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Technical Discussions
Steven
China, Joined Sep 2016, 1

Steven

China,
Joined Sep 2016
1
17:30 Sep-10-2016
Pulse Rise Time

Recently I sent a ultrasonic thickness gage for annual calibration. It's reported that the equipment was failed on the parameter called "rise time". The Rise Time was measured to be excess 2 times of required value, but the thickness measurement accuracy result was PASS. Logically, as a user we only concern whether the measurement accuracy of the equipment is well maintain or not. Hence I would like to enquire experts here how the parameter Rise Time affect the thickness gage performance? What is the potential risk to the measurement if that Rise Time parameter not correct immediately.

Thanks all.

 
 Reply 
 
Daniel Braun
Daniel Braun
08:03 Sep-11-2016
Re: Pulse Rise Time
In Reply to Steven at 17:30 Sep-10-2016 (Opening).

Every ultrasonic instrument is an electronic device and its parameters should match with the nominal values declared by the manufacturer - the annual verification is performed in order to ensure the said matching or finding the deviations. Beside the instrument there are other factors involved into getting the instrument readings: the probe, the operator's skills, etc. The rise time is the parameter characterizing the quality of the pulser, in some cases the deviation of the said parameter from the nominal value declared by the manufacturer may affect the precision of the thickness measurements, especially for the thin materials; but sometimes - not - that was your observation: probably the actual rise time that was found for your instrument has no sensible influence while working with the probe that you used

 
 Reply 
 
Linas Svilainis
R & D,
Kaunas University of Technology, Lithuania, Joined Nov 1998, 67

Linas Svilainis

R & D,
Kaunas University of Technology,
Lithuania,
Joined Nov 1998
67
21:51 Sep-11-2016
Re: Pulse Rise Time
In Reply to Steven at 17:30 Sep-10-2016 (Opening).

Pulse rise time defines max frequency achievable by pulser:
1/pi*tR

Simply speaking, if maximum operating frequency is fmax, then rise time should be 1/3 of 1/fmax (actually it's fall time, refer Fig.11 of EN 12668-1).
Example: if max 5MHz probe is used tR should not exceed 60ns. At least.
If not, excitation signal bandwidth does not match the probe used. SNR is reduced, effective bandwidth is reduced and measurement errors increase (see Cramer-Rao lower error bound).

Actually the steeper tR, the better, but EN 12668-1 requirement is plain simple: "pulse rise time tr shall be less than the maximum value quoted in the manufacturer's technical specification". Check the device specification.

 
 Reply 
 

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