CALIBRATION OF THE

TESTING SYSTEM
DR. ESTRELLA D. RELUNIA
 TOPICS TO BE COVERED
• Purpose of Calibration
• Standard Test Blocks
• Equipment Characteristics
• Calibration with Normal Probe
• Calibration with Angle Probe
• Calibration in Curved Work Pieces
• Construction of DAC Using Reference Block
• DGS(Distance-Gain-Size) Diagram
 PURPOSE OF CALIBRATION

Basic Functions of UT Equipment:

Generation of an elastic wave
Reception of ultrasonic signals
Signal conditioning and processing
Discontinuity signal
Gating and signal presentation
Codes usually specify the required
instrument capability. To ensure that this
can be achieved , equipment should be
calibrated.
 CALIBRATION
• In Ultrasonic Testing, Calibration
means the verification and
adjusting of ultrasonic equipment
characteristics so that reliable and
reproducible test results are
obtained.
 Classification of Ultrasonic
Procedure
• Equipment Characteristics
Verification
• Range Calibration
• Reference Level or Sensitivity
Setting
 STANDARD TEST BLOCKS
• Calibration and Reference Blocks
• In ultrasonic pulse echo testing test blocks
containing notches, slots or drilled holes are
used to:
– Determine the Operating Characteristics of the Flaw
Detector and Probes;
– Establish Reproducible test conditions
– Compare the height or location of the echo from a
flaw in the test specimen to that from an artificial
flaw in the test block.
I.I.W. CALIBRATION BLOCK
The most versatile calibration block is the
block made from medium carbon ferritic
and normalized steel described by the
International Institute of Welding (IIW) and
proposed by the International Standard
organization (ISO).
This block is called the I.I.W. V1 block.
The block is generally used for:
1.Calibration of the time base using 25
mm, 100mm and 200mm thickness with
the normal beam probes and 100 mm
quadrant for angle beam probes.
2.The determination of probe index using
100 mm quadrant.
3.The determination of the probe angle
using plastic wedge and degrees
stamped on the side of the block. The
angle beam transducer is subject to
wear in normal use.
4. The checking of performance
characteristics of the ultrasonic flaw
detector such as:
- Time base linearity
- Screen height linearity
- Amplitude control linearity
- Resolving power
- Penetrative power
- Dead Zone Check
- Pulse length
5. The setting of sensitivity.
6. The calibration of time base
and sensitivity setting for DGS
method.
7. The comparison of various
materials due to their different
acoustic velocities.
 ASME REFERENCE BLOCK
• Used to construct a distance-amplitude-
correction (DAC) curve on the CRT
screen by noting the changes in echo
amplitude from the hole with change in
scanning distance (multiple skip).
AREA-AMPLITUDE BLOCKS
• Provide artificial flaws of different
sizes at the same depth. Eight blocks
made from the same (50 mm)
diameter round stock, each 3-3/4”
(95.25 mm) in height, constitute a set
of area-amplitude blocks
DISTANCE-AMPLITUDE BLOCK
• Provides artificial flaws of a given size at
various depths (metal distance).
• From the ultrasonic wave theory, it is known
that the decrease in echo amplitude from flat
bottom hole using a circular probe is inversely
proportional to the square of the distance to the
hole bottom.
 DISTANCE-AMPLITUDE
BLOCKS
• Distance-amplitude blocks ( also known as Alcoa
series-B or Hitt blocks) can be used to check actual
variations of amplitude with distance for normal
beam inspection in a given material. They also serve
as reference for setting or standardizing the
sensitivity of the inspection system so that readable
indications will be displayed on the CRT screen for
flaws of a given size/larger, but the screen will not be
flooded with indications of smaller discontinuities
that are of no interest.
 ASTM BLOCKS
• ASTM blocks can be combined into various
sets of area-amplitude and distance-amplitude
blocks.
• The ASTM basic set of distance-amplitude
blocks consists of ten 2” (50mm) diameter
blocks, each has a 3/64” (1.19 mm) diameter
hole at a 3” (76.2 mm) metal distance.
 EQUIPMENT
CHARACTERISTICS
• Horizontal Linearity
• Screen Height Linearity
• Amplitude Control Linearity
• Resolution
• Dead Zone Estimation
• Maximum Penetrative Power
• Determining Pulse Length
 HORIZONTAL LINEARITY
• The horizontal linearity or time base linearity is
a measure of the degree of difference between
an actual distance and a distance read out on
the CRT screen. The I.I.W., Din 54122 or any
block of similar material and finish may be
used to measure horizontal linearity. The
choice of thickness is determined by the
requirement that a longitudinal wave probe
placed on the block produces several backwall
echoes (usually 4 or 5) within the chosen range.
 CHECKING LINEARITY
• For checking the linearity two of the backwall echoes
(say, the first and fourth in a five echo display)
should be set to coincide with appropriate scale
divisions. The position of each of the remaining
echoes is then carefully noted.
• An important precaution to take during the
assessment of time base linearity is that time base
readings are taken as each signal is brought to a
common amplitude. This is usually about ½ screen
height.
 SCREEN HEIGHT LINEARITY

• The screen height linearity or

amplitude linearity is a measure
of the degree of proportionality
between an input to the amplifier
and echo height displayed on the
CRT.
 Procedure for Determining
Linearity of Amplifier
• The time base of the flaw detector is calibrated for
a range of 250 mm and ten backwall echoes from
25 mm thick side of the V1 block are obtained. The
amplitude of the nth echo (usually the one which is
just outside the near field length) is set to a
particular amplitude (normally the screen height).
The amplitude of the subsequent echoes (n+1,n+2,
n+3 echoes, etc.) are noted. The amplitude of the
nth echo is then reduced to half its original values,
then the amplifier is linear, otherwise, it is not.
Deviation from linearity for any
particular echo can be expressed as a
percentage relative deviation from the
following equation:

Deviation = (Orig. Amp. Of Echo)-2 (Reduced Amp.)/

(Original Amplitude of the Echo) x 100%
 AMPLITUDE CONTROL
LINEARITY
• For checking of amplitude control linearity, the
time base is calibrated for a desired range and an
echo about midway along the time base is
obtained. The echo amplitude is set to a desired
height and the attenuator reading is noted. The
attenuator reading is then reduced by 6 dB, four
or five times in succession and the decrease in echo
amplitude is noted every time. If it decreases to
half the value of the previous setting then the gain
control of the equipment is properly calibrated.
 RESOLUTION
• The resolution of a flaw detector is the
ability to resolve minor differences in
distance and direction. To determine the
resolution of a flaw detector I.I.W. V1
block is used with normal beam probes.
This block has three target reflectors at
ranges of 85 mm, 91 mm and 100mm.
A rough estimate of the length of
the dead zone beneath a
compressional wave probe is
obtainable using the 1.5 mm hole
and the plastic insert of the V1
block. A special block is
recommended.
With this block the resolution is determined by
the minimum distance apart that flaws can be
indicated clearly and separately.
The probe is placed on the center line of the
block over the change in radius from one step
to the next. Its position is adjusted so that
echoes from the two radii are of the same
height and approximately ½ full screen height.
The steps are said to be resolved when their
echoes are clearly separated at half maximum
echo height or less.
 MAXIMUM PENETRATIVE
POWER
• A term used in British Standard BS
4331.
• Describes a check which is used to
compare the energy output for a
particular set and probe with its past
performance or with similar
equipment.
Procedure
A longitudinal wave probe is placed on the
plastic insert (methyl polymethacrylate
cylinder) of the I.I.W.V1 block. Having a
thickness of 23 mm which is equal to 50 mm
of steel and the gain for the instrument is set
to its maximum. The number of multiple
echoes and the amplitude of the last echo are
noted and are used to express the maximum
penetrative power of the set and the probe.
 DETERMINING THE PULSE
LENGTH
• Normal Probe
• Angle Probe
CALIBRATION WITH NORMAL
PROBE:
1.Calibration of Time Base
1.1 Using V1 Block
1.2 Using V2 Block
Using V1 Block
Using I.I.WV2 Block
Calibration With Angle Probes
1. Range Calibration Using V1 Block
Procedure
Get multiple backwall echoes
from the 100mm quadrant by
placing the probe at position
“E”.
Another method of calibrating
the time base for angle beam
probes is to position a normal
wave probe at “D” in Figure
5.11
Calibrating Time Base for 100 mm
with Angle Beam Probe:
Using V2 Block
 TIME BASE CALIBRATION
• METHOD 1
The probe faces the 25 mm radius quadrant. The
screen can be calibrated for 100 mm, 175 mm, 200
mm, 250 mm ranges. For 100 mm test range
calibration, facing the probe crystal to 25 mm
quadrant of V2 block, first echo is obtained from 25
mm quadrant, the same wave is then reflected from
probe index towards 50 mm quadrant. This wave is
reflected back to the probe crystal which is not
received by the crystal due to orientation of the
crystal as it is towards 25 mm quadrant.
Again it is reflected to the 25 mm
quadrant, this reflected wave from 25 mm
quadrant is received by the crystal. The
echo obtained now is at 100 mm on screen,
which means that fater first echo obtained
at 25 mm on CRT the other multiple
echoes will be obtained at an interval of 75
mm. The echo pattern for a 200 mm range
appera at 25 mm, 100mm, and 175 mm.
For a 250 mm range the echoes appear at
25 mm, 100 mm, 175 mm and 250 mm.
METHOD 2
The probe faces the 50 mm radius
quadrant. The CRT screen can be
calibrated for ranges of 125 mm
and 200 mm. In the CRT screen
pattern for a 200 mm range, the
echoes appear at 50 mm, 125 mm
and 200 mm. In this method the
echo from the n50 mm quadrant is
set at 10th scale division oc CRT
screen using the sweep control or
This echo is set at 5th scale division of
CRT screen using the delay control.
The procedure is repeated until the
echoes from 25 mm and 50 mm
quadrants respectively coincide with
5 and 10 scale divisions of CRT.
 DETERMINATION OF THE
PROBE INDEX
• Using V1 Block
The probe is placed at position L on the calibration
block and a backwall echo from the 100mm
quadrant is obtained. The maximum amplitude of
this backwall echo is determined by moving the
probe to and fro about the position L. When the
maximum amplitude is found then the point on the
probe which coincides with the point 0 (or cut
mark) on the block is the probe index.
•Using V2 Block
The probe is placed either facing the
25 mm quadrant or the 50 mm
quadrant to obtain echoes at 25 mm
or 50 mm on the CRT screen. The
probe is moved to and fro to
maximize the echo. When the echo
amplitude is a maximum, the probe
index is obtained by extending the
centre mark of the millimeter scale
 DETERMINATION AND
CHECKING THE PROBE ANGLE

• Using V1 Block
• Using V2 Block
 CALIBRATION IN CURVED
WORK PIECES
• Sensitivity
Two factors contribute to the reduction in
sensitivity when a test specimen with a curved
surface is ultrasonically tested. One is the
widening of divergence of the transmitted
beam because of refraction and the other is the
reduction in the contact area between the
probe and the test specimen.
 SKIP DISTANCE AND BEAM
PATH LENGTH CORRECTION
• The range calibration for a test with angle
probes is usually made with I.I.W V1
calibration block.
Refer to Figures 5.24 & 5.25
CONSTRUCTION OF DAC USING
REFERENCE BLOCKS
DGS (DISTANCE-GAIN-SIZE)
DIAGRAM
 COUPLING MEDIUM
• SURFACE FINISH OF THE MATERIAL
• TEMPERATURE OF THE TEST SPECIMEN
• POSSIBILITY OF CHEMICAL REACTION
BETWEEN THE TEST SPECIMEN AND
THE COUPLANT
• CLEANING REQUIREMENTS ( SOME
COUPLANTS ARE DIFFICULT TO
REMOVE)