Author Topic: Flow Proportional Counter Backflow Gas Regulation  (Read 3838 times)

Brian Joy

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Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #45 on: February 01, 2019, 08:43:14 am »
I’ve continued to add to my plot of GFPC anode bias versus atmospheric pressure over the past few months.  It appears that the addition of 22 feet of tubing to the channel 4 exhaust had little or no effect, and this is a little puzzling.  I expected the added tubing to at least reduce the amount of scatter present in the measurements on channel 4, but the magnitude of the scatter appears unchanged.



Clearly at least one other independent variable (in addition to atmospheric pressure) is important in order to account for scatter in the plot.  In the following plots, I’ve contoured indoor dew point temperature versus atmospheric pressure for anode bias in 4 V increments.  Although the plots would benefit from some more measurements, it seems clear that water content of the atmosphere affects the anode bias required to keep the distribution centered at 4 V (while maintaining count rate at 5000 s-1), even with the added tubing.  So I’m sticking to my claim that air is actually mixing with P-10 in the gas-flow counters.  On the plot above, as dew point decreases (to as low as ~-5°C in the past month) the anode bias for given atmospheric pressure also decreases.  (In the summer, dew point temperature ranges as high as ~+15°C.)





I’ll keep adding to these plots at least through next summer to see if my results from late last summer are reproducible.  By the way, I’m keeping track of dew point with the Lascar EL-USB-RT thermometer/hygrometer, which displays temperature and dew point in real time and also periodically dumps data to a file.  It can be gotten at Amazon for about $60 U.S.
« Last Edit: February 01, 2019, 08:59:26 am by Brian Joy »
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

dawncruth

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Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #46 on: August 29, 2019, 10:45:16 am »
Hey Karsten,
In October 2018 you said:

In any case, probably another reason to put something on the exhaust. I tried filling our bubbler with diffusion pump oil (to avoid the somewhat nasty dibutylphthalate) but it was way too viscous at room T. So now I'm trialling Alcatel 200 rotary pump oil, which seems to have the right sort of viscosity (with bubbler close to half filled 26 nicely shaped bubbles per minute, with P10 pressure regulator set at 16 kPa, flow regulator to around 1.15 ml/minute). From the specs it is hopefully fairly clean and long-term stable ("double distilled hydrocarbon fluid, low backstreaming ... strong oxidation resistance, ... for corrosive applications..."), so I'll see how that goes. It is a double chamber bubbler so even in the case of a detector window failure it shouldn't suck the fluid all the way back into the detectors (in theory, at least...). If anyone has a better idea what to use let me know. I'd still be interested in the back pressure regulator setup even at low altitude such as in our case.

Cheers,
Karsten

What was the outcome of your experiment? I'm going through the same process.
Dawn

Karsten Goemann

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Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #47 on: September 09, 2019, 11:13:42 pm »
Hi Dawn,

I still have the Alcatel oil in the bubbler and it seems very stable (no discolouring, no change in the level, constant bubble rates...). I don't have a dataset similar to what Brian has done to be able to verify if and how much it reduces drift in the bias settings. I'm hoping it does not only prevent air (with changing humidity) backstreaming into the detectors but also to have at least some backpressure regulating effect.

Cheers,
Karsten