This is what I purchase from Walmart. Steam distilled.
https://www.walmart.com/ip/Great-Value-Distilled-Water-1-Gallon/10315382
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They only had six gallons of that one available so I bought the rest in that other one without realizing that it won’t work.
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So, gotta buy more of the good stuff and find other uses for the “enhanced flavor” stuff?
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Or return it? I mean, I don’t know how much of the minerals they add, but I wouldn’t risk my laser tube on it. I think I paid 89¢/gallon for proper distilled water at the grocery store.
I guess you could be mini-prepper and store it all to be prepared for a very short apocalypse? 
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24 gallons is a four day emergency supply for my family. Better be a short apocalypse! I haven’t been in a walmart since March. I have enough plants to use this on.
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I have updated Cooling the K40 laser tube to mention the problem with distilled water with minerals added. Note that it has a recommendation for a specific algaecide, as well.
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I found that home depot carries distilled water (the good stuff) in one gallon containers in a store near me for a good price. I will get it there. Thank you for the help!
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At some point in time, it has got to become worthwhile to just have your own distiller, right? I don’t remember numbers, but I believe we were getting the PPM so low that the water would have counted as deionized, not just distilled.
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It would be useful to measure the conductivity of this mineralized & distilled water to see where its conductivity sits on the spectrum of fluids we have tested.
Consider we have some sense of the levels of conductivity that don’t work but really never identified the conductivity level that is “acceptable”.
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I found that there are a lot of different mineralized distilled waters out there, and I see no particular reason to think that they have similar conductivity. “Salt to taste” and all that…
The fact that we don’t know what the limits of acceptable conductivity are — only that it’s very clear that too high conductivity is bad — and the probability that it is different depending on the electrical characteristics of a machine, combined with how cheap it is to replace especially vs. the cost of tube and LPS, made me think that the easiest answer is “just don’t go there.”
All that said, I understand that you actually have the educational background and work experience here to do it right, and as a software guy who just dabbles here, I don’t. I just remember reading this and realizing that it is not as trivial as using a VMM to measure resistance…
I can’t remember whether you were using AgCl electrodes or what when you characterized conductivity, but it did seem to me like something where it was easy for someone like me to confidently come to the wrong conclusion from lack of knowledge. 
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Correct a VMM is not an accurate way to measure conductivity. I used a conductivity meter designed for this measurement.
My point was only that “someone” should measure it and add it to our data. I will if I find this type of water, I have not noticed it in on our walmart’s shelves.
Some day I will get an old tube and measure and characterize coolant conductivity effects systematically. For now the right approach is “don’t go there”.
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FWIW, I tested the mineralized distilled water with a TDS meter. It came out to 10ppm. The regular distilled was 0-1ppm. My tap water was 400ppm.
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Does your meter have a conductivity mode?
TDS and conductivity are related but not the same thing.
You can convert TDS to conductivity if you have the meters TDS to Conductivity conversion factor.
Note: I have been using a Com-100
Useful link: http://hmdigital.com/education-center/
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Thanks, Don. My current meter does not. However, it wouldn’t be terrible to pick one up that does. I will look at the model that you use.
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Here is where I got mine: https://amzn.to/2Xh5BGk
As an Amazon Affiliate, I earn from qualifying purchases.
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FYI: algaecide will eat away at the plastic in your pump fairly quickly. More than bleach in some circumstances. Better to just turn over the water more often.
Hi @Bailey_Steinfadt! Welcome to Maker Forums!
Do you have a citation on that damage for the one drop per gallon concentration of the tetra aglae control that’s recommended here? I know that folks here have been running that concentration for years without problems, so I’d like to know more about that.
No citation for Tetra… will look something up here soon if I can.
I worked in a multi-acre greenhouse and the Kleengro we used DESTROYED everything petroleum-based that it touched. Any plastics or oil based lubricants had to be replaced frequently, but at that scale, the tradeoff was worth it because nobody wants to go scrub out dozens of underground tanks. Bleach will do less to the plastics, but more damage to the metal in the system, so we stuck with Kleengro and constant pump-swaps.
For my lasers, and with that experience, I figure swapping the distilled water out and cleaning my reservoir on a regular basis is cheaper and less mental load than dealing with chemical agents. If you’ve cleaned properly, kept sunlight out, and used true distilled like you mentioned (now I have to go check what the maker space guys brought me last time…), you shouldn’t have too much growth anyway.
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Ah, I’m not the only one here who’s worked in a huge greenhouse—though that’s now decades ago. 
Kleengro is 2-Propanol, Ethanol, and Didecyl Dimethyl Ammonium Chloride (used in high concentrations), whereas Tetra Algae Control is Poly[Oxyethylene(Dimethyliminio) Ethylene (Dimethyliminio) Ethylene Dichloride] (used in extremely low concentrations).
Also, Kleengro is categorized as an acute health and fire hazard, whereas Tetra Algae Control is not categorized as any hazard (just don’t squirt it in your eyes; it’s a mild eye irritant).
So I think we’re quite safe here! 
Thanks!
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As a chemist I agree, the concentration we recommend for Tetra Algae Control is so low ( to minimize conductivity) it’s unlikely to have any effect on plastics.
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