Checkline TI-25DL-MMX User Manual download pdf (Page 10)

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4.1 Condition and Preparation of Surfaces

In any ultrasonic measurement scenario, the shape and roughness of the test

surface are of paramount importance. Rough, uneven surfaces may limit the

penetration of ultrasound through the material, and result in unstable, and there-

fore unreliable, measurements. The surface being measured should be clean, and

free of any small particulate matter, rust, or scale. The presence of such obstruc-

tions will prevent the transducer from seating properly against the surface. Often,

a wire brush or scraper will be helpful in cleaning surfaces. In more extreme

cases, rotary sanders or grinding wheels may be used, though care must be taken

to prevent surface gouging, which will inhibit proper transducer coupling.

Extremely rough surfaces, such as the pebble-like finish of some cast iron,

will prove most difficult to measure. These kinds of surfaces act on the sound

beam like frosted glass on light:, the beam becomes diffused and scattered in

all directions.

In addition to posing obstacles to measurement, rough surfaces contribute to

excessive wear of the transducer, particularly in situations where the transducer is

“scrubbed” along the surface. Transducers should be inspected on a regular basis,

for signs of uneven wear of the wearface. If the wearface is worn on one side

more than another, the sound beam penetrating the test material may no longer be

perpendicular to the material surface. In this case, it will be difficult to exactly

locate tiny irregularities in the material being measured, as the focus of the sound-

beam no longer lies directly beneath the transducer.

Setting the Zero Point of the TI-25DL-MMX is important for the same reason

that setting the zero on a mechanical micrometer is important. If the tool is not

“zeroed” correctly, all of the measurements the tool makes will be in error by

some fixed number. When the TI-25DL-MMX is “zeroed”, this fixed error value

is measured and automatically corrected for in all subsequent measurements. The

TI-25DL-MMX may be “zeroed” as follows:

4.2 Performing a Probe-Zero

1. Make sure the TI-25DL-MMX is on.

2. Plug the transducer into the TI-25DL-MMXL. Make sure that the

connectors are fully engaged. Check that the wearface of the transducer

is clean and free of any debris.

3. The metal probe-disc is on the top end of the TI-25DL-MMX DL.

Apply a single droplet of ultrasonic couplant to the face of this disc.

4. Make sure that the TI-25DL-MMX is in P-E ( pulse-echo mode ) by

pressing the Dual-Multi key to toggle the modes.

Note: The Probe-Zero feature is not used in Echo-Echo Thru-Paint mode,

and has been disabled. If the PRB-0 key is pressed, while in this mode,

“nO” followed by “Prb0” will be displayed.

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14.0 TRANSDUCER SELECTION

The TI-25DL-MMX is inherently capable of performing measurements on a wide

range of materials, from various metals to glass and plastics. Different types of

material, however, will require the use of different transducers. Choosing the cor-

rect transducer for a job is critical to being able to easily perform accurate and

reliable measurements. The following paragraphs highlight the important proper-

ties of transducers, which should be considered when selecting a transducer for a

specific job.

Generally speaking, the best transducer for a job is one that sends sufficient ultra-

sonic energy into the material being measured such that a strong, stable echo is

received by the TI-25DL-MMX. Several factors affect the strength of ultrasound

as it travels. These are outlined below:

Initial Signal Strength

The stronger a signal is to begin with, the stronger its return echo will be. Initial

signal strength is largely a factor of the size of the ultrasound emitter in the trans-

ducer. A large emitting area will send more energy into the material being meas-

ured than a small emitting area. Thus, a so-called “1/2-inch” transducer will emit

a stronger signal than a “1/4-inch” transducer.

Absorption and Scattering

As ultrasound travels through any material, it is partly absorbed. If the material

through which the sound travels has any grain structure, the sound waves will

experience scattering. Both of these effects reduce the strength of the waves, and

thus, the TI-25DL-MMX’s ability to detect the returning echo

Higher frequency ultrasound is absorbed and scattered more than ultrasound of a

lower frequency. While it may seem that using a lower frequency transducer

might be better in every instance, low frequencies are less directional than high

frequencies. Thus, a higher frequency transducer would be a better choice for

detecting the exact location of small pits or flaws in the material being measured.

Geometry of the Transducer

The physical constraints of the measuring environment sometimes determine a

transducer’s suitability for a given job. Some transducers may simply be too large

to be used in tightly confined areas. Also, the surface area available for contacting

with the transducer may be limited, requiring the use of a transducer with a small

wearface. Measuring on a curved surface, such as an engine cylinder wall, may

require the use of a transducer with a matching curved wearface..

Temperature of the Material

When it is necessary to measure on surfaces that are exceedingly hot, high

temperature transducers must be used. These transducers are built using special

materials and techniques that allow them to withstand high temperatures without

damage. Additionally, care must be taken when performing a “Probe Zero” or a

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