How do you figure out salinity with different temps and S.G.s? I've seen charts say 77F & 1.025 SG = 36.2 salinity. Is there any math I can do to figure out any salinity using any plugin numbers for temp & SG?
Specific gravity changes with temperature and it is a unitless number -- that is. since it is density divided by density (a ratio using the same units of measurement) the units of measurement divide out and leaves a number without units or dimensions. It is affected by temperature since the two numbers compared in the ratio are affected by temperature. That is, each number (numerator and denominator) is a density and density is affected by temperature. You have two 'moving targets' here because of many other (annoying?) facts.
The denominator is some standard AND that standard (pure water) does not change its density the same (at the same rate) as the salt water being measured. The measuring device is affected by temperature. No one can agree on what standard to use. Etc.
So your question is well put since it takes into account the need for knowing the temperature at which a specific gravity was read/recorded in order to convert it to a salinity. But the second missing part is to know what standard the water will be compared AND what temperature is that standard.
Unfortunately, there is no mathematical calculation the hobbyist can perform to convert the specific gravity to salinity. Above I mentioned that sp. gr. is the ratio of two densities. One is being measured, the other is a constant. The ratio is the sp. gr. So sp. gr. is related to the choice of what standard is being used. The world is in a bit of mess in this area, since there is no universal agreement on the standard.
The reasons why there is no formula that can be relied upon are complex and include: the choice of what to call a 'standard' (noted above), the temperature of the sample being measured, the fact that pure water expands with temperature at a different rate than salt water expands (see above and below), the materials used to construct the measuring device and how temperature affects the device, and the precision/accuracy of the device. This is only some examples of the complexity and to go further here would launch into an article, when there are many articles available on the subject (see below).
A formula would need to account for the choice of standard, behavior (expansion) of the standard with temperature, and you'd need to know how that behavior relates to how salt water expands with temperatures. Very complex -- especially since we know that pure water and salt water do not expand with temperature at the same rate.
We use tables set at specific temperatures based upon a noted standard to match the particular specific gravity (density) of water to salinity. Still, tables cannot take into account all the variables in the specific gravity measurement. The tables are only semi-practical and useful but handicapped by the choice of equipment and accurate information about the sample and standard. There are those above mentioned (at least) variables to account for.
aaah thanks for the detailed info! I didn't think there was any simple solution, had read many articles inc. the one you just posted about. I had some free time yesterday and thought to build a php script and database to store all my water parameter tests to view in the long term. Got to adding the salinity field and needed a formula/math or something to have the server do the calculations for me. man I'm loving this forum, with all the experience in here. thanks guys!
It would be easier just to obtain the table most close to your situation and temperatures, then have the spreadsheet/database column convert the read sp. gr. number to the salinity (from the table). Since for normal marine system operations the sp. gr. is in a very narrow range, the table could be put in for each 0.001 sp. gr. and not be a very extensive table. It would only likely become cumbersome if you had a means (and need) to measure salinity to 0.0001 and then need to have its corresponding salinity to those number of significant figures.
We have a post that has links to other articles and posts that you may find of interest: Table/Contents - Link List
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man I'm loving this forum, with all the experience in here. thanks guys!
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