Re: PCB's in liquid nitrogen

On May 30, 4:05 am, Okkim Atnarivik <Okkim.Atnari...@xxxxxxxxxxxxx>
wrote:
TheQuickBrownFox <thequickbrown...@xxxxxxxxxxxxxxxxxx> wrote:

:   That's why the right way is with an environmental chamber.

  I mostly do LHe work and there I just leave the circuit to the
top of the dewar neck and let it warm up before opening the neck flange.
The circuit gets flushed by evaporated, warmed-up dry helium.

  With LN (there is typically no flanged neck in LN dewars) you can
just quickly slip the circuit into a plastic bag and let it warm up
there. Some people like to evacuate the bag or fill it with dry
nitrogen, but IMO it is enough to just squeeze out most of the air.
The circuit catches a small amount of moisture anyway when moved from
dewar to the bag, but most of the frost (when kept in open air) comes
from the unrestricted airflow that goes by the circuit and deposits
its moisture on the cold surface.

  Regards,
            Mikko

Thanks Mikko, I'm definitely concerned about moisture, mostly leaking
into the probe when it’s inside the nitrogen dewar. The probe will
have at least one hole in the bottom. (I don’t want to make a
nitrogen bomb!) And will spend part of the time in the vapor above
the liquid. The current idea is to try to make a ‘soft’ seal around
the top of the nitrogen dewar and control the nitrogen boil off such
that no water can leak in. The fall back idea is to use a Plexiglas
tube, sealed at the top to the probe and open at the bottom, which
extends down into the liquid. (again sealing the probes environment)
Either way you’ll be able to lift the probe up apply heat and let it
warm up dry. (and maybe 1/2 of the people will follow our
instructions.) I like the plastic bag idea! In the past I’ve always
used probes that were vacuum tight with helium exchange gas.

George Herold
.

Relevant Pages

  • Re: Newbie - water detector circuit help please
    ... >>As the storage tank gets warm, any surface water will ... > be used for the latch circuit that stays on after being tripped. ... >>probe registers moisture another led on the box will be lit. ... supply for better sensor sensitivity and operation (74HC93 or 74HC132 ...
    (sci.electronics.basics)
  • Re: Hardening a simple op-amp circuit against EMI
    ... bandwidth to avoid EMI issues (circuit only actually needs kHz'ish ...
    (sci.electronics.design)
  • Re: E-field probe question
    ... on the charging part of the circuit. ... Rather, that probe capacitance ... diode capacitance is a moving target and even its average is ...
    (rec.radio.amateur.antenna)
  • Re: MOSFET switching of DC motor at 50V makes isolated 5V ground unstable...
    ... measurements you reported were reasonably accurate. ... > gator clip on the probe, I use a separate cable that plugs into the ... hookup leads soldered directly to the circuit board. ... Please describe how the signals physically get from the optos on ...
    (sci.electronics.design)
  • Re: Technical SNAFUs at last nights gig. Help? (long)
    ...  Are you running lights and such as well? ...   Takes a second ... Never a tripped circuit with this outfit, in fact before we added "extra" ... isn't excessive, line voltage is just low, resulting in the inability of the ...
    (alt.guitar.bass)