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Flowing CO2 Laser Flow Rates

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sngecko

Tue Apr 17 2012, 01:22AM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

As part of a flowing CO2 laser project, I would like to construct a gas system that allows me to select the partial pressures of each gas (CO2, N2, and He) on the fly. I've decided to purchase calibrated mass flow controllers for each gas. I'll probably have an Arduino run the whole thing ultimately, but that's a different fight.

I have run into what I think are conflicting accounts of the maximum power and optimum flow rates for flowing gas lasers.

(1) Laser Kinetics suggests that "Below about 10 cubic ft/min, the power decreases rapidly, and this region of operation is to be avoided for high-performance operation." I believe that Laser Kinetics is referencing a bore with a 50mm ID and 1m long. Scaling this down by volume proportionally (my laser is 2.4m long with a 15mm bore) amounts to a lower flow threshold of 3.2CFM, or 90LPM.

(2) Most home-built lasers that I've come across list an optimum flow rate around 2LPM, or scaling to my laser, again volumetrically, 7.4LPM.

So, what do I gain by increasing my flow rate from 7.4LPM to 90LPM??

Carbon_Rod

Tue Apr 17 2012, 06:50AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Hmmm, the tube diameter may become an issue as electrical impedance of gases does not proportionally scale is this way. Flow rate will likely be very specific to a given machine.... even if your power supply can light the tube without cooking the shells.

Most people go to a welding supply outlet to get their gas mixture in a single cylinder rental (i.e. accurately measured by mass). The equipment cost goes down by a third, it leaves only two regulators to calibrate, and the pumps much easier to control.

What brand of safety glasses do you wear?

sngecko

Tue Apr 17 2012, 11:37AM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

I see what you're getting at, but it's a bit premature for safety glasses, as I haven't begun fabrication of the laser head. I will certainly have a large Lexan shield, beamstop, and glasses when the time comes.

As the project stands so far, I've built a 20kVDC, 2kW power supply and two-channel current regulator (0-80mA per channel). See my project here. One important aspect of this project is the ability to adjust almost all of the parameters while running, or after a brief shutdown. So the power supply allows me to adjust the voltage and current as it is running. Since the laser will be folded, each branch will have an independent current control. Thus you can see why I would like each gases' partial pressure to be adjustable. I'm hoping to provide a lot of data to the online hobbyist community regarding gas composition, current densities, cathode materials, etc.

Anyway, regarding the widely disparate flow rate information, in the viscous flow region, I still don't see how wall effects alone can explain the huge difference in "optimum" flow rates.

Bored Chemist

Tue Apr 17 2012, 08:36PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Be sure to get MFCs that can cope with a partial vacuum at the output.

sngecko

Tue Apr 17 2012, 08:45PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

Thanks Bored Chemist- I recently noticed the minimum and maximum pressure differential ratings of the MFCs for sale on eBay at the moment.

It makes sense to me that if the MFC has a maximum pressure differential of 18 psi that I should be able to set the tank output pressure to, say, 2 psig, which should be about 16.7 psia pressure differential across the MFC at near vacuum. Is this really how it works, though?

Bored Chemist

Wed Apr 18 2012, 06:36PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I'm not sure of the details but when we wanted a whole bunch of MFCs regulating the flow of air we were pulling through them (from near ambient pressure) they needed to be "special order" types. You need to check with the manufacturer.

jpsmith123

Wed Apr 18 2012, 10:22PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

IIRC with a CO2 laser, the power output and efficiency are rather temperature dependent. If the tube wall is liquid cooled and the bore diameter isn't too big, then you probably don't need much gas flow...for heat removal purposes that is. In that case, I think the gas flow is mainly removing unwanted gas species produced by the discharge, e.g., CO, NO.

sngecko

Wed Apr 18 2012, 10:33PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

Thanks BC: I guess I was assuming a little too much regarding those things. I've never really worked with one, but they seemed perfect for this application. I guess before I spend all that money, I'll get in contact with the tech guys.

jpsmith: That's pretty much what I thought, too. I hadn't considered the relative difficulty in cooling such a mass of gas in a 50mm tube with its relatively small wall surface area, to which it could conduct it heat. Maybe I should compare the ratios of bore volume to surface area and see if it's more proportional to flow rates in the two lasers....

jpsmith123

Thu Apr 19 2012, 03:46AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I was considering building a CO2 laser a long time ago, but I just never got around to it. I did look into the design quite a bit though...but unfortunately I've forgotten most of what I learned. (I was interested in using a dielectric-barrier-discharge to drive the laser).

As I recall the main issue with a "conventional" CO2 laser is that there's a radial temperature gradient in the tube bore. And I think the object is to keep the gas temperature at the center of the tube from exceeding 300 degrees C or something like that.
IIRC this is why, for a conventional design, the power output is essentially independent of tube diameter.

Shrad

Thu Apr 19 2012, 07:44AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

hi,

the best approach for a homemade constructed CO2 laser, is to use a double concentric pyrex tube, with a cooling channel around the discharge channel, and use a double anode/single cathode like other long tube designs in order to reduce working voltage and get a physical support point at the cathode

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