Check-line TI-CMX User Manual page 18

4.0 P U M
RINCIPLES OF LTRASONIC
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 marginal. 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 thick-
ness.
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 determined
by the consistency of the material being measured
The range of thickness that can be measured ultrasonically depends on the material as well
as 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 transducer
to the test piece. Typically a high viscosity liquid is used as the medium. The sound fre-
quencies used in ultrasonic thickness measurement do not travel through air efficiently. By
using a liquid couplant between the transducer and test piece the amount of ultrasound
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.
EASUREMENT
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Probe Automatically Recognized (PECT only)
1. Press the OK key once to use the identified probe, or ESC to
display a list of optional transducers.
NOTE: If the TI-CMX recognizes a specific transducer, the user
should always select OK to use the identified probe. The only time
an alternative probe should be selected from a list is if the user
switched probes following initial power up and recognition.
2. Assuming the TI-CMX recognized the probe and the OK key
was pressed, the TI-CMX will advance to a Zero Probe menu. If
the transducer was identified as a special transducer capable of
measuring coating thickness, a menu will be displayed allowing
the user the ability to toggle the coating thickness display on/off
as follows:
3. Press the UP and DOWN arrow keys to toggle the coating
option on/off.
Multi Mode Key Pressed (PECT & CT)
1. Press the MULTI MODE key located
on bottom left of the keypad to display
the MEASURE MODE options menu.
2. Press the UP and DOWN arrow keys
to highlight the COATING ON (PECT)
or COATING ONLY (CT) menu option.
3. Press the ENTER key to enable the
coating option, or ESC to cancel
changing the measure mode, and
return to the main measurement screen.
Measure Mode (Tabbed Menus) – (PECT & CT)
1. 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 TUNE menu is highlighted
and displaying the submenu items.
2. Use the UP and DOWN arrow keys
to scroll through the sub menu items
until MEASURE MODE is highlighted.
3. Press the LEFT and RIGHT arrow
keys to scroll the modes.
4. When the correct MODE is being displayed, press the MEAS key to return to the
measurement screen and begin taking readings.
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Applied to Not Applied to
Metals Metals
Applied to Not Applied to
Metals Metals