4.2 EDDY CURRENT PROCESS CONTROL PROCEDURES.
4.2.1 General Process Control for Eddy Current Inspection Probes and Standards. For maximum reliability in ET, a high signal-to-noise ratio is desired. No specific signal-to-noise ratio is mandatory, but a minimum of 3-to-1 is desirable for flaw detection.
4.2.2 Probe Test. The following steps are designed for testing in-use surface eddy current probes 1/8-inch or smaller:
a. Attach probe and cable to the proper connector.
b. Place Teflon tape across the four surface slots on the Air Force general purpose eddy current standard.
c. Turn on the unit by pressing the green circular button. When the unit is first turned on it will display the
‘‘DIAGNOSTICS’’ self-test and the last setup program that was used.
NOTE
Step d - step j loads the ‘‘DEFAULT’’ program.
d. Press the [SETUP/SPECIAL] button until the ‘‘set up’’ menu appears. The ‘‘setup’’ menu displays REPORT CLOCK DEFAULT in the top row.
e. Press the 3rd soft key [ ] until DEFAULT is highlighted.
f. Press the [ENTER] button.
g. Press the 4th soft key [ ] to select LD DEFLT. There should be 3 highlighted areas: LD DEFLT lower right side above the 4th soft key [ ], LD DEFLT 4th row in right column, and DEFAULT above 5th soft key [ ].
h. Press the [ENTER] button.
NOTE
A 4th highlighted LD DEFLT will be displayed above the function row. To the right of the highlighted LD DEFLT there will be either CANCEL or CONFIRM.
i. Rotate the SMARTKNOB until it reads CONFIRM.
j. Press the [ENTER] button.
NOTE
The ‘‘DEFAULT’’ program is now loaded. The MAIN menu display should be in ‘‘flying dot’’ mode. The following values should be displayed.
k. FREQUENCY should be highlighted; if not, press 1st soft key [ ].
l. Rotate SMARTKNOB to set the minimum frequency identified on the probe.
m. Press the 3rd soft key [ ].
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NOTE
GAIN should be highlighted above the soft key and both the H-GAIN and V-GAIN should be highlighted on the right side.
n. Rotate the SMARTKNOB to set the gain at 50 dB horizontal and 50 dB vertical.
o. Place the probe on the Teflon tape away from slot and ‘‘null’’ the instrument by pressing the [NULL] button.
p. Press the [ERASE] button.
NOTE
After ‘‘nulling’’ the instrument it is always a good practice to press the [ERASE] button after pressing the [NULL]button. This clears the display and places the ‘‘flying dot’’ at the center position.
q. Press the 2nd soft key [ ]. ANGLE should be highlighted in two places.
r. Scan over the 0.050 inch slot while rotating the SMARTKNOB to adjust phase angle. The ‘‘flying dot’’ should be rotated to a vertical position.
s. Press the 3rd soft key [ ]. (See step m note)
t. While scanning the 0.050 inch slot, rotate the SMARTKNOB until the response from the crack moves the dot exactly three square divisions. To improve gain control use the [ERASE] button occasionally to clear the display.
u. Record the gain setting for that frequency.
v. Repeat the step k through step u to achieve a low, medium (pick a number that is between the low and high), and high frequency reading for each probe. {e.g., 60.3 at 50 kHz for a low frequency. 53.2 at 200 kHz and 59.6 at 500 kHz}
NOTE
Some probes have their frequency range (low to high frequencies) identified on the probe handle. Others may not. If the probe frequency range is not identified, use 200 kHz, 500 kHz, or 1 MHz. The “flying dot” will not behave properly when the probe’s frequency range is exceeded (either low or high). The dot should be relatively stable after “nulling” the instrument. There should be a definite response displayed when the 0.050 inch slot is scanned. If not, try another frequency. The optimum frequency for the probe is the frequency with the lowest gain setting and highest signal response.
Probes shall be tested after receiving them and periodically thereafter. If the gain settings are off by 20 percent or more the next time you test the probe, it should be replaced.
4.2.3 Slot Test. Attach probe and cable to the proper connector.
a. Place Teflon tape across the four surface slots on the Air Force general purpose eddy current standard.
b. Turn on the unit by pressing the green circular button. When the unit is first turned on it will display the
‘‘DIAGNOSTICS’’ self test and the last setup program that was used.
NOTE
Step c - i loads the ‘‘DEFAULT’’ program.
c. Press the [SETUP/SPECIAL] button until the ‘‘set-up’’ menu appears. The ‘‘setup’’ menu displays REPORT CLOCK DEFAULT in the top row.
d. Press the 3rd soft key [ ] until DEFAULT is highlighted.
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T.O. 33B-1-2
e. Press the [ENTER] button.
f. Press the 4th soft key [ ] to select LD DEFLT. There should be 3 highlighted areas: LD DEFLT lower right side above the 4th soft key [ ], LD DEFLT 4th row in right column, and DEFAULT above 5th soft key [ ].
g. Press the [ENTER] button.
NOTE
A 4th highlighted LD DEFLT will be displayed above the function row. To the right of the highlighted LD DEFLT there will be either CANCEL or CONFIRM.
h. Rotate the SMARTKNOB till it reads CONFIRM.
i. Press the [ENTER] button.
NOTE
The ‘‘DEFAULT’’ program is now loaded. The MAIN menu display should be in ‘‘flying dot’’ mode. The following values should be displayed.
j. FREQUENCY should be highlighted; if not, press 1st soft key [ ].
k. Rotate SMARTKNOB to set the frequency at 500 kHz.
l. Press the 3rd soft key [ ].
NOTE
GAIN should be highlighted above the soft key and both the H-GAIN and V-GAIN should be highlighted on the right side.
m. Rotate the SMARTKNOB to set the gain at 50 dB horizontal and 50 dB vertical.
n. Place the probe on the Teflon tape away from slot and ‘‘null’’ the instrument by pressing the [NULL] button.
o. Press the [ERASE] button.
NOTE
After ‘‘nulling’’ the instrument it is always a good practice to press the [ERASE] button after pressing the [NULL]button. This clears the display and places the flying dot at the center position.
p. Press the 2nd soft key [ ]. ANGLE should be highlighted in two places.
q. Scan over the 0.050 inch slot while rotating the SMARTKNOB to adjust phase angle. The ‘‘flying dot’’ should be rotated to a vertical position.
r. Press the 3rd soft key [ ]. (See paragraph 4.2.3, step n note)
s. While scanning the 0.050 inch slot, rotate the SMARTKNOB until the response from the crack moves the dot exactly three square divisions. To improve gain control, use the [ERASE] button occasionally to clear the display.
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t. Place a straight edge (ruler, triangle edge, etc) adjacent to and inline with the notch so that the probe’s center is on the center of the notch.
u. Press the [ERASE] button so that the dot is showing at three divisions.
v. Move the probe down the slot lengthwise using the straight edge as a guide.
w. The dot should not move more than 1/10th of a division until it comes to the edge of the standard or the edge of the slot (Edge Effect).
x. Repeat the step m through step u (see paragraph 4.2.3) for the 0.020, 0.010, and 0.005 slots. Remember the gain will have to be increased to align the dot at three divisions.
NOTE
For the 0.005″slot, use one division instead of three divisions. The aluminium general purpose eddy current standard SHALL be tested whenever the slots appear to be worn.
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