Ultrasonic thickness gauges are simple tools for measuring thickness on a wide variety of materials, both metallic (ferrous and non-ferrous) and non-metallic.
Measuring the thickness of a given material is particularly important when analyzing the corrosion of products that are continuously exposed to the elements or in contact with water, such as light poles, boats, reinforcing bars in concrete and almost all types of pipes.
Due to the principle involved, the ultrasound thickness measurement technique requires only one wall of the material, which is why it is indicated in situations where it is not possible to access the opposite side of the material to be measured. Therefore, it also finds a variety of thickness measurement applications not necessarily related to corrosion, but to the QA of plastic products (preforms, pipes, tanks, boats, cable insulation, etc.), ceramics, glass, rubber and even in biomedical applications, since it can determine thicknesses of soft tissues, skin, fat and walls of blood vessels , among others.
From the foregoing it follows that we can classify ultrasound thickness gauges into two large groups: corrosion meters Y precision meters.
They are designed for the measurement of the remaining wall thickness in pipes, tanks, structural parts and pressure vessels that are subject to internal corrosion impossible to see from the outside. To do this, they use signal processing techniques that are optimized to detect the minimum remaining thickness in a corroded part and for this purpose they use dual element transducers specials.
They are recommended for all other applications, eg, smooth-surfaced metals, plastics, fiberglass, composites, rubber, and ceramics. They use a wide variety of single element transducers and they are extremely versatile, since in many cases they can measure with a greater precision than is achieved with the corrosion meters, that is to say, of +/- 0.025 mm or more. The following figure shows an example of a dual element transducer used for corrosion and a single element used for precision measurements.
As we can see, ultrasound thickness gauges use different types of transducers (also known as probes), a fundamental component of these devices that determine their applicability.
Basically, there are two large groups of transducers for ultrasound measurement: the contact transducers and the immersion transducers. Below we will delve a little more into the types of transducers that come included with a thickness gauge kit and in which cases they are used.
a) Direct contact transducers: they are used in direct contact with the part, where the test materials are relatively flat and uniform. Direct contact transducer measurements are often the simplest to implement and are generally the first choice for thickness gauging applications.
Direct contact transducers can be of two types:
- Single Element Transducers: They are used on most metals and nonmetals that exhibit relatively uniform parallelism between their front and back faces. Depending on transducer selection and material conditions, they can measure from 0.13mm to 380mm on steel and plastic.
- Dual or Dual Element Transducers: They incorporate separate transmit and receive elements, mounted on a delay line at a small angle to focus energy to a selected distance below the part surface. Although measurement with dual elements is sometimes not as accurate as with other types of transducers, it often provides significantly better performance in corrosion inspection applications. For this reason, they are primarily used with corrosion meters on corroded rough surfaces, although they also have application in high temperature conditions, through coatings, thin, wear-resistant materials, limited access areas, difficult to penetrate materials (grain coarse / nonmetals), external pitting, boiler tubes, small diameter pipes and general purpose applications.
b) Transducers with delay line: incorporate a plastic, epoxy or fused silica cylinder known as delay line, between the sensitive element of the transducer and the part. They are mainly used for measurements on thin materials (eg sheet metal), where it is important to separate the recovery of the excitation pulse from the background echoes. A delay line can also be used as a thermal insulator, protecting the heat sensitive transducer from direct contact with hot parts, and its shape can also be adapted to optimize sound coupling in confined spaces or with sharp curves, as seen in the figure following.
Unlike the previous ones, these transducers do not make contact of any kind with the piece to be measured, but generally, both the transducer and the piece are immersed in a fluid (which also acts as a coupling) and is used with higher frequency in an automatic scanning system. Conventional immersion methods have the transducer and part submerged in the inspection tank. They can be used in in-line or in-process measurement of moving products, in digitized measurements or in optimization of coupling in sharp radii, grooves or channels.
The figure below shows the differences between contact transducers and immersion transducers.
What measurement modes do ultrasonic meters offer?
As we have already pointed out, the principle of an ultrasound meter is based on measuring the time of flight, that is, the time it takes for the path of an ultrasonic wave in a part when contact and immersion transducers are used. The type of transducer and the specific requirements of the application will determine the choice of the time of flight measurement mode.
There are three common modes of ultrasound measurement:
Mode 1 – Echo Pulse
Generally called mode pulse-echo, measures the time interval between the emission pulse and the first background echo. It is the most common and the most recommended, on which most meters that employ contact transducers, whether single or double element, are based. It has the advantage of typically offering the highest maximum thickness capability, and since only a single bottom echo is required, it has better penetration ability into difficult materials such as castings, low-density plastics, and rubber. The downside is that the minimum measurable thickness is higher than in other modes and the accuracy may be slightly lower due to variations in coupling. Furthermore, the direct contact transducers associated with Mode 1 can only be used on materials whose surface temperature is below 50 ° C, so measurement at high temperatures is not possible.
Mode 2 – Eco Eco
Measures the time interval between the first interface echo (after the excitation pulse) and the first background echo. Typically, this mode requires transducers with delay or dip line and is the most used in measurements in concave or convex acute radii, in confined spaces (by transducers with delay or dip line), in online measurement of materials in motion (using immersion transducers) and in high temperature measurements (using high temperature delay line transducers). The main disadvantage of Mode 2 is that the maximum thickness is limited by the length of the delay line.
When Mode 2 is applied to corrosion meters with dual element transducers, the mode known as eco-eco or mode about paint. This mode is typically used to measure the thickness of metals under a relatively thin layer of paint, resin, or similar non-metallic coating, not including the thickness of the coating. Since only metals produce multiple bottom echoes, the time interval measured between two bottom echoes represents only the thickness of the metal.
Measures the time interval between two successive bottom echoes after the first interface echo (after the excitation pulse). Typically, this mode offers the highest accuracy and best resolution measurement of minimum thickness in a given application, at the expense of penetration. Its most common use is in delay or dip line transducers, when precision and / or resolution requirements cannot be met in Modes 1 or 2. However, Mode 3 can only be used on materials that produce echoes. Multiple, clean backgrounds, typically on low-attenuation materials, such as fine-grained metals, glass, and most ceramics. It also offers the advantage of ignoring thin metallic coatings, such as paint, when measuring the thickness of coated metals.
While there are also other advanced techniques, for example, the patented ThruCoat ™ and ThruPaint ™ measurements, the most common measurement modes that we will find in mid-price meters are pulse-echo (Mode 1) and echo-echo (Mode two).
Precisely the mode used determines the measuring range of the instrument, that is, the interval between the maximum and minimum thickness that can be measured in a material, with a given transducer and configuration. Thus, for example, many meters have a typical range of 0.6 to 500 mm in the pulse-echo method (Mode 1) and 2.5 to 25 mm in the eco-echo method (Mode 2).
The measuring range and the accuracy of the device, added to the measuring conditions (temperature, thickness, type and geometry of material, etc.) and the transducers to be used are decisive factors when choosing an ultrasonic thickness gauge.
In the next article we will provide a detail of all these factors so that we can make a correct choice according to our needs.