13.1 Angle probes, initial checks and calibration
Before we can start to use an angle probe we need to find out something about it. For instance:
a Where is the sound coming out of the perspex shoe?
b Is it the angle that it is supposed to be?
c Has the angle changed since it was last used?
So we must check the probe before we can calibrate the time base to enable its use. There are also a number of other performance checks which should be carried out at specified intervals.
13.1.1 Finding the probe index
The point at which the centre of the beam leaves the probe and enters the test material is called the probe index or emission point. It should be marked on each side of the probe and checked regularly. As the probe surface wears down, the probe index can change. Stand-off measurements are taken from the probe index and are used to check the probe angle, another check that the UT technician must perform regularly, so it is the master reference point or datum.
To find the probe index place the probe on a V1 block and obtain an echo from the 100mm radius and establish at more than 50%FSH using the gain control. Maximise the echo by moving the probe backwards and forwards.
Mark a line on each side of the probe directly above the slots which indicate the centre of the 100mm radius. This is the probe index, where the axis of the beam leaves the perspex shoe.
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13.2 Checking the probe angle
For a 45 or 60° probe place it on the V1 block approximately adjacent to where the appropriate angle is inscribed, and directed at the plastic insert.
Obtain a signal on the screen from the plastic insert and maximise it. Find the position where the probe index coincides with the angle indicated on the side of the V1 block and this will tell you the probe angle.
This procedure can be repeated for a 70° probe but reflecting the energy from the plastic insert radius is unreliable. Therefore we suggest you use the 1.5mm hole as a target.
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13.3 Calibration of shear waves for range with the V1 block
By range in angle probe testing we mean the distance a reflector is from the probe index.
It is possible on some flaw detectors to calibrate the time base to 100mm range from the V1 block. However, this involves delaying the signal by 100mm and not all equipment can do this on the appropriate scale expansion setting, so we will confine ourselves to calibrating for 200mm full screen width.
Place the probe on the V1 block and obtain boundary echo from the 100mm radius. Establish this signal to more than 50%FSH using the gain control. Further maximise the echo by moving the probe backwards and forwards. Hold the probe stationary.
Wind in or out on the scale expansion/range control to establish a second boundary echo at 200mm range.
Place the signal from 100mm at 5 (half-scale) on the time base and the one from 200mm at 10 (full scale), using the delay and range controls. The time base is calibrated for 200mm. Longer ranges can be catered for in multiples of 100mm.
However, the V1 block is bulky, not convenient for site work and it is not always possible to calibrate for 100mm, so we tend to use the V2 block.
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13.4 Calibration of shear waves for range with the V2 block
The V2 block is the most convenient calibration block to use with angle probes. It has two arcs, at 25 and at 50mm.
13.4.1 Calibration for 100mm
Place the probe on the block and point it at the 25mm arc. Adjust the delay and range controls until you have two signals on the screen, the first will represent 25mm and the second will represent 100mm. Maximise the signals by sliding the probe forward and backward. Adjust range and delay until the first echo comes a quarter of the way across the screen at 2.5 and the second echo comes at the extreme edge of the screen on the righthand side at 10.
The time base now represents 100mm. Check it by turning the probe round and pointing it at the 50mm arc. If you have calibrated correctly, the signal when maximised will come up exactly in the middle of the screen at 5.
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13.4.2 Calibration for 200mm
Point the probe at the 50mm arc on the V2 block and obtain three echoes on the screen. These represent 50, 125 and 200mm. Maximise these signals by sliding the probe forward and backward. Adjust the range and delay until the first signal comes a quarter of the way across the screen at 2.5 and the third echo comes at the extreme edge of the screen at 10.
13.4.3 Calibration for 250mm
Point the probe at the 25mm radius arc on a V2 block and adjust the set until you get four echoes. These represent 25, 100, 175 and 250mm.
Maximise these signals by sliding the probe forward and backward. Adjust range and delay until the first echo comes one tenth of the way across the screen at 1 and the fourth echo comes at the extreme edge of screen at 10. Check on a V1 block. On the 100mm arc you should get one echo 4/10 across the screen and the other 8/10 across the screen.
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