Probe Software Users Forum

Hardware => JEOL => Topic started by: DavidAdams on July 19, 2018, 09:10:27 am

Title: Flow Proportional Counter Backflow Gas Regulation
Post by: DavidAdams on July 19, 2018, 09:10:27 am
Here at the USGS Denver Microbeam Lab we are in the process of having our new JEOL 8530F+ Microprobe installed (YAY!). Everything has been going smoothly up until a couple of days ago when the engineer was checking the performance of the WDS detectors. Without going into details, the GFP counters aren't really working the way they need to be.  Because of the altitude in Denver the atmospheric pressure is significantly different than at sea level and that has a very noticeable affect on X-ray detection using the GFP counters versus the sealed Xenon detectors. It screws with the gain and bias voltages and P/B ratios. Changes in barometric pressure here has a very noticeable effect on the PHAs on our system much more so than I have seen on other instruments at lower altitudes. This is something that we've noticed for years on our old 8900 and we've recognized it and just dealt with it, but with the new probe being installed I wanted to get people's opinions and thoughts on ways to eliminate or minimize the effect of altitude and barometric pressure on the GFP counters.

My questions are: has anyone with a JEOL probe attempted some form of back pressure regulation at the end of the P-10 gas line from the GFP counters? Is this even possible or feasible? Is there some why to safely do this without blowing the detector windows that are under vacuum?

Any input/ideas/help would be greatly appreciated!

Thanks,
Dave
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Owen Neill on July 19, 2018, 09:37:26 am
Hi Dave,

If you can hook the tube to a butyl phthalate bubbler, that will help a lot. The glass piece (usually two hollow, parralel advil-capsule-looking glass pieces with hose barbs on either end, about 2x2") usually comes with the probe, although the butyl phthalate won't. You can get 100ml on Sigma Aldrich for <$20, and that's more than you'll ever need.

If the don't give you the glass case anymore, you can make one with a couple of beakers, just make sure to have a second empty vessel in case the window pops so you don't backstream it.

Edit: Found a picture (photocredit, U. Utrecht): http://nanosims.geo.uu.nl/nanosims-wiki/lib/exe/fetch.php/eprobe:microprobe_april_2014.jpg
 (http://nanosims.geo.uu.nl/nanosims-wiki/lib/exe/fetch.php/eprobe:microprobe_april_2014.jpg). The bubbler I'm talking about is on the spectro on the left side (SP1 position), halfway between the black cap and the detector pre-amp. Connect the P-10 outlet hose from the last spectro to the barb on the left-hand glass tube, and fill the right-hand tube with butyl phthalate.

Good luck,
OKN
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: DavidAdams on July 19, 2018, 10:10:50 am
Hi Owen,

Thanks for the reply!

The dibuthyl phthalate in the bubbler is already something that I do on the the old 8900 probe and I also gave some to Ken for the new 8530 probe bubbler. Unfortunately, that does very little to curb the back pressure problems that I experience up here. I also experienced the atmospheric pressure effects when I was running the 8530F in Perth but the effect was less dramatic then what I'm seeing here in Denver.

Thanks again!
Dave
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman on July 19, 2018, 11:47:55 am
Hi Dave,
In Denver you are at roughly 85% sea level pressure, so I would expect that you can increase the back pressure by 15% and the detector windows should handle it fine.
john
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: DavidAdams on July 19, 2018, 12:02:30 pm
Hi John,

Thanks! That's definitely something I will try when JEOL hands over the 8530 to us. Hopefully that will be a simply easy fix if it works!!

Dave
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Owen Neill on July 19, 2018, 01:05:39 pm
Hi Dave,

OK, cool, and also interesting. The atmospheric effects I saw on the 8500 at WSU largely went away after I filled the bubbler, but Pullman is only ~2300ft ASL, so maybe not comparable. Maybe something simple like a de-humidifier for the lab?

Say hi to Ken for me,
OKN
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Karsten Goemann on August 15, 2018, 12:27:39 am
We got a bubbler from JEOL for our 8530F Plus as well.

I'm wondering why dibutyl phthalate is the fluid of choice (same for the bubblers on our Cameca SX100)? At least in Australia it is a class 9 dangerous substance, and the specified vapour pressure at room temperature is not particularly low either.

Wouldn't it be possible to use something like diffusion pump oil or even rotary pump oil? Many are non-hazardous, much lower vapour pressures. Santovac 5 is terribly expensive of course, but for a single small JEOL bubbler one wouldn't need much.

With the "clean vacuum system" of the newer generation machines, oil-free pumps, on-airlock plasma cleaners, cryo anticontamination etc, what would be the best bubbler fluid? Normally we probably wouldn't expect much back streaming, but if a counter window leaks or breaks...

Cheers,
Karsten

Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: D. on August 15, 2018, 05:31:54 am
Sorry to hijack, but I've always wondered why the FE instruments come with bubblers and the 8230's don't. ? . I assume there is no technical reason?

D.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: DavidAdams on August 17, 2018, 05:28:03 am
Karsten,

This is a good question and I return to my original post. If one put a back pressure regulator at the end of the gas flow chain between the last spectrometer and the bubbler I would guess that that would help solve any atmospheric pressure influences as well as help prevent any back streaming of any fluid (or anything else) in the bubbler in the event of a window failure.

D.

The FE 8530F in Perth, Western Australia didn't come with a bubbler either. I had to scrounge one up. Thankfully there was an old one in a drawer from a previous instrument. I just think JEOL randomly decides whether or not to send it with an instrument.

Cheers,
Dave
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Karsten Goemann on August 17, 2018, 05:40:04 am
Thanks Dave, we probably got the bubbler because we asked for it. The information given to us was that it is optional, so only supplied on request.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman on August 17, 2018, 09:13:34 am
I'm wondering why dibutyl phthalate is the fluid of choice (same for the bubblers on our Cameca SX100)? At least in Australia it is a class 9 dangerous substance, and the specified vapour pressure at room temperature is not particularly low either.

Wouldn't it be possible to use something like diffusion pump oil or even rotary pump oil? Many are non-hazardous, much lower vapour pressures. Santovac 5 is terribly expensive of course, but for a single small JEOL bubbler one wouldn't need much.

With the "clean vacuum system" of the newer generation machines, oil-free pumps, on-airlock plasma cleaners, cryo anticontamination etc, what would be the best bubbler fluid? Normally we probably wouldn't expect much back streaming, but if a counter window leaks or breaks...

I would think just some cheap silicone based diffusion pump oil would be fine to use as a bubbler fluid, but I have no special knowledge in this area.  Except that at room temperature, silicon oil is not going to be out gassing anything!
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman on August 17, 2018, 02:09:26 pm
This is a good question and I return to my original post. If one put a back pressure regulator at the end of the gas flow chain between the last spectrometer and the bubbler I would guess that that would help solve any atmospheric pressure influences as well as help prevent any back streaming of any fluid (or anything else) in the bubbler in the event of a window failure.

Cameca instruments have a bubbler on the exit of each spectrometer. Do JEOL instruments only have a single bubbler? 

The only advantage I can think of for separate bubblers is that it's easy to see which spectrometer has a broken detector window. Not that it happens very often, especially (never) since we added these "soft start" valves to our roughing pump line:

http://www.vatvalve.com/business/valves/catalog/H/311_1_A

https://probesoftware.com/smf/index.php?topic=120.msg487#msg487
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Karsten Goemann on August 17, 2018, 05:36:51 pm
John,

Yes, by default only one bubbler on the JEOL as normally all the P10 channels are in series. I assume one reason for having separate regulators and bubblers on the Cameca would be that they use different pressures ("low" and "high" pressure counters) for different x-ray energy ranges. On the JEOL all P10 counters are at the same pressure as sealed Xe is used for high x-ray energies.

But I've seen at least one JEOL where the P10 line is split upstream from the spectros to have an individual supply for each P10 counter (and I think also a separate bubbler).

I was curious and did some testing on our 8530F+ where the three P10 spectrometers are currently in series. I moved the last spectro in the line on a major element peak, and then did the same for the first and second spectro while monitoring the count rate on the last spectro in the line. There is no noticeable change even at very high count rates on the first two spectrometers. So "recycling" of the gas further down the line does not appear to be affected by what's happening on spectrometers upstream.

I've also heard that one may see differences in count rates when changing the order of the spectrometer in the P10 gas chain on a JEOL instrument, i.e. the spectrometers further downstream can have lower count rates. But I'd assume that may be due to P10 leakage somewhere in the chain, e.g. a leaky detector window, or a tube not attached properly...?


Dave,

I don't have any experience with these back-pressure regulators. I can see how it might help in your case at high altitude, but do you think it would  also make a difference for us, being pretty much at sea level? I.e. in addition to preventing potential backstreaming, could they even out fluctuations in atmospheric pressure? There seem to be a range of designs available. How much of a pressure difference to atmospheric pressure would one have to set for them to operate properly? Obviously too much pressure on the detector windows etc would be a concern.

Cheers,
Karsten
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: DavidAdams on August 20, 2018, 09:07:53 am
Karsten,

It would be interesting to see if a back-pressure regulator would be useful for JEOL probes at lower altitudes. I think we're going to try one here to see if it will actually smooth out the pressure effects we see. Have you monitored your PHA bias voltages on your P10 counters over time particularly when there is a big change in the atmospheric pressure? In Denver the voltages on our flow counters can often be ≥±10V during large changes in the weather, which, as you can imaging, has a noticeable affect on the analyses being performed. I don't remember what I was seeing when I was in Perth, but I don't remember it being quite so dramatic. The bias voltage on the Xenon counters is almost always the same. The greatest variation on the xenon counters that I've seen here is ±2V and that could be easily attributed to non-pressure related changes. Usually, however, the bias voltage is exactly the same for weeks and months at a time. A back-pressure regulator should, in theory, stop any atmospheric pressure related fluctuations. There are some very low pressure regulator options available (0-10 psi) and from what I understand there doesn't need to be a large pressure differential in those regulators to operate properly. I'm sure that the JEOL engineers will know what the maximum pressure before window failure would be.

Cheers,
Dave
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy on August 20, 2018, 10:28:16 am
Hi Dave,

In Kingston, at ~100 m AMSL (and no P-10 back-pressure regulator), I see variations in bias voltage required to keep pulse amplitude distributions centered at 4 V using the gas-flow counters (but not with the sealed Xe counters, at least over short periods of time).  These swings can be of similar magnitude to the ones that you report and appear to correlate with variation in atmospheric pressure -- but I need to keep a better record of this in my log file to be absolutely certain.  Even if centering the PHA distribution at 4 V for a given count rate doesn't solve the problem of variation in deadtime with X-ray energy, it sure does provide a useful reference for monitoring fluctuations in the distribution due to various causes.

Brian
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy 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.

(https://probesoftware.com/smf/gallery/381_31_08_18_7_33_44.png)

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.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Mike Matthews 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.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy 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
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman 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!
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: DavidAdams 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
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy 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.

(https://probesoftware.com/smf/gallery/381_12_10_18_4_17_37.png)
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman on October 13, 2018, 02:35:16 pm
Hey, it's a 1 million dollar barometer!   ;)
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy 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!

(https://probesoftware.com/smf/gallery/381_23_10_18_11_03_05.png)
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Mike Matthews 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
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: JonF on October 23, 2018, 12:55:25 pm
I wonder what the mechanism is: water vapour acting as a quench gas, perhaps?
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy 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.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman 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
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy 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?
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Karsten Goemann 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

Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman 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
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Karsten Goemann on October 23, 2018, 05:07:50 pm
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
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy on October 24, 2018, 07:32:18 am
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).
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy on November 24, 2018, 09:57:07 am
Below is a new plot of GFPC bias as a function of atmospheric pressure with ~22 feet of tubing added to the P-10 exhaust.  I've only had a chance to make a handful of measurements in the past few weeks, but it seems clear that the addition of the tubing makes little or no difference.  The cool-weather regression lines do not include the new measurements.

(https://probesoftware.com/smf/gallery/381_23_11_18_5_51_58.png)

EDIT 2018-11-28:  I may have made a conclusion based on too little data.  Measurements that I've made during the past few days suggest that bias actually has shifted to lower values (to get a distribution centered at 4 V at a count rate of 5000 s-1).  I'll post a new plot at some point during the winter.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Ben Buse on November 29, 2018, 09:58:11 am
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.

(https://probesoftware.com/smf/gallery/381_12_10_18_4_17_37.png)

Hi Brian,

This is really interesting what you've done, I'm hoping to repeat the measurements. I've just taken some measurements today and yes the exhaust spectrometer has a higher voltage. Pressure 997.8 mb, inlet spec 1710, outlet spec 1718

Ben
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: DavidAdams on December 18, 2018, 07:36:59 am
It's very interesting to see these plots! I'm glad I'm not the only one experiencing this.

What are everyone's P-10 pressures set to? The JEOL engineer here has set our 8530F gas pressure to 0.068 MPa. Does anyone know what the maximum pressure specification for the flow counters is?

Thanks!
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Ben Buse on January 10, 2019, 06:39:33 am
Hi,

What type of curve should you fit to a PHA spectra (bias scan) - e.g. Gaussian, etc. So far split pearson 7 seems to work.

Ben
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy on January 11, 2019, 05:03:39 pm
Hi,

What type of curve should you fit to a PHA spectra (bias scan) - e.g. Gaussian, etc. So far split pearson 7 seems to work.

Ben

Hi Ben,

The distribution is usually described as "quasi-Gaussian," with σ < √N, where N is the average number of primary ion pairs produced by an X-ray photon.  Fano (1947, Physical Review 72:26-29) showed that σ = √FN = √FE/ε, where E is the photon energy and ε is the mean ionization energy of the counter gas.  For argon, the "effective" F is usually set at 0.8 and takes into account widening of the distribution due to secondary ionizations produced during avalanching.  See Reed's "Electron Microprobe Analysis," 2nd ed., p. 86-87.

Brian
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Ben Buse on January 22, 2019, 08:19:13 am
Here's some initial results

Gas in at Sp3, out at Sp1.

(https://probesoftware.com/smf/gallery/453_22_01_19_8_18_43.png)

Also shown are the maximum height and the FWHM of the bias scan

(https://probesoftware.com/smf/gallery/453_22_01_19_8_22_14.png)
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman on January 22, 2019, 08:48:43 am
Here's a dumb question:  I know that the JEOL gas flow spectrometers are generally(?) connected in series as Ben mentions above (gas in at Sp3, out at Sp1), but I suspect that the Cameca (gas flow) spectrometers are connected in parallel.

From a recent visit to PNNL, I noticed that their P-10 bottle is tiny and yet they claim it lasts around a year. However on the Cameca there are 5(!) bubblers all running at the same time.  So even though we use a full size cylinder, our P-10 cylinder only lasts about 4 to 6 months.

Does anyone know if the Cameca spectrometers can be connected in series like the JEOL?  Are the any pros vs. cons on this question?
john

Edit by John: Edgar Chavez confirms the Cameca WDS spectrometer P-10 gas flow is in parallel.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Karsten Goemann on January 22, 2019, 02:27:02 pm
Does anyone know if the Cameca spectrometers can be connected in series like the JEOL?  Are the any pros vs. cons on this question?

I don't think doing in series for Cameca is possible as they run at different pressures ("low" pressure similar to the JEOL P10 channels, and "high" pressure for higher kV where JEOL uses Xe). You would need at least two "chains" one low, one high pressure?

That's probably also the reason that the JEOL P10 bottles last that long? Their overall P10 usage would be equivalent to 1 low pressure spectro on a Cameca?

Cheers, Karsten
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman on January 22, 2019, 06:47:05 pm
Does anyone know if the Cameca spectrometers can be connected in series like the JEOL?  Are the any pros vs. cons on this question?

I don't think doing in series for Cameca is possible as they run at different pressures ("low" pressure similar to the JEOL P10 channels, and "high" pressure for higher kV where JEOL uses Xe). You would need at least two "chains" one low, one high pressure?

That's probably also the reason that the JEOL P10 bottles last that long? Their overall P10 usage would be equivalent to 1 low pressure spectro on a Cameca?

Cheers, Karsten

That's a good point. One would need two systems, one for the 1 atm and one for the 2 atm detectors.  But still one would be using (theoretically) half the gas flow for the two high pressure detectors and 1/3 the gas flown for the others.

Based on the bubble rate I don't think the high pressure detectors flow any more gas than the low pressure detectors.  They're both about one bubble per second.
john
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Mike Matthews on January 23, 2019, 09:58:16 am
One benefit of the Cameca parallel plumbing is when you’ve got a leaking counter window it’s really easy to see which spectrometer it’s on.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman on January 23, 2019, 11:04:28 am
One benefit of the Cameca parallel plumbing is when you’ve got a leaking counter window it’s really easy to see which spectrometer it’s on.

That is a very good point.

That said, the 2 atm detector windows very rarely leak as they are 1.5 um(?) Be, and on the three 1 atm flow detectors that our instrument has, one can choose door #1, door #2 or door #3!    ;)

https://en.wikipedia.org/wiki/Monty_Hall_problem
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Mike Matthews on January 24, 2019, 10:24:05 am
One benefit of the Cameca parallel plumbing is when you’ve got a leaking counter window it’s really easy to see which spectrometer it’s on.

on the three 1 atm flow detectors that our instrument has, one can choose door #1, door #2 or door #3!    ;)

https://en.wikipedia.org/wiki/Monty_Hall_problem

Hmm, 3 doors and only one is leaking. All you need is three trolls that you’re only allowed to ask one question of and you’ve got a proper riddle.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Probeman on January 24, 2019, 02:34:57 pm
One benefit of the Cameca parallel plumbing is when you’ve got a leaking counter window it’s really easy to see which spectrometer it’s on.

on the three 1 atm flow detectors that our instrument has, one can choose door #1, door #2 or door #3!    ;)

https://en.wikipedia.org/wiki/Monty_Hall_problem

Hmm, 3 doors and only one is leaking. All you need is three trolls that you’re only allowed to ask one question of and you’ve got a proper riddle.

Hi Mike,
Well seriously, it is an interesting dilemma because it would be nice to see which spectrometer has a leaky detector window.  That said, we haven't had a bad detector window since we added these "soft start" values to our roughing pumps about 5 years ago:

https://probesoftware.com/smf/index.php?topic=120.msg487#msg487

And I sure wouldn't mind if our P-10 gas lasted several times longer!

I'll run it by our instrument engineer and see what he thinks...
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy 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.

(https://probesoftware.com/smf/gallery/381_01_02_19_8_45_22.png)

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.)

(https://probesoftware.com/smf/gallery/381_01_02_19_8_34_38.png)

(https://probesoftware.com/smf/gallery/381_01_02_19_8_36_34.png)

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.
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: dawncruth 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
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Karsten Goemann 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
Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: Brian Joy on October 15, 2019, 09:34:13 am
Below is a more or less final version of my plot of GFPC anode bias versus atmospheric pressure:

(https://probesoftware.com/smf/gallery/381_15_10_19_8_51_22.png)

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 * Patm [kPa] + 0.52 V/°C * Tdew [°C] + 909.4 V
Channel 4:  bias [V] = 7 V/kPa * Patm [kPa] + 0.76 V/°C * Tdew [°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.”

(https://probesoftware.com/smf/gallery/381_15_10_19_8_49_48.png)

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.

Title: Re: Flow Proportional Counter Backflow Gas Regulation
Post by: DavidAdams on October 16, 2019, 05:48:58 am
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