Making and Using a Hydrometer

Making and Using a Hydrometer ...

A Hydrometer is a device used to compare the densities of liquids. You can easily make your own Hydrometer using anything that can be made to float with part of itself projecting above the surface of the liquid. The instructions below outline how to make your own very inexpensive Hydrometer using readily available bits and pieces ...

Parts needed to make your Hydrometer.

Parts needed to make your Hydrometer

You will find it easiest if you mark a scale on your Hydrometer so that you can accurately judge how low it floats in different liquids.

Mark a scale to make reading easier.

Mark a Scale on your Hydrometer to make it easier to see how far it sinks in the liquid.

In the photographs below the Hydrometer is placed in Fresh and Salt water. The depth at which it floats indicates the relative densities of the two liquids.

The Hydrometer in Fresh Water

The Hydrometer in Salt Water

When measuring the depth to which the Hydrometer sinks you need to be aware of the Meniscus formed between the straw and the water surface. The accepted practice is to measure from the bottom of the meniscus in line with the liquid surface.

The Meniscus is the curve on the surface.

A close-up view of the straw in water showing the curved "meniscus" due to attraction between water molecules and the straw.

Adhesion between water and plastic molecules causes the Meniscus.

When measuring the depth of the straw in the liquid measure from the bottom of the meniscus in line with the water surface.

Commercial Hydrometers and Specific Gravity ("SG")

Two commercial Hydrometers.

The Hydrometers in the photograph are approximately 30 centimeters long.

Commercial Hydrometers are usually made of glass with lead shot to weigh them down. The red material above the lead is wax.

Hydrometers are used in all sorts of industries dealing with liquids. The wine industry, for example, uses density changes to follow the progress of the fermentation process.

The scale on these Hydrometers indicates the "Specific Gravity" of the liquid. Pure water is defined as having a Specific Gravity ("SG") of one. A liquid with a density twice that of water has a SG of 2.

Specific Gravity also applies to solids. The SG of some of the common metals are:

Aluminium - 2.55
Copper - 8.88
Iron - 7.2
Lead - 11.3

This ship has marks on the Bow indicating its depth in the water. In effect it is a giant Hydrometer similar to the one described above made out of a drinking straw, plasticine and nails.

Close-up of the RNZN TE MANA
The New Zealand Frigate TE MANA in Fremantle Harbour

Cargo ship in Fremantle harbour.

Cargo ships also have markers called the "International Loading Line", or "Plimsoll Line" indicating the maximum depth a vessel can be loaded to in different waters. It is illegal for a ship's master to allow their vessel to be loaded any deeper than the marks indicate. From your knowledge of Hydrometers you should understand why. Imagine, for example if the ship was loaded to the very top mark in the Northern Atlantic (ie to the TF mark) and then sailed to a fresh water port. It would sink lower down in the fresh water (just like your Hydrometer did). If there was a storm the ship could be in danger of sinking.

Plimsoll Line - The International Load Line for Shipping

(a) the Summer load line, shall correspond horizontally with the line passing through the centre of the ring of the load line mark, and shall be marked S;
(b) the Winter load line, which shall extend forward of the vertical line, and be marked W;
(c) the Winter North Atlantic load line, which shall extend forward of the vertical line, and be marked WNA;
(d) the Tropical load line, which shall extend forward of the vertical line, and be marked T;
(e) the Fresh Water load line, which shall extend abaft the vertical line, and be marked F; and
(f) the Tropical Fresh Water load line, which shall extend abaft the vertical line and be marked TF.

FROM: http://www.hmso.gov.uk/si/si1998/98224103.htm

DSRV mounted for transport on the USS Greeneville.

Submarines adjust their density to change their depth.

(NOTE: This page uses the English/Australian spelling of: Plimsoll. Samuel Plimsoll, born 1824 was a British member of parliament who introduced a bill to limit the amount of cargo that could be carried on ships.The American spelling is: Plimsol)

Hydrometers - QUIZ

Your Name and School -

Do you Certify the answers below to be all your own work? -

Use your Hydrometer to answer questions one and two...

1. Describe the level the Hydrometer sinks to in Tap Water, Salt Water and Wash-up Detergent.
(If you don't live near the ocean you can make up 'salt water' by dissolving about a handful of salt in a medium glass of tap water (about 250 ml). Stir the mixture well and let it settle before you begin experimenting.)

2. If we assume that Tap Water has a 'density measure' of "1" and Salt Water has a 'density measure' of 1.25, what is the 'density measure' of cooking oil? (Get permission to use cooking oil for before you conduct this experiment.)

3. What causes the "meniscus" when glass, or plastic are placed in water?

4. Why do ships have a "Plimsoll mark"?

5. If a cargo ship sails from a port in the North Atlantic Ocean to a port along the Amazon river (fresh water), what happens to the level it floats in the water? Why?

6. In relation to the "International Loading Line" why is the "S" mark higher than the "W" mark?

7. How could you find out if the human body is more, or less dense than Salt Water? -

8. What is the URL of a Web page containing information about Hydrometers (including a diagram of one)?

Feedback, comments, questions -

(Make sure your NAME and SCHOOL are included.)

This form will be emailed to a 'marker'. You will receive feedback shortly. Satisfactory performance on this and other Quizzes is required for the granting of a "Completion Certificate" for this Course.