Dear All
I thought I will share this recent experience in attempting to
measure He gas flow electronically at the outlet of NMR magnets and
what I found subsequently. Eager to hear your thoughts, of course...
I chose a product by a reputed company called Sensirion (pl. Google for
link). I quote from the manual about the principle of operation: "
The sensor has a heating resistor on a thermally insulated membrane,
which is kept above ambient temperature. In the presence of gas flow,
the temperature distribution up- and downstream is disturbed. This
asymmetry is then measured". Evidently this thermal gradient
measurement is converted into a volume transport by "some" means.
They supply sensors that can measure mass and/or volume flow of
gases like N2 or Air and "for other gases upon request for bulk orders
i.e. > 100 units, I presume" and each piece is a bout $350 a pop. So, I
had to figure out calibrating the sensor myself for He flow
measurement. They are pre-calibrated for N2/Air. The only thing
company could tell me was that, to convert the measured value (in milli
liters/min) to a value applicable to He gas, multiply the number by (Cp
of Air / Cp of He). Cp is the specific heat capacity at constant
pressure i.e. 1 bar pressure in our case.
The sensor has an intelligent chip with EEPROM and a simple RS232
interface thro' which I can make a script talk to the critter and spit
out values of flow in a stream. Nice scheme for automation....
Results: I used a primary calibration standard like a ball gauge flow
meter and tested with Air. With Air, things look ok. With He gas
though, the numbers I get (in milli liters/min) are (a) far from what
my primary calibration standard displays (b) the values don't have a
linear correlation with the input flow rate (I used a He gas bottle used
for liq. He fills for the calibration process). If I try to put in 2000
ml/min, I get a way small number i.e. 80 ml/min but when I put it
something like 15000 ml/min, I get about 2100 ml/min.
I conclude with two points :
(1) If you are thinking of using an Air or N2 flow electronic sensor
like this for NMR magnet He flow mesurement, save yourself
$350+shipping and just make sure that it is either pre-calibrated for
Helium (OR) wait for some enlightenment to arise for this post from
AMMRL before jumping into it !
(2) If you are already 'enlightened' on this issue in the past, pray,
please lead the rest of us ignorants to light ! Notwithstanding the
negative results, if I get to know what is it about Helium gas that
makes this sensor's measurement process useless, my efforts will be
worth it.
Thank you once again.
Regards
Rajan
*_______________________________
Rajan K Paranji, Ph.D.
*NMR Facility Manager
Department of Chemistry
Room 65, Bagley Hall
University of Washington
*Seattle, WA 98195*
*ph: 206 685 2581 pager:206 680 3779
fax: 206 685 8665
email: paranji_at_chem.washington.edu
___________________________________*
Received on Fri Oct 12 2007 - 14:40:54 MST