Volume-II of IV - GAIL

MECON LIMITED
REGD. OFF: RANCHI (BIHAR)
PROCESS & PIPING
DESIGN SECTION
NEW DELHI
TITLE PIPING FABRICATION AND
ERECTION
1.6 Calibration
STANDARD SPECIFICATION
SPECIFICATION NO. PAGE 53 OF 62
MEC/S/05/62/06 REVISION 0
For a precise check of the sound paths necessary for a full inspection of the
weld joint, the probe shall be moved (half skip and full skip distance) until
internal and external notches on the test piece are detected (see Fig. 5 of this
specification).
The relevant defect limits the path lengths on the time base. The calibration of
reference sensitivity is obtained by utilising the through drilled test hole in the
thickness of the reference block to draw the distance amplitude correction curve
relevant to the test probe.
Calibration shall be carried out according to the following procedure : place its
internal vertex until the maximum height of echo is displayed on the screen; this
echo is adjusted to 80% of full screen height by means of the sensitivity adjuster
set in dB. Without varying the amplification, the probe placed at full skip distance
from the hole is moved to detect the external vertex the hole until the maximum
height of echo is obtained. The straight line connecting the peaks of the two
echoes obtained by the above procedure, represents the 100% reference level,
while the one connecting the two points at half height of the same echoes
represents "50% reference level".
The two straight lines shall be marked on the screen with a pen. Calibration
shallbe repeated each time tests are re-started at intervals not longer than 30
minutes during normal operations; each time the conditions fixed in advance are
altered. This calibration is applicable provided that the crystal of the probe is 8 x
9mm size. Should this size of the crystal be different, the value of the sensitivity
obtained from the calibration by a crystal of a different size shall be brought to
the value of sensitivity obtained from the calibration by a 8 x 9mm crystal. The
sensitivities of the two different size probes shall be compared through the
echoes obtained on the notch of the test piece with the probe position at half
skip of the distance.
1.9 Regulation of Amplification during Production Testing
The amplification during production testing shall be obtained by adding 2-6 dB
(according to the surface condition of the pipe and its cleanness) to the
reference amplification.