Author Topic: Flow Proportional Counter Backflow Gas Regulation  (Read 13204 times)

sem-geologist

  • Professor
  • ****
  • Posts: 118
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #60 on: October 03, 2021, 05:39:46 AM »
The problem is that with sub-atmospheric pressure You will have no flow, as gas from lower pressure wont move to higher, but contrary. The only way for this to work would be to release sub-atmospheric pressure P10 into lower pressure (i.e. vacuum buffer). Of course we don't want to release it into current vacuum system (even primary) as that could contaminate vac system; however I think using a small low power pump (most important something not loud, as labs are already buzzing annoyingly) for such buffer for releasing gas from low-pressure counters could be very financially feasible and quite simple to achieve. This is really interesting concept, it could at last put some end to endless redoing of standard measurements forced by weather change... unless gas is not the only weakness (but probably it is as we see no such fluctuations on high-pressure spectrometers despite weather changes).

Probeman

  • Emeritus
  • *****
  • Posts: 2238
  • Never sleeps...
    • John Donovan
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #61 on: October 03, 2021, 09:00:42 AM »
Yes, exactly.  All these regulators (if you read the company details) are vacuum pump backed for positive flow.

I had a similar (homemade) differentially pumped detector flow system at UC Berkeley in the 1990's based on Henke's design from LBL. It ran pure propane gas at sub-atmospheric pressures for better detection of nitrogen. Obviously the (separate) vacuum mechanical pump was vented to a fume hood!  30" of vacuum is all one needs for such a system.

But I am hoping that these regulators will run at 15 or 30 psi (absolute) and as you say, because they are referenced to vacuum, might provide more pressure stability.

Of course what we really need are solid state detectors for our WDS spectrometers!
The only stupid question is the one not asked!

sem-geologist

  • Professor
  • ****
  • Posts: 118
Re: Flow Proportional Counter Backflow Gas Regulation
« Reply #62 on: October 04, 2021, 02:02:43 AM »
Quote
Of course what we really need are solid state detectors for our WDS spectrometers!

I am completely not sure about that. Why? (1) it probably could be ok replacement for low pressure counters; However providing very stable cooling of such detector is very challenging. We probably could use cooled nitrogen and same capillary to transport heat from the SDD. To cool SDD means there should be enough pressure of nitrogen, or lower pressure nitrogen but cooled to some low temperatures (then lots of liquid nitrogen is required). SDD and its counting electronics drift depends from temperature and humidity (We need to calibrate EDS SDD on SEM much more often than on EPMA, as the air conditioning is less sophisticated there, and SEM experiences more temperature swings (the SDD itself is cooled at stable set temperature, the decalibration is clearly a response to temperature and humidity variations by pulse counter unit (EDS SDD electronics))). This is why I am also not 100% sure that PHA drifts are directly produced alone only due to atmospheric changes, and rather suspect that electronics react to atmospheric changes too, in particularly that EPMA shaping and counting electronics is much less sophisticated from EDS pulse processing units, and low pressure counters most often works at lower BIAS and much higher GAIN (which means that gain-OPAMPs can swing the signal much more, than on high-pressure low gain spectrometers).

Partly problem with low pressure counters are the PHA drift. But if You use integral mode, there is no problem, as You still are counting same amount of counts (the counter bias can be set higher or lower in some margins which does influence pulse height, but not so pulse density).

SDD can look as that nice stuff, but there are many artefacts which would influence measurement linearity in particularly at low energies and is very hard to resolve:
incomplete-charge - no such thing on proportional counter
Si esc peaks - on proportional counter we have Ar esc, but for low energies it is not applicable.

Generally new design of proportional counter electronics could solve most of proportional counter problems (PHA resolution, pile-ups, increase linear throughput, intelligent bias offset depending from room temperature), while there would be no need to hardware changes inside spectrometer.

Actually there is no obstacle to use the last one NOW. Stick some digital thermobarometer (hygrometer) in the room. Log the values, apply some low-pass filter. Make correlation curves between bias and environment. Add some code to PfS to adjust bias depending from room environment -> profit.
   
« Last Edit: October 04, 2021, 02:09:13 AM by sem-geologist »