Application Notes
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Thickness Measurement of Glass

Application: Measuring wall thickness of commercial glass products including bottles, containers, tubing, sheets and plates, scientific glassware, bulbs and lamps, as well as thickness of glass coatings of glass-lined tanks.

Background: Glass is an inexpensive and highly versatile engineering material that can be cast, molded, or blown into a wide variety of shapes. It is also highly transmissive to high frequency sound waves, making it an excellent material for ultrasonic thickness gaging, and non-magnetic, making it measurable with the Magna-Mike Hall Effect thickness gages. The geometry of most common glass products is such that mechanical thickness measurement with calipers or micrometers is difficult or impossible, but virtually all common glass products can easily be measured with Olympus equipment. Testing does not damage glass, and thickness measurements are instant, accurate, and reliable.

Equipment: Magna-Mike The Magna-Mike 8600 Hall Effect thickness gage is an excellent instrument for glass measurement in applications involving bottles, containers and tubing with an open end that permits insertion of a target ball, and a thickness not exceeding approximately 25 mm (1 in.). The Magna-Mike uses magnetic field effects to measure the distance between the tip of a probe that is pressed against the outside surface of a test piece, and a small steel target ball that moves along corresponding points on the inside. The Magna-Mike is often the most convenient choice for measuring corners and tight radiuses, and it does not require material-specific calibration.

When the Magna-Mike is used in glass applications, no special setups or operating procedures are required. The instrument is simply used as described in its operating manual. The standard 86PR-1 probe is usually recommended for glass applications. The probe and target ball system will not normally scratch industrial glass. 

Ultrasonic Gages: Precision thickness gages such as Models 38DL PLUS and  45MG with Single Element software can be used to measure typical glass products over a thickness range from approximately 0.125 mm (0.005 in.) to 500 mm (20 in.). As in any thickness gaging application, the ultrasonic gage measures the round trip travel time of a high frequency sound pulse in the wall of that test piece, and then uses that measured pulse transit time and a calibrated material sound velocity to calculate wall thickness. Ultrasonic gages can be used to measure all common glass products, but they are particularly useful in situations where glass thickness exceeds the range of the Magna-Mike, where part geometry prevents insertion of a target ball (as in the case of sealed bulbs), and where very high levels of accuracy are required (approaching +/- 0.002 mm or 0.0001 in.).

Ultrasonic precision thickness gages 38DL PLUS and 45MG with Single Element software are pre-programmed with default transducer setups that can be used for most common glass setups, requiring only velocity and zero calibration as described in the instrument operating manuals. In a few specialized cases involving complex geometries or other challenging conditions, a custom transducer setup may be required, such as situations where a focused immersion approach is recommended due to sharp radiuses. In those applications, Olympus will provide specific setup assistance.

The following is a list of the most common glass measurement applications:

Bottles and containers: The Magna-Mike is the most commonly used instrument for glass bottle and container measurement, but ultrasonic gages used with small diameter contact transducers such as the M116 (20 MHz) also work well. Measurement is quick, simple, and completely nondestructive.

Glass tubing: Most short sections of glass tubing can be easily measured with the Magna-Mike. Tubing can also be measured ultrasonically using small diameter contact transducer such as the M116 (20 MHz). In the case of very small diameter tubing (diameter less than approximately 6.25 mm or 0.25 in.), focused immersion transducers such as the M316-SU F-.75 (20 MHz) are generally recommended in order to optimize sound coupling. These transducer will usually be used with a B-103 bubbler to maintain tubing orientation with respect to the sound beam.

Sheets and plates: While thinner sheets and plates (under 10 mm or 0.400 in.) can easily be measured with the Magna-Mike, thicker glass plates are measured ultrasonically with a Model 38DL PLUS gage or 45MG gage with Single Element software and contact transducers such as part numbers M109 (5 MHz) and M106 (2.25 MHz).

Scientific glassware: When part shape permits insertion of a target ball, scientific glassware can be easily measured with the Magna-Mike. Sealed bulbs and complex shapes that do not permit use of a target ball can normally be measured ultrasonically with precision thickness gages. In the case of very small diameter tubing (diameter less than approximately 6.25 mm or 0.25 in.) or small spheres (diameter less than 25 mm or 1 in.), focused immersion transducers such as the M316-SU F-.75 (20 MHz) are generally recommended in order to optimize sound coupling. These transducer will usually be used with a bubbler to maintain part orientation with respect to the sound beam.

Bulbs and lamps: The thin walls of sealed glass bulbs and lamps can be measured with precision thickness gages and an M208 delay line transducer (20 MHz). Some sharp radiuses may require use of focused immersion transducers with an appropriate bubbler. The same gage and transducer can also be used to measure the thickness of the plastic protective coatings that are sometimes applied to glass bulbs.

Glass coatings: Glass coatings, such as the protective liners of some chemical tanks, can usually be measured ultrasonically. The measurement is easiest if the test can be performed from the glass side, but in many cases glass liners can be measured through a steel wall if inside access is not available. In many cases the Model 38DL PLUS with the Multilayer Measurement software option can be used to measure both liner and tank thickness simultaneously. Transducer selection and instrument setup will depend on the specific materials and thickness ranges involved; consult Olympus for details.

Materials research: Physical properties of glass such as elastic modulus, residual stress, hardness, and density can often be correlated to acoustic properties such as longitudinal and shear wave sound velocity. Olympus offers a variety of instruments that can be utilized for ultrasonic materials research applications, including thickness gages with velocimeter readout, flaw detectors, and pulser/receivers, as well as a complete range of contact, delay line, and immersion transducers.

Olympus IMS

The handheld 45MG is an ultrasonic thickness gage packed with measurement features and software options. This unique instrument is compatible with the complete range of Olympus dual element and single element transducers, making this innovative instrument an all-in-one solution for virtually every thickness gage application.
The 38DL PLUS is an advanced ultrasonic thickness gage. Uses dual element transducer for internally corroded applications, features include THRU-COAT and Echo-to-Echo. Uses single element transducers for very precise thickness measurements of thin, very thick, or multilayer materials.
The Magna-Mike is a Hall Effect thickness gage that uses a magnetic probe to perform accurate measurements on nonferrous and thin materials such as plastic bottles.
The 27MG is a basic ultrasonic thickness gage designed to make accurate measurements from one side on internally corroded or eroded metal pipes and parts. It is lightweight, durable, and ergonomically designed for easy, one-hand operation.
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