Salinity is one of the most important chemical
parameters to monitor in reef aquaria. While there are a variety
of ways to measure salinity, including using refractometers
conductivity meters, hydrometers can be an accurate and
inexpensive method. Unfortunately, not all hydrometers are
suitably accurate, and last
month's column gave a recipe for a standard solution with
which to calibrate, or at least check the operation of, typical
In addition to such calibration issues, however, is the
significant problem of temperature effects on hydrometers.
This article describes how and why temperature affects some
hydrometers, and not others. It also provides a table and
a calculator for performing temperature corrections on one
of the most common types of hydrometers used by reef aquarists.
I won't address in this article the question
of what salinity values are "optimal" for keeping
marine aquaria. That has been addressed previously, such as
in this article
by Ron Shimek.
One further point on salinity: in this
article, as in the chemical oceanography literature, the salinity
of seawater is frequently defined as a dimensionless unit,
S (often referred to as PSU, Practical Salinity Units). In
older literature, salinity was traditionally expressed as
units of ppt (parts per thousand by weight), which is roughly
the correct way to think of it, but it is now defined as the
ratio of the seawater's conductivity to that of a potassium
chloride solution of defined composition. Consequently, seawater
has S=35 (or some similar number).1
Other solutions, like simple sodium chloride, are not defined
in this way, and are still reported in units of ppt.
Figure 1. The SeaTest swing arm hydrometer.
Summary of Temperature Corrections
For those who already have a good understanding
of how and why hydrometers are suitable for measuring salinity,
or who do not care, this section provides a simple way to
deal with temperature issues involving hydrometers. For those
with a greater interest in understanding what is being measured
and why, the subsequent sections of this article provide the
Swing Arm hydrometers (Figure
1). These types of hydrometers need no temperature
correction, both by their own claim, and by some limited
testing that I reported in a previous
article. Whether accurate or not, the salinity values
determined with such a hydrometer are fairly independent
Standard Floating hydrometers. These
hydrometers generally do need a temperature correction
when used at any temperature other than the one at which
the hydrometer is intended to be used. What correction
to use depends on the hydrometer, as indicated below.
For hydrometers calibrated at 77ºF
(such as the Tropic Marin, Figure
2), ), the correct relationship between the salinity,
the measurement temperature, and the hydrometer reading
can be found in Table
1 below (supplied by Johan Thelander) and in this
calculator (written by Simon Huntington).
For hydrometers calibrated at 15ºC
(or 60ºF; a common calibration temperature for scientific
hydrometers) many online
tables are available.
For those hydrometers calibrated at
other temperatures, corrections are more complicated (and
less accurate) since tables are not generally available.
In this case, my suggestion is to add 0.00035 to the hydrometer
reading for every 1ºC (or 0.00019 per 1ºF) by
which the measurement temperature exceeds the calibration
temperature, to get a corrected specific
gravity. Likewise, subtract that amount if the temperature
is below the calibration temperature.
Table 1. Click on image for larger Adobe Acrobat version.
Figure 2. The Tropic Marin floating hydrometer, showing
the calibration temperature of 77°F.
What is Specific Gravity?
Specific gravity is defined as the ratio
of a liquid's density to the density of pure water. Since
the density of pure water varies with its temperature, the
temperature of the pure water must be specified in order to
usefully define specific gravity. In many scientific endeavors
(such as mineralogy), the temperature standard chosen is 3.98°C
(39.2°F, the temperature of pure water's maximum density).
At that temperature, the density of pure water is 1.0000 g/cm3,
or 1.0000g/ml. If this is the standard chosen, it is easy
to see that the specific gravity is just the density of the
sample at 3.98°C when measured in g/cm3
(without any units since specific gravity is a unitless measure).
Why is specific gravity useful to aquarists?
Primarily because it is a simple and quantitative way to tell
how much of something is in water. If chemicals less dense
than water are dissolved in it, then its specific gravity
will drop. Ethanol, for example, is less dense than water,
and therefore lowers specific gravity. This fact is used by
brewers to gauge the amount of alcohol in their brews.
Likewise, if chemicals denser than water
are dissolved in it, its specific gravity rises. Nearly all
inorganic salts are denser than water, so dissolving them
in water raises the specific gravity. This increase can be
used by aquarists to gauge how much salt is in their water.
Of course, it cannot tell what is in the water, but for aquarists
using an appropriate salt mix, it can tell how much is there
and whether or not it approximates natural seawater.
Two fundamental types of hydrometers are
encountered by aquarists. The first is the standard floating
hydrometer, which consists of a glass "float" that
is put into the water. How high it floats in the water is
an indication of the specific gravity of the solution. The
second type is often called a swing arm hydrometer. It has
a plastic float attached at a pivot point, and that float
swings up to a certain position depending on the specific
gravity of the solution.
These two types of hydrometers are different
in some important aspects. In the context of this article,
the most important of these differences is that swing arm
hydrometers do not usually require any corrections for temperature
effects over the range normally used by aquarists, while standard
hydrometers often do.
How Do Standard Hydrometers Measure Specific
Standard hydrometers work on the Archimedes
Principle. which states that the weight of a hydrometer
(or any other object, such as an iceberg or a ship) equals
the weight of the fluid that it displaces. Consequently, the
hydrometer will sink only until it has displaced its own weight.
When it is put into solutions of different densities, it floats
higher or lower, just displacing its own weight. In denser
fluids it floats higher (displacing less fluid) and in less
dense fluids it floats lower. In essence, this principle is
a reflection of the fact that the gravitational potential
energy of the system is minimized when the hydrometer just
displaces its own weight. Any other displacement puts forces
on the water and hydrometer that cause them to move toward
the equilibrium position.
How Do Swing Arm Hydrometers Measure Specific
Swing arm hydrometers are a bit different
since no part of their arm is above the water line. Instead,
the swing arm responds to the density differential by rotating
an arm having nonuniform weight distribution. Typical hobby
swing arm hydrometers use an arm made of two different materials
(Figure 1). The density
difference between the water and one material forces the arm
to swing in one direction, and the density difference between
the water and the other material forces the arm to swing in
the opposite direction. At the equilibrium position these
forces cancel out, and the hydrometer gives a steady reading.
As with floating hydrometers, the final result is a minimization
of the gravitational potential energy of the system.
Do Ion Imbalances Impact Specific Gravity?
understanding of this effect, note that
it is reasonable to assume that all ions contribute to specific
gravity in an amount proportional to the percentage of their
weight in the seawater. For example, I looked up the specific
gravity of 15 different inorganic salts at the same "salinity"
(100 ppt at 20°C). All were very similar, with a difference
of less than a factor of two between the highest (zinc sulfate,
specific gravity = 1.1091 g/cm3)
and the lowest (lithium chloride; specific gravity = 1.0579).
In a sense, the more of any ion that is
present, regardless of its chemical nature, the larger its
effect on specific gravity. Since that's exactly what salinity
is (the weight of dissolved solids in the water), it is unlikely
that any normal ion variation seen by marine aquarists will
unduly skew specific gravity measurements. Since the four
most abundant ions in seawater (Na+,
SO4--) comprise 97% of the
total weight, any changes in other ions will not significantly
impact specific gravity.
What about changes in these four ions?
Let's take an extreme case where the salt consists of nothing
but sodium chloride. It turns out that a 37.1 ppt solution
of sodium chloride has the same specific gravity as S = 35
seawater. Thus, we see that even big changes in the ionic
balance result in fairly small changes in the relationship
between specific gravity and salinity. For these reasons,
it is safe for most aquarists to ignore any impact that differences
in the ionic constituents would have on the relationship between
specific gravity and salinity. Of course, if the seawater
mix were grossly inaccurate (consisting of only potassium
bromide or magnesium sulfate, for example) then the relationship
between specific gravity and salinity that is assumed for
seawater will be broken. A pure potassium bromide solution
with the same specific gravity as natural seawater (S = 35),
for example, has a "salinity" of about 36 ppt. A
similar pure magnesium sulfate solution has a "salinity"
of only 26 ppt.
Temperature of the "Standard"
Unfortunately, the world of specific gravity
is not as simple as described above. Different fields have
apparently chosen different standard temperatures. In addition
to the 3.98°C standard, others include 20°C (68°F)
and 60°F (15.6°C). A quick look through several laboratory
supply catalogs shows many examples of hydrometers using each
of these two, and a few odd ones thrown in for good measure
(such as 102°F for milk). Many authors writing about marine
aquaria assume that hobbyists are using the 60°F standard,
but in reality many are not, and probably in most cases they
don't even know what they are using. Many modern hobby hydrometers
use other standards, with 77°F (25°C) being quite
popular (used by Tropic Marin, for example).
The density of pure water at 20°C is
0.998206 g/cm3, and at 60°F
it is 0.9990247 g/cm3. While
these seem close to 1, and are often simply claimed to be
1.00 in many contexts, the difference can be substantial.
For example, the specific gravity of natural seawater (S =35)
is 1.0278 using the 3.98°C standard, 1.0269 using the
60°F standard, 1.0266 using the 20°C standard, and
1.0264 using the 77°F standard. [I calculated these based
on tables of the density of seawater; different tables may
present slightly different densities that might then result
in slightly different specific gravities]. While these differences
are small, they are real. They arise because the density of
pure water and seawater change in slightly different ways
as temperature changes. Seawater becomes less dense faster
than pure water as the temperature rises. This effect may
relate to the interactions between ions, and between ions
and water, in seawater, that are broken up as the temperature
rises, but that's beyond the scope of this article.
Unfortunately, many aquarists quoting a
specific gravity measurement for their tanks do not know what
standard their hydrometer is using. Most quality lab hydrometers
will have the standard used written on them or found in their
supporting documents. Some hobby hydrometers, however, make
no mention of the standard used. Note that there is NO "correction"
table that can convert readings at temperatures other than
the standard temperature unless the standard temperature is
known. If it isn't known, using such a table will not give
accurate values, and may give a value farther from the truth
than the uncorrected reading.
Temperature of the Sample
As if the confusion about the temperature
of the standard were not enough, the temperature of the sample
is also a variable. Many quality lab hydrometers (the standard
floating type) also have the expected sample temperature indicated
directly on them (Figure 2).
This is referred to as the "reference" temperature.
In a great many cases (although not all), the standard temperature
and the reference temperature are the same: either 60°F
or 20°C. This is often written as "60°F/60°F",
though it is sometimes written simply as "Temperature
of Standardization: 60°F". If a hydrometer is used
at the reference temperature, no special corrections are necessary
(though the measurement will depend a bit on the standard
temperature chosen by the manufacturer as described above).
Why does the temperature of the sample
matter? There are two reasons. One is that the hydrometer
itself may change its density as a function of temperature,
and thus give incorrect readings at any temperature except
that for which it is specifically designed (i.e., it floats
higher or lower as its density changes). Unfortunately, unless
there is a table for that specific hydrometer (which is rarely
supplied), this effect cannot be corrected by a table because
of the different materials of which hydrometers are constructed.
Various types of glass and plastic are used, and each will
have its own particular change in density as a function of
temperature. It should be noted that this effect is substantially
smaller for glass hydrometers than is the second effect described
below because the change in density of glass with temperature
is 8-25 times smaller than the change in density of aqueous
The second reason that the sample temperature
is important is that the sample itself will change its density
as a function of temperature. For example, the density of
seawater (S = 35) changes from 1.028 g/cm3
at 3.98°C to 1.025 g/cm3
at 20°C to 1.023 g/cm3
at a typical marine aquarium temperature of 80°F. Since
the density of the sample is changing with temperature, the
measured specific gravity will also change, unless this is
taken into account.
Temperature Corrections for Standard Floating
For standard floating hydrometers, the
impact of temperature on the density of the sample can be
corrected with a table, assuming that we know how the density
of the sample would change with temperature (which is well
known for seawater), and also that we know the hydrometer's
temperature of standardization. For example, a hydrometer
calibrated at 60°F/60°F needs to be corrected for
the difference in density between the sample at 60°F,
and the sample at the temperature at which it is tested. If
the actual sample were measured at 86°F, then the correction
is the ratio of seawater's density at 86°F (approximately
1.0217 g/cm3) divided by
the density at 60°F (approximately 1.0259 g/cm3),
or 0.996. Thus, a specific gravity reading, or more correctly,
a hydrometer reading, of 1.023 would be corrected to an "actual"
reading of 1.027.
If the temperature of standardization
is unknown, then a correction using a table is as likely to
cause bigger errors as it is to correct any. Likewise, using
a "correction" table that does not specify what
it is intended to correct is equally risky.
The corrections to use for standard hydrometers
are given in the summary at the beginning of this article.
Temperature Corrections for Swing Arm Hydrometers
Some marine hobby hydrometers, often called
swing arm hydrometers, claim to be accurate at all temperatures
(68 - 85°F; these include SeaTest, Deep Six, Instant Ocean
and eSHa Marinomat). Such a device could be designed, if its
change in density as a function of temperature were exactly
the same as seawater at all temperatures.
I have tested two of these swing arm hydrometers
and reported on the results in a previous
article. While one was not very accurate (reading S=32
when the solution was S=35), both of them did give results
that were roughly independent of temperature between 68 and
Consequently, swing arm hydrometers
should not be subjected to any temperature corrections in
the normal range of use for a reef aquarium. They may,
however, benefit from having their calibration checked with
How to Use a Standard Hydrometer
Here are a few additional tips for using
a standard hydrometer:
1. Make sure that the hydrometer is completely
clean (no salt deposits) and that the part of the hydrometer
above the water line is dry. Tossing it in so it sinks deeply
and then bobs to the surface will leave water on the exposed
part that will weigh down the hydrometer and give a falsely
low specific gravity reading. Salt deposits above the water
line will have the same effect. If any deposits won't easily
dissolve, try washing it in dilute acid (such as vinegar
or diluted muriatic acid).
2. Make sure that no air bubbles are attached to the hydrometer.
These will help buoy the hydrometer and yield a falsely
high specific gravity reading.
3. Make sure that the hydrometer is the same temperature
as the water (and preferably the air).
4. Read the hydrometer at the plane of the water's surface,
not along the meniscus (Figure
3; the meniscus is the lip of water that either rises
up along the shaft of the hydrometer, or curves down into
the water, depending on the hydrometer's hydrophobicity).
5. Rinse with purified freshwater after
use to reduce deposits.
6. Do not leave the hydrometer floating in the tank between
uses. If left in the aquarium, deposits may form that will
be difficult to remove.
Figure 3. The Tropic Marin floating hydrometer, showing
the meniscus rising to
about 1.0260, but the actual reading is about 1.0265.
How to Use a Swing Arm Hydrometer
In addition to those described above, here
are some special tips for using swing arm hydrometers:
7. Make sure that the hydrometer is completely
level. A slight tilt to either side will change the reading.
8. Some swing arm hydrometers recommend "seasoning"
the needle by filling it with water for 24 hours prior to
use. This presumably permits the water absorbed into the
plastic to reach equilibrium. In the case of the hydrometer
that I tested in a previous
article, the hydrometer became slightly less accurate
Hydrometers are an inexpensive and easy
way to measure salinity in marine aquaria. In order to most
effectively use hydrometers, aquarists need to know when they
should apply a temperature correction to the hydrometer reading
to get an accurate specific gravity reading, and when this
isn't necessary. This article should enable aquarists to properly
apply such corrections.