Caution: Raci 150W cracked by cold water or by cold weather!

Methods of cooling laser tubes

Caution: Raci 150W cracked by cold water or by cold weather!

Postby Speedythinker » Wed Feb 22, 2012 6:27 pm

Most people only concerning how to cool a laser tube. But some of those didn't pay attention how cool is cool!

A customer from Germany bought a Raci 150W(180W peak) A8 tube from me and it received it great conditiono. But after he power it on for 5 min (per his email), then he heard a 'pop' sound and the tube cracked. I told him that he may not have water flowing but he insisted he did. Well, then the problem was: water too cold! Germany now is 40'F (4'C) and if he feed water at that low temperture without warming up the tube.

The tube was tested before shipped by the manufacturer per our request as it's an expensive tube and ship to Germany. So, we paid extra caution to it and double packing. If it's cracked in the being, it won't fire for sure.

See my post: http://www.lightobject.info/viewtopic.php?f=47&t=1067

Bart, correct me if I'm wrong about this


Marco
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Re: Caution: Raci 150W cracked by cold water or by cold weat

Postby canadianavenger » Wed Feb 22, 2012 11:23 pm

The problem is thermal shock. Sudden changes in temperature from hot to cold, or cold to hot can cause the glass to crack.

5 degree water should be okay, as long as the tube glass and mirrors are near 5 degrees when the water is added, or the tube is turned on. If the tube is at or above room temperature and turned on before the water can bring the tube down in temperature, the tube is likely to crack with the sudden rise [and consequent fall] in temperature from the laser inefficiencies and the cold water. If the water is run through the tube for several minutes before the tube is turned on, it should be okay, as the rise in temperature of the glass will be more gradual then. The tube may still crack if the water and tube temperature are too far apart, even if the tube itself is never turned on. Glass is fragile, and does not like thermal stress.

Bottom line, the water and tube should be close to the same starting temperature. If very cold water is to be used [relative to the tubes starting temperature], it should be introduced slowly to bring the tube down to the water temp. Once the tube and water are at, or near, the same temperature, then the tube can be fired up.

Note that a poorly designed tube may still crack, if the cooling is uneven along the tube, due to uneven expansion of the glass.
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Re: Caution: Raci 150W cracked by cold water or by cold weat

Postby lasersafe1 » Thu Feb 23, 2012 1:38 am

A tube that powerful will be much more sensitive to mistakes because it has much more power to be removed in heat. I would say it could handle 5C water just fine, but if that water stops flowing (even momentarily by playing with the pump) during lasing, then there can be a rapid local increase in temperature near the High reflector (HR) or output coupler (OC). When water begins to flow again it will instantly crack the glass. That would be my guess for what happened. It is not something a warranty could cover because it is easy for a client to make this mistake. But they should be warned about how sensitive it will be.

It's kind of funny because I never told you Marco. I bought one of your tubes for my other job (non hobby) and I was trying to heat a piece of molybdenum in a vacuum through a ZnSe vacuum window. I did not know at the time that moly was a high reflector. The light bounced off the moly and came straight back at the laser where it melted the rubber tubing feeding the chill water. I was busy monitoring the data acquisition system and did not notice it until there was spraying water and glass cracking, and electrical arcing. Oops.
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Re: Caution: Raci 150W cracked by cold water or by cold weat

Postby Speedythinker » Thu Feb 23, 2012 5:06 am

It's a terrible accident and a terrible mistake. It's so bad to the customer and it's so bad to me also. So far, I really have no idea how to solve this case. Someone said it's the tube quality issue while other said it's customer fault. Funny, I could avoid the shipping damage on one hand but I coudln't avoid the accident on the other hand. The shipping cost alone cost me over $700 to ship to Germany + other cost. Return for a $3000 tube wasn't very good at all and I wourld rather to invest something else. So, I decided to drop the 150W tube. It's not worth to spend half a day to order and pack the tube and being worried for the whole journey of the shipment. I lost a lot of hairs and getting more white hair under stress.

Talked to the supplier and he didn't admit that it's a bad tube. He insisted that he tested it before shipment. Then customer insisted that water war running during firing up. So, whose fault is then, me?

I wish the tube was cracked instead so that I could blame on the custom or the DHL. And, I could just order one and pay for the accident and claim lost rather than scratching my head for a solution like now. Note, the DHL won't cover and damage or lost package as I'm not the sender and I don't have account with DHL. It's the 3d party in Hong Kong that deal direct with the DHL. The logistic company in China was nothing. They're workign as a sub-agent. DHL > Hong Kong agent > China agent. When trouble happened, China agent point us to -> Hong Kong agent point us to --> DHL point us to --> China agent point us to--> HK --> DHL -->... endless loop.

By the way molybde (MO) is good to make refleciton mirror but it's not the best. For power under 100W, it's better to use Si plated. For 120W or above, it's an good idea to use MO as the metling point of MO is better and it's harder so it last much longer. Some laser manufacturer actually using it for 80W machine to cut cost. But the drawback is that MO reflection index is lower than Si plated so you don't want to use it for 80W or smaller tube. Power lost is the big issue. I was told that the reflection rate for a MO mirror was 95% while the Si could does 99.5%. I do carry both type mirrors and quality made in case anyone interested(commerical time ha..ha..)

Anyway, let's see what the supplier say. But I bet they said the same thing I did. Plus, they did run a test before shipment. The chance they will take responsible is 0.1% if not 0%

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Re: Caution: Raci 150W cracked by cold water or by cold weat

Postby r691175002 » Thu Feb 23, 2012 11:32 am

It is unfortunately really hard to pin down fault here because any information given is questionable given the amount of money involved in the tube.

Clearly the issue is thermal shock, but if we assume the laser was running for 5 minutes before cracking I find it hard to believe it was caused by the water being too cold.

Glass doesn't have appreciable fatigue so if it didn't crack with the initial addition of water it should have had no effect. Plus the tubes are borosilicate (pyrex). Pyrex dishware routinely takes the freezer, boiling water and oven temperatures everyday.

My guess is manufacturing defect but its impossible to know for sure if the water pump was providing sufficient flow.
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Re: Caution: Raci 150W cracked by cold water or by cold weat

Postby Speedythinker » Thu Feb 23, 2012 5:05 pm

Right, it's really hard to tell as we couldn't see how they used it. The company was upgraded their machine from Reci 130W to the new Reci 150W (180W peak). I'm not sure if they have made consideration to determine whether the chiller for the 130W was good enough for the 150W. Or, did they check whether or not air-bubble appeared during fire up. For 150W tube, any visible bubble happened to have may caused inbalance heat dissipation. What he said was that his colleque powered it up with 20% trying to cut a plexiglas at 20mA. Then in 5 min, they heard a loud cracked sound. He opened the machine and found the cracked on the tube. Well, it's only one side story and we never know. I have my staff contacted the Reci see what could they do. The Reci asked us to tell the customer to take picture for the tube and if possible, to send more information about the machine with video.

Regardless, the customer still need to pay $695 to cover the shipping/packing should he Reci offer him a new tube.

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Re: Caution: Raci 150W cracked by cold water or by cold weat

Postby Liberty4Ever » Wed Jun 06, 2012 5:26 am

I hate to hear about this cracked laser tube, not only because of the suffering it causes the customer, distributor and manufacturer, but also because incidents like this make laser tubes more expensive for all of us. The cost to replace the tube or discount a replacement, or just the time spent dealing with an upset customer must be spread onto all customers eventually.

As I read through the posts in this thread, I wondered if there wasn't an air bubble in the tube. I wasn't too surprised to see that possibility mentioned in the previous post.

I'm new to all of this, but I'll make sure I absolutely do not have any air bubbles. I'm thinking about making the laser tube the lowest point in the cooling system to help avoid bubbles, and I'll probably have a float switch in the coolant tank and a thermal switch on the coolant output of the laser tube, in addition to the flow switch, to verify that the 60W laser tube I plan on using is being properly cooled.
Apparently, I didn't build that! :-)
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