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Hydrometer | measurement instrument- How To Use a Hydrometer

Hydrometer | measurement instrument- How To Use a Hydrometer

 Hydrometers is based on Archimedes' floating law, which is that if an object floats on a liquid surface, the weight of the submerged body is equal to the weight of the displaced fluid

What's a hydrometer?

What's a hydrometer?

It is a long glass tube, closed at both ends and consisting of three parts :

The first part: a long stalk numbered from top to bottom in solutions with a density higher than one and used in sugary and brine solutions ..or they are numbered from the bottom to the top so they are used in solutions with a density less than one and those used in alcoholic solutions and oils

The second part: a bulge in the center of the tube, which is filled with air, that helps the hydrometer to float

The third part: a lower bulge containing lead or mercury balls that gives the hydrometer a weight in order to remain stable in a vertical position
  • The hydrometer may contain a thermometer to measure the temperature of the solution

Types of hydrometers :

  1. Hydrometers measure concentration of sugary solutions (Baling, Brix
  2. Hydrometers measure concentration of saline solutions (Baume, Salometer)
  3. Hydrometers measure concentration of alcohol solutions (Trales
  4. Hydrometers measure specific gravity (density)

Hydrometers for measuring the concentration of sugary solutions:

  • Balling hydrometer later modified by Brix and added a thermometer inside it
  • The reading taken expresses the concentration of sugar in the solution as a percentage by weight%
  • Every 1 degree (Baling, Brix) = 1% of sugar in 100g of sugar solution

Note :
Hydrometer has been  gradated  from top to bottom (at 20 ° C) to increase accuracy to three types, which are (from 0 to 30), (30 - 60), (60 - 90)

Hydrometers for measuring the concentration of brine solutions:

 Baume hydrometers and the Salometer

Baume used to measure the salt concentration in brine solutions based on percentage by weight and it is gradated from (0 to 70) at a temperature of 20 ° C.
It gives readings of solutions concentration and intensity as it is gradated from the other side of the leg from top to bottom  (1 to 2)


The maximum concentration of the brine solution is 26.5% at a temperature of 25 ° C.
And it increases by boiling to reach 28-29% of the degree of saturation ... 
But why is the hydrometer gradate to 70  ? 
In order to be used to measure other solutions such as sugar oils and fats factories it is used to measure the concentration of sodium becarbonate solutions during the adjustment processes of free fatty acids in the oil.

Salometre: It is a hydrometer that measures the concentration of brine solutions gradate from top to bottom (0 - 100) (0 means distilled water, 100 means a saturated salt solution with a concentration of 26.5%)

Hydrometers for measuring the concentration of alcohol solutions:

It is a hydrometer, and its reading indicates the number of parts of the absolute alcohol present in the alcohol.

The immersion of the body in the solution is proportional to the density of the solutions. Therefore, we notice that the submerged part of the hydrometer in high concentration sugary and brine solutions is less due to the increased concentration of dissolved solids and vice versa.

Temperature is the main factor affecting the reading of the hydrometer because the specific weight of the solutions changes with the change of temperature, so it is necessary to perform a thermal correction for the reading of the hydrometer if the measurement temperatures  do not match the required degree, according to certain equations

Advantages of using a hydrometer:

1- Easy and fast, does not require experience and is used in many manufacturing processes

2 - Light weight and cheap compared to the refractometer

Disadvantages of using a hydrometer:

1- It is not the most accurate method and requires a large amount of tested solutions

2- Sometimes inaccurate readings are given in highly concentrated solutions due to the high viscosity of the solutions, and thus the hydrometer is not immersed to the level corresponding to the  certain concentrations.

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