So the chamber itself is actually taken through a "loop" around the critical
point. And this also indicates how it could be possible for particular
problems to occur in certain laboratory settings. If the ambient
laboratory temperature is too high, the
CO2 is always a vapor and never really resides
as a liquid. The way to "solve" this particular problem is to run cold
water through the unit, to cool it down, to ensure that at least, at the
beginning, the CO2 is going to be a liquid
and not a vapor.
The bottom line:
The units have essentially no moving parts and nothing really ever goes
wrong except for the occasional replacement of "O" rings and seals as with
any system of this type. The most demanding of researchers can manually (e.
g. carefully and precisely) control the inlet and outlet valves, which means
one can also control precisely the substitution of the dehydration agent
with the chamber completely closed. The temperature is controlled with
ordinary (warm) tap water, controlled with an ordinary thermostat-mixer.
The ultra-large viewing chamber permits easy viewing of everything one might
want to see while this is all going on, for example, confirming the absence
of turbulence, and also, confirming the level of the CO2 when flushing the
chamber. With an all-automatic operation of such a system, one loses the
all-important hands-on opportunity to precisely control what is going on.
And the risk of chamber overheating (and over pressurization) is greatly
reduced because of the temperature limitation imposed by the use of hot
water (instead of an electrically heated heating element). So the real
"bottom line" is that one can certainly pay much more for a CPD, in some
instances, much more, and for automatic operation that many don't use (for
the above reasons) but none will ever do a better job in the preparation of
critical samples for SEM than either of these two time-tested SPI units.
Comments about aggressive solvents:
The last few years have seen a dramatic increase in the use of the SPI-DRY
Critical Point Dryers in non-life science applications, such as for aerogel
and MEMS applications and this usually means the use also of aggressive
solvents, or at least solvents more vigorous as solvents than the otherwise
normally used CO2 and alcohol. The SPI
Critical Point Dryers are designed to withstand such solvents however,
one should be aware of special maintenance considerations.
This information might even serve as a worthwhile "heads up" for life science users as well.
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