How To Make Accurate Thickness Measurements!

March 1, 2007
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Ultrasound and Hall Effect (magnetic) are two important nondestructive methods for precisely measuring material thicknesses. The factors involved in choosing a method are generally dependent on the product to be tested.



Ultrasound:

Thickness gages provide an accurate, repeatable means of nondestructively measuring wall thickness from one side of the part. They work by measuring the time it takes for a pulse of sound waves to travel through the test piece. A transducer is placed on the surface of the part to be measured and acoustically coupled to the part using a liquid medium. The sound pulse travels from the contact surface to the opposite surface, and bounces back to the transducer as an echo that is detected by the gage.



Advantages and Limitations:

1. Measurements require access to only one side of the material.

2. It is necessary to calibrate the instrument for material sound velocity since sound travels at a different speed through different materials.

3. Ultrasound requires a drop of a liquid medium (couplant) between the transducer tip and the part.

4. Most engineering materials can be measured, except porous materials such as paper and styrofoam.

5. Wide thickness range from 0.003” to several inches.

6. Can measure closed containers, large sheets, and other geometries where access to both sides is difficult.

7. Certain material properties and part geometries such as temperature, porosity, and uneven surfaces may affect accuracy.



Hall Effect:

Measurements are made by simply placing a small target ball on the inside of a container while holding a magnetic Hall probe to the outside. When material thickness changes, the distance between the target ball and the probe also changes, which affects the magnetic field and changes the induced voltage in the Hall Effect sensor in a predictable manner. The gage displays the difference as a numeric value, which is wall thickness.

Advantages and Limitations:

1. Access to both sides of the material is necessary: small reference ball on the inside, probe on the outside.

2. Material sound velocity is not needed because measurements are independent of material properties.

3. This method does not require a liquid medium (couplant).

4. Only for non-magnetic materials such as plastic, glass, and aluminum.

5. Maximum measurable thickness is 10mm (0.400 inch).

6. Tightly radiused areas and extremely thin samples can be measured.

7. The probe can be scanned over an area to quickly locate minimum thickness.



Selecting a Gaging Method:

There are no hard and fast rules for choosing between the two methods. In general, if large rigid parts are to be measured, use an ultrasonic thickness gage such as the Olympus NDT Model 35DL. When small, thin wall (less than .100”) nonmagnetic parts with tight corners are to be measured, Hall Effect gages such as the Olympus NDT Magna-Mike® 8500 are preferred.

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