- I learned something today
that rocked my world as a hobbyist.
Refractometers don't measure salinity,
Hydrometers don't measure specific gravity
and salinity probes, well...
Salinity measures the concentration of salt in the ocean.
It is the measurement of the weight of dry salt
compared to the weight of the water.
The only way to actually measure this
to take a specific amount of sea water, measured by weight,
evaporate all the water away
until you're only left with dry salt
and then weigh the dry salt.
This is obviously not very practical for hobbyists.
We express salinity in parts per thousand.
So 35 parts for thousand actually means
that 3.5% of the weight of seawater
is due to the salt content.
Specific gravity, also known as relative density,
is the ratio of the density of seawater
compared to the density of fresh RODI water.
Since it is purely a ratio of two things,
there is no unit of measurement for specific gravity.
So while specific gravity cannot directly tell us
the concentration of salt in the water,
by telling us how dense the sea water is
when compared to RODI water,
we can infer how much salt is dissolved
in the water and thus approximate salinity.
Okay, so none of us are actually going to boil
down a fresh batch of seawater every week,
in order to weigh the remaining dry salt,
we're not directly measuring salinity.
And since I don't even know how to begin
to measure the ratio of the density of seawater
compared to the density of RODI water,
then we're clearly not directly measuring specific gravity.
So what in the name of Poseidon's Trident
are we actually measuring?
Well, there are three different tools
that we use in this hobby and each one of them
measures something a little different.
Hydrometers measure buoyancy.
These use some sort of floating mechanism
that is calibrated to a certain temperature.
The denser the liquid being measured,
the higher it will float.
But since the density of water changes with temperature,
it's important that your tank water temperature
matches the calibration temp in the hydrometer.
So with a hydrometer,
we are actually measuring how dense the water is,
meaning how many things are dissolved in it.
And we are assuming that the vast majority
of those dissolved things are actually salt.
And thus we're able to infer how much salt is in that water.
A refractometer uses the refraction
of light in water to approximate salinity.
Changes in the concentration of salt
affect the refraction of light.
A refractometer is able to measure
how much refraction is occurring
and display the results in a graduated scale.
So a refractometer merely measures light refraction.
Which, for measuring seawater,
we assume that the more refraction there is,
the more salt is in the water.
Thus again, allowing us to approximate
the salinity of the water.
And lastly, we have salinity probes,
which measures the conductivity of the water.
Meaning the degree to which it conducts electricity.
A salinity probe measures the conductivity
by passing an electric current between two electrodes.
The more salt that is dissolved in the water,
the greater its capacity to conduct electricity.
The greater the conductivity, the more salt.
So once again, a salinity probe does not actually
measure the amount of salt in the water.
Honestly, I can't say for certain
which tool is gonna be the most accurate.
The only way to be sure of its accuracy
is to use some sort of calibration fluid
like this Refracto Juice from BRS.
Hobby grade hydrometers can't be calibrated
and they require you to use a specific
temperature of water for accurate results.
Refractometers also require a certain temperature
but they can be calibrated
using something like the BRS Refracto Juice.
Salinity probes also require calibration,
but they're easy to use and adjust automatically
for temperature differences.
If I was a betting man, I would place my money
on either the salinity probe or the refractometer
to give you the most accurate results.
I think every hobbyist needs a refractometer.
They're super sturdy
and they hold their calibration really well.
Our refractometer here at BRS comes with calibration fluid
and it takes you less than a minute to calibrate.
And it may hold its calibration
for the rest of its usable life.
Although a little bit more expensive,
I use the Hanna Salinity Tester for my everyday needs.
It's 100% waterproof
but it does need to be recalibrated every month.
It is definitely easy to use,
and way more convenient than a refractometer.
But if you wanna go super fancy
then the Milwaukee Digital Seawater Refractometer
might be the one for you.
It's just like the refractometer we already mentioned
except this one reads the results for you.
The first thing you need to do
with your BRS refractometer is to calibrate it.
Lift the daylight plate and place two to three drops
of the calibration solution into the prism.
Close the daylight diffusion plate
so that the water spreads across
the entire surface of the prism.
Sometimes I find a gentle tapping can help this along.
Wait for 45 seconds for the sample
to get to room temperature.
Hold the refractometer toward a natural light source
and look into the eye piece.
Focus the eye piece so you can read the results clearly.
If the calibration fluid reads
35 parts per thousand, you're finished.
But if not, remove the cap from the adjustment screw.
Use the included screwdriver
to adjust the refractometer
until it reads 35 parts per thousand
or 1.026 specific gravity.
Replace the cap and you're done.
Always clean your refractometer
and pipette with RODI water
and allow to dry completely before storing it.
To calibrate the Hanna Salinity Tester,
remove the battery cover and pop out the battery
and remove the safety strip on the battery
before putting it back into place.
Rinse your new probe and RODI water and dry it.
Cut open the top of the Hanna calibration solution.
With the salinity tester on, press the cal button.
Once the 35 part per thousand use is displayed,
insert probe into the calibration solution.
Tap the tester a couple times to remove any air bubbles.
Leave in place until after the REC is no longer displayed
and the tester returns to measurement mode.
Hanna does recommend a monthly calibration
for your salinity probe.
To use the salinity tester, merely insert
the entire probe into the sample water.
Leave it in place until the hour glass stops blinking,
and you're done.
Then just rinse it in RODI water,
and give it a couple shakes to dry it before storing it.
Moving on to the Milwaukee Digital Seawater Refractometer.
I already made a video about how to calibrate and use it.
So we'll put a link in the description below.
I have made some accidental, yet deadly mistakes,
when it comes to salinity in my tanks.
For example, I accidentally filled up
my ATO reservoir with seawater instead of RODI water.
And didn't catch my mistake until the tank almost crashed.
There is but one piece of equipment I could've purchased
that would've prevented that entire mess.
Check it out by watching this video right here.
And as always, thanks for watching.
Happy reefing, be well.
We'll see you next time.