Specific Gravity

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 Water Parameter Specific Gravity Unit(s): none Target: 1.024 - 1.027 Danger: < 1.020 > 1.031 Tendancy: Increase Increased: Evaporation Decreased: Top off Testing: Weekly Maintenence: Pure water

Introduction

Specific gravity is a common way to approximate and measure the salinity of the water. Normal ranges for seawater are between 1.024-1.027. However often results can be obtained from devices that vary greatly from this. More often than not its due to improperly calibrated devices. Sometimes it can be a difference in they way its measured.

Specific gravity is the ratio of density of the sample being measured (seawater in our case) to the density of pure water. That part is easy. The problem is that the temperature of the sample determines the density of the sample and the temperature of the pure water influences its density. Different devices for measuring specific gravity are based on different temperatures for both the sample and the pure water. The exact same sample of water can have a different specific gravity depending on what standards are used for the device - and to complicate matters further, the actual temperature of the sample may influence the reading for specific gravity.

For example, most laboratory grade floating glass hydrometers give the specific gravity as the ratio of the density of the sample at 15.6ºC over the density of pure water at 4ºC. As the density of pure water at 4ºC (or more accurately 3.98ºC), is 1.000 kg/L, the specific gravity of the sample is equivalent to the density of the sample at 15.6ºC. However, if the sample is actually at 25ºC, the reading has to be corrected because water at 25ºC is less dense than at 15.6ºC. Confused?

Measurement

Refractometers are a method of determining the specific gravity. Most, if not all, refractometers used in the hobby give specific gravity as the ratio of the density of the sample at 20ºC to the density of pure water at 20ºC (d20/20). Seawater with a salinity of 35‰ (35 parts per thousand) will have a specific gravity (@ d20/20) of 1.0266 which is close enough to 1.027. The aim should be 35‰, so a specific gravity of 1.027 is desirable, but a bit above (1.028) or a bit below (1.026) is fine, depending on errors in reading or even a bit of evaporation. If you are using a refractometer, 1.026 to 1.028 would be a valid range, although 1.027 would be ideal.

With a hydrometer that gives specific gravity as d25/4 (ratio of density of sample at 25ºC to pure water at 4ºC), the same sample of water above (35‰) would have a specific gravity of 1.0234.