DI vs. Distilled

[planters]

I have several lasers that require DI water in their cooling loops. They all have DI filters to maintain the water quality. I can easily obtain distilled water with a resistance of 1 Meg Ohm to fill and replenish the cooling loops, but to obtain fresh DI water is much harder. How necessary is the use of DI water if the filter is in place? Really. Not just "you should use the purest water you can obtain". How much ionic content does that jump in resistance represent re the DI filter capacity?

[NobleGas]

I was reading about this a few days ago; it was suggested that 'steam distilled' water is ideal; DI water that has a very high resistance is actually corrosive and can strip metals out of the system - even the gold in a lamp pumped YAG can dissolve! I have a vague feeling the document was published by Lee Lasers.

[aktopjian]

I was just wonder if something like this would work to make water http://www.bulkreefsupply.com/brs-4-...m-75gpd-1.html . I have one for my saltwater tanks and get a TDS of 0 out of it. I have one more stage though and my house water measures at 35 tds.

[NobleGas]

This is the article I was reading: Lee Lasers Water

[Photonbeam]

Is the water in contact with an electrode? I would think this is the main reason the resistivity is important - to protect the power supply from shorting to ground through the water. Deionized water starts out as distilled water but distilled water is not necessarily deionized. Deionizing cartridges are usually in a bypass loop of the cooling system. Your change of resistivity measurement while in use gives the answer.

[planters]

Yes, the water is in contact with the electrodes. In one case the potential is 12V (micro-channel cooler). In an other it is a LS with a 4KV trigger followed by several hundred continuous volts. In one it is 4 KV pulsed to Xenon flash lamps.

The 1 meg water through 20 or so cm of water would be no more than a watt of conduction in the worst situation. I suspect that electrolysis may be a bigger threat. I don't know, but would the DI filter be much used up to bring 1 meg water to its equilibrium state?

I think the Lee laser link might be the answer. They are filling with distilled and maintained with the DI filter.

[kecked]

RO water is pretty close to DI water. TDS is usually around 1-2 ppm coming from the cheap fishtank water filters. IF there is a DI filter after that it is called RO/DI. So if you have a DI filter in the laser and you load distilled you end up with RO/DI water anyway. Once the water flows through the filter it should be clean. It should last a long time unless the laser is dumping ions into the water from the electrodes. If it is the DI filter will pull those ions from the water and refresh it. So the rate of use depends on the lasers rate of ruining the water. No matter what you put in the system in terms of water, the laser is the limiting factor.

[Fearless]

Hey Eric, I've got 18 meg Ohm DD, Di HOH--all the water you could possibly want out of the tap here at the lab, but I suspect the hospital has pretty good water. We have Water's filtration units next to our mass spec equipment which brings up the water to the absolute limit of 18 meg Ohm. By the way, it tastes pretty good as well.

[mixedgas]

This really depends on how the laser cooling was designed.
!
DI must be used on equipment with submerged lamp electrodes, PERIOD. While the usual DI filter cartridge costs 80-120$, I have a source of the damn things direct from the OEM at 30$. Which are quite good. In fact better then the "factory specified " cartridges. A Laserist friend has a 80 gallon aquarium, and he located the OEM filters. Which when placed in his Laserscopes, eliminated the sizzle sound from the electrodes.
!
!
I take care of my present department's 18 Meg DI/RO system, and I can say without a doubt that 18 Meg is down to 8 Meg after setting in plastic or glass for a week. 18 Strips ions from ANYTHING it can. I've found there is no standard for grocery store distilled water, and in fact, without a conductivity probe, you cant tell a damn thing.
!
For my last employer, a major maker of flashlamp pumped laboratory lasers, we tested a couple of brands of grocery store water, and found they were anything but clean. Three store brands that were advertised as distilled were nothing more then filtered tap water, no distillation at all. Our goal was to stop algae that build up in lamp pumped lasers that do not use DI, and many cities tap city water contains live algae. Much to our amazement, our lasers often contained anaerobic Algae that thrived on plasticizer in the flexible tubing, and off what ever biomaterial made it thru a 2 mm air vent in the cooling tank. These globs of Algae would build up in a year or so, and clog the flow sensors we used. Beautiful red algae, but often resulted in a needless service call for our less skilled customers. So we went to a tiny DI cartridge receiving less then 5% of the coolant flow, and a better filter cartridge. We then specified a 180$ a gallon low phosphorous, no acid, pH controlled cleaning fluid, to be used every six months.

!
I can personally attest that the water around arc lamps must be 8 Meg or Higher, and at 6-7 Meg you can hear the arc current sizzle or hiss into the cooling water at high currents. This strips the gold off the spider clips that go around the lamp, ensuring they will corrode and fail. Then your out 120-240$ for Spider clips, if you can find a source. If the spiders fail, you'll blow a lamp. We just ordered 280$ worth of clips for two LEEs, here at work, as the previous user let the DI conductivity take a dive and ran the laser till the water quality warning light came on, then ran more it till the water quality interlock tripped.
!
I've been in the room when another friend's Laserscope ALE supply failed due to poor water, it arced in a swirl around the lamp at 35 amps, shattering it, then arced across the gold, ruining the reflector, then the ALE failed dead. Laserscope specs a minimum of 8-10 Meg in some of their manuals.
!
On professional lasers that do not have submerged electrodes, a filter and an oxygen removal cartridge are often used.
!
DI/RO corrodes aluminum like there is no tomorrow. Conversely, a few years ago I had to mill a whole new cooling block for a $80,000 Coherent Verdi, because the Post-Doc who maintained the system for a year filled all lasers in the lab with 18 Meg DI weekly. That laser was designed for anything but DI. The chiller plumbing was coming apart at the seams, and I had to rebuild much of the chiller.

!
Coherent puts a tiny cartridge into their closed loop lasers that strips oxygen, and its worth every penny.
!
Without a conductivity probe, you have no idea what your getting from store bought water. !

Steve

[planters]

Steve,

I do have a conductivity probe and the store bought distilled water varies, but not wildly. I see 800K Ohm to 1.5M Ohm. As you say, the 18M Ohm water drops quickly during storage. My Laserscope water tests at 3-4M Ohm in the tank. There is a DI filter loop, but at what measured point does it need replacement? I haven't noticed the fizzle, but it may be very quiet? Also, how does this work? The operating current for one of these lamps is about 40A which means between 100 and 200 V at the nominal 4-5kW. 150V at 1.5M Ohm is 0.1mA or 15mW. Is this a threat?

Hey Eric, I've got 18 meg Ohm DD, Di HOH--all the water you could possibly want out of the tap here at the lab

I thought about installing a small unit (had a good sized one at one point for a marine aquarium), but I suspected that the practical issues such as storage and the ion build up in the operating laser as Kecked suggested were the drivers (so to speak) and the actual ion load from 1 meg to even 18 meg might be a small fraction of the capacity of the DI filter in the loop, but I don't know.