co2 tube problems

[peterklos]

Hi All
I hope you can help
I bought an 80 watt CO2 laser tube and I am having problems and hope you can help.

The tube is brand new but has been boxed waiting for install for 12 months
I loose power after about 20 minutes with the pump running even if I don’t fire the laser
When I press the Test button on the power supply I get 14 mA current and I can see a pink glow in the tube , no arcing of the voltage
The laser has power then after about 5 minutes of running the power almost disappears and almost no mark on thermal paper at the end of the tube

My set up is as follows
Tube is 80 watts and 80 watt power supply
I have a water chiller and the temp is set to 16 deg C
Anode and cathode connections OK
Water flow correct and is dematerialized water
The pump is a water fountain pump
I have a water flow meter connected and the flow is 7 to 10 L per hour

Any suggestions.
Regards peter

[turbohead]

Sounds like the tube is loosing mirror alignment due to thermal expansion.

[peterklos]

Any suggestions on a solution , If I shut down the pump ( power supply mains still on and no power to tube to eliminate power supply problem) for about 30 min and start up again it works for about 5 min then power is lost.
Peter

[turbohead]

Aim a laser pointer on the rear mirror, have the reflection of the laser pointer shine a spot on a far wall, mark the spot (when the tube is cold), and then see if the spot moves as the tube is operating. Do the same on the front mirror as well. If the spot is moving, then that verifies that the mirrors are moving due to thermal expansion.

If the spots are stationary, then it may be a power supply problem.

[The_Doctor]

Any suggestions on a solution , If I shut down the pump ( power supply mains still on and no power to tube to eliminate power supply problem) for about 30 min and start up again it works for about 5 min then power is lost.
Peter

Is the water temperature (after 30 minutes to reach ambient) rising or falling in the first 5 minutes of operation?

I'm not familiar with cooling CO2 lasers but until someone who is comes up with something better I tried a bit of crude reckoning myself:

http://www.seai.ie/Schools/Post_Prim...Test_yourself/
...states that 1.5Kg (so 1.5 litres) of water will go from 15°C to 100°C in 4 minutes if heated by 2221W.

If the laser is 80W output, and efficiency is 30% (as I read once in a book for a DIY CO2 laser), the power would be 186W of heat, and if not lasing, there could be another 80W because the only way out is waste heat. (If your PSU is 80W total, not designed for a tube with 80W output, that's a different matter entirely, but I'm assuming it IS enough for an 80W tube output).

10 litres per hour is just 167 ml per minute. As 167 is slightly more than a tenth of 1.5 litres, 4 minutes of 220W would raise it by nearly 85°C, and by about 20°C in one minute. This is ok so long as it all works right.

So how powerful is your chiller? To pump about 200W it will have to consume more. Most fridge compressors don't consume more than about 150W.

I can't go any deeper into this because I haven't got enough info or experience to do it, but it looks to me as if your water cooling may be marginal at best, and maybe something isn't as it appears to be.

[peterklos]

Hi Thanks for the response , I have done some calcs also and think the water flow / temp /disapated heat is a little low , also noticed the surface area for colling the mirror is small .
i will increase the water volume over time and reduce the temp to see what effect that has on it
thanks Peter

[mixedgas]

If it fades after 5 minutes its most likely alignment or gas pressure. The Co2 mixture is sensitive to heat, so ensure cooling is good first.

Then Do the laser pointer test mentioned below but see that the tube is firmly clamped at the recommended clamping points. The distance to the wall to see a error occur will be several meters minimum. It will take several repeatable tests to ensure its mirror alignment. It is unlikely you will see the small deflection, but you may try.

Placing slight pressure on the ends of the mirror mounts (Caution! Danger ! High Voltage) with a piece of stiff PVC plastic tubing may result in lasing. Very slight pressure is all that is needed. The direction you may need to apply pressure on will be random. Do not apply too much pressure or you WILL crack the seals. How much pressure is needed is a learning experience. I cant tell you how much force you may need. It is up to you if you want to try this. Far IR optics are brittle and will crack under too much force. It thus requires thinking to know where to place the pressure. Shoot us a picture looking down on the mount end from a 45 degree angle and wait for my reply.

Is your tube under warranty? If so they may need to send another tube.

Is the tube dimming in brightness at five minutes? if so.... Power Supply Issues.

If you apply pressure, and it lases, you will have to shoot me a picture of that mirror mount. I MAY or May Not be able to tell you how to flex it back into permanent alignment. These tubes are constructed of minimum cost materials with low cost labor, so results are often unpleasant.

Steve

[peterklos]

Hi Steve
Thanks , I have tried to see if the mirrors are missaligned with a laser pointer however , the are buried in the tube with lots of glass around so tricky to get any spot to check.
I have been running it with a constant 17 dec temp and flow and I can get a beam but weak and after time it fades , quicker with higher power longer time fade with low power , full power at 80 watts only marks thin plastic so I think you guys are correct with the mirror issue , also I have slightly pushed the rear glass on the tube and the beam dissapears!
any sugestions to rectify , what is the function of the 4 allen screws on the out put end of the tube , I dont get any response from teh chinese company I bought it from.
appreacite your help
peter

[turbohead]

what is the function of the 4 allen screws on the out put end of the tube

I think we need pictures.

[peterklos]

This is the Anode end and cathode end , the tube is 1600mm long and I guess any small change in miror angle will have an effect , do you think the 4 screws adjust the front mirror ?