Re: Tritium Bubbler

[FRAMEP@ORAU.GOV]

Bob Shannon wrote :

>Does anyone have information or experience regarding HTO sampling
>using silica
>gel versus bubblers ?

Bob, I've never used silica gel, but I have run molecular sieve  alongside
bubblers with ethylene glycol and obtained exactly the same results. 

To some extent, which approach you prefer (absorber vs adsorber)
depends on whether you are managing the samplers or doing the
analysis.  The bubblers are easily broken and more prone to leaking than
an adsorber cartridge but the analysis is much easier - just transfer a
few mls into a cocktail.

An adsorber like silica gel or molecular sieve should buy you a little
sensitivity over the bubbler..

WRT silica gel vs molecular sieve.

The general theory is that silica gel might be better in high humidities
because its adsorptive capacity is greater at high humidities: 0.26 and
0.18 for silica gel and molecular sieve respectively at 50% humidity. But
at lower humidities(10%) the respective capacities are 0.05 and 0.15.

You can get indicating silica gel to provide early warning that the
capacity is being reached. But a potentially big problem with the stuff is
that it is almost impossible to completely dry - if it is being reused, there
can be carry over. This is not a problem with molecular sieve.

The silica gel (or molecular sieve) method for collecting HTO is described
in detail in ASTM D 3442 "Tritium Content in Air" The latter recommends
flow rates of 0.1 to 0.5 lpm for one week using a glass or metal cartridge
2-5 cm in diameter and 25-35 cm long.  Most folks I've seen using silica
gel run with shorter tubes - the longer narrower tubes though should
provide more uniform air flow. Also run them with the long axis of the
cartridge running vertically - channeling is less likely to be a problem.

Best wishes

Paul Frame
Professional Training programs
ORISE
framep@orau.gov
http://www.orau.gov/ptp/ptp.htm