by calibrating salinometers and conductivity–temperature– In this article, a new density–salinity relation is presented, whereby . natural seawater (as used for the preparation of standard sea- water) to correct the sample density (of seawater). .. have a salinity of less than 35 is derived from b0 = SP ·. and for estimating seawater density from salinity, temperature, and pressure were made on Natural seawater densities can differ from computed density by as much as kg/m 3 review the relationship of the equation of state to chlorinity, .. trical conductivity and, less commonly, specific gravity or re-. Density of standard seawater solutions at atmospheric pressure McCartney, F. CulkinThe specific gravity/salinity/temperature relationship in natural seawater.
For example, to measure the salinity of seawater at 35 ppt, calibrate a refractometer using a standard with the same refractive index, and the slope miscalibration error disappears when measuring seawater samples near that salinity Figure In those cases, the measured and true salinity or specific gravity relate to one another in exactly the same way that measured and true specific gravity relate to each other in Figures 13 and Figure 15, for example, shows the relationship between the measured and actual specific gravity for a refractometer with a slope miscalibration.
Figure 16 is an expansion of the region of specific gravity of interest to reef aquarists. The relationship between the real actual specific gravity and the measured specific gravity for an incorrectly calibrated seawater refractometer red and a perfectly calibrated seawater refractometer green.
This red refractometer has a slope error, with values far from the calibration point freshwater with a specific gravity of 1. The amount of error in measuring seawater is indicated. This figure is an expansion of Figure 15 in the region of most interest to reef aquarists. Similarly, Figure 17 shows the relationship between the measured and actual salinity for a refractometer with an offset miscalibration. Figure 18 is an expansion of the region of salinity of interest to reef aquarists.
It is clear that seawater 35 ppt reads much lower in this case, at about 30 ppt. The relationship between the real actual salinity and the measured salinity in ppt for an incorrectly calibrated seawater refractometer red and a perfectly calibrated seawater refractometer green.
This red refractometer has a slope error, with values far from the calibration point freshwater with a salinity of 0 ppt reading higher than the actual value. This figure is an expansion of Figure 17 in the region of most interest to reef aquarists. Just as was shown for refractive index, recalibration of a refractometer with a slope error can be discussed in terms of specific gravity and salinity. Figure 19 shows what happens when adjusting the calibration screw so that the specific gravity of a 35 ppt seawater standard with a known specific gravity of 1.
Figure 20 is an expansion of the region of salinity of interest to reef aquarists. In this figure, the miscalibrated red line moves onto the green line, and the refractometer is then good to go at specific gravity values near 1.
Refractometers and Salinity Measurement by Randy Holmes-Farley - az-links.info
The refractometer of Figure 15 and 16 red has a slope error, with values far from the calibration point reading incorrectly. In this figure it has been recalibrated with seawater and so is adequately accurate over the range of specific gravity from 1.
This figure is an expansion of Figure 19 in the region of most interest to reef aquarists. Similarly, Figure 21 shows what happens when adjusting the calibration screw so that the salinity of a 35ppt seawater standard really reads 35 ppt. The refractometer of Figure 17 and 18 red has a slope error, with values far from the calibration point reading incorrectly.
In this figure it has been recalibrated with seawater, and so is adequately accurate over the range of salinity of ppt despite the slope error. This figure is an expansion of Figure 21 in the region of most interest to reef aquarists.
This type of slope correction turns out to be important for reef aquarists, as slope miscalibration errors seem to abound in inexpensive refractometers. Many aquarists have found that when calibrated using pure freshwater, their refractometers do not accurately read 35 ppt seawater standards. Many read 1 ppt, which is likely acceptable to most aquarists, but some read much further from the actual value.
These inaccuracies may be partly because many of these may actually be salt refractometers and not actual seawater refractometers see next section. Correction of slope miscalibration errors should be carried out using a fluid that approximately matches the refractive index of the water being tested, so for reef aquarium water, calibration with 35 ppt seawater solves this problem, while calibration with pure freshwater does not.
Scale Misunderstanding and Salt Refractometers Refractometers can lead to incorrect readings in additional ways and, again, these issues abound for reef aquarists. One is that many refractometers are intended to measure sodium chloride solutions, not seawater.
These are often called salt or brine refractometers. Unfortunately, many refractometers used by aquarists fall into this category.
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In fact, very few refractometers used by hobbyists are true seawater refractometers. Fortunately for aquarists, the differences between a salt refractometer and a seawater refractometer are not too large. A 35 ppt sodium chloride solution 3. This error is significant, in my opinion, but not usually enough to cause a reef aquarium to fail, assuming the aquarist has targeted an appropriate salinity in the first place.
Figure 23 shows the relationship between a perfectly calibrated and accurate salt refractometer and a perfectly calibrated and accurate seawater refractometer when the units are reported in salinity. This figure shows the measured salinity reading for seawater being about 1. The relationship between the real actual salinity and the measured salinity in ppt for a perfectly calibrated seawater refractometer green and a perfectly calibrated salt refractometer red.
This salt refractometer effectively has a significant slope error, with values far from the calibration point freshwater with a salinity of 0 ppt reading roughly 1. Salt refractometers reading in salinity can be recalibrated using seawater to eliminate nearly all of this error just as the refractometer in Figures 17 and 18 was recalibrated in seawater to give Figures 21 and It turns out that this is a slope miscalibration in the sense that a perfectly made sodium chloride refractometer necessarily has a different relationship between refractive index and salinity than does seawater.
This type of problem with a refractometer IS NOT at all corrected by calibrating it with pure freshwater. If you have this type of refractometer, and it was perfectly made and calibrated in freshwater, it will ALWAYS read seawater to be higher in salinity than it actually is misreporting an actual Even more confusing, but perhaps a bit less of a problem in terms of the error's magnitude, salt refractometers sometimes read in specific gravity. In older literature it has the units of ppt parts per thousand by weightand that is roughly the way to think of it, but it is now defined as the ratio of the seawater conductivity to that of a potassium chloride solution of defined composition.
Other solutions, like simple sodium chloride, are not defined in this way, and are still reported as ppt. This definition of salinity is described in detail in "Chemical Oceanography" by Frank Millero What is specific gravity?
Specific gravity is defined as the ratio of the density of a liquid compared to the density of pure water.
Temperature Corrections for Hydrometers by Randy Holmes-Farley - az-links.info
Since the density of pure water varies with temperature, one needs to specify the temperature of the pure water to usefully define specific gravity. For many scientific endeavors such as mineralogythe temperature standard chosen is 3. At that temperature, the density of pure water is 1. If this is the standard chosen, it is easy to see that the specific gravity is just the density of the sample at 3.
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 things less dense than water are dissolved in it, then the specific gravity will drop. Ethanol, for example, is less dense than water, and makes the specific gravity drop.
This fact is used by brewers to gauge the amount of alcohol in their brews. Likewise, if things denser than water are dissolved in it, the specific gravity goes up. Nearly all inorganic salts are denser than water, so dissolving them in water makes the specific gravity rise. This rise can be used by aquarists to gauge how much salt is in their water.
Of course, it cannot tell you what is in the water, but if you are using an appropriate salt mix, it can tell you how much is there and whether it approximates natural seawater or not.
Standard hydrometers work on Archimedes Principle. This principle states that the weight of a hydrometer or other object, like an iceberg or a ship equals the weight of the fluid that it displaces. Consequently, the hydrometer will sink until it displaces its own weight. When it is put into solutions of different densities, it floats higher or lower, until it just displaces 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 it's own weight. Any different displacement puts forces on the water and hydrometer that cause them to move toward the optimal position. Swing Arm Hydrometers Swing arm hydrometers are a bit different since none of the arm is above the water line. In this case, the swing arm responds to the density difference by rotating an arm with nonuniform weight distribution.
Typical hobby swing arm hydrometers use an arm made of two different materials Figure 2. The density difference between the water and one of the materials forces the arm to swing in one direction, and the density difference between the water and second of the materials forces the arm to swing in the opposite direction.
At the equilibrium position these forces cancel out, and the hydrometer gives a steady reading. Again, the final result is a minimization of the gravitational potential energy of the system.
A SeaTest swing arm hydrometer made by Aquarium Systems, showing the arm made of two different materials. One question often asked is whether changes in various ions impact specific gravity. The answer is that, to a hobbyist using a normal salt mix, they do not. To get a ballpark understanding of this effect, it is reasonable to assume that all ions contribute to specific gravity in an amount proportional to their weight percentage in seawater.
In a sense, the more of any ion that is present regardless of chemical nature, the larger is the effect on specific gravity. Since that's exactly what salinity is the weight of solids in the waterit is unlikely that any normal ion variation seen by marine aquarists will unduly skew specific gravity measurements. What about changes in these top four ions?
Let's take an extreme case where the salt consists of nothing but sodium chloride. Thus, one can see that even big changes in the ionic balance result in fairly small changes in the relationship between specific gravity and salinity.
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. 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 waterit is unlikely that any normal ion variation seen by marine aquarists will unduly skew specific gravity measurements.
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 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.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 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. While these seem close to 1, and are often simply claimed to be 1. 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 allthe standard temperature and the reference temperature are the same: 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.
Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!
Unfortunately, unless there is a table for that specific hydrometer which is rarely suppliedthis 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 times smaller than the change in density of aqueous fluids.
The second reason that the sample temperature is important is that the sample itself will change its density as a function of temperature.
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 Hydrometers 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 seawaterand also that we know the hydrometer's temperature of standardization.
Thus, a specific gravity reading, or more correctly, a hydrometer reading, of 1. 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.