How to Use a Hydrometer For Your Lab Operations

Hydrometers are indispensable for lab applications in various sectors, including chemical processing, brewing, and marine engineering. Because they play a crucial role in determining precise liquid density, it is necessary to understand how to correctly read and calibrate a hydrometer and explore the significance of salometers.

In this guide, we've detailed essential points you need to know to maximize the use of your hydrometer in gathering accurate readings for your lab processes.

What Is a Hydrometer Used For?

Scientific Research and Education

Hydrometers measure the density of liquids in various scientific experiments and demonstrations. This information can help explain concepts like buoyancy, specific gravity, and solution concentration.

Brewing and Winemaking

Alcohol hydrometers are essential for monitoring beer, wine, cider, and mead fermentation. They measure the sugar content of the liquid, which the yeast converts into alcohol during fermentation. By taking readings at different stages of fermentation, you can track the progress, ensure proper alcohol content, and determine when your beverage is ready to bottle.

Aquarium Keeping

Salt brine hydrometers or salometers measure water salinity in saltwater aquariums. Maintaining the correct salinity level is crucial for the health of fish and invertebrates in the tank.

Battery Maintenance

For lead-acid batteries, like those found in cars, a digital hydrometer can measure the electrolyte density, which indicates the battery's charge level and health.

Antifreeze Testing

Antifreeze or coolant hydrometers are specifically designed to measure the concentration of antifreeze in coolant mixtures, ensuring your vehicle's engine is correctly protected against freezing temperatures.

How to Read a Hydrometer

Figure 1: Hydrometer in Glass Cylinder

Pour your sample into a smooth, clear cylinder or jar that is dry or well-rinsed with a portion of the sample.
Make sure your sample is thoroughly mixed before testing.
Immerse the hydrometer in the liquid to a point slightly below the place where it naturally floats. Ensure the hydrometer and liquid are at rest and free of air bubbles.
Measure the sample's temperature. Ideally, it should equal the hydrometer's temperature standard, generally 60°F. If temperature differences are unavoidable, correction tables can help adjust readings (see link under image). You can also call our Applications Specialists for information.
Take your reading at the point where the surface of the liquid touches the hydrometer. See Figure 1.

How to Calibrate a Hydrometer

While calibrating a typical glass hydrometer used in home applications is not feasible, scientific research often demands higher precision. Cuckow's method is often used for specialized equipment and expertise. This method involves:

Using a Weight Balance

The hydrometer is weighed in the air to determine its mass.

Submersing the Hydrometer

It's then submerged in a liquid of known density at specific depths corresponding to its scale markings. These depths are usually chosen at 10%, 50%, and 90% of the scale.

Weighing the Submerged Hydrometer

The hydrometer is weighed while submerged at each specific depth.

Calculations

The volume of liquid displaced by the hydrometer at each depth is calculated using Archimedes' principle and the liquid's known density.

Correction Factor

A correction factor is determined for each point by comparing the calculated volume with the actual volume indicated by the hydrometer scale at each depth.

Other Options for Calibration

Invest in a High-Precision Hydrometer

Some manufacturers offer pre-calibrated hydrometers with higher accuracy specifications. These instruments often come with certificates of calibration from accredited laboratories.

Use Alternative Density Measurement Techniques

Depending on the specific research needs, alternative methods like pycnometers, digital density meters, or vibrating tube densimeters might be more suitable and offer higher precision.

How to Find the Density of a Liquid

Using a Hydrometer

A hydrometer is a weighted, calibrated instrument that floats in the liquid. The depth to which it submerges is related to the liquid's density. Means of hydrometers, weighted glass floats that sink in the liquid to a depth dependent on the density, which is read at the liquid line on a calibrated stem extending above the liquid

Specific Gravity 60º / 60ºF

Compared with water at 60ºF

In percentage of a substance in a solution or mixture
In arbitrary divisions, such as degrees Baumé (Be.), degrees Twaddell (Tw.), degrees Salometer (S.)

Using a Pycnometer

Weighing a known volume, or weighing equal volumes of water and liquid and comparing them using a pycnometer, determines liquid density.

Using a Westphal Balance

A Westphal balance determines liquid density by considering the loss in weight of a plummet of known volume weighed in air, and the liquid or by comparing the weight of a plummet of unknown volume weighed in water (at 4ºC) and in the liquid

For Liquids Heavier than Water

Degrees Salometer	It is the most common hydrometer scale used for testing brines. The scale indicates directly the percent saturation of the brine, reading 0ºS in pure water and 100ºS in fully saturated brine. Since saturated brine contains 26.395% salt by weight, each salometer degree represents 0.26395% salt
Degree Barkometer	They are commonly used for testing the density of tanning liquors. ºBk = 1000 (Sp. Gr. – 1.000)
Degrees Twaddell	A scale similar to the Barkometer scale, and widely used in England ºTw = 200 (Sp. Gr. -1.000)
Degrees Baumé	An arbitrary scale that indicates % salt in brine. ºBe = 145 145 Sp. Gr.
Degrees Brix	(Balling) used in the sugar industries; each degree Brix represents percent sugar (sucrose)

What Does a Salometer Measure?

The hydrometer is the most common industrial instrument for measuring liquid density, and the salometer is the most common hydrometer used to measure brine strength.

Salometer Scaler

The salometer scale directly indicates the percent of brine saturation, reading 0º in pure water and 100ºS in fully saturated brine. By definition, the salometer degree indicates the percent of saturation, i.e., a 70ºS is 70 percent saturated.

Therefore, in calibrating salometer scales or computing brine tables based on this scale, it is necessary first to establish the percent value of a fully saturated brine as the fundamental unit and then divide this percent into 100 parts. The salometer degree and the percent salt are thus rigidly tied together by the formula:

Degrees S = % salt – brine X 100
% salt – saturated brine

Occasionally, a special salometer is used in the canning industry to test brine in quality grading. It is graduated on a scale where 100ºS represents brine containing 25% salt instead of saturated brine containing 26.395% salt.

Ordinarily, salometers are scaled for readings at 60ºF, but special salometers are available for the meat packing industry scaled for readings at 38ºF.

Proper Procedure for Reading a Salometer

Select a clean, straight-walled glass cylinder with a diameter at least twice that of the salometer bulb and sufficient height to allow complete immersion of the salometer scale. The cylinder should also be tall enough for the salometer to float at an O°S reading.
Place the cylinder on a level surface. Fill it with a sufficient volume of brine to raise the liquid level near the top of the cylinder after the salometer is immersed.
Record the brine temperature. If it is significantly different from 60°F, a temperature correction to the reading will be necessary (see chart).
Carefully immerse a clean, dry salometer into the brine. Ensure that the salometer is floating and not touching the cylinder walls. After the salometer has stabilized, take a reading at the brine surface at eye level at the bottom of the meniscus, which is the concavity formed on the cylinder walls at the brine surface (see illustration).
Check new salometers by first placing them in clear water. The reading should be nearly O°S at 60°F. Empty the cylinder, rinse it with saturated salt solution, and then refill with saturated brine at 60°F. The reading should be near 100°S And not less than 98°S.

Errors in Reading a Salometer

If proper procedure is not used, readings made with the correct salometer at the correct temperature may result in considerable errors. The following suggestions will help secure correct salometer readings.
The temperature of the brine should be the same as specified in the brine table being used. A 60ºF salometer will not give a correct reading at 38ºF and vice versa.
The brine should be tested only in a straight-walled cylinder of clear glass set solidly on a level surface. Any moisture that collects outside the cylinder should be wiped off.
Ensure that the salometer stem is dry, clean, and free from grease or caked salt crystals and that the salometer does not touch the sides of the cylinder when readings are taken.
Check new salometers by placing them first in clear water. The reading should be 0ºS at 60ºF. Empty the cylinder, rinse with a saturated salt solution, and then refill with saturated brine at 60ºF. The salometer should read 100ºS.
Care must be taken to read the scale marking at the actual surface of the brine when the salometer has come to rest. This brine surface is not level, as brine tends to rise along the sides of the cylinder and along the stem of the salometer, forming a concave surface known as a meniscus.

For a correct reading, bring the eye to a point level with the bottom of the meniscus.

Explanation of Hydrometer Scales
Density is weight per unit volume (pounds per gallon, grams per milliliter, pounds per cubic foot, etc.). The specific gravity of liquids and solids is the density compared with that of water at 4ºC.

This guide will help you master the steps for using a hydrometer, whether for testing coolant, measuring brine strength, or monitoring beer fermentation. Ready to put your knowledge to the test? Choose from our wide range of hydrometers and find the model you need.

Information provided by Thermco Products