ASTM E797 provides the guidelines for measuring thickness by the contact pulse-echo method, at temperatures not exceeding 93 °C.
- ✓ Current edition: ASTM E797/E797M-21
- ✓ Contact pulse-echo method
- ✓ Declared temperature limit: 93 °C (200 °F)
- ✓ Intended above all for flat components with parallel surfaces
In English-language practice the reference for contact ultrasonic thickness measurement is ASTM E797, whose current designation is ASTM E797/E797M-21. The document «provides guidelines for measuring the thickness of materials using the contact pulse-echo method at temperatures not to exceed 93 °C (200 °F)».
Two indications in this scope have an immediate effect on field work. The first is the temperature limit: above 93 °C one is outside the scope declared by the practice, and measurement on a hot wall must be set up with dedicated probes and couplants, checking the conditions permitted by the documentation of the components used. The second is the condition on the material: the practice is applicable to any material in which ultrasonic waves propagate at a constant velocity and in which backwall reflections can be obtained, a criterion that includes many materials and excludes others for physical reasons.
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There is then a note on geometry that explains many measurement discrepancies. ASTM E797 is intended principally for flat components with parallel surfaces; on non-parallel or concentric surfaces its applicability is limited. This does not mean that measurement on a small-diameter pipe is impossible, but that the result must be interpreted taking account of the curvature and the geometry of the backwall reflector, and that the conditions of the measurement must be stated in the report.
The fixed points of the practice
| Aspect | Confirmed content |
|---|---|
| Designation | ASTM E797/E797M-21 |
| Method | Contact pulse-echo |
| Temperature | Not exceeding 93 °C (200 °F) |
| Materials | Constant propagation velocity and obtainable backwall reflections |
| Geometry | Flat components with parallel surfaces; limited on non-parallel or concentric surfaces |
How this translates into the choice of instrument
Since the practice describes the contact pulse-echo method, what is needed is an instrument that works in that way and a transducer suited to the part. The Dakota NDT range covers both typical cases: corrosion measurement on an industrial wall and precision measurement on machined components, with one- or two-point calibration on test pieces of known thickness.
On walls where the contact reading can be disturbed, instruments with A-Scan display make it possible to observe the echo rather than trust the number alone. Several models state declared conformity to ASTM E797 in their documentation: this must be checked on the data sheet of the individual model, because the declaration refers to the product and not to the brand.
Recommended instruments
| Instrument | Typical use |
|---|---|
| Dakota PMX3-DL | Precision measurement with A-Scan display |
| Dakota CMX2-DL | Contact corrosion measurement with A-Scan |
| Precision transducers | Probes for machined components with parallel surfaces |
| Couplants | Probe-to-part coupling in contact measurement |
Frequently asked questions
The scope states temperatures not exceeding 93 degrees Celsius, equal to 200 degrees Fahrenheit.
To any material in which ultrasonic waves propagate at a constant velocity and in which backwall reflections can be obtained.
The practice is intended principally for flat components with parallel surfaces and its applicability is limited on non-parallel or concentric surfaces.
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