Hydrometers and Barometers

303. Hydrometer. A hydrometer is used in gaging the density of liquid. This instrument consists of a closed glass tube which is enlarged at the lower end and filled with some heavy material like mercury or shot, to keep it in an upright position when in liquids.

The tube or stem contains a paper on which divisions called degrees are marked. The 0 mark is usually the point reached by the surface of distilled water when the hydrometer is placed in this liquid. The less the density of the liquid, the lower the hydrometer sinks, for it displaces an amount of liquid equal to its own weight. The density of the liquid then can be determined by observing the mark to which it sinks. Specific-gravity hydrometers used in the household show the ratio of the weight of a given volume of liquid to the weight of the same volume of water at a definite temperature. Arbitrary scale hydrometers are used to indicate the concentration or strength of syrup, brines or milk. These are defined as lactometers and Baume hydrometers. A brine hydrometer is called a saltometer, and a syrup gage a sacchrometer. A jellometer, especially for making jelly, is sometimes used instead of a sacchrometer. The scale on this tells how much sugar to use in proportion to the amount of solids in the fruit juice without having to refer to a table. Some hydrometers are constant-volume hydrometers, and on these weights are placed always, to sink the hydrometer to the same depth in the liquid.

TABLES FOR BRIX AND BALLING HYDROMETERS WHEN USED AT 20° C.*

When the reading for the fruit juice is determined the table shows how much sugar is used for juice of that specific gravity.

TABLE SHOWING AMOUNT OF SUGAR PER GALLON

In the second table the readings show the specific gravity of the syrup, and from that may be ascertained the proportion of sugar to a gallon of water in it.

A 250 cc. cylinder, or other tall vessel deep enough to float the sacchrometer, is suitable for making the measurements. Be sure to have the eye on the level of the liquid when making the readings. If no sugar is in the water, the reading on the hydrometer will be near zero. If there is sugar in the proportion of seven ounces to a gallon of water, the reading will be at the line marked 5.

SYRUPS FOR CANNING

304. Hygroscopes. Hygroscopes are devices for measuring humidity. Forty-five to sixty per cent humidity is desirable in a house. This means forty-five to sixty per cent as much water as the air is capable of taking up at room temperature. Cold air is usually dryer than warmer air because cold air cannot take up as much humidity as warm air. This is analogous to the fact that warm water will dissolve more of some salts or of sugar than cold water.

305. Barometers. Barometers (Fig. 162) are devices which show changes in pressure and currents of air. Changes in the barometer usually indicate changes in the weather, and thus they are of interest to all persons. A decided fall in the mercury of a barometer usually precedes foul weather, while a rise indicates the approach of fair weather. When the pressure is low in any locality, air begins to rush toward that point as it would to fill a vacuum. So a fall in the barometer precedes the coming of a high wind or a rainstorm. A rise in the barometer precedes a calm, and since most rain is accompanied with wind, the calm is a time of fair weather.