Hydrometers

Hydrometer standards

• BS718
• BS 5470 (Alcohol)
• BS734
• ASTM E100
• ISO 649
• DIN 12791

Components of
a hydrometer

A hydrometer is an instrument used to determine the specific gravity of liquids and is manufactured primarily from glass although there are some low accuracy hydrometers manufactured in plastic.

It consists of a thin glass tube with a graduated paper scale fixed inside and welded to a larger diameter bulbous tube which is accurately weighted with either fine lead shot sealed into a solid mass using sealing wax or mercury.

WHAT IS A HYDROMETER USED FOR?

Many industries and sciences measure the densities of both liquids and solids for products that need to be measured by either mass (weight) or volume.

Hydrometers are used to determine the specific gravity of liquids and operate on the Archimedes principle that a solid body displaces its own weight within a liquid in which it floats.

Hydrometers can be of two types:

• Liquids heavier than water
• Liquids lighter than water.

The standard hydrometer scale is the specific gravity scale which when used in distilled water the scale reading is 1.000 at the point of measurement. Therefore liquids lighter than water are measured on a scale below 1.000 specific gravity and those liquids heavier than water are scaled above 1.000 specific gravity.

The standard calibration method used on hydrometers is the Cuckow method. It is very exact and involving, designed by F.W Cuckow using sophisticated equipment and precise cleaning practices.

HOW TO USE A HYDROMETER

The hydrometer is placed in a glass tube known as a hydrometer jar and which is filled with the liquid that is to be measured. The specific gravity of the liquid is indicated when the level of the liquid in the jar aligns with a point on the hydrometer scale. If the liquid is heavier than water then the hydrometer sits higher in the liquid and gives a higher reading.

Conversely a liquid lighter than water sits higher in the liquid and gives a lower reading.

METHOD OF READING A HYDROMETER

It is most important that hydrometers are read correctly. So as to prevent errors in results. And the following method is standard.

For transparent liquids the eye should be placed slightly below the plane of the surface of the liquid then raised slowly until this surface forms an ellipse appears as a straight line. Where this point sits on the hydrometer scale is the actual reading.

When measuring an opaque liquid that is not sufficiently clear to allow the previous method of reading it must be read from above the surface and estimated as accurately as possible where the liquid rises to on the hydrometer.

Accuracy depends on Cleanliness, Temperature and Proper Immersion.

All equipment must be cleaned properly especially the hydrometer bulb and stem and the hydrometer jar so that the liquid can travel up the stem uniformly. The temperature of both the liquid and the hydrometer should be the same temperature as the surrounding atmosphere. The Hydrometer jar should have an internal diameter at least 1inch (25mm) greater than the outside diameter of the hydrometer.

TYPES OF HYDROMETER

Different scales are used on hydrometers depending on the types of liquids being measured.

• Brix or Balling Scale: This scale is used to read the percentage of pure sucrose (sugar) by weight.
• Baume Scale: This scale reads degrees of Baume or percentage of NaCL (salt) by weight.
• Alcohol Scale: This measures a standard “specific Gravity” and is used to measure specific gravity before and after a liquid has fermented. The difference between the two readings is referenced to an alcohol scale to determine the percentage of alcohol by weight.
• Twaddle: The specific gravity of pure water is 1.000 at 60 degrees Fahrenheit and the twaddle scale is a simple scale based on specific gravity where 0 degree Twaddle is equal to SG60/60 degrees F of 1.00 (that of water and each degree of Twaddle equals 0.005 degrees SG
• API Hydrometers: The American Petroleum Institute , The United States Bureau of Mines and the National Institute of Standards and Technology selected the API scale in 1921 as the standard for petroleum products in the United States of America ASTM hydrometers are also used in many other applications and procedures set forth by ASTM all are calibrated at 60 degrees Fahrenheit

Where accuracy is of paramount importance hydrometers should be calibrated to UKAS standards. And the results used to compensate for any inaccuracies.