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- since 1996 -

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Technical Discussions
William Blum
Consultant, Training, Level III Services
NDT Consulting Group Inc., USA, Joined Nov 2000, 92

William Blum

Consultant, Training, Level III Services
NDT Consulting Group Inc.,
USA,
Joined Nov 2000
92
05:13 Sep-12-2007
ET Frequency

What happens when you try to operate a 100 kHz to 500 kHz eddy current probe at a frequency well above or well below the designated probe frequency?


 
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Joe Buckley
Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT, United Kingdom, Joined Oct 1999, 528

Joe Buckley

Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT,
United Kingdom,
Joined Oct 1999
528
06:35 Sep-13-2007
Re: ET Frequency
----------- Start Original Message -----------
: What happens when you try to operate a 100 kHz to 500 kHz eddy current probe at a frequency well above or well below the designated probe frequency?
------------ End Original Message ------------

Normally (assuming a bridge circuit or similar) an eddy current probe is operated so that its impedance is roughly equivalent to the bridge impedance, If its operated outside this area two things may happen.

1. The Bridge circuit (instrument) may not balance - usually this can be fixed by reducing drive or pre-balance gain.

2. The sensitivity will be lower by a factor of approximately the actual to optimal impedance ratio.

This may or may not render the probe unusable - It depends on how marginal the application is to start with. The 'acceptable range' of 1/3x to 3x nominal frequency is a good rule of thumb- but I have used probes well outside this in some cases.

Joe




 
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John Hansen
John Hansen
06:39 Sep-13-2007
Re: ET Frequency
Depending on how far off this frequency range you are and what instrument you are using then you will find the sensitivity decreases and noise increases but it may be more than adequate for your requirements.

You will be suprised at how well it might work particularily if you are doing a low gain application like sorting.

A properly designed instrument should be able to do this with no harm to the circuitry.

It is always worth trying something out.

ATB

John Hansen

----------- Start Original Message -----------
: What happens when you try to operate a 100 kHz to 500 kHz eddy current probe at a frequency well above or well below the designated probe frequency?
------------ End Original Message ------------




 
 Reply 
 
Tom Bipes
NDT Inspector
Progress Energy, USA, Joined Sep 2007, 1

Tom Bipes

NDT Inspector
Progress Energy,
USA,
Joined Sep 2007
1
04:20 Sep-18-2007
Re: ET Frequency
----------- Start Original Message -----------
: What happens when you try to operate a 100 kHz to 500 kHz eddy current probe at a frequency well above or well below the designated probe frequency?
------------ End Original Message ------------

Each probe has a frequency bandwidth, of which some are wide, some are narrow. If you run outside of the bandwidth, you will have decreased amplitude and increased noise. As mentioned earlier, you may have to experiment with what frequencies will work for your application. You may be able to sacrifice sensitivity if you are only looking for gross changes.


 
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