Specific Gravity Article

[Randy Holmes-Farley]

Following up on the announcement that James posted in the main forum, I'd like to provide a link to my specific gravity article:

http://www.advancedaquarist.com/issues/jan2002/chemistry.htm

Should anyone want to, I'm happy to discuss it here or at the Forum specific for the article at AAOM:

http://www.reefs.org/ubb/ultimatebb.php?ubb=forum&f=44

Anything that I write for them will be chemistry, so it is suitable for this forum. The AAOM forum is only for things relating to the article itself.

I've already submitted my second article to them on alkalinity. I actually think it is a much more interesting article than the specific gravity one, but probably won't get posted there for a couple of months.

[Green Lantern]

Hi Randy. Thanks for your article. I have a Seatest swing arm hydrometer and a refractometer that both read within .001 of each other. Do you think that they are both inaccurate. I calibrate the refractometer using RO/DI water that is close to 78 degrees (temp that I keep my tank at).

[Randy Holmes-Farley]

Troy:

<< I have a Seatest swing arm hydrometer and a refractometer that both read within .001 of each other. Do you think that they are both inaccurate. >>

Well, it's impossible to know for sure, but having two different devices give essentially the same reading would give me significant confidence in that reading. Unless I had some other reason to doubt the value, I'd accept it for reef keeping purposes.

[Green Lantern]

Thanks Randy, re-reading my question I realized the answer was fairly obvious. Do refractometers have a standard temperature? My guess is that they don't because you can calibrate them but I've been wrong before :) .

[Randy Holmes-Farley]

The index of refraction of water is a function of temperature and wavelength. Some refractometers correct automatically, for others you should use a table if you read at any temp other than the design temp (often 68 deg F).

The index of refraction of pure water at 589 nm (the wavelength)changes as follows:

10 deg C 1.33408
20 deg C 1.33336
30 deg C 1.33230

[piercho]

I approximated my own Marin Tropic hydrometer correction value, and would like to know if this method is valid:

At 83F, as hot as my aq gets, I removed 5g of water and recorded the SG, SGI. I let this container cool to 77F, stirred the water, and measured SG again, SGF. From this, the correction value I obtained is SGC=[(SGI-SGF)/(TI-77)] per degree F.

When I take a SG reading, I add the correction value SGA=SG + SGC*(T-77). From a table in R. Shimek's booklet "Pacific Host Anemone Secrets" I match SGA to the T the reading was made at and read S off of the table.

Right? Wrong? Sideways? Thanks for writing the article, this is one I've been puzzling over a while.

[Randy Holmes-Farley]

Piercho:

Two concerns:

1. I don't know what Ron's table is, or whether it is accurate. If it is a table of 60 deg F/60 deg F specific gravity vs salinity, then what you are doing isn't perfect, but it is close enough for government work. The reason it isn't perfect is that the 77 deg F/77 deg F sg of normal seawater, which is what your formula yields(i.e., is what the Tropic Marin yields) is a little different than the 60 deg F/60 deg F sg, but not by much (the article gives the difference).

2. I think your equation is slightly backwards?

SGC=[(SGI-SGF)/(TI-77)]

let's rewrite that as

SGC = [(SG83-SG77)/(TI-77)]

Now you have

SGA=SG + SGC*(T-77).

and if we measure at 83 deg to get

SGA = SG83 + [(SG83-SG77)/(TI-77)] * (TI-77)

SGA = SG83 + SG83 - SG77

Which i don't think is what you want (is it? maybe I haven't taken enough time to think this through).

If instead your equation was

SGC=[(SGF-SGI)/(TI-77)]
then what you end up with for an 83 deg measurement is

SGA = SG83 - SG83 + SG77

So

SGA = SG77

Which is, I think, what you want it to end up as.

Does that make sense, or nonsense?

[piercho]

Let's say the reading at T=77 was 1.0245, and the reading at T=88 was 1.025. Now, in this case, as you correctly derived: SGA = SG83 + SG83 - SG77. 1.025 + 1.025 - 1.0245 = SGA = 1.0255, where SGC=0.0001. My actual SGC found was 0.00013/1F. So if I measure SG at 82F I add a correction of 0.00065 to the reading.

Shimek's table gives acceptable salinities versus temperature for 81 to 85F. For instance, at 83F the acceptable salinities are 35.2 to 36.5, which corresponds to specific gravity readings of 1.024 to 1.025. The y axis of the table is SG in units to the 3rd place (1.0XX), the x axis of the table is temperature in whole numbers. I doubt that Dr Ron or his publisher would be really inflammed if I just emailed you a scan of the page with the table, if you are interested.

[Randy Holmes-Farley]

One of the main reasons that I wrote the article was to clear up confusion about how sg does or does not change with temperature. Many hobby articles have made the incorrect assumption that one can simply talk about sg values at different temperatures. That method is simply too ambiguous to be useful. Be very careful when using tables such as you describe.

<< Let's say the reading at T=77 was 1.0245, and the reading at T=88 was 1.025 >>

Well, it couldn't be, as a floating hydrometer will always read lower at higher temps, and that's why the correction as you write it is backward, I think.

When you take a reading at 83, and apply your correction according to your formula, you should get exactly the same value you get for the same water cooled to 77. If you don't, the correction is being applied incorrectly.

[piercho]

Well, let me try to back up and push. S=35, regardless of T, if I read your article right. My hydrometer is calibrated at 77F, and the sg of SW at 77F is 1.024. So, when I make a measurement at 83F I want to correct it to 77F, and I want to maintain my tank at a sg of 1.024. This is part of what I was trying to do (without understanding it) by figuring a linear correction value and adding it to my reading at 83F (even though in my example I got it back-assward). By then correlating my adjusted sg to the table of salinities VS T in the book I was probably double correcting, or something, at any rate I probably am NOT maintaining S at 35.

Following your example in the "Temperature of the Sample" section of the article, I need to know 3 values to correct my sg reading to 77F. I need to know sg of SW at 77F (1.024), T, and sg of SW at T (I need a table of SW densities for this). Then I multiply the reading taking by the ratio of 1.024/(sg@T). The idea is, if I understand right, to always adjust the sg of the reading to the T the hydrometer is calibrated at.

So, assuming I got the above right, I have one last question. Can you refer a source for these tables of SW densities? Working in a shipyard you would think that this is a no-brainer for me but floating the boat is not MY job. It would help if the reference was fairly generic because I don't have direct access to a University library here.

Thanks for your patience.

[Randy Holmes-Farley]

<< Well, let me try to back up and push. S=35, regardless of T, if I read your article right. My hydrometer is calibrated at 77F, and the sg of SW at 77F is 1.024>>

If S=35 and your read the specific gravity with a Tropic marin hydrometer at 77 deg F, the reading will be about 1.026 (assuming that it is functioning correctly).

If you read at temperatures higher than 77 deg F, the reading that you get from the hydrometer will be lower, and can be corrected to give the correct, higher specific gravity value (1.026) with a correction figure.

If you have some S=35 tank water, you can determine he necessary corrections as I indicated in the earlier post, and you don't have to resort to tables if you don't wan t to. FWIW, corrections between 77 deg F and 78-83 deg F are all fairly small. You might even be safe ignoring them.

Corrections are most important for hydrometers calibrated at 60 deg F, and used at 83 deg F. The Tropic Marin hydrometer is calibrated at 77 deg F (I got confirmation of this from Trpoic marin before writing my article).

[piercho]

Ok 1.0264, not 1.024, I dropped a 6 there in the 3rd place. Thanks, I understand now, and should have understood just from the article itself. Also, thanks for the limewater article links. One was right on the mark for me.