Reef Crystals... UGH!

[sfsuphysics]

And I'm not talking about some massive algae bloom/die-off/etc.

According to the directions it says 1.4 pounds brings 5 gallons of water up-to a specific gravity of 1.021.

I have 30.6 gallons of water, and I want to bring it up to a specific gravity of 1.024.

So I figure 30.6g * 1.4 lbs / 5g = 8.568 lbs to bring it up to 1.021, but I want it to 1.024 (and I assume it's a linear relationship?) 8.568 lbs * 1.024 sg / 1.021 sg = 8.593 lbs. (So I added 8.6 pounds, measured out on a scale)

Now it hasn't been 24 hours since I tested it , but I did a preliminary test to see if it's at least close and I got 1.016! Even if I screwed up on my assumption that specific gravity rose in a linear fashion with weight of added salt mix it should at least be about 1.021 (we'll see again tomorrow though).

The purpose is I'd like to find a way to automate this water change, by knowing exactly how much salt I need to add for a specific level, so I can make some sort of container that has "the level" to fill with salt rather than doing multiple measurements where I might screw up with +/- here or there.

So my question is.... Is this normal? A known issue with RC salt? Or will it actually stabilize by tomorrow?
I did stick a heater in there to bring it up to 77-78 degrees, it might be a smidge under (unsure how long it takes to heat up 30.6g of water) but could that cause a sg difference that high?

 

[tuberider]

Dunno, I whip it up with the 1 cup per 2 gallons rule, heaping if I'm making salt for a reef, or level for fish only. That usually puts me in the right place.

I've not had that happen with reefcrystals or any other salt for that matter, I'd try measuring by volume and see what happens and save the scale for baking.

 

[sfsuphysics]

option B is that my refractometer is really out of whack too! Which would worry me because my tank is reading 1.024 right now.

 

[Mr. Ugly]

[quote author=sfsuphysics link=topic=3849.msg45004#msg45004 date=1212970077]
So I figure 30.6g * 1.4 lbs / 5g = 8.568 lbs to bring it up to 1.021, but I want it to 1.024 (and I assume it's a linear relationship?) 8.568 lbs * 1.024 sg / 1.021 sg = 8.593 lbs. [/quote]

First calculation is correct.

The second one should be:

8.568 * 24/21 = 10.2

In actual practice, the volume will increase a bit when you dissolve the salt, but it should be pretty close.

 

[Mr. Ugly]

Or...

.024/.021, but same difference.

The .024 is contributed by the salt. The 1 is from the water.

 

[Mr. Ugly]

Also, volume measurement is probably better than weight.

Weight measurements are affected by hydration of the salt from moisture in the air.

 

[Gomer]

Also, since the salt is hygroscopic, as it "ages" an equal weight will result in a lower salinity.

 

[Mr. Ugly]

Yup!

 

[sfsuphysics]

Ahhh thanks Norman, I thought that calculation was a little off since it didn't seem like much of a change in weight for a significantly higher specific gravity, seems I was off by almost 2 pounds of salt! However I'm guessing that whole hygoscopic dohickey stuff is the reason it wasn't even at 1.021 which it technically should have been since I added a smidge more than what was what it said would bring it up to that level.

Also I need to whip up some reference solution to test my refractometer, I can make it read 1.000 with RO/DI water, but I'm taking a big assumption in that the scale of any calibration will be linear.

 

[Gomer]

Well, the thing with a refractive index device is, it is good for 1.000 and the markings are accurate from the factory (or good enough when you initially bought it), then 1.000 will always be all you need to calibrate.

 

[sfsuphysics]

Hmm, good enough for me!

 

[sfsuphysics]

The plot thickens!

I used my RO/DI water on my refractometer, low and behold, it's reading below the 1.000 mark! Now this worries me because my tank is set at 1.024, which going by how much it might actually be at 1.031! (explains why a few corals are doing kind of crappy!)

I tried to calibrate the refractometer but the little screw is frozen tight, might be rust from years of taking readings, but if the little bit of liquid wrench doesn't fix it, I'm going to be getting a new one.

So here's the next brilliant question, how fast should I drop the specific gravity? I know I can basically do a small water change with freshwater but I don't want to go too fast down, plus I want to make sure I can accurately read the specific gravity before I go to quickly.

 

[Gomer]

if you have a slow flow ATO, I would just start removing some salt water from your tank over time.

 

[stubbsz]

[quote author=sfsuphysics link=topic=3849.msg45059#msg45059 date=1213025581]
The plot thickens!

I used my RO/DI water on my refractometer, low and behold, it's reading below the 1.000 mark! Now this worries me because my tank is set at 1.024, which going by how much it might actually be at 1.031! (explains why a few corals are doing kind of crappy!)

I tried to calibrate the refractometer but the little screw is frozen tight, might be rust from years of taking readings, but if the little bit of liquid wrench doesn't fix it, I'm going to be getting a new one.

So here's the next brilliant question, how fast should I drop the specific gravity? I know I can basically do a small water change with freshwater but I don't want to go too fast down, plus I want to make sure I can accurately read the specific gravity before I go to quickly.

[/quote]
Eek, I wonder how long you were running high salinity. I know i did it for a long time... it was a slow decline of all Hard corals and some zoa. Also wiped out my Aiptasias.

 

[sfsuphysics]

Tony, no ATO here

Stubbz, yeah I have aiptasia that are alive, so maybe it's not TOO bad however my corals seem to be doing ok, with a couple exceptions... either way I'll try to bring it down maybe .001 per day or something.

 

[Gomer]

well, you can MTO remove a pint, slowly replace a pint. IIRC, your tank is atleast 50g, so this would slowly bring it down with no big spikes. You can probably do this every 1-2 hours over the course of 2-3 days.

 

[sfsuphysics]

the tank is 100g, the sump is 50g (maybe 3/4 of that is water), hell when I do a top off I usually dump in 5g of freshwater at a time

 

[Gomer]

then you can easily remove and replace 1g every hour and slowly and safely shift your SG.

 

[Mr. Ugly]

Mike,

Read this article. Especially the part on refractometers and slope error.

http://reefkeeping.com/issues/2006-12/rhf/index.php

I made some calibration standard a while back. I can bring it to the meeting for you.

 

[sfsuphysics]

Great Norman, I'd love some of your standards that I keep hearing you talk about

I went ahead an ordered a new refractometer from marinedepot anyways, mine has run it's time, the plastic cover broke off to the point where I was simply using the top of a deli container to hold the water flat, and there was a huge crack in the glass somewhere... it's time for a replacing, but I'll definitely want to calibrate the new one.