Flow Proportional Counter Backflow Gas Regulation

[Brian Joy]

Excellent plot Brian, mind if I use it in my WDS lecture?

This is one of the reasons why the SDD-WDS that Ken Moran and Ric Wuhrer have been developing in Australia is a good idea - no pressure sensitivity.

Hi Mike,

Feel free to use it as you wish.  It should look much more interesting in a few months.

Brian

[Probeman]

This is one of the reasons why the SDD-WDS that Ken Moran and Ric Wuhrer have been developing in Australia is a good idea - no pressure sensitivity.

And no more lugging around of P-10 gas cylinders!

[DavidAdams]

Thanks, Brian! That's a great plot. I'm going to start doing the same thing on my instruments too. I'll be interesting to compare what you're seeing to what I see. I'm hoping to buy a back-flow regulator too and try it out on one my my instruments to see if that'll have a positive effect.

Dave

[Brian Joy]

I’ve continued to add data to my plot of GFPC bias versus atmospheric pressure.  Lately we’ve had some larger oscillations in pressure as is typical of the non-summer months.  I’m curious to know if other JEOL users who have gas flow counters arranged in series see the same pattern that I do:  the spectrometer that serves as the P-10 exhaust 1) requires higher counter anode bias and 2) shows a smaller slope on the plot of bias versus pressure than the one into which the P-10 enters.

[Probeman]

Hey, it's a 1 million dollar barometer!   

[Brian Joy]

I’ve been continuing to record GFPC anode bias as a function of atmospheric pressure, and it’s becoming apparent that another variable is influencing the bias necessary to keep the PHA distribution centered at 4 V for Si Kα count rate = 5000 s^-1.  During September and early October -- up through October 11th -- the outside temperature oscillated between summer- and fall-like values but was generally above 15˚C and was often above 20˚C.  Since October 12th, though, the temperature has been no higher than 15˚C.  Even though the outdoor relative humidity has been high at certain points since then, dew point temperatures have been much lower than they were before the 12th.  Apparently due to the lower absolute water content of the air, the bias required to keep the distribution as specified above is now noticeably lower for a given value of atmospheric pressure.  In the plot below, I’ve separated the values I obtained prior to the 12th from those I collected after the 12th.  Note that the effect of atmospheric water is more pronounced for the spectrometer that serves as the P-10 exhaust.  Perhaps some of the scatter in the late summer values is due to variation in dew point temperature, though I haven’t checked this.

So the gas-flow counters appear to be both barometers and hygrometers!

[Mike Matthews]

I can understand the pressure sensitivity, but humidity’s a surprise.

If you use tight pha windows they make fantastically sensitive room thermometers too. Four detectors for the price of one

[JonF]

I wonder what the mechanism is: water vapour acting as a quench gas, perhaps?

[Brian Joy]

But if the water were behaving as a quench gas, then wouldn't I be getting lower bias values rather than higher ones as air water content increases?  Since H₂O is a strongly polar molecule, maybe it's deflecting the paths of electrons as they approach the anode and is thus reducing the voltage drop at the anode?

To me the variation in counter behavior with room air water content makes sense, as the composition of the counter gas is changing.  Note that channel 1 is more isolated from the atmosphere than channel 4, but both spectrometers are subject to the same variation in room temperature, which does range to a couple °C higher in the summer.

[Probeman]

I'm missing something here.

I can see how barometric pressure can affect the detector response simply due to the change in detector gas density, but how can room humidity affect the detector?

I mean the detector gas is coming from the P-10 bottle and constantly flowing so there should be no atmospheric gases getting into the detector.  Right?
john

[Brian Joy]

Maybe I'm wrong, but doesn't the difference in behavior between channels 1 and 4 during both warm and cool weather suggest that atmospheric gases are getting into the counters?  If the effect were due just to increase in counter gas density, wouldn't both spectrometers behave essentially the same?

[Karsten Goemann]

Awesome stuff. If the humidity in the room is substantial I can see some water vapour diffusing into the detectors against the gas flow, even though it should be constantly purged out again by the P10? But there would be a strong concentration gradient for the water vapour. It might be possible to calculate this from thermodynamics? Water molecules are very effective for charge compensation in variable pressure/environmental SEM. Any water in the detector will probably have some effect.

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

[Probeman]

If the humidity in the room is substantial I can see some water vapour diffusing into the detectors against the gas flow, even though it should be constantly purged out again by the P10?

Hi Karsten,
That is exactly my question.  How could there be air getting in if it's constantly flowing?  Since the gas is flowing, the lines should be very slightly higher than the ambient pressure just due to the resistance to flow by a small diameter long distance line.

Could the room humidity instead be affecting the bias electronics slightly?  Can the bias voltage be monitored?
john

[Karsten Goemann]

Well the P10 flow is very low (around 1ml/min). One could try using a longer exhaust tube and see if the issue becomes less apparent. But if water vapour can diffuse against the flow due to the concentration gradient the same should be true for air...

I was wondering as well if the humidity could affect the detector electronics. But then the effect should also be visible for the Xe detectors, right?

Cheers,
Karsten

[Brian Joy]

One could try using a longer exhaust tube and see if the issue becomes less apparent.

I have ~6.7 m of spare tubing.  I'm going to attach it to the channel 4 P-10 exhaust and resume making measurements.  Currently the length of tubing attached to the exhaust is only 30 or 40 cm (not including the length inside the spectrometer).