Checkline TI-CMX - Manual (Page 17)

4.0 P RINCIPLES OF U LTRASONIC M EASUREMENT
4.1 Time versus thickness relationship
Ultrasonic thickness measurements depend on measuring the length of time it takes for
sound to travel through the material being tested. The ratio of the thickness versus the
time is known as the sound velocity. In order to make accurate measurements, a sound
velocity must be determined and entered into the instrument.
The accuracy of a thickness measurement therefore depends on having a consistent
sound velocity. Some materials are not as consistent as others and accuracy will be mar-
ginal. For example, some cast materials are very granular and porous and as a result
have inconsistent sound velocities.
While there are many different ultrasonic techniques to measure thickness, which will be
discussed below, all of them rely on using the sound velocity to convert from time to
thickness.
4.2 Suitability of materials
Ultrasonic thickness measurements rely on passing a sound wave through the material
being measured. Not all materials are good at transmitting sound. Ultrasonic thickness
measurement is practical in a wide variety of materials including metals, plastics, and
glass. Materials that are difficult include some cast materials, concrete, wood, fiberglass,
and some rubber.
4.3 Range of measurement and accuracy
The overall measurement capabilities, based on the wide variety of materials, is deter-
mined by the consistency of the material being measured
The range of thickness that can be measured ultrasonically depends on the material, the
technique being used and the type of transducer. Thickness measurements can be made
from a minimum of 0.010 inch to 9.999” in steel. However, the maximum attainable
thickness is much less for more attenuative materials (materials that absorb sound).
Accuracy, is determined by how consistent the sound velocity is through the sound path
being measured, and is a function of the overall thickness of the material. For example,
the velocity in steel is typically within 0.5% while the velocity in cast iron can vary by
4%.
4.4 Couplant
All ultrasonic applications require some medium to couple the sound from the transduc-
er to the test piece. Typically a high viscosity liquid is used as the medium. The sound
frequencies used in ultrasonic thickness measurement do not travel through air efficient-
ly. By using a liquid couplant between the transducer and test piece the amount of ultra-
sound entering the test piece is much greater.
4.5 Temperature
Temperature has an effect on sound velocity. The higher the temperature, the slower
sound travels in a material. High temperatures can also damage transducers and present
a problem for various liquid couplants.
Since the sound velocity varies with temperature it is important to calibrate at the same
temperature as the material being measured.
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2. Apply a drop of couplant on a piece of metal and place the coating sample on the
metal over the couplant. Apply a drop of couplant on the transducer and place the
transducer in steady contact with the coating and sample or actual test material. Be
sure that the reading is stable and the repeatability indicator, in the top left corner of
the display, is fully lit and stable. Press the MENU key once to activate the menu
items tab. Press the MENU key multiple times to tab right and the ESC key multiple
times to tab left until the CAL menu is highlighted and displaying the submenu
items.
3. Use the UP and DOWN arrow keys to scroll through the
sub menu items until COATING 1PT is highlighted.
4. Press the ENTER key to display the Digits Edit Box.
5. Press the UP and DOWN arrow keys to scroll the highlighted
value.
6. Press the LEFT and RIGHT arrow keys to scroll the digit
locations.
7. Repeat steps 5 & 6 until the known thickness value is correctly
displayed.
8. Press the OK key to calculate the velocity and return to the menu
screen, or ESC to cancel the coating one point calibration.
9. Finally, press the MEAS key to return to the measurement screen and begin taking
readings.
NOTE: CHECK YOUR CALIBRATION! Place the transducer back on the calibration
point. The coating thickness reading should now match the known thickness. If the
thickness is not correct, repeat the steps above.
9.4 Introduction to Coating Measurement (CT)
In the previous sections we’ve discussed how to setup and use the coating feature for use
in conjunction with material thickness and flaw and pit detection. The TI-CMX also has
the capability to be used for general coating measurements. This measurement mode is
called Coating (CT) and can be enabled using the same methods as described in a
previous section above.
When the Coating Only (CT) mode is enabled, a two point calibration on the coating
samples must be performed. This is to ensure linearity over the coating measurement
range will be achieved. Important note: If coating measurements will be made with the
coating applied to a metal surface, the calibration must be done in the same manner,
with the samples coupled to a metal surface. However, if the coating will be measured as
a stand alone material, the calibration must be performed the same way.
9.5 Two Point Coating Calibration (CT)
Known Thickness
The following section will demonstrate the two point coating calibration procedure. This
example demonstrates a coating thickness range of .040” - .120” ( 1 – 3 mm) as follows:
NOTE: It’s always handy to carry a set of mechanical calipers to use in conjunction
with the TI-CMX for calibration in the field:
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