Thickness gages are used to make precise dimensional measurements on coatings and materials such as steel, plastic, glass, rubber, ceramics, paint, electroplated layers, and enamels. They differ in terms of performance specifications, applications, and features. There are three main performance specifications to consider when selecting thickness gages: thickness range, thickness resolution, and number of channels or probes. In terms of applications, these instruments are used to measure coating thickness, corrosion thickness, material thickness, and wet film thickness. Form factor or mounting style are also important to consider. Thickness gages are available as gage heads, gage sets, and gaging systems; automatic or in-line and machine-mounted systems; handheld or portable devices; and bench, rack, or floor units. 

Types of Thickness Gages

There are many different types of thickness gages. Examples include acoustic emission instruments, air gages, and infrared (IR), nuclear, and beta gauge instruments. Acoustic emission devices are used in non-destructive testing (NDT) and measure specific acoustic or vibrational responses. Air gages can be used to measure the thickness of metal sheets or strips, plastic webs or films, and other thin materials or sections of components. IR, nuclear, and beta gauge instruments use the absorption of radiation to measure the thickness or basis weight of webs, sheet materials or coatings. On nonmetallic materials such as paper or plastic films or webs, the amount of radiation is reflected back or transmitted through the material is measured to determine absorption levels.  

Thickness gages may use eddy current or various electromagnetic methods, or techniques such as electromagnetic acoustic resonance (EMAR). In addition to measuring thickness, eddy current and penetrating radar techniques are used to detect flaws, bond or weld integrity, and electrical conductivity. The eddy current method is the most widely applied electromagnetic NDT technique. Electromagnetic acoustic resonance (EMAR) devices use non-contact electromagnetic transducers to impart signals for resonant acoustic analysis. The non-contact EM-induced signal produces a pure resonant response. Electronic and mechanical gages for thickness measurement are also available. 

Thickness gauges include laser micrometers, and laser micrometers, magnetic devices (e.g., Hall effect, induction), optical-based instruments, X-ray equipment, snap gages, and ultrasonic (UT) inspection techniques. Micrometers are instruments for precise gaging.  They consist of a ground spindle and anvil mounted in a steel frame. In addition to traditional micrometers, calipers and laser micrometers are used to gage thickness. Optical instruments use methods such as laser shearography, magneto-optical techniques, and holographic interferometry. X-ray equipment uses penetrating X-ray or gamma radiation to capture images of the internal structure or a part or finished product. Snap gages are functional or attribute gages. UT thickness gages often use the pulse-echo technique.