Dear all,
This is a summary on PFG safety limits. I apologize for the delay posting it.
And Happy New Year to everyone!
Thanks: Iain Day, David VanderVelde, Rajan Paranji, Christine
Hofstetter, Clemens Anklin, Andy Soper
Contents:
(1) Varian
(1.1) Limits (no responses yet)
(1.2) Fuses - check them :)
(1.3) Software protection.
(2) Bruker.
----------------------------
(1) Varian:
(1.1) I still didn't find a direct answer on what is the highest safe level
and duration of PFG at a set duty cycle. Probably section 1.3 can give
some hints,
but I also got impression from some previous conversations that software
is not guaranteed to catch unsafe PFG pulsing.
(1.2) According to Christine Hofstetter, newer Inova consoles are
safeguarded by 1.25A slow-blow fuses, older ones may have 5A slow-blow
fuses installed which WILL NOT protect the pfg coils. I've checked our
PFG module - indeed we have 1.25A sb fuses.
Slow blow fuses are tolerant to short spikes in current above the
specification -
that is key because PFG most often operate in that way - short intense
pulses of
current interleaved with longer delays.
If anyone is curious what markings are on fuses installed in our
console, please take a look here:
http://nmrwiki.org/wiki/index.php?title=UCI800:Maintenance_documents#Fuses
(1.3) Software protection.
There is a routine called testgrad()
/vnmr/psg/miscfuncs.c
which limits maximum pfg strength pulse to 10ms at 2% duty cycle.
The routine does not seem to account for heat dissipation during pulses,
it only substracts (time*LOSSPERSEC) during delays.
If anyone has ideas on what is maximum tolerable RMS pfg strength
for Varian probes - please let me know.
Again - someone suggested to me that software is not guaranteed to
catch unsafe PFG pulsing. Had anyone had experience of damaging PFG coils
where software did not issue warning? Please let me know - I'll
repost an update.
(2) Bruker:
According to Clemens Anklin:
model: Bruker room temp probe any type
max gz (G/cm): > 50 (typ 65)
gztime at max strength (ms): 20 ms _at_ 2% duty cycle
max gx/y (G/cm): > 35 (typ 40) gxytime at max strength (ms): 20 ms _at_
2% duty cycle
can go DC at 10% (~6 G/cm on Z)
model: Bruker cryo probe any type
max gz (G/cm): > 50 (typ 65)
gztime at max strength (ms): 10 ms
max gx/y (G/cm): NA
gxytime at max strength (ms): NA
--
Evgeny Fadeev, Ph.D.
Director, BioMolecular Spectroscopy Facility
1212 Natural Sciences 1
University of California Irvine
Irvine, CA 92697
telephone: 949-824-5842
www.physics.uci.edu/~biomolenmr
nmrwiki.org
Received on Fri Dec 26 2008 - 16:05:11 MST