T.O. 33B-1-1 5-81 5.5.3 Equipment and Materials. 5.5.3.1 Instruments. Some  instruments  are  designed  specifically  for  thickness  measurements.    However,  a  basic  ultrasonic  instrument  can also be used for thickness measurements.  Most instruments of both kinds have a digital display to provide the thickness being measured.  Detailed operating instructions shall be obtained by consulting the manual for the specific instrument being used. 5.5.3.2 Search Units. Search units recommended by the instrument manufacturer shall be used with the thickness measurement instruments. A  narrow  dead  zone  and  a  good  resolution  are  required  for  measurements  in  the  lower  ranges  (thin  materials). Therefore, dual search units and/or search units with plastic delay lines are used (see paragraphs 5.2.2.4 and 5.2.2.6.). Instruments dedicated to thickness measurements often are supplied with compatible search units.  With a dual search unit, the ringing of the transducer element is not detected by the instrument.  Therefore, received signals close to the initial  pulse  can  be  clearly  resolved.    A  plastic  delay  line  bonded  to  the  face  of  the  transducer  element  separates  the initial   pulse   from   the   front   surface   signal;   this   improves   near-surface   resolution   (shortens   the   dead   zone).      For measurements in the higher ranges (thick materials), a conventional straight beam contact search unit may be used. 5.5.3.3 Reference Standards. Reference standards are required to calibrate the instruments prior to the inspection.  This calibration is performed by the   instrument   operator;   calibration   by   laboratories   is   not   generally   required.      For   the   thickness   measurement instruments, check the operator's manual to see if one or two reference standards are required.  If two are required it is best  to  have  one  50-90%  of  the  nominal  thickness  to  be  measured  and  one  110-150%  of  the  nominal  thickness  to  be measured.  Only one reference standard is required when using a basic pulsed instrument for thickness measurement. Direct,  accurate  readings  of  thickness  can  be  obtained  only  when  the  acoustic  velocity  in  the  reference  standards  is equal  to  the  acoustic  velocity  in  the  test  part.    For  this  reason,  the  material  and  heat  treat  condition  of  the  reference standards should be the same as the test part.  If reference standards of a different material or heat treat condition is used;  the  resultant  thickness  readings  must  be  corrected  by  a  correlation  factor.    The  correlation  factor  may  be established in two ways: a. Use the ratio v v 2 1 when the velocities of the test part and reference standard are known. Where: v₂ = acoustic velocity in the test part material v₁ = acoustic velocity in the reference standard material Example: Assume the calibration blocks are made of 2014-T4 aluminum and the test part material is 410 stainless steel. v₂ = longitudinal wave velocity in 410 stainless steel = 2.91 x 10⁵ inches/sec from Table 5-2. v₁ = longitudinal wave velocity in 2014-T4 aluminum = 2.46 x 10⁵ inches/sec from Table 5-2 v v x x 2 1 5 5 29 1    1 0 2 1 0 1 1 8 =  =  = . . 4 6 . the correction factor. All readings on the test part are now multiplied by 1.18 to obtain the actual thickness. If a test part reading is 0.110 inch, correct this by multiplying by the correction factor: 0.110 inch x 1.18 = 0.130 inch = the actual test part thickness