#DENSITY OF WATER AT 20C FULL#

If you trawl the internet and scientific literature you will find that the specific gravity measurements and tables for seawater which you come across were first developed long ago by shipping companies and oceanographic bodies like the NOAA. I suspect, rightly or wrongly, that the specific gravity levels that we as hobbyists use in our aquariums may be historical from general oceanographic studies and it is not immediately clear when or where the 1.025 at 25C standard originates from. reading that you obtain may be more specific to the equipment you have chosen to use to measure it rather than to the actual salt concentration in your tank. NOT QUITE TRUE - At a simple level this is correct however as you get further into the subject you will find that the S.G. SIMPLES - to use the words of a well known Russian meerkat - All we need to do is to ensure that our sample water is at that temperature and the specific gravity will be exactly right. This is the temperature at which the hydrometer was designed/calibrated to measure the water sample at – Many people may have noticed on their hydrometers that there is a temperature quoted on the side, normally in small writing. by quoting it at a specific measured temperature, e.g. We should always therefore qualify the S.G. This of course can be quantified quite easily, however as the density of a liquid is calculated as the weight of a unit volume of that liquid and because liquids expand with temperature, then the volume that we measure and subsequently the weight of that volume will also change. Specific Gravity, or relative density, is expressed as the ratio of the density of seawater relative to the density of the same volume of pure water. Obviously this is not a convenient technique for the average hobbyist, so we use other methods to determine the salinity indirectly. It is not possible to measure the salinity of the ocean or your aquarium directly unless you take a precise weight of seawater and evaporate all of the H20 from it then measure the weight of the residue, (mass solids analysis). The generally accepted standard salinity for natural seawater, (NSW), is 35ppt.

Salinity is a pure weight per weight measurement and as weight is not influenced by temperature then neither is salinity.

Salinity is a true measurement of the concentration of salt in the ocean and is calculated as the total weight of ‘dry’ salt dissolved in a total of 1000 weight units of water or parts per thousand, (ppt). The salinity of the world’s oceans is not constant and varies substantially from one location to the next with concentrations of just 10-15ppt in the Baltic Sea to around 40ppt in the Red Sea.īut stop a minute!! Why are we talking about Salinity now instead of S.G. is 1.025 at 25C, but where does this figure come from, and is it the best way to measure or express the amount of salt in your aquarium, is it even the correct level to be running at? Those with greater understanding may go into further detail and tell you that the S.G. Most people, if asked, would probably tell you that their reef aquarium is running at an S.G.

#DENSITY OF WATER AT 20C FULL#

measurement and has prompted me to write this article which tries to illustrate some potential problems when using specific gravity as a unit of measurement without a full understanding of what is actually being measured. not enough salt in their water.įurther investigation brought to light a number of anomalies and misunderstandings regarding the whole issue of S.G. The main reason that we found for this discrepancy, other than general mixing and test kit errors, was due to low specific gravity, i.e. Of course once we gave people levels to test against we received a steady stream of emails and threads from individuals who were measuring readings outside these guaranteed parameters.

When we brought out the D-D H2Ocean Pro+ reef salt a few years ago, we were adamant that we wanted to print the exact levels and tolerances for the main important elements that we would expect to find in our salt on every bucket information that should be readily available to allow an informed choice for every good reef salt.

Density of water at 20c