensioning is possible, that has to be stated in the scope and the method chosen accordingly.
Can GPR detect plastic conduit in concrete?
Often, where the conduit is large enough and contrasts with the surrounding concrete — an air-filled duct is a strong reflector. Small-diameter tubing, or a conduit sitting directly beneath dense reinforcement, may produce nothing usable. A cover meter will not detect it under any circumstances.
Do I need both instruments on the same job?
Sometimes, and it is a reasonable thing to ask a provider about. Using radar to screen an area for anything unexpected and a cover meter to confirm the bar grid and cover plays to each one's strength. How to locate rebar in concrete walks through how a combined scan is run on site.