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Technical Discussions
Terry White
Terry White
04:29 Dec-17-2002
U/T rep rate

We test nickel alloy billet material using basic pulse echo techniques. our method spec calls for a minimum pulse repetition rate of 500PPS. We often experience "wraparound" at this rate and have to lower the rep rate to eliminate the unwanted signals. My question is, why is it so important to maintain that rep rate? What harm could dropping it to 450 - 400 PPS do? Max surface scanning speed is 10 lin. inches per/sec. Transducer is 10 or 5MHz. Beamwidth as measured at .150" metal travel on a #1 FBH cal block is @ .040".

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Udo Schlengermann
Consultant, -
Standards Consulting, Germany, Joined Nov 1998, 183

Udo Schlengermann

Consultant, -
Standards Consulting,
Germany,
Joined Nov 1998
183
06:46 Dec-17-2002
Re: U/T rep rate
reply:
Hello Terry,

If your scanning speed v is 10"/s and your beam width b is only 0.040", i.e. per shot you cover only 0,040", you need a pulse repetition rate for complete coverage of
frep = v/b.
For your example that means frep = (10"/0.04"s)=250Hz at least.
If you want a factor n for redundancy, i.e. n pulses per beam diameter b, the equation goes to
frep = n v/b.
In your case n=2 means frep = 2 10"/.040"s) = 500 Hz.

You are correct, that this repetition frequency may be too high for low attenuating materials as nickel based alloys are.
The only solution then is to look for the maximum repetition rate fmax you can work with (static), and for scanning to lower the scanning speed until you get full coverage including a safety factor n:
vmax = (b fmax/n)
If you get fmax = 50 Hz, with n = 2 you will get the maximum allowable scanning speed vmax = (0.04" 50/2s)= = 1"/s only.
May be, for lower repetition rates you need a digital ultrasonic instrument, because they have separate screen refreshing rates independent from pulse repetion frequency, whereas with analog instruments the screen refreshing rate is the same as the pulse repetition frequency and for low rates you will not see a constant screen display.

Kind regards
Udo Schlengermann
Agfa NDT GmbH, Huerth, Germany
+++

: We test nickel alloy billet material using basic pulse echo techniques. our method spec calls for a minimum pulse repetition rate of 500PPS. We often experience "wraparound" at this rate and have to lower the rep rate to eliminate the unwanted signals. My question is, why is it so important to maintain that rep rate? What harm could dropping it to 450 - 400 PPS do? Max surface scanning speed is 10 lin. inches per/sec. Transducer is 10 or 5MHz. Beamwidth as measured at .150" metal travel on a #1 FBH cal block is @ .040".
.

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