Appendix D. Ultrasonic Thickness Gauge Theory of Operation
Transport® Model PT878 Portable Liquid Flowmeter User’s Manual 251
D.1.1 Transducer Positioning/Alignment
For proper sound coupling the transducer must be pressed firmly against the
test surface. On small diameter cylindrical surfaces such as pipes, hold the
transducer so that the sound barrier material visible on the probe face is
aligned perpendicular to the center axis of the pipe. See Figure 157 below.
Figure 157: Proper Alignment of Transducers for Cylindrical Surfaces
While firm hand pressure on the transducer is necessary for good readings,
the probe should never be scraped along or twisted against a rough metal
surface. This will scratch the face of the transducer and eventually degrade
performance. The safest technique for moving a transducer along a rough
surface is to pick it up (not slide it) and reposition it for each measurement.
Remember that an ultrasonic test measures thickness at only one point
within the beam of the transducer. In corrosion situations, wall thicknesses
often vary considerably. Test procedures usually call for making a number
of measurements within a defined area and establishing a minimum and/or
average thickness. Ideally, data should be taken at increments no greater
than half the diameter of the transducer, to insure that no pits or other local
variations in wall thickness are missed. It is up to the user to define a pattern
of data collection appropriate to the needs of a given application.
It is possible that on some severely corroded or pitted materials there will be
spots where readings cannot be obtained. This can happen when the inside
surface of the material is so irregular that the sound energy is scattered
rather than being reflected back to the transducer. The lack of a reading may
also indicate a thickness outside the range of the transducer and instrument
being used. Generally, an inability to obtain a valid thickness reading at a
particular point on a test specimen could be a sign of a seriously degraded
wall which may warrant investigation by other means.
Transducer
