Hydrometer | Specific Gravity, Salinity & Density (2024)

FAQs

Hydrometer | Specific Gravity, Salinity & Density? ›

The device consists essentially of a weighted, sealed, long-necked glass bulb that is immersed in the liquid being measured; the depth of flotation gives an indication of liquid density, and the neck can be calibrated to read density, specific gravity, or some other related characteristic.

With the hydrometer, you will collect a specific gravity reading. You also need to take the temperature of the water. With the temperature and specific gravity values, you use a a table to determine the salinity in parts per thousand (ppt).

Serious hobbyists know that the best way to measure the salinity and specific gravity of aquarium water is to use a refractometer. These precision optical instruments are incredibly accurate even at very low salt levels and are equally easy to use.

How to Use a Hydrometer. Pour the sample liquid into the hydrometer jar. Place the hydrometer in the jar and give it a quick twirl to dislodge any air bubbles. Once the hydrometer has settled, take the reading from the appropriate scale at the lowest level of the liquid's surface.

Using a Hydrometer to Determine Density and Salinity

The more dissolved salts there are in a body of water, the greater the density of the water. If the density and temperature of a water sample are known, salinity can be determined from a graph (Fig.

A hydrometer, such as this one from Instant Ocean, is a quick and inexpensive method of measuring salinity. While they are not as accurate as a refractometer, they are less expensive and some find them simpler to use. A hydrometer operates on the principle that salt water has a higher density than freshwater.

Water and soil salinity are measured by passing an electric current between the two electrodes of a salinity meter in a sample of soil or water.

Two common ways are by using an electrical conductivity (EC) meter or by measuring how much salt is in a solution of soil or water. An EC meter measures how much electricity moves through a solution—the saltier the solution, the more electricity moves through it and the higher the conductivity reading.

Read the gravity reading from the scale on the hydrometer where it crosses the water-air boundary. There will be a slight curve along the water-air line (called the meniscus), so if you want to be really accurate you should take the reading at the lowest point in that air-water curve (the bottom of the meniscus).

How do you measure density with a hydrometer? ›

The liquid to test is poured into a tall container, often a graduated cylinder, and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer correlates to relative density.

The analysis of pure water at 20°C - 68°F will have a gravity of 1.000. (This reads one point zero zero zero). Add sugar to the water, and the gravity will increase. Adding alcohol to the pure water the gravity will go down as alcohol is less dense than water.

Salinity is often derived from electrical conductivity (EC) measurement. EC is measured by passing an electric current between two metal plates or electrodes in the water sample and measuring how readily current flows between the plates.

In modern oceanography, salinity is determined in part from conductivity measurements done on water samples using laboratory or on-board salinometers, or in situ using an oceanographic instrument called a CTD. The CTD is named for the three variables it measures: conductivity, temperature, and depth or pressure.

Put simply, it refers to the 'saltiness' of a given body of water. It is the concentration of dissolved mineral salts therein — and is often measured either directly by its concentration or through electrical conductivity, which it affects.

Traditionally chloride has been the ion measured because it is the most abundant, and thus the simplest to measure accurately. Multiplying the concentration of chloride by 1.8 gives the total salinity. For example, looking at Figure 5.3. 1, 19.25 g/kg (ppt) chloride x 1.8 = 35 ppt.

To calculate the salinity, divide the mass of salt by the mass of water, then multiply by 1000 to express as parts per thousand.

Marine aquarium owners should aim for a salinity of 1.025 s.g. which is about 35 parts per thousand. That means that for every thousand grammes of freshwater you should add 35 grammes of salt, or 35 grammes of salt per litre of water.