Author Topic: Flow Proportional Counter Backflow Gas Regulation  (Read 4789 times)

Brian Joy

  • Professor
  • ****
  • Posts: 147
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #15 on: August 31, 2018, 08:02:57 am »
For the past ten days I’ve been collecting data on P-10 gas flow counter anode bias as a function of atmospheric pressure; I’ve used the pressure recorded hourly at the Kingston airport (93 m AMSL).  I’ve measured Si Kα on wollastonite while keeping the count rate at 5000 s-1 and adjusting the bias until I get the PHA distribution centered at 4 V.  I’ve done this in the PC-EPMA “base level” window using a step of 0.1 V and dwell time of 1 s.



Barometric pressure (corrected/adjusted?) averages about 100.5 kPa and usually stays within about 1 kPa of this value.  However, during intense storms in the fall, winter, and spring, it can range as low as ~97.5 kPa.  On the opposite end, after passage of a cold front, the pressure can rise as high as ~103.5 kPa.

I’m going to keep adding data to this plot.  I’m curious to see how much change occurs when barometric pressure is at more extreme values.
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Mike Matthews

  • Professor
  • ****
  • Posts: 110
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #16 on: August 31, 2018, 08:59:24 am »
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.

Brian Joy

  • Professor
  • ****
  • Posts: 147
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #17 on: August 31, 2018, 09:43:28 am »
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
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Probeman

  • Emeritus
  • *****
  • Posts: 1955
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #18 on: August 31, 2018, 12:08:05 pm »
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!
The only stupid question is the one not asked!

DavidAdams

  • Professor
  • ****
  • Posts: 27
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #19 on: September 04, 2018, 07:42:06 am »
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

  • Professor
  • ****
  • Posts: 147
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #20 on: October 13, 2018, 01:52:47 pm »
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.


Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Probeman

  • Emeritus
  • *****
  • Posts: 1955
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #21 on: October 13, 2018, 02:35:16 pm »
Hey, it's a 1 million dollar barometer!   ;)
The only stupid question is the one not asked!

Brian Joy

  • Professor
  • ****
  • Posts: 147
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #22 on: October 23, 2018, 11:09:00 am »
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!

Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Mike Matthews

  • Professor
  • ****
  • Posts: 110
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #23 on: October 23, 2018, 12:32:07 pm »
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 :P

JonF

  • Professor
  • ****
  • Posts: 38
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #24 on: October 23, 2018, 12:55:25 pm »
I wonder what the mechanism is: water vapour acting as a quench gas, perhaps?

Brian Joy

  • Professor
  • ****
  • Posts: 147
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #25 on: October 23, 2018, 01:05:42 pm »
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.
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Probeman

  • Emeritus
  • *****
  • Posts: 1955
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #26 on: October 23, 2018, 01:25:53 pm »
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
The only stupid question is the one not asked!

Brian Joy

  • Professor
  • ****
  • Posts: 147
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #27 on: October 23, 2018, 01:37:00 pm »
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?
Brian Joy
Queen's University
Kingston, Ontario
JEOL JXA-8230

Karsten Goemann

  • Global Moderator
  • Professor
  • *****
  • Posts: 194
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #28 on: October 23, 2018, 04:09:13 pm »
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

  • Emeritus
  • *****
  • Posts: 1955
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #29 on: October 23, 2018, 04:19:24 pm »
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
The only stupid question is the one not asked!