Flow Proportional Counter Backflow Gas Regulation

[Brian Joy]

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.

[dawncruth]

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]

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

[Brian Joy]

Below is a more or less final version of my plot of GFPC anode bias versus atmospheric pressure:



In contrast to previous versions of the plot, I’ve now contoured it (by hand/eye) for dew point temperature, and I’ve also color-coded the data.  The curves that I’ve drawn on the plot are given by the following equations, which work well for interpolation:

Channel 1:  bias [V] = 7 V/kPa * P_atm [kPa] + 0.52 V/°C * T_dew [°C] + 909.4 V
Channel 4:  bias [V] = 7 V/kPa * P_atm [kPa] + 0.76 V/°C * T_dew [°C] + 929.7 V

Some unexplained scatter is still present on the plot, and so additional variables are likely significant in addition to atmospheric pressure and dew point temperature.  For instance, I don’t really have a good handle on time required for equilibration, which obviously would be particularly important when atmospheric conditions are changing rapidly.

When determining the appropriate anode bias, I should note that I used the JEOL “base level” scan rather than the “high voltage” scan and then adjusted the bias in 2 V increments until I got a distribution centered at/near 4 V.  I made the final scan using a step of 0.1 V and dwell time of 1 s; generally I repeated the slow scan at least once in order to assure reproducibility.  This is a tedious process, but it produces better results than the “high voltage” scan, which tends to have a broad, gently sloping “peak.”



So I guess now I need to do something about this problem.  I’m reluctant to add liquid to the bubbler at the exhaust, as I worry about backstreaming in the event of counter window failure.

[DavidAdams]

Wow, Brian!! This is a fantastic data set! I'm really happy that you took the time to do such intensive testing and compile all this. This definitely shows VERY similar behaviour to what I have observed for a lot of years. As to a fix, I'm still scratching my head on that myself.

I don't think you should be too concerned about backstreaming. I've had fluid in all of my instruments and have also had window failures. I've never had any fluid pulled back into the system. The key seems to be to only fill the bubbler to just where the bubbles start and no more.

-dave

[Brian Joy]

I've attached a copy of the measurements in an Excel file in case anyone wants to examine them more carefully and/or plot them differently.