Checkline TI-CMXDL - Manual (Page 19)

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 ti
is known as the sound velocity. In order to make accurate measurements, a sound veloci
must be determined and entered into the instrument.
The accuracy of a thickness measurement therefore depends on having a consistent soun
velocity. Some materials are not as consistent as others and accuracy will be marginal. F
example, some cast materials are very granular and porous and as a result have inconsist
sound velocities.
While there are many different ultrasonic techniques to measure thickness, which will b
discussed below, all of them rely on using the sound velocity to convert from time to thi
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 determi
by the consistency of the material being measured
The range of thickness that can be measured ultrasonically depends on the material as w
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 attainab
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,
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 transduce
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. B
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 sou
travels in a material. High temperatures can also damage transducers and present a prob
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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Important Note: For the duration of this chapter, all references to GRIDS and SEQ
LOGS should be considered synonymous with references to FILES.
11.2 Creating a new Grid or Sequential Log (File)
Important Note: This entire section is a step by step guide to successfully create a grid or
sequential log. The instructions must be used in the sequential order specified, as follows:
Creating a Name
Grid/Seq Log Name: Can contain a combination of up to 20 numeric, alpha, or special
characters listed in the first section of this chapter.
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 DATA menu is
highlighted and displaying the submenu items.
2. Use the UP and DOWN arrow keys to scroll through the sub menu items until NEW is
highlighted.
3. Press the LEFT & RIGHT arrow keys to toggle the data formats – GRIG LOG &
SEQ LOG.
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Sequential Log Formats
Grid Log Sequential Log