NOAA Technical Memorandum GLERL-139

CONVERSION OF SPECIFIC GRAVITY TO SALINITY FOR BALLAST 

WATER REGULATORY MANAGEMENT

David F. Reid

NOAA, Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105

Great Lakes Environmental Research Laboratory

Ann Arbor, Michigan

November 2006 (updated March 2008)

UNITED STATES

DEPARTMENT OF COMMERCE

Carlos M. Gutierrez

Secretary

NATIONAL OCEANIC AND

ATMOSPHERIC ADMINISTRATION

Conrad C. Lautenbacher, Jr.

Administrator
2

NOTICE

Mention of a commercial company or product does not constitute an endorsement 

by the NOAA. Use of information from this publication concerning proprietary 

products or the tests of such products for publicity or advertising purposes is not 

authorized. This is GLERL Contribution No. 1409.

This publication is available as a PDF file and can be downloaded from GLERL’s 

web site: www.glerl.noaa.gov. Hard copies can be requested from GLERL, 

Information Services, 2205 Commonwealth Blvd., Ann Arbor, MI 48105.

pubs.glerl@noaa.gov.

Errata

Section 1.2, Specific Gravity (page 6): the stated reference temperature for most 

hydrometers used aboard ships was corrected to 60o

F. Previous versions of this 

report had it incorrectly listed as 60o

C.
3

TABLE OF CONTENTS

ABSTRACT..............................................................................................................................5

1. INTRODUCTION ..............................................................................................................5

1.1 Background and Purpose ...........................................................................................5

1.2 Specific Gravity .........................................................................................................6

1.3 Equation of State of Seawater....................................................................................6

2. RELATIONSHIP BETWEEN SPECIFIC GRAVITY AND SALINITY AT THE 

OCEAN SURFACE............................................................................................................7

2.1 Calculation of Specific Gravity .................................................................................7

2.2 Measuring Specific Gravity with a Hydrometer – Importance of Temperature ........7

2.3 Salinity-Specific Gravity Curves - How to Use Figures 2-16 ...................................7

3. REFERENCES ...................................................................................................................8

4. ACKNOWLEDGEMENTS................................................................................................8

5. COMMENTS AND CONCLUSIONS ...............................................................................8

LIST OF FIGURES

Figure 1. Relationships between salinity and specific gravity at 5o

C and 35oC ...................9

Figure 2. Relationships between salinity and specific gravity at 0oC.................................10

Figure 3. Relationships between salinity and specific gravity at 2.5o

C..............................11

Figure 4. Relationships between salinity and specific gravity at 5o

C.................................12

Figure 5. Relationships between salinity and specific gravity at 7.5oC..............................13

Figure 6. Relationships between salinity and specific gravity at 10o

C...............................14

Figure 7. Relationships between salinity and specific gravity at 12.5oC............................15

Figure 8. Relationships between salinity and specific gravity at 15oC...............................16

Figure 9. Relationships between salinity and specific gravity at 17.5o

C............................17

Figure 10. Relationships between salinity and specific gravity at 20oC...............................18

Figure 11. Relationships between salinity and specific gravity at 22.5o

C............................19

Figure 12. Relationships between salinity and specific gravity at 25o

C...............................20

Figure 13. Relationships between salinity and specific gravity at 27.5o

C............................21

Figure 14. Relationships between salinity and specific gravity at 30o

C...............................22

Figure 15. Relationships between salinity and specific gravity at 32.5o

C............................23

Figure 16. Relationships between salinity and specific gravity at 35o

C...............................24
4
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Conversion of Specific Gravity to Salinity for Ballast Water

Regulatory Management

David F. Reid

ABSTRACT. To reduce the risk of new aquatic species introductions to coastal 

ecosystem via the ballast tanks of ocean-going ships, both the United States and 

Canada have established regulatory and/or policy requirements based on assuring 

that the salinity of incoming ballast water, including residual ballast water, is 30 

ppt or greater. However, common shipboard practice for management of ballast 

water is to determine the specific gravity of the water, not the salinity. Thus there 

is a technical disconnect between the information the ship typically records and 

what the regulatory agencies need. In 1981 a new equation of state for seawater 

was established, including a highly accurate mathematical relationship between 

density, salinity, temperature, and pressure. The equation is valid for salinity 

from 2 to 42 (practical salinity) and temperature from -2 to 35o

C. The equation of 

state for seawater was used to calculate a set of tables relating salinity to density 

and specific gravity, which were then converted into a related series of graphs, 

presented in this report, that can be used by ship’s crews and regulators alike to 

convert between salinity and specific gravity.

1. INTRODUCTION

1.1 Background and Purpose

While a universal solution to the transoceanic dispersal of aquatic nuisance species through 

ship’s ballast water is being developed, ballast water exchange and, more recently, ballast tank 

flushing and the resultant potential for salinity shock to fresh- and brackish-water biota have 

been and are the only practical and available defense for coastal waters and internal water bodies 

such as the Great Lakes-St. Lawrence Seaway system. Mid-ocean ballast exchange to achieve 

a ballast water salinity of at least 30 ppt is required for ships coming to U.S. and Canadian 

ports with or in ballast (U.S.: 33 CFR Part 151 subpart C; Canada: Canada Gazette, Part II, 

Wednesday, June 28, 2006). In August 2005 the U.S. Coast Guard implemented a new policy 

aimed at ships entering the Great Lakes with no pumpable ballast (NOBOB ships) to encourage 

the use of ballast water exchange or ballast tank flushing on the high seas to assure that the 

salinity of all water in their ballast tanks, including residual water, is at or above 30 ppt (U.S. 

Coast Guard, 2005). Similar regulations were established in Canada in June 2006 (Transport 

Canada, 2006). Thus it has become important for both mariners and U.S. and Canadian 

enforcement officials to know or be able to determine the salinity of water in ballast tanks. 

Unlike the scientific community, mariners may not typically measure salinity while ballasting. 

Ships are primarily concerned with and measure the specific gravity of the water in which the 
6

ship floats, not the salinity. Enforcement officials typically measure the salinity in a water 

sample drawn from a ballast tank, via either a Refractive Salinometer or a hand-held electronic 

Conductivity-Salinity-Temperature Meter. Thus there can be a technical disconnect between 

the information a ship records (specific gravity) and what the regulatory agencies require and 

measure (salinity). This report provides a means to easily convert between salinity and specific 

gravity (or density) for purposes related to ballast water regulation.

1.2 Specific Gravity

Specific gravity is defined as the ratio of the density of a liquid to the density of pure water at 

a given reference temperature and atmospheric pressure (pressure at sea level). The standard 

instrument used on ships for measuring the specific gravity of water is a hydrometer. Modern 

hydrometers are usually made of glass, although marine hydrometers may also be made of brass. 

A hydrometer is a hollow tube with a thin cylindrical stem marked with a calibrated specific 

gravity scale and a wide bulb weighted at the base to make it float upright. The hydrometer is 

gently lowered into the water sample until it floats freely, and the reading where the surface of 

the water touches the stem is the specific gravity. For most hydrometers used aboard ships, the 

reference temperature is 60o

F (15.56oC), although it may be another temperature depending on 

where the hydrometer was manufactured. Some hydrometers include a built-in thermometer to 

measure the sample temperature. The temperature of the sample at the time the specific gravity is 

measured is an important consideration (see section 2.2, below).

Specific gravity (or density) affects the depth to which the hull of a ship will submerge and 

thus the amount of cargo the ship can safely carry on her voyage as well as the depth of water 

that the ship requires for safe navigation. In particular, specific gravity determines the weight 

of the volume of ballast water taken on board and its consequent effect on trim, stability, and 

the stresses to which the hull is subjected. Although specific gravity is a ratio and therefore 

technically a unitless number, it is often reported as metric tonnes per cubic meter (t/m3 = kg/m3) 

for ballast-related purposes. 

1.3 Equation of State of Seawater

In 1978 a new definition of salinity was established by the Joint Panel on Oceanographic Tables 

and Standards (UNESCO, 1981) called “practical salinity.” Practical salinity is defined and 

measured in reference to electrical conductivity of the seawater sample compared to that of a 

potassium chloride solution of a specific concentration. Practical salinity is a unitless number, 

but is essentially equivalent to the traditional “parts per thousand” unit of salinity that was used 

until the new definition was adopted in 1978. Salinity is now often reported as “psu” (practical 

salinity units) to make it clear that what is being reported is based on the practical salinity scale, 

although in some uses it is still reported as “parts per thousand” (ppt).

A new equation of state for seawater was established in 1981 by the Joint Panel on 

Oceanographic Tables and Standards (UNESCO 1981) and is considered valid over a 

temperature range of -2 to 35o

C and a practical salinity range of 2 to 42 psu. Related equations 

allow calculation of the density of seawater and the corresponding specific gravity as a function 
7

of practical salinity, temperature (oC), and pressure (bars). However, ballast water is taken from 

approximately the sea surface, where p = 0 (bars), which simplifies the calculations. Millero 

and Poisson (1981) calculated a standard error of 3.6 x 10-3 kg/m3 for the one-atmosphere (p=0) 

equation of state, which is insignificant for the present purpose.

2. RELATIONSHIP BETWEEN SPECIFIC GRAVITY, SALINITY, 

AND TEMPERATURE AT THE OCEAN SURFACE

2.1 Calculation of Specific Gravity

For this report, UNESCO (1981) Equations 9 and 10 were used to calculate values of seawater 

density (kg/m3

)for given values of temperature and salinity over the ranges 0-35oC in 2.5oC 

intervals and, for each temperature, 2-42 psu, in 1 psu intervals. Specific gravity was then 

calculated for each value of salinity by dividing the calculate density of the water sample (S) at 

the selected temperature (T) by the density of pure water (0) at the reference temperature of 60o

F 

(15.56oC) and multiplying by 1000. 

 

Specific Gravity = (ρs

(T)/ρ0(15.56)) X 1000

2.2 Measuring Specific Gravity with a Hydrometer – Importance of Temperature

When specific gravity is measured with a hydrometer, a water sample is drawn into a tube or 

other suitable container, and the hydrometer float is placed in the water and allowed to stabilize. 

The specific gravity is then read as described in Section 1.2. Since density is a function of 

temperature as well as salinity, it is important to measure the temperature of the water sample 

at the time the specific gravity is measured. This is illustrated by Figure 1, which shows the 

relationships between salinity and specific gravity at 5oC and 35oC. Note that for the same 

specific gravity, the salinity of a seawater sample at 5oC would be ~8-10 psu less than a sample 

with the same specific gravity but at 35oC.

Figures 2-16 show salinity vs. specific gravity (x 1000) for temperatures ranging from 0 to 35oC 

in 2.5o

C increments and salinities ranging from 2 to 42 psu. For the purpose of ballast water 

characterization for regulatory purposes, 1 psu = 1 ppt.

2.3 Salinity-Specific Gravity Curves - How to Use Figures 2 - 16

Figures 2-16 may be used to find the equivalent salinity of ballast water for which both the 

temperature and the specific gravity at that temperature are known. The charts are provided 

with 1 psu and 1 t/m3 grids. Values falling between whole units can be visually interpolated with 

accuracy of 0.5 psu or less, which should be sufficient for ballast-related purposes. When the 

temperature of the sample falls between charts, the chart with the closest temperature should be 

used. 
8

Example 1: ballast water at 21oC has a specific gravity (x 1000) of 1028; using the chart for 

20o

C, find the specific gravity grid line for a value of 1028 on the vertical axis, read across to the 

plotted line and then read down to the corresponding salinity on the horizontal axis, estimated as 

38.0 psu (= 38.0 ppt).

Example 2: ballast water at 9o

C has a specific gravity (x 1000) of 1018; using the chart for 

10o

C, find the specific gravity grid line for a value of 1018 on the vertical axis, read across to the 

plotted line and then read down to the corresponding salinity on the horizontal axis, estimated as 

22.2 psu (= 22.2 ppt).

3. REFERENCES

Miller, F.J. and A, Poisson. International one-atmosphere equation of state of seawater. DeepSea Research, 28A, 625-629 (1981).

Transport Canada. Ballast Water Control and Management Regulations. Canada Gazette, Vol. 

140, No. 13, June 28, 2006 (2006).

UNESCO. The practical salinity scale, 1978 and the international equation of state of seawater, 

1980. Tenth report of the joint panel on oceanographic tables and standards. Technical Papers in 

Marine Science, 36, 25 pp., UNESCO, Paris (1981).

U.S. Coast Guard. Ballast Water Management for Vessels Entering the Great Lakes That Declare 

No Ballast Onboard. Federal Register, Vol. 70, No. 168, Wednesday, August 31, 2005, Notices, 

51831-51836 (2005).

4. ACKNOWLEDGEMENTS

A query from Capt. Philip T. Jenkins of Philip T. Jenkins and Associates, Ltd (Fonthill, Ontario, 

Canada), led to the preparation of this document. Discussions with Capt. Jenkins were quite 

helpful in framing the text and reviewing the content. In addition, I am indebted to Capt. Jenkins 

for coordinating review of this report and to the anonymous reviewers who evaluated it and 

provided comments and suggestions. 

5. COMMENTS AND CORRECTIONS 

Please send any comments or corrections to:

Dr. David Reid

National Oceanic and Atmospheric Administration (NOAA)

Great Lakes Environmental Research Laboratory (GLERL)

Ann Arbor, Michigan, USA

Phone: 734-741-2019

Email: david.reid@noaa.gov
9

Hydrometer Specific Gravity vs. Salinity at 5oC and 35oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

995

1000

1005

1010

1015

1020

1025

1030

1035

1040

0.05.0

10.015.020.025.030.035.040.045.0

Salinity (psu)

Specifc Gravity x 1000 (t/m3

) Figure 1 5o

C and 35oC Comparison 5
10

Hydrometer Specific Gravity vs. Salinity at 0oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, M

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

Figure 2 0o

C 6
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Hydrometer Specific Gravity vs. Salinity at 2.5oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Specifc Gravity x 1000 (t/m3

)

 Figure 3 2.5o

C 7
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Hydrometer Specific Gravity vs. Salinity at 5oC

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Specifc Gravity x 1000 (t/m3

)

Figure 4 5o

C 8
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Hydrometer Specific Gravity vs. Salinity at 7.5oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Specifc Gravity x 1000 (t/m3

)

 Figure 5 7.5o

C 9
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Hydrometer Specific Gravity vs. Salinity at 10oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Specifc Gravity x 1000 (t/m3

)

 Figure 6 10o

C 10 
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Hydrometer Specific Gravity vs. Salinity at 12.5oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

 Figure 7 12.5oC 11
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Hydrometer Specific Gravity vs. Salinity at 15oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Specifc Gravity x 1000 (t/m3

)

 Figure 8 15o

C 12 
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Hydrometer Specific Gravity vs. Salinity at 17.5oC

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Specifc Gravity x 1000 (t/m3

)

 Figure 9 17.5o

C 13 
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Hydrometer Specific Gravity vs. Salinity at 20oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

 Figure 10 20o

C 14
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Hydrometer Specific Gravity vs. Salinity at 22.5oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

 Figure 11 22.5o

C 15
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Hydrometer Specific Gravity vs. Salinity at 25oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

Figure 12 25o

C 16
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Hydrometer Specific Gravity vs. Salinity at 27.5oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

 Figure 13 27.5o

C 17
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Hydrometer Specific Gravity vs. Salinity at 30oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

 Figure 14 30o

C 18
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Hydrometer Specific Gravity vs. Salinity at 32.5oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

 Figure 15 32.5o

C 19
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Hydrometer Specific Gravity vs. Salinity at 35oC

NOAA-Great Lakes Environmental Research Laboratory, Ann Arbor, MI 48105 USA

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Salinity (psu)

Specifc Gravity x 1000 (t/m3

)

 Figure 16 35o

C 20