Dakota Ultrasonics MVX User Manual

OPERATION MANUAL

DAKOTA ULTRASONICS

MMVVXX

Visual A / B Scan Thickness Gauge

P/N P-149-0002 Rev 1.10, January 2008

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CHAPTER ONE INTRODUCTION.......................................................................1

CHAPTER TWO QUICK STARTUP GUIDE........................................................2

CHAPTER THREE KEYBOA RD, MENU, & CONNECTOR REFERENCE.......13

CHAPTER FOUR PRINCIPALS OF ULTRASONIC ME ASUREMENT .............23

CHAPTER FIVE SELECTING THE MEASUREMENT MODE..........................28

CHAPTER SIX MAKING MEASUREMENTS.....................................................31

CHAPTER SEVEN USING THE A-SCAN & B-SCAN DISPLAYS.....................45

CHAPTER EIGHT THRU PAINT MEASUREMENT TECHNIQUE....................70

CHAPTER NINE ADDITIONAL FEATURES OF THE MVX...............................73

CHAPTER TEN DATA STORAGE – SETUP, EDIT, & VIEW GRIDS...............82

CHAPTER ELEVEN SETUPS – CREATE, STORE, EDIT, & RECALL..........100

CHAPTER TWELVE USING THE FLAW MODE FEATURE...........................107

CHAPTER THIRTEEN USI NG THE UTILITY SOFTWARE.............................111

APPENDIX A - VELOCITY TABLE ...................................................................112

APPENDIX B - SETUP LIBRARY.....................................................................114

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CHAPTER ONE
INTRODUCTION

The Dakota Ultrasonics model MVX is a visual A / B scan ultrasonic thickness gauge.
Based on the same operating principles as SONAR, the MVX is capable of
measuring the thickness of various materials with accuracy as high as ± 0.001
inches, or ± 0.01 millimeters. The principle advantage of ultrasonic measurement

over traditional methods is that ultrasonic measurements can be performed with
access to only one side of the material being measured.

Dakota Ultrasonics maintains a customer support resource in order to assist users
with questions or difficulties not covered in this manual. Customer support may be
reached at any of the following:

• Dakota Ultrasonics Corporation,
1500 Green Hills Road, #107 Scotts Valley, CA
95066 USA

• Telephone: [phone]

• Facsimile: [phone]

• www.dakotaultrasonics.com

1.1 Disclaimer

Inherent in ultrasonic thickness measurement is the possibility that the instrument will
use the second rather than the first echo from the back surface of the material being
measured. This may result in a thickness reading that is TWICE what it should be.
Responsibility for proper use of the instrument and recognition of this phenomenon
rest solely with the user of the instrument.

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CHAPTER TWO
QUICK STARTUP GUIDE

Turn the MVX on and off using the switch located on the bottom right corner of the
keypad. When MVX is initially turned on, a flash logo and blinking lights will be
displayed prior to entering into the main measurement screen. Note: This section is
primarily written as a basic startup guide only.

2.1 Selecting The Transducer Type

The first step in using the MVX is to select the transducer type according to
frequency and diameter. By selecting the transducer type from a predefined list, the
MVX can recall specif ic properties about the transducer. Note: Once the transducer
has been selected, the MVX will store and recall this transducer type every time the
MVX is powered on/off. The type will only change if the user physically selects
another transducer type from the list, or selects a previously saved setup. Therefore,
if you have previously gone through this section and selected the transducer you are
using, proceed to the next section. Use the following steps to select your transducer
type:

Selecting the Transducer Type

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 PROBE menu is highlighted and displaying the submenu items.

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MVX High Performance Thickness Gauge

2) Use the UP and DOWN arrow keys to scroll through the sub menu items
until TYPE is highlighted.

3) Press the ENTER key to display the list of transducer types.

4) Press the UP and DOWN arrow keys to scroll through the transducer list
until the appropriate type is highlighted.

5) Press th e ENTER key to display the confirmation screen.

6) Press the OK key to select the transducer and return to the menu screen, or

ESC to cancel the selecting the transducer.

2.2 Probe Zero & Calibration

The next step is to calibrate the MVX to the material and transducer being used. If a
sound velocity is not known, the MVX can be calibrated to a known thickness sample.
This demo will briefly explain both of these techniques.

In either case, the transducer MUST be zeroed on the probe zero disk (battery cover
located on the top of the unit as follows:

Performing a Probe Zero

1) Apply a drop of couplant on the transducer and place the transducer in
steady contact with the probe zero disk, and obtain a steady reading.

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2) Assuming the probe menu is still displayed from the previous type selection,
use the UP and DOWN arrow keys to scroll through the sub menu items
until ZERO TRANSDUCER is highlighted.

3) Press the ENTER key to display the confirmation screen.

4) Press the OK key to complete the probe zero function, or ESC key to cancel
the probe zero function.

5) Remove the transducer from the probe zero disk, and proceed to the
calibration section.

Note: The value that is displayed will change depending on the current velocity
setting in the MVX. Disregard the number that is displayed. It is not
important. What is important is accurately performing the steps outlined above
to insure reliability of the probe zero calculation.

Known Velocity

If the material velocity is known, the user may wish to simply enter the velocity
number into the MVX, rather than have the MVX calculate the velocity value on using
a know thickness. The steps for entering the velocity are outlined below:

Using a Known Material Velocity

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MVX High Performance Thickness Gauge

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 CAL menu is highlighted and displaying the submenu items.

2) Use the UP and DOWN arrow keys to scroll through the sub menu items
until VELOCITY is highlighted.

3) Press the ENTER key to display the Digits Edit Box.

4) Press the UP and DOWN arrow keys to scroll the highlighted value.

5) Press the LEFT and RIGHT arrow keys to scroll the digit locations.

6) Repeat steps 4 & 5 until the velocity number is correctly displayed.

7) Press the OK key to set the velocity and return to the menu screen, or ESC
to cancel entering the velocity.

8) Finally, press the MEAS key to return to the measurement screen and begin
taking readings.

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Known Thickness

Sometimes the sound velocity of a material is not known. In this case a sample with
a known thickness can be used to determine the sound velocity. It would be very
handy to carry a set of mechanical calipers to use in conjunction with the MVX for
calibration in the field:

Using a Known Thickness

Note: Be sure that the probe zero procedure has been performed prior to

performing this calibration procedure.

1) Physically measure an exact sample of the material or a location directly on
the material to be measured using a set of calipers or a digital micrometer.

2) Apply a drop of couplant on the transducer and place the transducer in
steady contact with the 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.

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MVX High Performance Thickness Gauge

3) Use the UP and DOWN arrow keys to scroll through the sub menu items
until ONE POINT 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 entering the velocity.

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 thickness reading should now match the known
thickness. If the thickness is not correct, repeat the steps above.

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Basic Material Type

If the material velocity is unknown, and a sample thickness cannot be taken from the
material, the user may opt to choose a basic material type from a list with
approximate velocity numbers. It’s important to note that these velocities will not
always be an exact representation of the material being tested. Use these values
only if a close approximation is acceptable. Follow the steps below to select a basic
material type:

Selecting a Basic Material Type

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 CAL menu is highlighted and displaying the submenu items.

2) Use the UP and DOWN arrow keys to scroll through the sub menu items
until MATERIAL is highlighted.

3) Press the ENTER key to display the list of material types.

4) Press the UP and DOWN arrow keys to scroll through the material list until
the appropriate material is highlighted.

5) Press the ENTER key to display the confirmation screen.

6) Press the OK key to sel ect the material and return to the menu screen, or
ESC to cancel the material selection.

7) Finally, press the MEAS key to return to the measurement screen and begin
taking readings.

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MVX High Performance Thickness Gauge

2.3 Measure

The MVX is now ready to measure. There are four different measurement view
options, each with a specific purpose. The steps below outline how to toggle
between the different view mode options:

Selecting the Measurement View Option

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 DISP menu is highlighted and displaying the submenu items.

2) Use the UP and DOWN arrow keys to scroll through the sub menu items

until VIEW is highlighted.

3) Use the LEFT and RIGHT arrow keys to scroll the view options.

4) Once the view is displayed, press the MEAS key to return to measurement

mode.

RF: Is useful to see exactly what the signal looks like directly around the detect
point. Use this view only when viewing smaller ranges (zoomed in) around the
detection point. This can be done manually by adjusting the delay and width
settings.

RECTIFIED (RECT): Displays the entire range being scanned. This screen is useful
to “get the big picture” when viewing wide ranges (zoomed out). The point which is

triggering the digital thickness reading (called the detect) is displayed as a vertical
dashed line.

BSCAN: The Time Based B-Scan provides the user with a cross sectional view of
the material being tested. This mode is useful when there is concern regarding the
profile of the blind surface. This can also be a useful view when scanning for pits and
flaws.

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DIGITS: Displays the digital thickness value using a larger font size. This view is
useful when the MVX is being used as a basic thickness gauge.

Once the view has been selected according to the application requirements, the
delay and width of the screen will potentially need to be adjusted, if the view has

been set to RF, RECT, or BSCAN. Use the following steps to adjust these settings
directly from the measurement screen as follows:

Adjusting the Delay & Width

1) Press the MEAS key once to activate the measure menu items. Press the

MEAS key multiple times to move right and the ESC key multiple times to

move left, until the either the DELAY or WIDTH cell is highlighted.

2) Use the UP, DOWN, LEFT, or RIGHT arrow keys to scroll the DELAY and

WIDTH values.

3) Repeat steps 1 & 2 until the range is correctly being displayed.

Alternatively, the DELAY and WIDTH values can be changed using the Digit
Edit Box as follows:

1) Press the MEAS key once to activate measure menu items. Press the

MEAS key multiple times to move right and the ESC key multiple times to
move left, until the either the DELAY or WIDTH cell is highlighted.

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MVX High Performance Thickness Gauge

2) Press the ENTER key to display the digits edit box.

3) Press the UP and DOWN arrow keys to scroll the highlighted value.

4) Press the LEFT and RIGHT arrow keys to scroll the digit locations.

5) Repeat steps 3 & 4 until the DELAY or WIDTH value is correctly displayed.

6) Press the OK key to set the DELAY and WIDTH value and return to the
measure screen, or ESC to cancel entering the DELAY or WIDTH value.

7) Finally, press the MEAS key to return to the measurement screen and begin

taking readings.

Note: The DELAY & WIDTH can also be adjusted from the menu tab items.
However, using the hot menu keys is the easiest method.

RF RECTIFIED

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B-SCAN DIGITS

In the upper left corner of each of the mode photos above, is the repeatability
indicator. The repeatability indicator is represented by six vertical bars and
represents how repeatable the measurements are. In regular measurement mode,
the MVX makes 4 measurements a second. In scan mode, the MVX makes 32
measurements a second. When the MVX is idle, only the left vertical bar and the
underline will be displayed. However, when the MVX is making a measurement, five
or six of the bars should be displayed on the repeatability indicator. If fewer than five
bars are showing, the MVX is having difficulty achieving a stable measurement and
the thickness value displayed is potentially be unstable. Another thing to note is the
vertical broken line in the RF and RECTIFIED photos above. This line represents the

actual point of detection. The MVX uses a zero crossing or flank detection method,
and presents the actual point of measurement on the display.

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CHAPTER THREE
KEYBOARD, MENU, & CONNECTOR REFERENCE

3.1 Menu Key (Operation & Sub Menus )

The Menu key activates the primary menu structure containing 9 menu tab groups.
These tab groups then contain sub menu items, or functions. The sub menu items
have been organized in tab groups according to how closely they are related to the
individual tab group names. Let’s first get familiar with how to move around in these
tabs before continuing on to the sub menu functions. This procedure is outlined
below:

Activating and Getting Around in the Menu Items

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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 desired tab group is highlighted and displaying the submenu items.

Now that your familiar with activating and moving amongst the tab groups, let’s have
a look at how to move around in the sub menu items as follows:

Getting Around in the Sub Menu Items

1) Use the UP and DOWN arrow keys to scroll through the sub menu items
until the desired function is highlighted.

2) Depending on which function is highlighted, use the LEFT, RIGHT, and
Enter keys to scroll the options or activate the Digit Edit and List Box

options.

The sections to follow will provide the user with an explanation of the sub menu
functions:

3.2 Probe – Menu

Zero: The MVX is zeroed in much the same way that a mechanical micrometer is
zeroed. If the MVX is not zeroed correctly, all of the measurements made using the

MVX may be in error by some fixed value. Refer to the section on page 35, for an
explanation of this important procedure.

Pulse: The MVX has an adjustable pulse width for both high penetration and
resolution applications. The pulse width refers to the duration of time the pulser is

on. The options are Spike, Thin, and Wide. Refer to page 73 for a further
explanation.

Type: Enables the user to select the type of transducer being used from a chart of
transducer types. This provides increased linearity between transducers. Refer to
page 31 for a further explanation.

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MVX High Performance Thickness Gauge

Flaw Mode: Activates the flaw detection mode and view. This feature is for use with
single element angle beam transducers and used as a general prove-up flaw
inspection mode. Refer to page 107 for a further explanation.

3.3 CAL – Menu

Units: Toggle between English or Metric units. The readout will change from inches

to millimeters.
Velocity: Function to calibrate the MVX by setting the velocity to a known material

velocity. Refer to page 37 for further info.
One Point: Performs a single point calibration. This option allows the user to

automatically calculate the velocity by entering a known sample thickness. Refer to
page 39 for further info.

Two Point: Performs a two-point calibration. This option allows the user to
automatically calculate the velocity by entering a second known sample thickness.
Refer to page 40 for further info.

Material: Select the material velocity from a chart of basic material types, when a
known sample thickness, or material velocity cannot be obtained. Refer to page 44
for further info.

3.4 DISP (display) – Menu

View: Selectable RF wave, RECT (rectified) wave, BSCAN (cross section), and

DIGITS (large digits) views. Refer to page 46 for further info.

Backlight: Selectable OFF, ON, AUTO, or INVERT backlight option.
Contrast: Adjustable display contrast for variable light conditions.
Delay: Provides the user the ability to change where the left side of the display

window starts according to thickness, in inches or millimeters. Refer to page 55 for
further info.

Width: Provides the user the ability to change the overall size of the viewable
measurement area. It functions a lot like a zoom on a camera. Refer to page 52 for
further info.

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3.5 TUNE – Menu

Gain: Increases or decreases the overall amplitude of the signal. Much like turning

the volume up or down on a stereo receiver. Refer to page 57 for further info.
AGC: The MVX is equipped with an automatic gain control when operating in echo-
echo mode only. This is much like turning the volume up or down on a stereo

receiver. However, the MVX will automatically control how much the volume is
turned up or down. Alternatively, the AGC can be manually controlled using the
same procedures as GAIN described above. Refer to page 57 for further info.

Threshold: Enables the user to set the sensitivity level of the MVX. The amplitude
of the signal must reach and exceed the threshold level before a measurement is

detected. Refer to page 60 for further info.
Polarity: The MVX operates on a zer o crossing detection principle. This feature
toggles which stroke of the cycle the crossing detection uses, either positive or

negative. Refer to page 78 for further info.
Rect Waveform: This option provides the user an outlined or filled view option when

the display setting is in RECT (rectified) wave mode only. Refer to page 80 for
further info.

3.6 GATE – Menu

Measure Mode: Used to select the measurement mode for different application

requirements. The modes are P-E (pulse-echo), P-E GT(pulse-echo w/gate), and E­E(echo-echo). Refer to page 24 for further info.

Gate 1: Gates allow the user to view areas or sections of the waveform and ignore
others. Gate 1 can be used in both pulse-echo and echo-echo measurement modes.
Refer to page 63 for further info.

Gate 2: Gates allow the user to view areas or sections of the waveform and ignore
others. Gate 2 is only used in echo-echo measurement mode. Refer to page 67 for
further info.

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MVX High Performance Thickness Gauge

3.7 SETUP – Menu

Open: Displays a list of factory and user defined setups currently stored in memory.

These setups can be recalled and used at any time. Refer to page 100 for further
info.

Save: Provides the user with the ability to save a custom setup that has been
modified or created by the user. Refer to page 102 for further info.

Default Setup: Loads a basic default setup. Use only as a last r esort when the
setups in the MVX have been corrupted and a computer is not accessible. Refer to

page 105 for further info.

3.8 DATA – Menu

New: Allows the user the ability to create a new alpha numeric grid with custom

parameters, rows, and columns depending on their application reporting
requirements. Refer to page 82 for further info.

Edit: Gives the user the ability to change parameters of grid that have been
previously saved. Note: Pre-defined coordinates cannot be changed once they have

been created. Refer to page 95 for further info.
Open: This function provides the user with the ability to recall grids that currently

exist in memory from a list of grids. Refer to page 97 for further info.
Delete One Grid: This function provides the user with the ability to delete one
individual grid from a list of multiple grids previously saved in memory. Refer to page

93 for further info.
Delete All Grids: This function provides the user with the ability to delete all grids

currently stored in memory. Refer to page 93 for further info.

3.9 UTIL (utilities) – Menu

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Auto Find: Automatically locates the detection point if the measurement is out of the
viewable display area. Refer to page 74 for further info.

Scan Mode: This function enables a hi speed scan mode that increases the overall
sample rate from the standard 4 measurements a second to 32 measurements a
second. Refer to page 75 for further info.

Alarm Status: Toggles alarm mode on or off. Refer to page 76 for further info.
Alarm LO Limit: Gives th e user the ability to set the LO limit parameter. If the

measurement falls below this value, a red light will illuminate and sound the internal
beeper. Refer to page 77 for further info.

Alarm HI Limit: Gives the user the ability to set the HI limit parameter. If the
measurement exceeds this value, a red light will illuminate and sound the internal
beeper. Refer to page 78 for further info.

3.10 XFER (transfer) – Menu

Backup Setups: Enables the user the ability to backup the setups currently stored
in the MVX to a PC via RS232 port. Refer the help section of the DakView 3 or
MVXview software for a complete electronic manual.

Restore Setups: Enables the user the ability to restore the setups currently saved
on a PC to an MVX via RS232 port. Refer the help section of the DakView 3 or
MVXview software for a complete electronic manual.

Backup Grid: Enables the user the ability to backup grids currently stored in the

MVX to a PC via RS232 port. Refer the help section of the DakView 3 or MVXview
software for a complete electronic manual.
Restore Grid: Enables the user the ability to restore grids currently saved on a PC
to an MVX via RS232 port. Refer the help section of the DakView 3 or MVXview
software for a complete electronic manual.
About: Provides the user with Dakota Ultrasonics contact information and the MVX
software version. Refer the help section of the DakView 3 or MVXview software for
a complete electronic manual.

3.11 CLR (clear) Key

The primary functions of the CLR key, is to clear a measurement from a grid cell
location or set obstruct, and backspace in an Alpha Edit Box. If a user has already

saved a measurement and waveform to a cell location, use this key to clear the
measurement at any time.

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3.12 MEAS (measurement mode) Key

The MEAS key puts the MVX into it’s primary mode of operation. In this mode, the
user has a complete view of the LCD, as well as control of the Hot Menu Functions.
These hot functions provide the user with the ability to make crucial adjustments to
the display, waveform settings, and grid storage locations without having to search
through the menu and sub menu items. It’s important to first get familiar with moving
around in the hot functions as follows:

Getting Around in the Hot Menus

1) Press the MEAS key at any time to return to the primarily measurement

mode.

2) Press the MEAS key multiple times to tab right through the menu fields, and

the ESC key multiple times to tab left through the menu fields, until the
desired hot function is highlighted.

Now that you’re familiar with activating and moving amongst the hot function fields,
let’s have a look at how to adjust or change the values of these fields:

Adjusting the Values of the Hot Menus

1) Use the UP, DOWN, LEFT, and RIGHT arrow keys to increase and
decrease the values of the hot function fields.

2) Repeat step 1 until the desired value has been achieved.

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Alternatively, the delay, width, gain, and threshold hot function fields can be
changed using the Digit Edit Box as follows:

1) Press the ENTER key to display the Digits Edit Box.

2) Press the UP and DOWN arrow keys to scroll the highlighted value.

3) Press the LEFT and RIGHT arrow keys to scroll the digit locations.

4) Repeat steps 2 & 3 until the delay, width, gain, or threshold number is
correctly displayed.

5) Press the OK key to set and return to the measure screen, or ESC to cancel
entering the delay, width, gain, or threshold value.

3.13 OK Key

The primary function of the OK key is confirmation of a change or selection. This key
also has a secondary function: pressing the OK key while in the main measurement
screen, toggles large digits to be displayed in place of the hot menu items. This will
provide the user with a larger digital readout of the measurement, while in RF or
RECT display modes.

3.14 ESC Key

The ESC key is used in the MENU, MEAS, and EDIT functions as a back or escape
function. This key also has a secondary function: pressing the ESC key, while in the

main measurement screen, toggles between all of the possible display view options –
RF, RECT, B-SCAN, and DIGITS.

3.15 Arrow Keys

The Arrow Keys are used to navigate through the menus, increase/decrease values,
and toggle specific function keys.

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3.16 ENTER key

The ENTER key is used in the overall menu selection process, to activate list and
edit boxes, and save measurements to grid locations.

3.17 MULTI MODE Key

The MULTI MODE key toggles the measurement mode of the MVX, from pulse-echo
(flaw mode), to echo-echo (thru paint) mode. This is a quick automatic preset mode

that enables the user to change modes without having to manually adjust the view
and gate parameters. Both modes have been configured for use with any transducer
type selected.

3.18 ON/OFF Key

The ON/OFF key simply powers the unit either ON or OFF. Note: Unit will
automatically power off when idle for 5 minutes. All current settings are saved prior
to powering off.

3.19 Top & Bottom End Caps

The top & bottom end panels are where all connections are made to the MVX. The
diagram above shows the layout and description of the connectors:

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Transducer Connectors

Refer to Diagram: The transducer connectors, and battery cover/probe zero disk are
located on the MVX’s top end cap. The transducer connectors are of type Lemo
“00”. Note: There is no polarity associated with connecting the transducer to the
MVX.

Probe Zero Disk & Battery Cover
Refer to Diagram: The Battery cover is the large round disk shown in the diagram.

Note: This same disk is also used as a probe zero disk. Simply remove the cover

when replacing the batteries (3 AA cells). When performing a probe zero function,
simply place the transducer on disk making firm contact. Important: Be sure to
follow the polarity labels located on the back label of the MVX. Note: Rechargeable
batteries can be used, however they must be recharged outside of the unit in a stand
alone battery charger.

RS-232 Connector
Refer to Diagram: The RS-232 connector, located on the bottom end cap, is a 2 pin

female Lemo connector. It is designed to connect directly from the MVX to a
standard AT serial port on a PC. The cable supplied with the MVX is a Lemo to 9 pin
serial cable. Note: This connector is also used to upgrade the MVX with the latest
version of firmware.

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CHAPTER FOUR
PRINCIPALS OF ULTRASONIC MEASUREMENT

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 bel ow, 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 sou nd.
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 det ermined 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 frequencies used in ultrasonic thickness measurement do not travel

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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.

Normal temperature range

Most standard transducers will operate from 0°F to 180°F.

High temperature measurements

Special transducers and couplants are available for temperatures above 180°F up to
650°F with intermittent contact. It is necessary to cool the transducer, by submerging
the transducer in water between readings, when measuring high temperatures.

Modes and temperature errors

In addition to errors caused by velocity changing with temperature, some modes
(measurement techniques) are affected more than others. For example, dual
element mode has larger errors due to changes in temperature of the delay line.
However, multi-echo techniques help to minimize these errors.

4.6 Measurement Modes

In this section we will discuss the different measurements modes the MVX is capable
of operating in, the transducers required, and the reasons for using specific modes:

Pulse-Echo Mode (Flaw & Pit detection)

Pulse-echo mode measures from the initial pulse (sometimes referred to as an
artificial zero) to the first echo (reflection). In this mode, the transducer is placed on a
reference disk, located on top of the MVX, and a key is pressed to establish a zero
point for the particular transducer.

In this mode errors result from surface coatings and temperature variation s.
Since pulse-echo only requires one reflection, it is the most sensitive mode for

measuring weak reflections (flaws) typically found when measuring heavily corroded
metals.

V-Path Correction

Dual element delay line transducers have two piezoelectric elements mounted at an
angle on one end of the delay line. One element is used for transmitting sound, while
the other element only receives sound. The two elements and their delay lines are
packaged in a single housing but acoustically isolated from each other with a sound

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MVX High Performance Thickness Gauge

barrier. This allows the transducer the ability to achieve very high sensitivity for
detecting small defects. Also, the surface of the test material does not have to be as
flat in order to obtain good measurements.

Dual element transducers are normally used in pulse-echo mode for finding defects,
and in echo-echo mode for through coating measurements.

Dual element delay line transducers are usable over a range of 0.025 inches to 20
inches depending on the material, frequency, and diameter.

A limitation of dual element delay-line transducers is the V shaped sound path.
Because the sound travels from one element to another, the time versus thickness
relationship is non-linear. Therefore, a correction table in the instruments software is
used to compensate for this error.

Dual Element Transducer showing V-path of signal

Echo-Echo Mode

The echo-echo mode measures between two reflections. This technique is
commonly used to eliminate errors from surface coatings and also to make
measurements in multiple layered materials. The disadvantage is that two echoes
are needed which requires a much stronger echo (reflection).

Dual Element Transducer in Echo to Echo mode

Searching for small defects

Dual element delay line transducers are especially useful in searching for small
defects. In the pulse-echo mode with high amplifier gain, very small defects can be
measured. The A-Scan display of the MVX can be used to see the defect and
optimize the transducer placement.

Sometimes a grain particle or air bubble will cause a reflection. Without the A-Scan
display it is impossible to know if this is a thin area or just a harmless defect. The A­Scan allows the user to see the backwall as well as the defect similar to a flaw
detector.

Dakota Ultrasonics

Waveform showing echo from corroded backwall

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Waveform showing flaw with solid backwall