Needing more than a spark test?

[WobblyHand]

A couple of notes, one regarding the PCB design. One application note I found suggested putting a ground trace underneath the TIA feedback resistor, to shunt-out some of the parasitic capacitance that's across the resistor. This is meant to improve the circuit bandwidth. There wasn't enough room to do that between the 0805 SMT resistor pads so I placed traces on the two inner layers. I _could_ go to a larger resistor like a 1206 but they have higher parasitic capacitance anyway. And there's nothing that can be done regarding parasitic capacitance across the top of the resistor, anyway. The feedback resistor is R5 on the layout. A thru-hole resistor that has a wrap of thin wire around its middle would _really_ knock down that parasitic capacitance, but do I actually want to go there??

The second note is that I found a surprisingly inexpensive flow sensor whose range matches the low flow rates I'd want for a continuous-flow gas proportional counter. It could be used to make a relatively cheap mass flow controller in the 0-30 SCCM range. Digikey has them for $45 but unfortunately they drop-ship directly from the German manufacturer so shipping is a bit high. I also have been looking into DIY flow sensors that use cheap NTC thermistors. Thing is, I'd still need a "real" flow sensor to calibrate the home-made one.

Smaller SMT parts have lower parasitic capacitance but are harder to manipulate.

Since I have a ball flow meter for my TIG, I'd be tempted to use it for something like this. However, the scale is different for different gases, since their properties differ, at least for argon and helium. Absolute calibration is hard, but is it necessary for the application? Is it possible to discover the "window" experimentally? How wide is the window of operation? I'd think if it could operate for a minute or two statically, then there's some window width.

 

[homebrewed]

Smaller SMT parts have lower parasitic capacitance but are harder to manipulate.

Since I have a ball flow meter for my TIG, I'd be tempted to use it for something like this. However, the scale is different for different gases, since their properties differ, at least for argon and helium. Absolute calibration is hard, but is it necessary for the application? Is it possible to discover the "window" experimentally? How wide is the window of operation? I'd think if it could operate for a minute or two statically, then there's some window width.

The main determiner of the tube's operating characteristics (other than the Ar/CO2 mix) is the pressure, and that will be fixed at 760 torr -- atmospheric pressure. This is the simplest setup to achieve, and also considerably eases the choice of _real_ window material, since it won't need to be robust enough to hold pressure or withstand a partial vacuum. Like thin aluminum foil or even the stuff used for gilding. That's VERY thin but hard to handle.

I have some .5 mil stainless-steel shim that would make a dandy window, in terms of robustness -- but I'm not sure if I want a window made out of the very same things we're trying to analyze. It MIGHT work out OK due to its energy-filtering characteristics, but since it's an alloy it wouldn't have just one "jump" in its x-ray absorption spectrum. Single-element metal foils seem to be hard to come by, unless you're willing to spend a lot of money. Check out goodfellow.com to see what I mean -- a 10 x 10mm piece of 12.5 micron-thick nickel foil will "only" set you back $241.39!! For that price, I'd buy some nickel chloride and try my hand at electroplating some instead.....

I'm thinking that the tube could be operated statically for at least a few hours, once the oxygen has been purged from it. A ball-type flowmeter would work fine, as long as it goes low enough to keep the gas consumption low. Calibration, in terms of the actual flow rate, really won't be an issue in this situation.

The reason I'm looking at flow sensors is that the ideal Ar/CO2 blend probably isn't going to be one that a TIG owner would want. Two separate bottles -- Ar and CO2 -- and two MFC's could whip up whatever you want, in either case. But hopefully a commonly-used TIG blend will be good enough.

 

[rwm]

Just to clarify: TIG uses straight argon. MIG uses the CO2 blend. Or am I missing something here?

 

[homebrewed]

Just to clarify: TIG uses straight argon. MIG uses the CO2 blend. Or am I missing something here?

Just shows you how (little) I know about inert-gas welding! But in my defense I was going with some information I found on the web and just repeated that part. The internet nonfactual menace strikes again!

And: thanks for the education. It's a good day when you learn something new.

 

[WobblyHand]

TIG also uses helium, it was initially called heliarc. Helium is expensive compared to argon. I've only used argon. Helium has better heat transfer. My Harris ball flow meter is calibrated for helium and argon.

 

[Larry$]

MIG uses the CO2 blend. Or am I missing something here?

Is the reason for using the blend, rather than straight argon, price?

 

[rwm]

The CO2 is actually helpful for MIG. I don't know the physics but someone here does. I believe it is preferred due to better heat transfer. I think Argon is now used for TIG because it is dense and stays in the weld area better than He. Also price as noted. Argon can be distilled from air. Helium cannot. It is too light for the earths gravity to hold it and it boils away to space so very little is found in air. Helium is a byproduct of crude oil and occurs from radioactive decay. (An alpha particle is a helium nucleus so it is germane to this thread!) The price is regulated by the government or it would be even more expensive.

 

[WobblyHand]

Is the reason for using the blend, rather than straight argon, price?

If I recall there's better heat transfer with that mix. There's lots of gas blends available for MIG, depending on the mode of operation. There's blends for better penetration, etc.

 

[homebrewed]

If I recall there's better heat transfer with that mix. There's lots of gas blends available for MIG, depending on the mode of operation. There's blends for better penetration, etc.

I have read that there are various performance reasons for the different blends. I do like the fact that argon is basically a renewable resource; and it apparently is a "byproduct" of the process for making LN2 and LO2. According to Wikipedia, the concentration of argon in the atmosphere is about twice the average concentration of water vapor! So there's a LOT of it out there. It's a good thing it's not a greenhouse gas.....

FYI, the ideal percentage of CO2 for a GP detector is around 8%. The nearest "standard" blend is 10%, probably close enough. I'm not sure how common that blend is. If it's rarely used that might impact the cost for a fill.

Many commercial GP detectors use a 90/10 Ar/CH4 (methane) blend but the methane renders the mix flammable. Isobutane has been used, too, but it's got the same problem. I'd stick with CO2 if I was to try this. FAR safer.

Also in line with the general topic of this thread, argon is produced by the radioactive decay of potassium-40. That's got to be the main reason for the relatively high abundance of Ar.

 

[homebrewed]

One paper I found on the subject of detector blends was extolling the performance of an Ar-Acetylene blend (!). Now THERE's a way to get some excitement in your life! Anyone for a little RUD (Rapid Unplanned Disassembly)?