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Impedance Ratios for Ultrasonic Thickness Testing

Impedance Ratios for Ultrasonic Thickness Testing

Impedance Ratios for Ultrasonic Thickness Testing

(OP)
Is there a general rule of thumb for impedance ratios of different materials for UT?  My question is if I have a UT meter that is calibrated to a carbon steel cal block, can I expect reasonable results if the material to be examined is copper?  The impedance ratio of steel to copper is 1.4.

Thank you for any input, it is appreciated.

RE: Impedance Ratios for Ultrasonic Thickness Testing

Adjust your meter for the velocity of sound through copper, table of materials below, copper 4700 m/s. Unfortunately if you are sizing flaws, you are going to need a calibration block of copper with known indications of a certain size, becasue as you mention the impendence ratio is different for copper (different size/structure grains), and the same flaw in these materials won't show exactly the same response on the meter.

Material        Metres/second
Air        330
Aluminium        6300
Alumina Oxide        9900
Beryllium        12900
Boron Carbide        11000
Brass        4300
Cadmium        2800
Copper        4700
Glass(crown)        5300
Glycerine        1900
Gold        3200
Ice        4000
Inconel        5700
Iron        5900
Iron (cast)        4600
Lead        2200
Magnesium        5800
Mercury        1400
Molybdenum        6300
Monel        5400
Neoprene        1600
Nickel        5600
Nylon, 6.6        2600
Oil (SAE 30)        1700
Platinum        3300
Plexiglas        1700
Polyethylene        1900
Polystyrene        2400
Polyurethane        1900
Quartz        5800
Rubber, Butyl        1800
Silver        3600
Steel, Mild        5920
Steel, Stainless        5800
Teflon        1400
Tin        3300
Titanium        6100
Tungsten        5200
Uranium        3400
Water        1480
Zinc        4200
 

RE: Impedance Ratios for Ultrasonic Thickness Testing

(OP)
Thank you very much; however, would thickness measurements be adversely affected (and to what degree) if the unit was calibrated to steel?  

For example, would a thickness reading on the UT meter of the copper piping at 0.120 inches be somewhat close, or just way off?  The relative flaw size makes perfect sense though.

RE: Impedance Ratios for Ultrasonic Thickness Testing

Not if you can adjust your meter for the velocity in copper (which even the most basic meters can do) and take the reading. If your meter was functining perfectly, you would get a correct reading, although you cannot use the steel block anymore to calibrate. If you have a piece of copper, you can measure the thickness with a ruler and use this as a calibration piece.

RE: Impedance Ratios for Ultrasonic Thickness Testing

(OP)
Thank you,

That was my concern, the meter was calibrated to steel then used to measure thickness for copper (unfortunately, this already occurred).     

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