Setting-Up Distance Amplitude

Correction Curve (DAC Curve) on the

Page 1 of 4
 1. Introduction

1. This document was written using components from the GE Measurement & Control USM
36 Technical Reference and Operating Manual Issue 2 (12/2013). This issue, in conjunction
with this document, applies to software version 4.00 (August 2013). Any changes to the
software and/or the referenced document, renders this document obsolete. Obsolete
documents SHALL not be used and must be revised in order to continue to meet minimum
requirements.
1.1. The intent of this document is to assist an operator in the generation of an accurate
D.A.C. curve in a comprehensive manner.
1.2. This document is nonbinding and the responsibility for verifying the D.A.C. curve’s
adherence to applicable requirements is charged to the operator.

2. Definitions

2.1. Operators should be familiar with the following terms that will be referenced
throughout this document.
2.1.1. Attenuation- The phenomena of soundwave energy’s decay that can be
attributed to different factors. In this document it will be referring to distance.
2.1.2. Couplant-Substance(s) used to couple the transducer to the material of interest
allowing ultrasonic sound to pass through the interface.
2.1.3. Distance Amplitude Correction Curve (DAC)-A gradual slope plotted by
recording known reflectors (simulated defects) at varying depths. This slope aids in
determining rejectable indications with respect to distance based attenuation.
2.1.4. Frequency-The amount of times something occurs within a given interval. In the
context of this document, the amount of ultrasonic waves generated per cycle
(Hertz).
2.1.5. Full Screen Height (FSH)-Vertical divisions of the display referred to in
percentages within this document.
2.1.6. Gain-The “volume” of the soundwaves, measured in dB (decibels)
2.1.7. Hertz-The unit of measurement of cycles per second.
2.1.8. Horizontal Linearity-When Ultrasonic signals of varying depths corresponds with
equally spaced horizontal divisions on screen.

Page 2 of 4
2.1.9. Initial Pulse-Also known as the “Main Bang”, the initial group of sound that is
generated. This signal is always displayable without the need of the transducer to
be coupled to a surface.
2.1.10. Interface-The surface forming the boundary between 2 materials of different
properties.
2.1.11. Main Bang-(see 2.1.9 “Initial Pulse”)
2.1.12. Material Velocity-The speed at which sound waves travels through a given
material.
2.1.13. Piezoelectric-Property of a material to convert electrical energy into mechanical
energy. In the context of this document, generating ultrasonic sound waves.
2.1.14. Probe-(See 2.1.20 “Transducer”)
2.1.15. Probe Delay-A variable that adjusts where the transducer interprets the
ultrasonic signal.
2.1.16. Range-Amount of the ultrasonic signal displayed on screen.
2.1.17. Reflector- With respect to this document, artificial flaws used in calibration of an
instrument.
2.1.18. Screen Delay-Adjustable variable which the operator may manipulate to display
a portion of interest of the ultrasonic signal.
2.1.19. Time Corrected Gain (TCG)-program created from an already established D.A.C.
curve (See 2.1.3) that visually compensates the gain for distance based
attenuation.
2.1.20. Transducer -Apparatus containing a piezoelectric crystal for the generation and
receiving/interpretation of ultrasonic soundwaves.

3. Procedure

3.1. Make sure instrument is running in the configuration “DAC/TCG” in the “EVAL
MODE”. This is done by holding down the “Home” button and using the “Gain”
rotary knob to get to the “EVAL” screen. Use the “Enter” button to select “EVAL
MODE” use the right rotary knob to change the value to “DAC/TCG”. Hold the
“Home” button to return to the A-scan screen.
3.2. Selection of transducer size, frequency and angle is outlined in T9074-AS-GIB-
010/271 REV 1 DATED 11SEP14 REQUIREMENTS FOR NONDESTRUCTIVE TESTING
METHODS Section 6.6.4.3.2.
3.3. With appropriate transducer configuration obtained, place an adequate amount of
couplant (Vaseline or equivalent coupling recommended) in the threaded opening
of the wedge.

Page 3 of 4
3.4. Place threaded end of transducer into the threaded end of wedge. Take care to line
up threads. Excess couplant may or may not come out of the weep hole. Take care
to not go beyond a snug fit. Excessive tightening may cause damage to the wedge.
3.5. Connect transducer to micro-dot end of applicable cable.
3.6. Connect BNC connector to USM 36 unit in one of the available connections.
3.7. Power on instrument.
3.8. Adjust Gate A to 80% FSH (Full Screen Height) apply couplant to applicable test
block as needed.
3.9. Place transducer on test block. Adjust gain, range, material velocity, display delay
and probe delay so the signal form 1/8” reflector is observed and maximized to
break Gate A at 80% FSH. NOTE: If the AUTOCAL function is used, “Probe Delay” and
“Material Velocity” SHALL NOT be adjusted.
3.10. Put maximized peak for the 1/8” reflector on the first vertical division on the left
of the screen.
3.11. Repeat obtaining 80% FSH (adjust Gain as necessary) of reflectors from lesser to
greatest depth, ensuring intervals of each reflector lands on the consecutive vertical
divisions up to 2 times the material thickness of the item to be inspected. This is to
establish “Horizontal Linearity”.
3.12. Once obtained, select the DAC/TCG option on the bottom of the screen.
3.13. Maximize the peak of the 1/8” reflector at approximately 85-82% FSH and
record. Excessive FSH may result in false rejectable indications.
3.14. Record the peak by selecting “Record” and pushing the “Enter” button until the
next sequential point is displayed.
3.15. Once points have been recorded, select the “Finish” option and push the “enter”
button. The resulting slope SHALL have a gradual and uniform curvature. If so, the
DAC curve is completed. If not, repeat steps 3.9 through 3.14.
3.16. Select “TCG” mode so that the instrument can visually compensate for distance
based attenuation.

Page 4 of 4