THEORY AND APPLICATION OF PRECISION ULTRASONIC THICKNESS GAGING :
Appendix I

by By Kenneth A. Fowler, Gerry M. Elfbaum, Karen A. Smith and Thomas J. Nelligan *
Figure 3a
Summary of Thickness Measurement Applications Where Direct Contact Transducer Measurement (Mode 1) is Recommended
Thickness Application Normal Thickness Range (Approx.) Normal Accuracy Limit (+/-) Recommended Transducer Notes
Flat Plate (metal or ceramic) 1.25 to 500mm 0.050 to 20 in 0.02 to 0.25mm 0.001 to 0.010 in. B, D Accuracy depends on surface finish.
Flat Plate (plastic) 6 to 50mm 0.250 to 2 in. 0.02 to 0.25mm 0.001 to 0.10 in. D, B Accuracy limited by temperature homogeneity & attenuation.
Flat Sheet (metal or ceramic) 0.5 to 6mm 0.020 to 0.250 in. 0.01mm 0.0005 in H Surface - 1 micron rms or better.
Flat Sheet (plastic) 0.15 to 6mm 0.005 to 0.250 in. 0.01 to 0.04mm 0.0005 to 0.002 in. F, H Attenuation may limit range. Temperature variation limits acc.
Pipe (metal) 1 to 12.5mm 0.040 to 0.5 in. 0.02mm 0.001 in. F Surface roughness may limit range and accuracy.
Pipe (plastic) 1 to 20mm 0.040 to 0.75 in. 0.04mm 0.002 in. D, F Temperature variation may limit accuracy.
Tubing (metal, glass) 0.5 to 3mm 0.020 to 0.125 in 0.02mm 0.001 in. H Min. dia. - 1.5mm
Rods, Shafts - Diameter 6 to 250mm 0.25 to 10 in. 0.02 to 0.2mm 0.001 to 0.010 in. D, F, H Transducer selection depends on diameter
Containers (glass) 0.5 to 6mm 0.020 to 0.250 in. 0.02mm 0.001 in. F, H 6mm min. radius of curvature.
Containers (large plastic) 0.5 to 6mm 0.020 to 0.250 in. 0.02mm 0.001 in. F Surfaces must be approx. parallel. Temperature variation limits accuracy.
Containers (small plastic) 0.15 to 2mm 0.005 to 0.075 in. 0.02mm 0.001 in. H 1mm min. radius of curvature.
Castings (iron, stainless) 2 to 25mm 0.075 to 1.0 in. 0.04 to 0.25mm 0.002 to 0.010 in. B, E Cast surface may have to be sanded or ground.
Investment Castings (turbine blades & vanes) 0.5 to 5mm 0.020 to 0.200 in. 0.04mm 0.002 in. H Surfaces must be approx. parallel.
FRP (layup) 1 to 20mm 0.040 to 0.75 in. 0.2mm 0.010 in. C, D, E Depends on homogeneity
Coatings (plastic, rubber) 0.3 to 20mm 0.012 to 0.75 in. 0.04mm 0.002 in. D, F May produce phase reversal of echo signal.
Rubber (tire tread) 5 to 20mm 0.200 to 0.75 in. 0.2mm 0.010 in. B, A Usually high attenuation in rubber.
Liners (plastic pipe) 1 to 6mm 0.040 to 0.250 in. 0.04mm 0.002 in. G Measure from ID.


Transducer Code
Description
A 1 MHz, 13mm dia. (0.5 inch dia.)
B 2.25 MHz, 13mm dia. (0.5 inch dia.)
C 2.25 MHz, 25mm dia. (1 inch dia.)
D 5 MHz, 13mm dia. (0.5 inch dia.)
E 5 MHz, 6mm dia. (0.25 inch dia.)
F 10 MHz, 6mm dia. (0.25 inch dia.)
G 10 MHz, 3mm dia. (0.125 inch dia.)
H 20 MHz, 3mm dia. (0.125 inch dia.



Figure 3b
Summary of Thickness Measurement Applications Where Mode 3 Measurements Utilizing Delay Line or Immersion Type Transducers Are Recommended

THICKNESS APPLICATION NORMAL THICKNESS RANGE (APPROX.) NORMAL THICKNESS ACCURACY (+/ ) RECOMMENDED TRANSDUCER

A. Delay Type Transducers

Thin Sheet-Plate (metal, glass, ceramic) 0.1 to 6mm (0.004 to 0.250 in.) 0.002mm (0.0001 in.) B, C
Thin Wall Tube (metal, glass, ceramic) 0.2 to 6mm (0.008 to 0.250 in.) 0.002mm (0.0001 in.) B, C
Thin Wall Tube (small dia.) 0.2 to 3mm (0.008 to 0.250 in.) 0.002mm (0.0001 in.) C
Flat or Slightly curved Thin Metal, glass, ceramic with smooth surface. (Dry Coupled) 0.25 to 6mm (0.010 to 0.250 in.) 0.002mm (0.0001 in.) A, B, C with compliant layer on delay line

B. Immersion Type Transducers

Sheet (Metal, Glass, Ceramic) 0.2 to 3mm (0.008 to 0.125 in.) 0.005mm (0.0002 in.) E
Sheet (Metal, Glass, Ceramic) .5 to 6mm (0.020 to 0.250 in.) 0.005mm (0.0002 in.) D
Tubing (Metal, Glass, Ceramic) Min. dia. 50mm 1 to 6mm (0.040 to 0.250 in.) 0.01mm (0.0005 in.) D
Tubing (Metal, Glass, Ceramic) Min. dia. 25mm 1 to 3mm (0.040 to 0.125 in.) 0.01mm (0.0005 in.) D
Tubing (Metal, Glass, Ceramic) Min. dia. 15mm 0.5 to 2mm (0.020 to 0.080 in.) 0.002mm (0.0001 in.) E



Transducer Code
Description
A 10 MHz, 6mm dia. Delay Type
B 15 MHz, 6mm dia. Delay Type
C 20 MHz, 3mm dia. Delay Type
D 10 MHz, 6mm dia. Focused at 50mm
E 20MHz, 3mm dia. Focused at 25mm



Figure 3 includes an attempt to summarize the major applications where Mode 1 measurements with direct contact transducers are recommended or preferred. The thickness range, accuracy, and transducer recommendations are intended only as a guide and should not be considered firm. Because of possible variations in material acoustic properties and the effects of geometry, the exact range and accuracy in a given application should always be verified with the aid of reference standards of the material in question. In some cases measurement will be possible over greater ranges and to greater accuracies than indicated in the table, and in other cases less. And although transducer recommendations are shown, it will often be possible to use two or more different transducers with essentially equivalent results over a specified range.

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© Copyright 1. Oct 1997 Rolf Diederichs, info@ndt.net