A crawler robot is a delivery platform, not the inspection itself — the actual data comes from the NDT (non-destructive testing) sensor payload it carries. Here’s what the most common payloads actually measure, where each is used, and how to choose the right one for your asset.
Ultrasonic thickness measurement (UT / UTM)
A UT probe sends a high-frequency sound pulse (typically 500 kHz–20 MHz) into the material and measures how long it takes to bounce back off the far wall or a defect. That time-of-flight converts directly into a thickness reading, which is why it’s the standard method for tracking corrosion and wall loss on tanks, pipework, pressure vessels and structural steel. Measurement range can span roughly 0.08mm to 635mm depending on the material and transducer used.
Automated ultrasonic testing (AUT)
AUT is UT inspection carried out by a mechanised scanner rather than a hand-held probe, moving systematically across a surface to build a full coverage map instead of spot readings. Mounting this on a crawler robot is what turns single-point UT checks into a full-surface corrosion map, which is the main reason robotic AUT surveys report closer to 100% coverage versus the much lower sampling rate typical of manual spot-check UT.
Electromagnetic acoustic transducers (EMAT)
EMAT is a non-contact ultrasonic method: it generates the sound wave inside the material itself using electromagnetic induction, rather than coupling a probe to the surface. That means no couplant gel and less sensitivity to surface prep, which makes it well suited to automated weld inspection and thickness checks on hot or rough surfaces where standard UT probes struggle.
Ground penetrating radar (GPR)
GPR sends high-frequency radio waves into a solid structure and reads the reflections as they bounce off boundaries with different density — reinforcing steel, voids, or delamination inside concrete. It’s the standard method for locating rebar, measuring concrete cover depth, and flagging internal defects without any drilling or coring.
Half-cell potential (HCP) testing
Half-cell potential is the standard method for estimating the likelihood of active corrosion in steel-reinforced concrete. It measures electrical potential at the concrete surface to build a map of where reinforcing steel is most likely corroding, and is typically most reliable when combined with other methods like GPR.
| Technique | What it measures | Typical asset |
|---|---|---|
| UT / UTM | Wall thickness, corrosion, wall loss | Tanks, pipework, pressure vessels, steel structures |
| AUT | Full-coverage thickness/corrosion map | Large tank shells, pipe runs |
| EMAT | Thickness and weld integrity, without couplant | Welds, hot or rough surfaces, pipelines |
| GPR | Rebar location, concrete cover, voids, delamination | Concrete tanks, silos, cooling towers, bridges |
| Half-cell potential | Corrosion risk mapping in reinforced concrete | Concrete tanks, digesters, bridges, silos |
Choosing the right payload for your asset
Metal assets (tanks, pipework, pressure vessels) generally call for UT, AUT, or EMAT, depending on whether you need spot checks, full-coverage mapping, or weld-specific inspection. Concrete assets (digesters, silos, cooling towers, bridges) call for GPR and half-cell potential, usually combined, since GPR locates the rebar and half-cell potential assesses its corrosion risk. Most real programmes end up combining two or more of these rather than relying on a single technique.
Where this applies: industry use cases
Energy & Nuclear
Mixed steel and concrete assets need both metal and concrete-specific techniques.
- UT/AUT for turbine halls, pressure vessels and steel structures
- GPR and half-cell potential for concrete cooling towers and containment structures
- EMAT for weld inspection on pipework in hard-to-couple locations
Oil & Gas
API 653 tank integrity programmes rely heavily on UT and AUT data.
- UT/AUT thickness readings feed directly into API 653 corrosion-rate calculations
- EMAT suits pipeline and weld inspection where couplant isn’t practical
- GPR is less common here, mainly relevant to concrete secondary containment
Water
Concrete-dominant assets like digester tanks need GPR and half-cell potential (as used on our Digester Tanks case study).
- GPR located reinforcement and assessed cover depth on Northumbrian Water’s digester tanks
- Half-cell potential assessed corrosion risk on the same structures
- UT applies to any steel pipework or fittings on site
Cement & Quarrying
Concrete silos need the same GPR/half-cell combination as other concrete assets.
- GPR and half-cell potential for concrete silo shells
- UT for steel conveyor structures and supports
- AUT for large-scale corrosion mapping across multiple silos
Chemicals
Stainless steel reactors and vessels are the dominant asset type.
- UT/AUT for reactor and vessel wall thickness
- EMAT where couplant contamination is a process concern
- GPR relevant only where concrete secondary containment is present
Food and Beverages
Hygienic stainless steel vessels require non-contact or minimal-residue methods where possible.
- UT for fermenter and tank wall thickness
- EMAT’s no-couplant approach suits hygienic environments
- Visual inspection payloads for internal seal and cleanliness checks
Honest limitations of suction-and-airflow crawlers
No single NDT technique answers every question. UT and AUT measure thickness but don’t locate rebar; GPR locates rebar but doesn’t directly measure corrosion; half-cell potential estimates corrosion probability rather than confirming an exact defect. Most credible inspection programmes combine at least two techniques, and the choice of payload also depends on what a robot can physically carry — HB2’s 6kg payload suits lighter sensor packages, while HB3’s 25kg payload is needed for heavier scanning or multi-sensor rigs.
What you get: reporting & deliverables
No single NDT technique answers every question. UT and AUT measure thickness but don’t locate rebar; GPR locates rebar but doesn’t directly measure corrosion; half-cell potential estimates corrosion probability rather than confirming an exact defect. Most credible inspection programmes combine at least two techniques, and the choice of payload also depends on what a robot can physically carry — HB2’s 6kg payload suits lighter sensor packages, while HB3’s 25kg payload is needed for heavier scanning or multi-sensor rigs.
Frequently asked questions
Can HausBots' robots carry these NDT payloads?
AEROGRIP is a platform, not a fixed sensor package — it’s designed to integrate with the UTM, AUT, GPR, half-cell potential, and visual inspection payloads most NDT teams already use, so existing workflows and data formats don’t need to change. If EMAT is part of your inspection programme, talk to us about your specific payload requirements.
What's the difference between UT and AUT?
UT usually refers to spot readings taken with a hand-held probe. AUT (automated ultrasonic testing) mounts the same underlying technique on a mechanised scanner that moves systematically across the surface, building full coverage instead of individual points.
Why use GPR instead of just drilling a core sample?
GPR locates rebar, cover depth, and internal defects without any drilling, which matters most on tanks or structures where coring isn’t practical or safe — for example, tanks in Ex-rated (potentially explosive) zones.
Does half-cell potential testing tell you the exact location of corrosion?
It maps the probability of active corrosion across a surface rather than pinpointing a single exact location, which is why it’s typically combined with GPR (to locate the rebar itself) and other techniques for a full picture.
What's the measurement range of a typical UT thickness gauge?
Roughly 0.08mm to 635mm, depending on the material and transducer selected — though the practical range for most tank and pipework inspection sits well within that.
Why would I choose EMAT over standard UT?
EMAT doesn’t need couplant gel and is less sensitive to surface preparation and probe positioning, which makes it well suited to automated weld inspection, hot surfaces, and rough or hard-to-couple locations where a standard UT probe would struggle.
Do these payloads work on both HB2 and HB3?
Both platforms are designed to integrate with the same range of NDT payloads — the main difference is payload weight capacity (6kg on HB2 vs up to 25kg on HB3) and the platform’s overall downforce, not which sensor types are supported.
Can one robot carry multiple NDT payloads on the same survey?
Payload capacity is limited by the platform’s weight allowance (6kg on HB2, up to 25kg on HB3), so it depends on the combined weight of the sensors you want to carry in a single pass.
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