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 gage. 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 Panametrics-NDT equipment. Testing does not damage glass, and thickness measurements are instant, accurate, and reliable.

Equipment: Magna-Mike The Magna-Mike 8500 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 10 mm (0.400 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.

Standard probes such as the 801PR and 802PR, and the 802PR-109 extended range probe, are usually recommended for glass applications. Use of the 80SG1 scratch guard is also recommended when testing glass. This is a protective plastic cover that fits over the top of the Magna-Mike probe. The probe and target ball system will not normally scratch industrial glass, however if the possibility of scratching test pieces is a concern, the plastic scratch guard will cushion the probe tip. The scratch guard also protects the probe tip from possible excess wear caused by long term sliding contact with hard glass.

Ultrasonic Gages: Precision thickness gages such as Models 35 and 35DL 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").

Procedure: When the Magna-Mike Model 8500 is used in glass applications, no special setups or operating procedures are required. The instrument is simply used as described in its operating manual.
Ultrasonic thickness gage Models 35, 35DL, and 38DL PLUS are all 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, Panametrics-NDT will provide specific setup assistance.

Bottles and containers: The Magna-Mike 8500 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. A V-block adapter (part number 80FXV) is available to help hold tubing in the proper orientation with respect to the Magna-Mike probe. 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") can easily be measured with the Magna-Mike, thicker glass plates are measured ultrasonically with a Model 25 series gage 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 a 35 Series gage. 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 a 35 Series gage 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 Panametrics-NDT 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 NDT offers a variety of instruments that can be utilized for ultrasonic materials research applications, including thickness gages, velocimeters, flaw detectors, pulser/receivers, and high precision laboratory test systems, as well as a complete range of contact, delay line, and immersion transducers.