| Saltwater Basics, How Your Hydrometer Works |
More than 70% of the Earths surface is covered with it. You are literally filled with it. It is what you should be
drinking instead of all that beer. It is the only substance that we can find in abundance in all three states here
on earth (gas, liquid, solid). But you may have never given much thought to it…
Water, specifically saltwater is more complicated than many people think. Here we will discuss just one small
aspect of water, its density.
We (saltwater aquarists) all use a hydrometer to measure the specific gravity (density of the water) in our
tanks, it tells us how much salt is actually in the water. Do we have too much, not enough, or is the balance just
right? We think to ourselves that 1.026 is ideal, but what does that even mean? How does that hydrometer work
anyway? Density is literally mass divided by volume. How much stuff fits into a given space, stuffing more stuff
into a given space make the density go up (gets “more dense”).
Water in its purest state is of course, two hydrogen atoms and one oxygen atom. But you will never encounter it
like that in nature, there will always be other substances mixed in. Anything that mixes with water without
significantly contributing to its volume increases its density. If that seems a bit far out, consider the following
example. You place marbles into a glass until it is full, the marbles represent molecules of water. Now the glass
is full of marbles (water) and no more can fit, but if you take another substance that will fit between the gaps
created by the round surface of the marbles then you can actually fit more inside the glass…more stuff in the
same space. So you pour sand into the glass, this represents the salt. You can again fill the glass, all the gaps
can take in the sand. More material fits in the same size glass and nothing overflows. This is how salt mixes with
water. It literally fills the gaps between the water molecules, the salt makes the water more dense.
So that is all fine and dandy, but then how does the hydrometer actually measure this change? It is actually
very simple, but we have to understand buoyancy. Any item, even a block of concrete, will float if it meets one
simple criterion. If it is able to displace an amount of water equal in weight to itself before it is completely
submerged, it will float. In other words, when you put a canoe in the water, the amount of water that has to
“move out of the way” to make room for the canoe is exactly equal in weight to the canoe and all its contents. If
you were to force the canoe down, it is actually capable of displacing an amount of water that weighs more than
itself…it floats! A hydrometer works in exactly the same way. As you lower it into the water, the water has to
“move out of the way” to make room for it. When a volume of water that is equal in weight to the entire
hydrometer is displaced (moved out of the way), the hydrometer quits sinking and holds its place in the water.
Now go back to our marbles and sand example. We have added salt to the water, more stuff in the same size
glass. We again lower the hydrometer into the water. This time, the water AND the salt that is filling the gaps
between it have to move out of the way to make room for the hydrometer. Since there is more stuff in the same
space here we actually have to displace less total volume to equal the weight of the hydrometer, but the same
principal applies. When equilibrium is met the hydrometer floats. This is why the hydrometer actually takes a
higher floating position in saltwater than it does in freshwater.
So what do the numbers mean? What exactly is 1.026? Well, let’s start with pure water. It has a specific gravity
of “1.0000”. That means that it weighs one gram per cubic centimeter. Simple right? So a specific gravity of
1.026 must mean that the water in your aquarium weighs 1.026 grams per cubic centimeter. Remember how we
stuffed more particles into the same space? Yep, it is just that simple. Scale it up to a size that is easier to
relate to and you will see that one cubic meter of pure water weighs 1000 kilograms, and one cubic meter of
your saltwater aquariums water weighs 1026 kilograms. So adding 26 kilos of salt to cubic meter of water
makes it ready for fish…now that is a tank! (for reference 1000 kilos is 2200 pounds, 26 kilos is 57.2 pounds
and one meter is approximately 3.2 feet)
Not to muddy the water (pun intended) but it does get a little more complicated than this. Density actually
changes with temperature as well as with anything dissolved in the water. For our purposes however, the
hydrometers we use are calibrated by the factory for use at normal aquarium temperatures. For all practical
purposes, the instruments we use are not precise enough to measure the extremely tiny density change due to
the temperature changes we would be dealing with anyway. As a matter of fact, even a 100 degree Fahrenheit
swing is considered small when considering density...although not so small when trying to keep fish alive!
So there you have it. That device that magically senses how much salt is in your water isn’t really magic at all. It
is just measuring how much stuff it has to move out of the way to equal its own weight. Who would have thunk it
was just that simple?
drinking instead of all that beer. It is the only substance that we can find in abundance in all three states here
on earth (gas, liquid, solid). But you may have never given much thought to it…
Water, specifically saltwater is more complicated than many people think. Here we will discuss just one small
aspect of water, its density.
We (saltwater aquarists) all use a hydrometer to measure the specific gravity (density of the water) in our
tanks, it tells us how much salt is actually in the water. Do we have too much, not enough, or is the balance just
right? We think to ourselves that 1.026 is ideal, but what does that even mean? How does that hydrometer work
anyway? Density is literally mass divided by volume. How much stuff fits into a given space, stuffing more stuff
into a given space make the density go up (gets “more dense”).
Water in its purest state is of course, two hydrogen atoms and one oxygen atom. But you will never encounter it
like that in nature, there will always be other substances mixed in. Anything that mixes with water without
significantly contributing to its volume increases its density. If that seems a bit far out, consider the following
example. You place marbles into a glass until it is full, the marbles represent molecules of water. Now the glass
is full of marbles (water) and no more can fit, but if you take another substance that will fit between the gaps
created by the round surface of the marbles then you can actually fit more inside the glass…more stuff in the
same space. So you pour sand into the glass, this represents the salt. You can again fill the glass, all the gaps
can take in the sand. More material fits in the same size glass and nothing overflows. This is how salt mixes with
water. It literally fills the gaps between the water molecules, the salt makes the water more dense.
So that is all fine and dandy, but then how does the hydrometer actually measure this change? It is actually
very simple, but we have to understand buoyancy. Any item, even a block of concrete, will float if it meets one
simple criterion. If it is able to displace an amount of water equal in weight to itself before it is completely
submerged, it will float. In other words, when you put a canoe in the water, the amount of water that has to
“move out of the way” to make room for the canoe is exactly equal in weight to the canoe and all its contents. If
you were to force the canoe down, it is actually capable of displacing an amount of water that weighs more than
itself…it floats! A hydrometer works in exactly the same way. As you lower it into the water, the water has to
“move out of the way” to make room for it. When a volume of water that is equal in weight to the entire
hydrometer is displaced (moved out of the way), the hydrometer quits sinking and holds its place in the water.
Now go back to our marbles and sand example. We have added salt to the water, more stuff in the same size
glass. We again lower the hydrometer into the water. This time, the water AND the salt that is filling the gaps
between it have to move out of the way to make room for the hydrometer. Since there is more stuff in the same
space here we actually have to displace less total volume to equal the weight of the hydrometer, but the same
principal applies. When equilibrium is met the hydrometer floats. This is why the hydrometer actually takes a
higher floating position in saltwater than it does in freshwater.
So what do the numbers mean? What exactly is 1.026? Well, let’s start with pure water. It has a specific gravity
of “1.0000”. That means that it weighs one gram per cubic centimeter. Simple right? So a specific gravity of
1.026 must mean that the water in your aquarium weighs 1.026 grams per cubic centimeter. Remember how we
stuffed more particles into the same space? Yep, it is just that simple. Scale it up to a size that is easier to
relate to and you will see that one cubic meter of pure water weighs 1000 kilograms, and one cubic meter of
your saltwater aquariums water weighs 1026 kilograms. So adding 26 kilos of salt to cubic meter of water
makes it ready for fish…now that is a tank! (for reference 1000 kilos is 2200 pounds, 26 kilos is 57.2 pounds
and one meter is approximately 3.2 feet)
Not to muddy the water (pun intended) but it does get a little more complicated than this. Density actually
changes with temperature as well as with anything dissolved in the water. For our purposes however, the
hydrometers we use are calibrated by the factory for use at normal aquarium temperatures. For all practical
purposes, the instruments we use are not precise enough to measure the extremely tiny density change due to
the temperature changes we would be dealing with anyway. As a matter of fact, even a 100 degree Fahrenheit
swing is considered small when considering density...although not so small when trying to keep fish alive!
So there you have it. That device that magically senses how much salt is in your water isn’t really magic at all. It
is just measuring how much stuff it has to move out of the way to equal its own weight. Who would have thunk it
was just that simple?