Specific Gravity vs Salinity

[nemmy]

Ok, so i just ordered a reefkeeper lite with the salinity probe and module.

Heres the confusion but i think i have it worked out I'm just looking for a confirmation.

 

I run my tank at 1.025 at 79 degrees

So my salinity should be roughly 33.6-34ppt?

 

Heres how i came to this assumption. I took my temperature at 79 and converted it to Celsius which is roughly 26.12. I did this because the chart i found was only in Celsius.

 

Using the chart they had:

26.0 ºC (78.8 ºF) @ 1.025 = 33.64 ppt

26.5 ºC (79.7 ºF) @ 1.025 = 33.86 ppt

 

So giving a few .x ppt for variations in temperature and evaporation i should be safe staying in the 33.6-34.0 ppt range correct?

 

The reason i want to figure this out is i dont 100% trust my hydrometer, ive had no problems with the specific gravity it reads at the temperature i run so i havent changed anything. If the SG is off by some its consistently off and if it aint broke dont fix it.

So i want to make sure 100% with this salinity probe that my tank is in the correct range, therefore my salt is mixed to its 1.025 that it suggests. Thus giving my trace elements their "advertised" numbers.

 

Feel free to correct me if you see something wrong in these numbers, or you think i should allow for more or less fluctuation.

I also may be missing something here.

 

The RKL wont be here for awhile, i just want to get this sorted out ahead of time.

[skimlessinseattle]

It's correct.

[Mr. Fosi]

Your math and chart matching skills appear to be in tact.

 

I'd say that shooting for a 0.01-0.4 ppt accuracy and precision is a pretty tight tolerance. If you can do it, good, but if you find that the best you can do is +/- 1 ppt, I still wouldn't sweat it since many people's tanks and changewater fluctuate more than that on a regular basis... Like mine. My NSW ranges from 35-40 ppt and I don't generally amend it with FW. I adjust the Mg, Ca and Alk and call it even.

[nemmy]

Your math and chart matching skills appear to be in tact.

 

I'd say that shooting for a 0.01-0.4 ppt accuracy and precision is a pretty tight tolerance. If you can do it, good, but if you find that the best you can do is +/- 1 ppt, I still wouldn't sweat it since many people's tanks and changewater fluctuate more than that on a regular basis... Like mine. My NSW ranges from 35-40 ppt and I don't generally amend it with FW. I adjust the Mg, Ca and Alk and call it even.

 

My chart matching skills are top notch! I can also use the hell out of a calculator .

 

My main concern was the fluctuations of the salinity I will try to stay within .5 but +/- 1 does seem a bit more realistic.

 

So do "Specific Gravity" swings really affect the livestock? Or is it the Salinity that affects them and SG is just easier to monitor?

 

I've always kept my SG in check but after all of this salinity SG stuff im starting to wonder why the majority always talks about SG and never salinity and their swings.

[jeremai]

im starting to wonder why the majority always talks about SG and never salinity

I imagine it's because the floating glass hydrometers, swing arm hydrometers and refractometers used in the hobby tend to read in SG and not salinity (well, I guess most refractometers show both). also, since SG is independent of temperature, it's a more relevant comparison between systems.

 

I'm probably wrong though. fosi will come along with the right answer. or lakshwa will google it. either way.

[Mr. Fosi]

Well, first you need to know what "salinity" is and how you can directly measure it. If you do some digging, I think you'll find that even among chemical oceanographers, the actual definition of salinity is a bit sticky. Most people will go along with you if you talk about g salt/kg water, chlorinity, conductance, refractive index or specific gravity.

 

If you want a non-wikipedia/internet def you can go with the one in Knauss, J.A. (2005). Introduction to Physical Oceanography. Waveland Press Inc. p 3: The total amount of dissolved non-organic and non-volatile material in seawater... given in parts per thousand, grams of salt per kilogram of seawater...

 

If you go with that definition, how would you measure it? Can the average hobbyist do this? Do you think you could do so as quickly as estimating it using a refrac or hydro?

 

Of course, a measure of salinity using mass/mass will be independent of temperature (or as is important in oceanography) pressure, it is difficult and time consuming to do. Better to have some other correlative measurement that gives a reasonable approximation of actual salinity and does so quickly.

 

EDIT to fix i tags

[nemmy]

Of course, a measure of salinity using mass/mass will be independent of temperature (or as is important in oceanography) pressure, it is difficult and time consuming to do. Better to have some other correlative measurement that gives a reasonable approximation of actual salinity and does so quickly.

 

Good way to put it Fosi, that solves it for me!