How to Choose an Ultrasonic Thickness Gauge

The one decision that matters 

Most people choosing an ultrasonic thickness gauge start by comparing specifications. Measurement range, resolution, display size, memory capacity. Those matter eventually, but they are not the decision. The decision is whether the steel you are measuring is coated. 

Everything else follows from that. 

Why coatings change the instrument you need 

A standard ultrasonic thickness gauge sends a pulse into the material and times the first echo back from the far wall. Simple, accurate, and fast. The problem is that paint is also a material. The pulse travels through the coating before it reaches steel, and the gauge counts that travel time as thickness. A 300 micron coating system can add roughly a millimetre to the reading depending on the coating's acoustic velocity, which on a 6mm plate is a substantial error in the direction that makes corrosion look less severe than it is. 

The traditional answer is to grind back to bare metal at every test point. On a handful of readings that is fine. On a survey with four hundred test points across a coated tank, it is most of the job. 

Multi-mode gauges solve this with echo-to-echo measurement. Instead of timing the first return, the instrument times the interval between two successive back-wall echoes. Because both echoes have travelled through the coating equally, the coating cancels out of the calculation entirely. You measure steel thickness through paint without touching the surface. 

Our Cygnus 4 Mk5, Cygnus 2+ Mk5 and Cygnus 6+ PRO all work this way, as does the Dakota CMX Series. 

When a general purpose gauge is the right call 

If your work is on bare metal, new fabrication, machined components or uncoated pipework, multi-mode capability is paying for something you will not use. A general purpose instrument like the Cygnus 4+ Mk5 measures faster, handles thinner sections and costs less. 

The same logic applies to the Dakota range. The ZX Series is built for corrosion work on bare steel, the CX Series covers a broader set of materials including plastics and composites, and the QTG IV suits occasional general measurement. 

Do you need to see the waveform? 

An A-scan display shows the returning signal rather than just the number derived from it. For most routine measurement it is unnecessary. It becomes valuable in two situations: when readings are inconsistent and you need to know whether the problem is the surface, the couplant or genuine wall loss; and when the result is going into a report someone may challenge. 

The Cygnus 6+ PRO is the instrument in our range with this capability. 

The probe matters as much as the gauge 

This gets overlooked more than anything else in the selection process. A good instrument with the wrong probe will give you poor readings, and no amount of specification will fix it. 

Single crystal probes offer better resolution and lower minimum thickness on smooth, machined or lightly weathered surfaces. Twin crystal probes use separate transmit and receive elements, which keeps a usable signal on pitted, scaled and heavily corroded plate where a single crystal probe simply drops out. If you survey weathered structures, this is the probe that will live on the gauge. 

For measurement on hot surfaces - live steam lines, heated vessels, process pipework that cannot be shut down - a high temperature probe is not optional. Standard probes are limited by what the couplant and housing tolerate. 

A short checklist 

Before you request a quote, know the answers to these: 

Is the surface coated, and can it be ground back at each test point? Is it smooth or pitted? What is the thinnest section you will measure? Will one person use the instrument or several? Does the result go into a report that might be challenged? 

Those five answers narrow the field faster than any specification comparison. 

Where to go next 

Browse the full range of ultrasonic thickness gauges. If the job also involves weld inspection, our ultrasonic flaw detection equipment covers subsurface defects rather than wall loss. 

Tell us what you're looking for and we will provide supplier-recommended instruments and probe combinations. All gauges are supplied by quote with free shipping throughout New Zealand.