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I've found that one of our probes lost ~1.5-2 dB of power on 13C, but nutation curve looks OK. There are no random spikes in the intensity of the peak seen on arcing probe. So I assume there is no "serious" arcing.

Is it OK to continue using it with a bit longer 90 pulses and slightly higher power for decoupling till arcing becomes a real issue or have it serviced right away?


asked Jan 17 '10 at 15:06

Evgeny%20Fadeev's gravatar image

Evgeny Fadeev

updated Jan 17 '10 at 23:17

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If (1) the probe remains well "tuned and matched", that is, forward-to-reflected voltage (not power) greater than 10-to-1; (2) the forward voltage, measured at the probe connector, remains the same and (3) you haven't suddenly switched to lossy samples then it is hard to imagine just how this can all occur simultaneously.

What exactly do you mean when you write "...lost ~1.5-2 dB of power on 13C..."? Do you mean for the same incident voltage (not power - we measure voltage on an oscilloscope and there's often little reason to convert it to power) and pulse duration that the flip angle is smaller?

And what about the 1H decoupling? Do you know that you need more incident 1H voltage to get the same decoupling field strength? Or is this an assumption?

Now seriously, all arcing is serious. ;) Arcing (if it occurs under the conditions of the measurement) is immediately evident on a nutation plot. See for example an excellent article written by Paul Keifer (Application Scientist at Varian) on the reading of nutation plots at Concepts Magn Reson 11: 165-180, 1999 for more details on this and related issues.

Best regards,


answered Apr 19 '10 at 18:07

Joseph%20DiVerdi's gravatar image

Joseph DiVerdi

Thank you Joseph. I think I now understand why this happened - I forgot to remove directional coupler inserted before the probe! Maybe it will account for 2db. Will test when I come back from the ENC. - Evgeny Fadeev (Apr 20 '10 at 02:18)

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