HI Matt,
I have experienced valco valves loosing "calibration" and not rotating
completely. This resulted in stopped flow. The valve actuator can be
"recalibrated" and the instructions should be in the paperwork that came
with the valve. If I remember correctly, the procedure is simply toggling
the valve rapidly from position to position a number of times until it
senses the home position. I would imagine these instructions can be found on
the valco website or you could get more detail from a technician by phone.

Another thing that I have encountered that resulted in stopped flow was
blockage in the valco valve ports themselves. Depending on your type of
valve, some have micro volume ports that are easily plucked by small
particles (in my case a granule of Porapak). This requires dismantling the
valve and cleaning the post with a small wire (I used the cleaning wire for
a syringe). After reassembly you will need to do the home positioning
"recalibration" I mentioned above.

Hope this helps. Good luck. 

Brian Jones
Stable Isotope Ecology Lab
Texas A&M University
College Station, Tx. 77843-3146
979-845-8916
 
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Matt Black
Sent: Thursday, June 08, 2006 9:40 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Multiflow flow changes

Hello everyone,

I'm running into problems running our multiflow with soil headspace samples.
After about 10-15 samples the He flow through the sample loop drops
drastically (from 20ml/min to 7ml/min), and the column head pressure
increases from 17psi to >30psi. Thinking it may be a water issue, I plumbed
in an additional water trap after the nafion dryer to pick up any residual
water, and swapped in a new column - no improvement. Also tried swapping in
a shorter sample loop in the event there was a blockage of some sort.

Has anyone experienced issues with the valco 6-way vale not rotating
completely, resulting in the pressure increase and flow reduction that I am
describing? The 6-way appears to be rotating completely....

Any help would be appreciated.
Matt