Description: Floating Thermometer Sub Hydrometer Floating Thermometer Sub Hydrometer Floating 0 to 40oC temperature and 1.000 to 1.060 SG. Glass construction. NORMAL VALUES AND DEFINITIONS When measuring how “salty” sea water is, we measure the “specific gravity”. Salinity, a term that is often used when meaning specific gravity, is really a measurement of the sodium chloride component of sea water. SPECIFIC GRAVITY is the ratio between the weight of a given volume of sea water and the weight of an equal volume of distilled water. It is usually measured with the aid of an hydrometer. The value of the normal specific gravity of sea water at a temperature of 20°C is 1.024. SALINITY is a measure of the total salts in a given weight of sea water. It is measured in parts per thousand, Normal value is 34%. Do not confuse salinity with specific gravity. CONSIDERATIONS IN MEASURING SPECIFICGRAVITY There are several common methods used to measure specific gravity. The most common are (1) hydrometers, (2) conductivity meters and (3) refractometers. Hydrometers are the most common instrument used in the aquarium industry. With each type of measurement there are interfering factors (the main factor that affects specific gravity measurements is temperature). TEMPERATURE Temperature, when measured with a thermometer, greatly affects specific gravity and this can lead to a misunderstanding of the correct values for aquarium use. For instance, in a colder latitude, where water temperature could be as low as 12°C, the specific gravity would be 1.026. The specific gravity of sea water lapping a coral reef would be 1.023 at a temperature of 24°C. The effects of temperature on specific gravity when measured with a hydrometer, are charted over. Specific gravity when measured with a conductivity meter is also drastically affected by temperature. It is recommended to use a conductivity meter that is temperature compensated. USING SPECIFIC GRAVITY TO ADVANTAGE The raising or lowering of specific gravity has an effect through osmosis on marine fish that may be used to advantage when faced with certain problems. Marine fish continually loose fresh water through their skin into the sea water, via osmosis. This loss is compensated by their extraction of fresh water from seawater. By raising or lowering the specific gravity the fishes ability to extract freshwater is also raised or lowered.
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