Using The Phase Angle Method
This procedure can be used as a general procedure to calibrate the ect MAD 8D eddy current system for nonferrous heat exchanger tubing inspection. This procedure was written using the example of calibrating to inspect a one inch diameter by .055 inch wall 90/10 copper nickel tube (CuNi).
Choose probes suitable for inspecting the tube. The probe outside diameter should be approximately .03 inches less than the inside diameter of the tube. The probe frequency specification should be within a factor of three of the frequency selected for Channel 2 below.
Cables may be connected to the ect MAD 8D eddy current system either before or after power has been applied to the unit.
Connect the ect MAD 8D amplifier to the main unit using the cable that has four 9-pin connectors and one 25-pin connector Interface Cable. The four 9-pin connectors are labeled Channel 1 through Channel 4. Connect these to the frequency cards in the main unit, which are also labeled Channel 1 through Channel 4.
Connect the inspection probe to the probe connector on the ect MAD 8D amplifier. This probe will be used for scanning the calibration tube and for inspecting the tubes in the heat exchanger. Connect a second identical probe to the reference connector, and place it in a flawless tube. This is the reference probe for the Absolute Channel. It is recommended that this probe be secured in the flawless tube with tape to prevent accidental movement of the probe. The flawless tube must be identical to the new condition of the probes to be inspected.
Set the Channel 1, 2, and 4 switches up to the DIFF (differential) position. Set the Channel 3 switch down to the ABS (Absolute) position. Set the Normal Send/Receive switch up to the NORM (normal) position. The normal position is used for impedance probes. The send/receive position is used for send/receive probes, which are not normally used for nonferrous heat exchanger tubing inspection.
Select the frequency for Channel 2 from the ect Electrical Conductivity of Materials Manual, Report ect R8418-R1, published by Eddy Current Technology Incorporated. Using the table labeled Tabulated Calculation for the Value F, identify the alloy you are using (or an alloy with similar electrical resistance) in the first column. Identify the wall thickness of the tube to be inspected in the first row of the table. At the intersection of the alloy and thickness is printed the frequency for Channel 2 in kHz. This frequency can be rounded to convenient whole numbers.
In the case of a 90/10 copper nickel tube with a .056 inch wall, the value at the intersection point is 22.17 kHz. Round this to 22 kHz.
The most convenient way to set all four frequency channels to the correct frequency is in the Setup Menu. From the Main Menu, type <U> for setup and <Y> to select the frequencies for change.
Set Channel 2 to 22 kHz. Set Channel 1 to twice this value, 44 kHz. Set Channel 3 (the Absolute Channel) to 44 kHz. (Under some conditions, it may be desirable to set Channel 3 to a frequency approximately ten per cent less than Channel 1.) Set Channel 4 to half the frequency of Channel 2, 11 kHz.
In Channel 1, the high frequency 44 kHz channel, check the phase angle spread between the through wall hole and the 20 per cent outside defect. A phase angle spread of 90 degrees generally works well. If the phase angle spread is significantly lower than this, then increase the frequency in all channels. If the phase angle spread is significantly greater than this in Channel 1, then decrease the frequencies in all channels. (NOTE: It is important to keep the factor of two difference between Channels 4 and 2 and 2 and 1 in order to achieve high speed, low noise performance.) (This phase angle spread test can be performed after setting the frequency channel phase controls if desired, but this will make it necessary to adjust the frequency channel phase controls after adjusting the frequencies.)
FREQUENCY CHANNEL PHASE ADJUSTMENT
For phase adjustment, it is recommended that you select 4 XY mode (from the Main Menu, type <M>, followed by the number <1>). It is recommended that you type <S> to select screen assignments, and assign Channels 1, 2, 3, and 4 to screens A, B, C, and D.
For phase and gain adjustments (and most calibration adjustments), it is recommended a probe speed of about one foot per second be used, and that for this probe speed, a sample rate of 1,000 Hz be used. In the Setup Menu, set the sample rate to 1,000 Hz and return to the Main Menu.
Adjust the dent signal in all channels to be horizontal. This is the most critical adjustment to be made. Make sure that the dent signal is accurately set to horizontal, specifically for the Differential Channels, the straight line portion of the dent signal between its two left and right extremes must be exactly horizontal.
While pulling the probe slowly through the tube past the 100 per cent through wall hole, confirm that the signals in the three differential channels go down and to the right first before continuing up and to the left. For any channel in which the signal is in the opposite direction, select the phase control adjustment and rotate it 180 degrees. This can conveniently be done by typing the or keys twice after selecting the appropriate phase control for adjustment.
This phase adjustment meets the requirement of the ASME code as written in Section 3.5.
FREQUENCY CHANNEL GAIN ADJUSTMENT
For each of the Differential Channels 1, 2, and 4, set the volts per division to .5 and adjust the gain controls so that the amplitude of the signal from the through wall hole is two volts peak-to-peak minimum. Note that due to the clearance between the outside diameter of the probe and the inside diameter of the tube, the distance between the probe coils and the defect will vary each time the probe is pulled past the defect. For this reason, the amplitude of the signal will vary somewhat. This amplitude adjustment is not as critical as the phase control adjustment.
When the through wall hole signal has been adjusted to two volts, it will be four divisions, or one half of the screen size. This meets the requirements of the ASME code.
Mixer 1 will be used to mix the signals from Channels 1 and 2 in order to eliminate support plate signals. Enter the Setup Menu and make sure that Mixer 1 source is Channel 1 and Channel 2. Return to the Main Menu.
Select screen assignments for change. Set Screen D to Mixer 1. The signal from the support plate can be canceled automatically. To accomplish this, it will first be necessary to record the support plate signal alone in random access memory. To accomplish this task, first erase random access memory with the key <F8>, and then perform the following three steps quickly:
1. Press <F3> to begin the Record function.
2. With the probe positioned in the tube away from defects, move the support plate past the probe at a speed of about one foot per second. Note that the support plate should be moved in a direction such that it is being moved away from the cable end of the probe, as it passes over the probe in the tube.
3.Press <F2> to enter the Playback mode.
Request the ect MAD 8D eddy current system to cancel the support plate signal by pressing <X> to enter the Mixer Menu and then <C> for cancel. The cancellation process should take a few seconds. During this period of time, you will see the support plate signal drawn over and over again as it diminishes in size. You will also note the system adjusting the following mixer input phase controls: In Phase, H Weight, and V Weight.
MIXER OUTPUT PHASE CONTROL
Enter the Record mode by pressing and move the probe past the dent in the calibration tube. Adjust the output phase control so that the signal from the dent is horizontal. This is a very critical adjustment, which must be carried out precisely. Output phase control settings normally fall in the range of 10 to 20.
Check to make sure that the signal from the hole in the calibration tube moves down and to the right first as the probe is pulled past the hole in the calibration tube.
MIXER GAIN CONTROL ADJUSTMENT
Set the mixer volts per division controls to .5. Adjust the gain control so that the signal from the through wall hole is two volts peak-to-peak. This will be four divisions on the screen, or half the screen width.
Now that the calibration procedure is almost complete, it is a good idea to save the settings. Type for <F> file services and then <S> for save. Enter a suitable DOS file name up to eight characters long and press the key and then to confirm save to disk.
To prepare the calibration table, it is recommended to have the Auto Stop function ON. Enter the Setup Menu and press <A> followed by the<Home> key and then <Enter>. Auto Stop will now indicate ON. Return to the Main Menu.
Enter the Analyze Menu from the Main Menu by typing <A>. Erase RAM by typing <F8>. Position the probe in the tube so that the end of the probe barely protrudes from the end of the tube. Orient the tube so that the dent signal will be the first signal encountered, followed by the hole. Press <F3> and pull the probe through the tube at a speed of approximately one foot per second. Pass all the defects and then press <F1> to stop the recording function. Observe the signals on the screen.
The diagonal portion of each defect signal between the end points should be red, and preferably only the straight portion of the signal should be red. If the red portion of the signal extends partway through the curved portion of the defect, then the threshold value should be set to a higher number. If only the central portion of the straight line portion of the diagonal defect signal is red, then a lower threshold setting should be used. Threshold settings between four and 10 normally work best. Another reason for the incorrect portion of the diagonal signal being red could be due to the speed of the probe being outside of the range of one half to two feet per second. If this was the case, then rescan the tube. Another reason for this problem could be that the sample rate is not set to 1,000 Hz in the Setup Menu.
If the curved portion of the defect signals are red instead of the straight line diagonal portion, this could be caused by one of two possibilities:
The probe was pushed past the defects and not pulled past the defects.
Some or all phase controls were not set correctly, and the signals go up and to the left when the probe is pulled past defects. If necessary, correct any erroneous phase adjustments at this time.
Measure the angles from all these signals. To accomplish this, it will be necessary that the Don’t Analyze Less Than values in the Setup Menu are set to zero per cent O.D., zero per cent I.D., and zero volts. If these values are not set correctly, then some defect signals may not be analyzed.
On a pad of paper prepare a Table of Values. This should appear similar to the Calibration Table which you can view by typing <C> from the Analyze Menu. Make columns for Per Cent Wall Loss, Channel 1, Channel 2, Channel 3, Channel 4, and Mixer 1. In the Per Cent Wall Loss column, as a minimum enter the values for zero per cent I.D., 100 per cent O.D., 80 per cent O.D., 60 per cent O.D., 40 per cent O.D., 20 per cent O.D., and zero per cent O.D. The remaining values will be obtained by measuring angles in the Analyze Menu. Type <A> to return to the Analyze Menu.
Select Channel 1 for Analysis. Type <F2> to begin playback from RAM. The defect analyzer will scan up to the first defect, which should be the hole. The angle of this signal will be displayed on the left on the sixth line in the Analyze Menu. Record this value under column Channel 1 for the value 100 per cent in the table you have prepared. Press <F1> and the system scans to the second defect. Record the angle for the 80 per cent defect in the appropriate place in the table. Press <F1> three more times and record the values from the 60, 40, and 20 per cent O.D. defects.
Select Channel 2 for analysis and type <F2> to begin analysis from the beginning. Record the values for the 100, 80, 60, 40, and 20 per cent outside defects and record these in Channel 2. In the same manner, record the values for Channel 3 and Mixer 1. Recording this information for Channel 4 is optional, as Channel 4 is too low of a frequency to be used for phase analysis of signals. Channel 4 is used as a confirmation channel to distinguish between defects and other anomalies, such as deposits.
Enter values in your table for zero per cent O.D. Do this by extrapolating the values for 40 and 20 per cent O.D. For example, if the angle for the 40 per cent O.D. defect was 120 degrees, and the angle for the 20 per cent outside defect was 130 degrees, then record the value of 140 degrees for zero per cent O.D. It may be slightly more accurate to use a slightly lower angle for this extrapolation. For example, increase the angle by only 80 per cent of the difference between the angle for the 20 and 40 per cent defects, and enter the value 138 for the zero per cent O.D. defect. Note that the value to be entered here is not critical because it only affects the analysis of shallow outside defects that are normally considered to be insignificant.
Values for zero per cent I.D. must be entered in the Calibration Table. It is recommended that zero per cent I.D. be set to approximately 20 per cent of the value for the 100 per cent O.D. defect. This value is not critical as it only is used for measuring shallow I.D. defects which are normally considered to be insignificant. It is also used to distinguish between shallow I.D. defects and dents.
Move to the Calibration Table by typing <C>. You may clear the existing table with the Clear Table function if you wish. In the Per Cent Wall Loss column, enter as a minimum the values you entered in this column on your note pad. Enter the numeric values for angles for all of the defects and zero per cent I.D. and O.D. indications. To do this, use the arrow keys to move the question mark cursor to the cell you wish to change. Enter the new number directly from the keyboard and then move the question mark cursor with the arrow keys to the next cell to be edited. It is acceptable to leave values for the 25, 50, and 75 per cent I.D. defects blank. If you have chosen not to enter values for Channel 4, then position the cursor over each of these cells in turn and press the <Delete> key to delete any values which may be in this Table at this time.
View this information graphically by typing the letter <V> . If you have incorrectly entered a value, it will probably show up in this graph by an irregular shape of the graph. Go back to the Calibration Table by typing a <Q> and then type an <A> to go back to the Analysis Menu. Calibration is now complete. Use the procedure given above to again save the Settings File together with the Calibration Table.
Normal values for the defect angles will range between 20 and 140 degrees.If any of the angle values are negative, then this indicates that some or all phase controls have been incorrectly set. If negative angle readings are received for Channel 3, the Absolute Channel, then change this phase control by 180 degrees. Then rescan the tube and use the new values which will now have positive angles for the Calibration Table.