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View Full Version : Extending shelf life of glass laser tubes



Rich Harman
05-18-2011, 4:26 PM
I have read that the life of laser tubes is used up even in storage due to the gasses leaking. An obvious solution, but I have not heard of it being used, is to place the spare tube into a pipe which is sealed and then put under some pressure. If the pressure in the pipe is equal to the pressure in the tube it should be able to last indefinitely.

Has anyone heard of this being done?

Is there some reason why it would not work?

Dan Hintz
05-18-2011, 6:08 PM
I have read that the life of laser tubes is used up even in storage due to the gasses leaking. An obvious solution, but I have not heard of it being used, is to place the spare tube into a pipe which is sealed and then put under some pressure. If the pressure in the pipe is equal to the pressure in the tube it should be able to last indefinitely.

Has anyone heard of this being done?

Is there some reason why it would not work?
Unless the pressurizing gas is the same as what's already in the tube, you'll eventually contaminate the gas in the tube with your pressurizing gas.

Whomever told you that was either pulling your leg, making a WAG, or had no clue whatsoever about gas and equilibrium.

Rich Harman
05-18-2011, 6:44 PM
Hmm, I don't understand why the gas in the tube would be contaminated if there is no pressure differential across the two.

Dan Hintz
05-19-2011, 7:02 AM
If there is a location for gas to escape (usually between the electrical post and the glass), that's a location for gas to pass through. Even at perfectly matched pressures, gas molecules wander. Now, you might mitigate the issue somewhat by pressuring the container to a level somewhat less than what's inside the tube, but it's kinda like building a mansion to house a Yugo.

Tammy Larrabee
05-19-2011, 9:30 AM
Molecules are not static, they tend to wander around and bump into each other (good thing for the universe as a whole) best to just plan for the time when you will need to replace your tube, like taxes, it is inevitable, some sooner, some later.

Gary Hair
05-19-2011, 2:28 PM
but it's kinda like building a mansion to house a Yugo.

I just lost a lot of respect for you Dan... Who can resist a project that would result in the creation of an enclosure that is so airtight and pressure regulated that it can, reliably, keep a laser tube from losing gas. It's probably going to take many hours and quite a bit of money to do it, and the results won't be known unless you buy two tubes, one control tube and the other to be encased, but how could you pass up this challenge?

I'm disappointed in you Dan.

Gary

Rich Harman
05-19-2011, 2:46 PM
Wow, tough crowd!

Let's go through this step by step.

Molecules move from high pressure to low pressure.

They take the path of least resistance.

The greater the pressure differential the faster they will move from high pressure to low pressure through that path. Conversely, they will move slowest through the path when the pressure differential is the least.

If a thousand molecules per second are pushed through that path of least resistance when the pressure differential is the greatest, how many will pass through when it is the least? I am sure that the difference is substantial.


I don't think the mansion for a Yugo analogy fits. Were talking about $20 worth of materials and an hour of labor to extend the service life of a several hundred dollar tube that cannot be sourced quickly when you need it.

Scott Shepherd
05-19-2011, 2:57 PM
and the results won't be known unless you buy two tubes, one control tube and the other to be encased, but how could you pass up this challenge?


Probably need a heck of a lot more than 2 tubes to do that test. How would you know if you placed a lesser quality sealed tube in the chamber and a better quality sealed one outside the chamber? Or the opposite? You'd have to have some control point prior to going into the chamber.

I have no idea if it'll work or not, I never made it that far in school :)

Neil Pabia
05-19-2011, 4:37 PM
The guys in the shop next door to me have a spare tube and one in their laser, I'll volunteer their tubes and they can send all work my way. It's a win-win for most.

Michael Hunter
05-19-2011, 5:16 PM
How do you know what pressure to set your container to?
Do the Chinese give a value for the pressure inside the laser tube, or will you have to break one to find out?
What if the pressure is less than atmospheric? (Gas discharge tubes are often partially evacuated, but I don't know about laser tubes).

Not trying to be difficult - just intrigued.

Paul Brinkmeyer
05-19-2011, 5:55 PM
Just put the tube under water in a pressure container instead.
then just watch for bubbles and pressure
If bubbles come out of the tube you need to add more water pressure.
If the water pressure drops, you might have too much water pressure.

Notice I said "might" Could be something else.

Note: You can always use different fluids as water might not be the best.

Rich Harman
05-19-2011, 6:17 PM
I suspect the tubes are under low pressure but that is one piece of information that would need to be known, as least approximately. If they are partially evacuated then instead of the storage tube being under pressure it would also be partially evacuated.

Rodne Gold
05-19-2011, 6:45 PM
It's a bit of a pointless excercise , a 80w cheap glass tube is about $200 or less and freight would be around that mark too, so you might as well order a "fresh" one than spend a fortune developing a device that keeps an old one fresh. You could also spring for a Reci tube which is WAY better made , is better sealed and has better engraving for $500 or so (8000 hour life vs 800-1000 hrs)

Richard Rumancik
05-19-2011, 6:50 PM
Rich, it probably could be done if you had enough info about the tube itself and had the equipment to do it.

Some questions I would ask: 1) what is the pressure in the tube, and the tolerance on it? What kind of seal is used on the glass tube - epoxy, rubber o-ring, metal? Does the seal require internal pressure to make it effective? (or will it lose some sealing integrity as outside pressure rises, thereby increasing gas transfer?) How would you control the pressure you put into your enclosure and how accurately can you set it? The pressure you use would presumably be slightly less than the tube pressure. Would you just use air in the enclosure? (You would not want to get any air (N2/O2) into the tube.) Or would you use helium or laser gas? How would you monitor the pressure in the containment cylinder?

I really don't know the pressures involved but suspect that they are very low. If you were able to extract enough data about the tube manufacture it might be possible to devise something but it is probably not an easy matter to pressurize the containment cylinder without creating a new problem with the tube. I'm thinking that there is a chance that if something isn't just right you might end up accelerating the problem. Not trying to discourage you from experimenting but you just need to get some technical info first and work through it with someone who has experience in the design of glass laser tubes.

Bill Cunningham
05-19-2011, 10:17 PM
In my past life, I used to deal with this problem everyday.. Bounce dives and surface decompression.. Gas transfers/exchanges would control your life, sometimes for days! And you even get to sound like Donald Duck sometimes:D I wouldn't wish this on a 'laser tube':p

Dan Hintz
05-20-2011, 8:03 AM
I know, Gary, it's a tough row to hoe... so many challenges, so few of me to attack them with. I get all jittery just thinking about something that still needs to be proved. ;-)




Molecules move from high pressure to low pressure. They take the path of least resistance.

The greater the pressure differential the faster they will move from high pressure to low pressure through that path. Conversely, they will move slowest through the path when the pressure differential is the least.

If a thousand molecules per second are pushed through that path of least resistance when the pressure differential is the greatest, how many will pass through when it is the least? I am sure that the difference is substantial.

I don't think the mansion for a Yugo analogy fits. Were talking about $20 worth of materials and an hour of labor to extend the service life of a several hundred dollar tube that cannot be sourced quickly when you need it.
Rich,

The point is, as Rodney pointed out, these tubes are cheap cheap cheap. Spending several hours of your life to make a cocoon that will have dubious effects is your call, but I don't see the benefit in it.

Let's say the tube has a small leak, so you throw it in the pressurized container... what pressure do you set it to. If you're too high, you'll contaminate the gas in the tube. Okay, let's pressurize with Helium, the tube's gas with the highest concentration. You introduce more helium into the tube... likely not a big deal as concentration of helium is (relatively) loose. Now when you go to use the tube and it's no longer pressurized by the container, the pressure is even higher than before, so it quickly outgasses to the atmo not just the extra helium, but the other precious gases we need, the ones with a lower overall concentration and the ones we cared more about the concentration levels. Tube dies more quickly than if you had left it alone and started using it immediately.

Same tube, same leak, but this time we under-pressurize the container. It slows the leak and extends the period of time for which the tube's gas concentration will lase properly. It doesn't die sitting on the shelf for four years... but it still dies as fast as if you had started using it the day it came in, you were simply able to postpone by initial "sell-by" date. Mission accomplished... I guess. But at what cost of time (building, measuring proper gas pressure), materials (don't forget your pressurization gas), etc.?

Trust me, I admire your inquisitiveness and desire to try something unique (one of the reasons people poke me in the ribs here from time to time, like Gary did). All I'm saying is you would get very little benefit, one that is likely not even measurable.

AL Ursich
05-20-2011, 9:20 PM
Then there is Getter flash.... working in a Sony CRT plant my job was to fire off the Getter Flash with a small Fanuc robot and a 4 turn RF Coil and a RF Generator. The Getter inside the CRT would glow and burn further reducing the vacuum.... Yes different concept from a CO2 Laser but lots going on in a Laser.... Still dealing with Glass and Metal Pins....

I am in the "It's better to buy the Tube when you need it Camp...." Rather than make a pressure device to slow the natural aging process of the laser.

AL