Are we incorrectly evaluating specific gravity?

[timdanger]

debated putting this in the equipment forum, but it's really more of a water chemistry issue.

 

reefbuilders has a new blog entry out about the new D-D h2o salinity refractometer. it supposedly measures the salinity of seawater as opposed to sodium chloride/brine solution, which is what most hobbyist refractometers measure. briefly, they are saying that in order to get seawater salinity to 35ppt, you would have to get a typical sodium chloride/brine solution refractometer to measure 36.5ppt. this discrepancy is causing people to get lower numbers than they expect for ca/alk/mg when they mix up salt.

 

very interesting stuff. anyone else have comments? does this mean i should be measuring for 36.5 instead of 35 with my regular old marine depot refractometer?

[johnmaloney]

what salinity do you want? changes all the time in the ocean, I guess just pick one and keep it stable....I wouldn't change the salinity in your tank to increase the other parameters - for me it would/is easier to have a separate method of handling them.

[cruiZe]

so run ur tank at 1.026 instead of 1.025... If its only $50 that sounds cool I guess...

 

agreed with John (of course) stability is key. and more important than if 1.024 1.025 1.026 is the best

[franklypre]

I quit using refractometers because they weren't accurate as well got me a pinpoint, LOVE IT. I definately agree with using other methods for alk/calc control, unless you want to change water every other day or two.

[timdanger]

i feel like my point is different than what's being responded to. my point is that, when we mix up our water to 35ppt, and wonder why we get lower than expected Ca/Alk/Mg readings, maybe it's because we aren't mixing to the recommended specific gravity. so, maybe everyone who is saying "my SG is 1.026" shouldn't be so confused as to why they then have to adjust the calcium, magnesium, etc. after mixing up to what their refractometer reads as 1.026/35ppt.

 

to answer your question, john, i don't have a particular salinity that i "want" -- i actually really don't care, so long as it's stable -- however, i do have Ca/Alk/Mg numbers that I want. so, that's why it's relevant. sure, you can adjust your Ca/Alk/Mg after the fact, but maybe it would be easier if we just mixed up to the recommended salinity to get a salt's "advertised" parameters. also, if we're not using enough salt, we also may not be getting the trace elements we might be expecting.

[doctaq]

im pretty sure the difference in concentration is negligible

 

if 35ppt gives you 400 ppm ca just for the sake of calculation

36 ppt would give you 411 ppm, and that 11 isnt even going to show on a test kit for the most part

[Amphiprion1]

Using a known seawater standard skirts this problem pretty much entirely. It will get you very, very close with minimized error. That's what should be done on any refractometer, seawater or NaCl specific. The associated error (ideally speaking) is also pretty slight to begin with, as doctaq stated.

 

Edit: more specifically, something along the lines of .7 ppt or about .0003 specific gravity units. Not very significant, IMHO.

[star27624]

And as long as you are making your saltwater from a bucket/bag of mixed salts, your levels will vary batch to batch. The salt mixes we buy to make our salt water are heterogeneous mixtures, so each batch we mix from it will be slightly different. To achieve true consistency, you either have to mix the entire batch up at once, or make homogeneous mixtures (solutions) of the components (various salts) in our salt mixes. Only one salt manufacturer has done this so far, and not completely as they still have some dry components to their mix.

[timdanger]

im pretty sure the difference in concentration is negligible

 

if 35ppt gives you 400 ppm ca just for the sake of calculation

36 ppt would give you 411 ppm, and that 11 isnt even going to show on a test kit for the most part

 

fair point, but also keep in mind that it's 36.5, so it would be more like 417 compared to 400. and, if i'm keeping my water at, say, 420 -- i'm having to add calcium chloride every time to supplement to get to that number.

 

and, regardless, that's enough of a difference that, over enough water changes (not that many), you're going to have a significant reduction in calcium concentration. and, in fact, this is exactly what happened to me with magnesium concentration, requiring me to supplement pretty significantly (ended up using about 0.5gal of the BRS magnesium component of their 2-part solution).

 

Using a known seawater standard skirts this problem pretty much entirely. It will get you very, very close with minimized error. That's what should be done on any refractometer, seawater or NaCl specific. The associated error (ideally speaking) is also pretty slight to begin with, as doctaq stated.

 

Edit: more specifically, something along the lines of .7 ppt or about .0003 specific gravity units. Not very significant, IMHO.

 

i understand that using a known standard it skirts the problem entirely -- however, what i'm pointing out is that we're not doing that, and we should be. instead, we're calibrating with RO/DI water (which will lead to definitely being 1.5ppt off), or we're using what appears to be calibration fluid for brine/NaCl.

 

And as long as you are making your saltwater from a bucket/bag of mixed salts, your levels will vary batch to batch. The salt mixes we buy to make our salt water are heterogeneous mixtures, so each batch we mix from it will be slightly different. To achieve true consistency, you either have to mix the entire batch up at once, or make homogeneous mixtures (solutions) of the components (various salts) in our salt mixes. Only one salt manufacturer has done this so far, and not completely as they still have some dry components to their mix.

 

it's not that i necessarily disagree with what you're saying, but i don't see how that's really relevant. yes, your variation from advertised parameters MAY be from non-homogeneous salt mixtures. nothing you can really do about that. however, your variation from advertised parameters WILL be due to shorting the amount of salt mix you use -- there's no question about that. and, the idea would be to limit variation as much as possible to achieve stability.

[doctaq]

the amount you are dealing with here is the issue, or really, the non issue.

your levels will never drop below 400 if that is what you are getting out of a new batch, if your tank is not consuming calcium, really, you should just account for the level of calcium that your organisms are consuming, which in reality is not that much.

[timdanger]

the amount you are dealing with here is the issue, or really, the non issue.

your levels will never drop below 400 if that is what you are getting out of a new batch, if your tank is not consuming calcium, really, you should just account for the level of calcium that your organisms are consuming, which in reality is not that much.

 

if you have no calcium consuming organisms in your tank, i agree, it will never drop below 400. however, i (and everyone else, whether it is a clam, coral, other invertebrate or just algae) do have calcium consuming organisms in our tanks, and to differing degrees. and, you can't just blindly assume that i don't use "that much" calcium in my tank. in fact, in the example of the 420ppm water vs. the 400ppm water, if i use more than 4ppm of calcium per 20% water change, i will deplete the calcium in the water over time, as follows: if i have 100% of my water with Ca at 420ppm, and I change out 20% of my water with water that has Ca at 400ppm, my water will now only have 416ppm of Ca. i'm not replenishing Calcium that my calcium-consuming organisms are using, and I'm actually taking even more calcium out of the water with every water change until the Ca concentration falls below 400ppm, which will happen within or at 5 water changes. And, at that point, my calcium-consuming organisms will continue to deplete my calcium unless they consume less than 4ppm per 20% water change, which may be the case in some situations, but not to any invert-filled reef tank.

 

I guess if i want my Ca at 400ppm, that's not a problem. but, if i'm mixing it to get to 420ppm, and i'm driving myself crazy trying to figure out why it's not coming out right when I think i'm mixing up the correct amount, this is an obvious explanation. why not skip the whole calcium deficiency issue and just mix it to the recommended specific gravity?

 

frankly, though, we're just not focused on the relevant issue, which is that by undermixing your salt, you're forcing yourself to supplement with more Ca/Alk/Mg/etc. I guess I'm confused as to why you believe this is a debatable point, because the way i see it, it's just math. Maybe i'm wrong, but i don't see what i'm missing.

[yardboy]

I think what is missing here is the realization that the amount of sodium chloride relative to calcium or bicarbonate is so much greater that calibrating your refractometer to a solution of sodium chloride is accurate enough to not have the kind of effects that are being debated. And that is assuming that the caslibration solution you buy is only sodium chloride. The Pinpoint and VMR salinity calibration standards do not state that they are sodium chloride only.

Here's what Randy Farley has to say:

It turns out that an aqueous solution's refractive index is relatively insensitive to small changes in the solution's ionic makeup. For example, the usual changes in seawater's major ions that are encountered in a reef aquarium do not greatly alter the measured salinity. However, large differences in the big four ions (chloride, sulfate, sodium and magnesium) will alter the relationship between refractive index and salinity or specific gravity.

 

Ends up sounding like a marketing ploy to me.

[timdanger]

now we're getting somewhere with this discussion!

 

 

I think what is missing here is the realization that the amount of sodium chloride relative to calcium or bicarbonate is so much greater that calibrating your refractometer to a solution of sodium chloride is accurate enough to not have the kind of effects that are being debated. And that is assuming that the caslibration solution you buy is only sodium chloride. The Pinpoint and VMR salinity calibration standards do not state that they are sodium chloride only.

 

i understand what you're getting at, and you're right that i'm making some assumptions about the calibration fluid, but what about the fact that the alleged 1.5ppt difference (according to D-D) is going to make a ~16ppm difference in Ca in the example we're using?

 

if the calibration fluid is calibrating you to "seawater" salinity, that's fine -- but if it's calibrating to sodium chloride salinity, according to D-D, it's going to leave you 1.5ppt short, right?

 

Here's what Randy Farley has to say:

It turns out that an aqueous solution's refractive index is relatively insensitive to small changes in the solution's ionic makeup. For example, the usual changes in seawater's major ions that are encountered in a reef aquarium do not greatly alter the measured salinity. However, large differences in the big four ions (chloride, sulfate, sodium and magnesium) will alter the relationship between refractive index and salinity or specific gravity.

 

Ends up sounding like a marketing ploy to me.

 

first, it could definitely be a marketing ploy. my LFS is a big D-D distributor, so i might ask him about it.

 

here's my concern, though: maybe i'm misunderstanding, but i don't think it's necessarily the ionic makeup that we're concerned about -- on a basic level, it seems to me that we're concerned about mixing the salt to the manufacturer's recommended concentrations so that we get (closer to) the advertised parameters for Ca/Mg/Alk/etc. i know that when my marine depot refractometer, calibrated with ro/di water, measures the specific gravity of D-D H2O salt at 1.025, my elos calcium test kit measured Ca at 400ppm. i recognize that i'm working with imperfect measurements to measure imperfect substances an imperfect number of times to be making the broad generalizations i'm making here, but doesn't that seem significant? if i use the conversion they give in the advertisement, my math says i'd get much closer to the advertised parameters of 430-460ppm.

[yardboy]

While I am a chemist, the number of times I've used a refractometer at work is few, and with so coarse of a need that any small errors were inconsequential, but I do know this. A refractometer measures refractive index, which changes with the concentration of a salt in solution. Thus, even a sodium chloride solution could be made up with a refractive index equivalent to the accepted average salinity of seawater.

Read the article I referenced above and it should answer any questions you may have.

In my opinion, if I made up a saltwater solution that gave me the correct concentration of calcium in that solution, I'd be disregarding the salinity of the solution but assuming that the salt manufacturer was measuring out his proportions of chemicals and mixing them correctly. I got into measuring my water change saltwater after wondering if I needed to change brands of salt from IO to say ReefCrystals, which advertises that they have added calcium and alkalinity. The lower calcium and alkalinity levels of IO relative to ReefCrystals could much more cheaply be made up using 2-part or kalkwasser than it could from changing salt brands. Also, my bags of IO salt say to add 1/2 cup/gallon to get an approximate salinity then add water/salt and measure the salinity to get it correct.

Your error is as likely to be from using pure water for a standard as it is to be from using a different brand of refractometer.

Lastly, I often bring my refractometer with me when I go collecting/diving. I've found that salinity in the open water varies from 1.024 to 1.027 (my measurements) so I run 1.025 and am happy with the results.

[johnmaloney]

I see what you are saying...I would just correct with the dosing, I just find it easier. I usually try to get water change water to fix parameters rather than match them though.

 

Besides - you think the salt manufacturers would have the trace elements and calcium/mag/alk in line based on refractometers, because that is what their consumers use. Instead of 100000 people switching over, those 6 companies should hit up Aqua Cave if that isn't the case.