Session 11 – The Laser Tube Cooling System

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The Concise RDWorks Learning Lab with Russ Sadler. Session 11: The Tube Cooling System.

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You have a very valuable machine here and one of the most vulnerable parts of the machine is this bit here.

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And in the winter, we’ve got water in the cooling jacket and a cold, possibly getting close to freezing temperature in this unheated workshop.

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And the two things just don’t go together.

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So what I’m going to do today is to show you how I have protected this machine with what I call a winterizing kit.

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People have tackled this freezing water problem in different ways. One of the most obvious ways is to put antifreeze in your system.

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Can I recommend you don’t? There are several good reasons why you wouldn’t want to put antifreeze in here,

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but the most important one comes down to and let me just take you to here.

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This is the brass water jacket that’s being used to protect the partially reflecting mirror that’s mounted on the end of this tube here.

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Now, the mirror itself is attached to the tube with, as you can see, something like an epoxy resin.

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Two reasons why you wouldn’t want to put any sort of slightly corrosive or dodgy chemicals into the water.

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Brass, which is 90 percent copper and 10 percent zinc, is in itself quite vulnerable to chemical attack.

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And that’s one good reason why you wouldn’t want to put anything like an ethyl glycol or a propyl glycol mixture of water into this tube.

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Now, yes, it will stop it freezing because it pushes the freezing point of water down to probably something like about minus 20 or minus 30 degrees C.

Transcript for the Laser Tube Cooling System (Cont…)

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So it does a fantastic job, but it also, over time, will corrode any metal parts that are in the system.

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The other thing that I’m worried about is it causing damage to this seal just here, this epoxy seal.

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Now, I’ve got no idea what the chemical reaction between epoxy and the glycols are, but it’s not a risk that I’m prepared to take.

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Now, if you go on to various user groups, you will find people claiming the advantages of using things like ethyl glycol and propyl glycol.

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But as I said, disadvantage number one is it is corrosive.

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They also claim that it has the great advantage of assisting the cooling effect.

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Well, that’s both true and untrue.

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They’ve misunderstood the physics of what is actually going on when they make that claim.

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If you’re using it in a car as an antifreeze, it does a fantastic job of preventing your radiator from freezing.

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But two bits of physics that I need you to understand, first of all, water boils at 100 degrees C.

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If you raise the pressure of the water and you do that with the radiator cap,

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which holds the pressure in the tank at about 15 psi above atmospheric pressure. What that does,

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that causes the temperature of the water, the boiling point of water to go up by probably about 10 degrees C.

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So that automatically raises the point at which your water will boil.

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And the second thing is, if you add a water glycol mixture to your radiator, that will raise the temperature by another 10 or 15 degrees C.

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So you could possibly get up to about 130 degrees C before your water boils in your radiator.

Transcript for the Laser Tube Cooling System (Cont…)

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Now, the great advantage of that is, because the water temperature can go higher.

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The amount of energy that you can dissipate,

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the amount of heat that can be lost gets greater because the bigger the difference in temperature between the radiator and the ambient air,

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the greater the heat flow and the more efficient the heat flow. That works fine at 130 degrees C, but we’re not going to run this at 130 degrees C.

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We’re going to run this around about 20 or 25 degrees C, and there you’re reliant upon the thermal transfer rate of the fluid itself.

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In the center of this tube. We’ve basically got lightning in a bottle. There’s an incredibly high temperature in this pink beam that you see in here.

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We’ve got heat that we’re trying to transfer away from this inner tube to the outer tube where the water is

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flowing. Now to squash the myth that you get better cooling because of having an antifreeze mixture in your water.

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I need you to just understand a little teeny weeny bit of physics, something called thermal conductivity of materials.

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There are two components that go into an antifreeze mixture. One of them is water and the other one generally is ethylene glycol.

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Sometimes you can use propylene glycol. It generally depends on which antifreeze you choose.

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Now, what I’ve got down here are numbers. I’m not going to bother about the units because they just confuse.

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But basically these numbers down here show you the conductivity, the thermal conductivity or the ability of a material to let heat travel through it.

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These are all relative numbers from very poor,

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to excellent. Now, you wouldn’t think the diamond was the most effective conductor of heat, but,

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hey, the figures are what they are and that’s what is says. Water is in itself,

Transcript for the Laser Tube Cooling System (Cont…)

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0.6, ethylene glycol is 0.258.

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So if you mix those two together, you’re going to actually reduce the thermal capability, the thermal transfer of water.

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So it’s not going to be able to pick up heat as easily if you mix it with ethylene glycol.

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Now, the other thing is, which is very interesting, and I have to admit that I didn’t understand that this was quite as bad as this.

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I always thought the glass was actually a worse conductor of heat, than water.

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But it turns out not to be the case. Actually, glass conducts the heat better than water.

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So that lightning in a bottle that we talked about, transferring heat through the glass jacket is actually quite efficient,

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not very efficient, but just quite efficient. But the water on the outside needs to be kept as cool as possible.

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And the way you keep the water cool is to make sure that it flows. So that it doesn’t get a chance to pick up huge

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amounts of temperature before you whip it away and then you put more cool water in to help draw the temperature away.

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And that’s the mechanism for drawing the heat away from the tube.

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So risk of corrosion and reduced thermal conductivity of the water are two good reasons why you would not use antifreeze in your system.

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We just talked about how conductive this stuff is.

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Now, all tube manufacturers recommend that you use distilled water, not deionised water, which has had the particles removed from it,

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but distilled water. Because distilled water has been heated and all the bugs in the water, all the microbes have been killed.

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And that’s an important fact. I’ve had water in one of my machines for about five years.

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I changed it recently. It still looked like this.

Transcript for the Laser Tube Cooling System (Cont…)

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But if you want to keep your water clean and stop airborne bugs from getting into the water and contaminating it.

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Make sure you use a sealed lid and put holes through, quite snug holes for your tubes and cables to come through.

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Now, the other reason why you would want to use distilled water,

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remember what I said about the chemicals the glycols, etc., having an effect on the epoxy seals around the mirror.

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This is pure. There is no chance there’s any chemical reaction going to take place within your tube and there’s no

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likelihood of corrosion taking place in the brass or the metal caps that are on the end of your tube.

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One of the other things that I’ve just mentioned is keeping that water flow across

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the tube as fast as possible because water is a very poor conductor of heat.

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And if you allow the water to build up temperature by it moving slowly, it’s going to carry less heat.

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So the cooler you can keep the water by moving it quickly across the surface of the tube. So that

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it doesn’t have a chance to pick up much heat, the more efficient the cooling is going to be.

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That brings us to another point, and that is about the flow.

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If you’re using a little system like this, which is typical for most starter machines.

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You’ve got a little pump here which has got very low pressure capability. It might be able to deliver two, three PSI.

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If you put this on the floor in a bucket of water and you’ve got your machine here on a bench,

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three foot tall, and then the tube is another maybe a foot above the base of your machine.

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That means you got a four foot head that you’re trying to drive the water uphill to, before

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it gets to your tube. You are throwing away some of the pressure capability of this pump.

Transcript for the Laser Tube Cooling System (Cont…)

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Always try to keep your pump, at least on the level of your machine, and maybe even higher.

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So that there is the smallest pressure difference between the pump and the entrance to your tube.

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That way you’ll keep maximum pressure and keep maximum flow through your tube.

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Pressure loss in this hydraulic system is a big problem.

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One of the problems is what I’ve just described, loss of head.

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So you’re losing all the pressure, pumping the water uphill before it even gets to your tube.

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And the second thing is things in the system which are essential, things like this and this, which you may or may not even realize are there.

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They’re called flow switches. It’s very important,

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that we monitor the flow of water in this system. Because if the water stops and we turn the tube on, the tube will very, very, very quickly get hot,

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burst the glass, and you’ve got a very expensive accident on your hands as well as maybe a rather messy one. When you get your machine,

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it may well be fitted with one of these. It may be fitted with one of these.

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They look completely different, but they are roughly one in the same sort of thing.

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What you’ve got in here is a two wire system.

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This is just a little reed switch in here and inside here,

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what we’ve got, we’ve got a little bobbin that moves backwards and forwards under the action of a spring, which holds it one way.

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And when the water flows this way, then the bobbin moves against the spring.

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The bobbin has got a magnet in it and it causes this reed switch to close.

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When the switch closes, it basically tells the controller, the brain, that we’ve got water flow and when the water flow stops,

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the spring takes over the bottom and drops away and the switch opens. Which says, we’ve got no flow.

Transcript for the Laser Tube Cooling System (Cont…)

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And it’s a very important part of the protection system on your machine, to make sure you don’t have an expensive accident.

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And after two or three years, you might be cutting and all of a sudden the beam stops.

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The machine keeps cutting, but the beam just stops. What has happened?

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The tube is broken. You go into panic.

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Well, first thing you do is you go and have a look at your keyboard and on the keyboard it will say water error.

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The first thing to do is go to your pump and check that it is doing what you expect it to do.

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It’s pumping and therefore you realize that the only thing that’s wrong is the switch is telling you lies.

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Now, it does mean to say that you need to replace your flow switch. If you know that your pump is still working and everything is OK,

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and the only thing that’s wrong is this. These two wires here go back to your controller, turn the machine off first, snip the wires and then do this.

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You’ve just told the machine now the flow switch is working.

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That’s now your flow switch, which obviously isn’t a flow switch, but it completes the circuit and tells the brain everything’s OK, carry on.

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Now, you must be very, very, very careful when you do this, to keep an eye that your machine is still actually working.

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Your pump is working, because in the meantime, you can go out and buy a new flow switch to replace the one that you’ve got.

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These are only probably four, five, six, seven dollars or something like that.

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I’ve just had a failure on one of my machines as well, and I bought one of these as a replacement for one that I replaced about five years ago.

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This time I stopped and I looked at it very carefully. In this end here, I’ve got a little spool that goes in and

Transcript for the Laser Tube Cooling System (Cont…) out.

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But the problem is when I blow on this, I realize that it takes maybe two, maybe two psi or more to actually cause that bobbin to move out the way.

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So I’m actually using some of my valuable pressure to just open the switch.

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And losing pressure means that I’m actually reducing the flow rate through the system.

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Lower pressure means lower flow rate.

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So instead of fitting that back on my machine and my machine is currently like this, oops, I decided I would look at an alternative system.

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And here is the alternative system. It’s exactly the same format as this one with two 1/2 inch threads on the end.

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So it will go in exactly the same place that I’m going to take this one out of. This time,

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it has got no internal restriction. It’s a straight through passage with a little rotor inside, a little fan, a little turbine.

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That’s spinning round. I have actually now doubled the flow rate through my tube.

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I’ve removed the pressure from the system.

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And this is an amazing advantage as far as I’m concerned ok. Because it improves the cooling ability of the water that I’m pumping past my tube.

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The only problem is it’s not a simple,

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two wire switch system. This is, I’m afraid, an electronic Hall effect switch, which has got five volts going into it,

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and the yellow wire that’s coming out of it, is supplying a digital square wave signal that is proportional to the speed of the rotor.

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Let me just show you on the oscilloscope. So there’s a signal that’s being generated by this piece of electronic equipment.

Transcript for the Laser Tube Cooling System (Cont…)

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And as I strangle the flow, the number of signals per second changes.

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And what we have to do is to run this into a special circuit.

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If it drops below a certain pulse count, there’s insufficient flow, shut off the machine.

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So that’s full flow. So we’ve got lots of pulses to count.

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So this requires special electronics, which I do not have at the moment, but I’m in the process of getting something designed.

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This pump is capable of delivering more than four liters a minute and what’s coming out of here at the moment is four liters a minute.

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So the pressure loss or flow loss through this turbine is nil.

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When I try and run this, in this same test circuit, this delivers one point eight liters a minute.

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There’s a huge pressure loss caused by the spring inside this type of valve.

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Now, this here you’ll find often on machines, this has got exactly the same sort of system inside.

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It might not look the same, but it is exactly the same system.

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So both of these are very inefficient. I will show you on my other machine what we do to protect the tube against freezing during the winter months.

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If we’re not using antifreeze, we’ve somehow got to make sure that the machine stays warm.

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Well, there are two ways that you can do that depending on your climate.

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I have found here in the U.K. that all I’ve got to do is to leave the pump on.

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So when it’s a cold night coming up, I’ll just leave the pump on and the pump generates about 30 or 40 watts and there’s enough heat coming from the

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pump into the water to stop it from freezing and circulating the water through the tube keeps everything safe.

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If your temperature gets a bit colder than that, then we need to look at option two,

Transcript for the Laser Tube Cooling System (Cont…)

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and that is to add some physical heating. Something like a little greenhouse heater inside your machine.

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So remember the reason why I’m showing you this little turbine here, as we’re trying to maximize the flow rate of water through the tube.

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Now, these tubes here are about 10 millimeters diameter.

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But if we take a look in the end of the tube itself, we see we’ve only got about five millimeter bore.

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So we’ve already got a restriction there. That’s quite a big hole.

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But in the other end of the tube where the water flows in, we’ve got a very,

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very small inlet there that might be four millimeters, maybe three millimeters diameter.

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So that’s by far and away the biggest restriction in the system.

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So we got to make sure that we have the maximum amount of pressure running into the inlet of this tube,

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which is what you don’t have don’t want to throw any pressure away with this

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being at the wrong height or loss of pressure because of your flow switch.

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You could go silly and put a really big pump in.

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Look, this is a 60 watt pump and a 60 watt pump is capable of getting up to three meter, 10 foot head. Well 10 foot is still only four, just over four PSI.

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So even if you put a monster pump in like this, you’re still not going to generate huge amounts of pressure.

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So don’t try and increase the pump, try and decrease the pressure loss in your system. If you’ve got one of these industrial chillers.

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This is called an industrial chiller, but it’s not! Basically all that is, is a pump, a reservoir, a fan and a radiator.

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It’s a cooling system like you have on your car. This is a CS-3000.

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If you go for the CW-5000, that has a proper refrigeration cooling system built into it as well.

Transcript for the Laser Tube Cooling System (Cont…)

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Both of those systems have got a very powerful and strong pump which is capable of delivering probably the four or five liters a minute that these

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expensive and high powered tubes require. And high powered tubes normally would

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be supplied or be requesting to use something like an industrial chiller.

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This is servicing a 70 watt tube and it works perfectly ok.

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But here I am in the UK, where most of the year, the temperatures about 20 degrees C or less, it rarely gets below maybe minus five and only on a few

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odd days does it get to plus 30. Now, at plus 30,

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this is actually taking heat out of the air and adding it to the water.

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So this is no good at all, if you’re in a very hot climate, you must have a CW-5200 or 5000 with a refrigerator built into it. Most of the year,

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I can get away with something as simple as this.

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I don’t need to spend three or four hundred dollars on a chiller. If I put something like around about 12 to 15 liters of water in this tank.

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It’s a very thin plastic tank. The heat lost through the side will be sufficient most of the year to service my machine without any trouble at all.

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Winterproof in the UK means we have to survive occasional very cold weather spells,

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but we’re talking about maybe at the most, a week of subzero temperatures that could go down minus three, minus five.

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Not every year. But I can’t take the risk that that tube full of water there will freeze. So consequently, that’s why I’m winterizing this machine.

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Winterizing comprises of two things, really.

Transcript for the Laser Tube Cooling System (Cont…)

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First of all, I’ve added some halogen lights and that’s a 100 watts of power there, which will be immediately in this tube enclosure.

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But because the enclosure is not completely sealed,

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the heat will percolate into the main body of the machine as well and prevent condensation in there.

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The other part of the exercise is to. Winterize, the chiller, or the water system, and I’ve done the same thing there.

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If you look inside the chiller unit, I’ve installed a couple of those halogen lights as well.

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And if we look at it from the other direction, you can see that those halogen lights are, in fact, pointing at the cooling coil to heat it.

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I’m also producing a little control box, which I’ll show you as well.

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Okay, well, now we’ve got the winterizing kit controller mounted on the back here.

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And basically what we’ve got, we’ve got a cable that comes in here from an external power supply.

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Now, this power supply here, which is regulated by the machine itself,

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in other words, when the machine is on, this power supply is on. And when the machine is off,

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there’s no power in this line. So the machine is off at this moment in time.

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So there is no power in this line. But this line would normally go to the chiller.

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But this time, I’ve intercepted that line here and it goes into the normally open contacts on the relay.

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And it also goes on to the coil for the relay as well.

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So when I turn the machine on, the coil will energize and it will connect this,

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through here, through that relay and out to the chiller and the chiller will operate as normal.

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OK, so you can see the controller coming on and now it says the temperature is too low.

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Switch on the lighting, the heating. I mean, I’ve got this thing set at the moment, to far too high.

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The temperature in here measured by this probe says that it’s 10.4 degrees C.

Transcript for the Laser Tube Cooling System (Cont…)

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Well the intention is to put that probe inside the main body of the machine.

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And when the temperature in the main body of the machine, the air temperature in that body, drops down to six degrees C, it will do just this.

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The machine will be off the pump is on, the water is circulating through the tube and the lights are on in here.

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So there’s heat inside this cabinet. There’s heat down there where the coils are likely to get cold near the tank.

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So in effect, we’ve got 200 watts of energy that could be going into the water system to keep it warm.

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And if I turn the machine on, the power on and then I turn the machine on itself.

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Nothing changes, because the water temperature is still too low. So what I’ll to do at the moment,

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I will just reset this because I’ve set this purposely high to demonstrate.

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So the intention would be to have this set to something like about six degrees C. So,

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and I will run that down to six degrees C. Six degrees C.

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And the lights go off. That’s because the air temperature in here is 12 degrees C, so we don’t need the heating and if I turn the machine off.

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Although that system is nowlive, nothing’s happening because it’s controlled by this temperature.

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So when the air temperature drops down to six degrees C, the heating in there and the pump will go on and everything will circulate.

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And they we are, now that the machine is in place. Carefully sitting there.

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Watching my machine doesn’t freeze.

Transcript for the Laser Tube Cooling System