[nvshooter2276]

One ounce per five gallons of water. In metric measurements, that's 28.35 cc in 18,935 cc of water.

28.35 grams divided by 18,935 grams = 0.0014972. We'll call it 0.0015. Exactly what is that measurement? Is it in reality 1.5 grams, which would be 1.5 cc? I bought a small-volume eyedropper that's used for dosing liquid medicine to infants; goes from 1 cc to 5 cc. The eyedropper came with a graduated cylinder that goes from 1 cc to 10 cc. Would I use 1.5 cc of Streisand in one gallon of water? No; 28.35 x 0.2 = 5.67 cc in one gallon. So nice that we cleared that up...

I have read that mixing-up Star*San well in advance is not a good idea because it loses its strength over time. Any truth to that ugly rumour? Does it turn back into just water if it sets for long periods of time?

[mainebrewer]

If you mix Starsan with distilled water and keep it in a covered container, it will stay good for a long time.

[KellerBrauer]

I put 2oz. of StarSan in a bucket with 5 gallons of water for sanitizing small parts, tubing, etc. and it lasts for a few months. I know this because the mixture foams when agitated.  It may even last longer.  But I replace the mixture after a few months.

[Bob357]

6 ml/gallon is what most folks use.
I mix with RO or distilled water and use it for 4 to 6 months.

[CounterPressure]

One ounce per five gallons of water. In metric measurements, that's 28.35 cc in 18,935 cc of water.

28.35 grams divided by 18,935 grams = 0.0014972. We'll call it 0.0015. Exactly what is that measurement? Is it in reality 1.5 grams, which would be 1.5 cc? I bought a small-volume eyedropper that's used for dosing liquid medicine to infants; goes from 1 cc to 5 cc. The eyedropper came with a graduated cylinder that goes from 1 cc to 10 cc. Would I use 1.5 cc of Streisand in one gallon of water? No; 28.35 x 0.2 = 5.67 cc in one gallon. So nice that we cleared that up...

I have read that mixing-up Star*San well in advance is not a good idea because it loses its strength over time. Any truth to that ugly rumour? Does it turn back into just water if it sets for long periods of time?

Not to nit pick, but you're conversion is incorrect. Or maybe better stated, you're doing a conversion on numbers that are already in the stated units.  .0015 GRAMS IS the amount per CC.  So when you ask about 1.5g, yes, 1.5g is indeed 1.5cc, but you're not asking about 1.5g, you're asking about 0.0015g. 
Water is ~ 1g / CC.

[Drewch]

6 ml/gallon is what most folks use.

This is what I do, too.

The directions are given as "1 oz" per 5 gallons.  It's ambiguous as to whether that's fluid ounce or weight, but since the little dosing chamber on the bottle is graduated in "ml" and "oz", I've always assumed they meant "1 oz" volume and hence fluid ounce.

1 US fluid ounce is 29.57 milliliters.

29.57 ml / 5 gal = 5.91 ml / gal ≈ 6 ml / gal

29.57 ml / 18.93 liters = 1.56 ml / l  or 0.00156 l / l

[Drewch]

Water is ~ 1g / CC.

We can't really jump back & forth between volume and mass, though, because we don't know the specific gravity of StarSan concentrate.

[CounterPressure]

I have read that mixing-up Star*San well in advance is not a good idea because it loses its strength over time. Any truth to that ugly rumour? Does it turn back into just water if it sets for long periods of time?

I think there's a fair bit of conjecture in the whole discussion.  For me, out of pure ignorance of the mfgr recommendation, I kept the stuff for months in a 5 gallon bucket with a lid, and would just change it out when the mood struck me.  I never knew there was a recommendation to change it every batch.  Did that hurt anything? No.  But that might be because I'm really fussy about cleaning.  After stopping in here again after many years, and reading a bunch, lots of people think cleaning is more important than sanitizing.  Maybe that's why my old sanitizer works so well is that it has nothing to sanitize.  I do what's worked, and replies above seem to indicate plenty of people do the same thing.  Maybe we're all wrong about letting it sit around... As long as the beer isn't contaminated, I'm going with it till someone convinces me there's a danger otherwise.

As to the concentration for a small batch, I never cared. When I make a new 5 gal bucket of it I dump what's in my countertop spray bottle and refill it from the new bucket and that's worked for me.  I am never without a 5 gallon bucket of it sitting with the lid on. I simply use it too much.

When I run a bottle tree (rarely bottle) , I use saniclean for the low foaming.  The recommended concentration is higher on that.  1.6oz/5gal if I recall correctly.  You need more than a full measure on the container anyway.

[CounterPressure]

Water is ~ 1g / CC.

We can't really jump back & forth between volume and mass, though, because we don't know the specific gravity of StarSan concentrate.

Which is why I included the "~" (meaning roughly) is because I knew someone would mention the SG of Starsan infinitesimally influencing the 1G/CC assumption, would get corrected if I didn't. Lol. It's the internet!

Let's just say that in school I was taught the metric system G and CC units were based upon a conversion with water and bla bla bla.  I'm not interested in debating the minutia of how pure the water must be in a lab and that sort of thing, or even if that was indeed the reason they used CCs and G and etc...  For my sanitizing, I'll just assume the gravity of my star san solution in the bucket is close enough to 1.000G/CC that I'm not concerned with carrying the empty fermenter with the extra weight.  Lol.

Sorry, couldn't resist that.

[Richard]

If you mix StarSan with RO or distilled water it will last a long time. If you mix it with tap water you may find that it only lasts a day before it becomes cloudy and slimy. StarSan is an acid and if you mix it with water that has high alkalinity it gets neutralized. If you want to see whether it is still good you can measure the pH. If it is still below ~3.5 it is probably OK, but even then its ability to coat surfaces degrades with time.

[Drewch]

I'll just assume the gravity of my star san solution in the bucket is close enough to 1.000G/CC that I'm not concerned with carrying the empty fermenter with the extra weight.  Lol.

Sorry, couldn't resist that.

We're talking at cross purposes, I think. Yes, the mixed StarSan solution is going to so near to 1.000 that it makes no difference.

But the OP was jumping between volume & mass with the StarSan concentrate which is so dense my hydrometer barely sinks into it. Their 1.5 g of StarSan concentrate is going to be significantly different from 1.5 ml.

[CounterPressure]

I'll just assume the gravity of my star san solution in the bucket is close enough to 1.000G/CC that I'm not concerned with carrying the empty fermenter with the extra weight.  Lol.

Sorry, couldn't resist that.

We're talking at cross purposes, I think. Yes, the mixed StarSan solution is going to so near to 1.000 that it makes no difference.

But the OP was jumping between volume & mass with the StarSan concentrate which is so dense my hydrometer barely sinks into it. Their 1.5 g of StarSan concentrate is going to be significantly different from 1.5 ml.

I just saw he was trying to measure with an eye-dropper gizmo and was still in volume units, so that was plenty close enough.  But he had made a conversion error of 3 orders of magnitude on grams which was most certainly not ok.  Well, maybe it was OK because he reversed the error in the other direction without mentioning another conversion.  

I didn't put it together that he was trying to measure the concentrate by weight, I just saw volume.  Even if he did, the concentration is still probably so tolerant of error as to be insignificant.  I've never bothered to check the gravity of my StarSan, I'm LOL now even thinking about it.  What is the gravity?  I'm not sure there's enough left in my bottle to fill a hydrometer jar.   I see what you're talking about now, and honestly didn't make the connection that he (or someone) might attempt to weigh the stuff.  I do have a scale where I can do that (1mg resolution).  I don't use it for such things!  I do use it for weighing mineral additions after using BrunWater.  I think mixing a bucket with 1oz and stealing a spray bottle full, before sticking a lid on the bucket is a great way to avoid all this mathematics.

[Drewch]

I've never bothered to check the gravity of my StarSan, I'm LOL now even thinking about it.  What is the gravity?

Yeah, I tried once because I'm a nerd through and through.

Refractometer was useless; it doesn't change the refraction index the same way dissolved sugar does.  My hydrometer floated with only about half the bulb submerged.  I never got around to just weighing a ml directly.

[CounterPressure]

Yeah, I tried once because I'm a nerd through and through.

Refractometer was useless; it doesn't change the refraction index the same way dissolved sugar does.  My hydrometer floated with only about half the bulb submerged.  I never got around to just weighing a ml directly.

Fair enough, from one nerd to another then!

Mine is about empty. (Probably still last another decade!).   I'll write a note on it to save the bottle and weigh it empty, then compare to the new bottle with the old one as the tare weight.  We'll get to the bottom of this!  lol.

[nvshooter2276]

If you mix StarSan with RO or distilled water, it will last a long time. . .

I did not know that. I have multiple 2.5-gallon bottles that once held Diesel Exhaust Fluid. A half-ounce of Star*San in that volume of distilled water would be nice to have at-the-ready for whatever purpose confronting me at whatever time. I buy distilled water for my CPAP machine, so its purchase is not something that would give me pause.