What is a hydrometer? How it works and what is it for


During the maintenance of the car, it is periodically necessary to measure the density of the electrolyte and antifreeze. Visually, this parameter cannot be determined. For such purposes, there is a hydrometer.

How does this device work, how does it work, what types are there and where else is it used? Answers to these questions will help novice motorists use a hydrometer correctly.

What is a hydrometer?

The density of a liquid is the concentration of an additional substance in the main medium. Knowledge of this parameter helps to determine at what point the technical fluid needs to be replaced or makes it possible to find out whether the manufacturing technology has been followed in production.

Motorists use a hydrometer to measure the quality of electrolyte and antifreeze. A low content of additional substances in the main environment can lead to freezing of the liquid in the cold or a decrease in its level due to the rapid evaporation of water in hot summer.

What is a hydrometer? How it works and what is it for

In the case of a battery, this will lead to difficulty starting the engine, reduced service life, or decay of the lead plates. Low density coolant can boil at a lower temperature.

To prevent the occurrence of problems, it is necessary to measure these liquids in a timely manner using a hydrometer - a glass float with a scale. It is very easy to use, but there are some factors to consider.

Principle of operation

According to legend, the ancient Greek scientist Archimedes plunged into an overflowing bathtub, which caused the water to overflow. This situation prompted him to think that in the same way it is possible to measure the volume of gold from which the crown of Tsar Heron II was made (the inventor was tasked with determining whether a precious piece of jewelry was made of pure gold).

Any hydrometer operates according to the displacement principle discovered by Archimedes. According to the hydrostatic law, when an object is immersed in a liquid, a buoyant force acts on it. Its value is identical to the weight of the displaced water. Since the composition of the liquid is different, then the buoyancy force will be different.

What is a hydrometer? How it works and what is it for

The sealed flask is placed in the main container with liquid. Since the weight is fixed at the bottom of the device, the flask does not turn over, but remains upright.

In the case of local measurement, as in determining the density of antifreeze or electrolyte, hydrometers are used with a reservoir in which a float is placed. During aspiration, the liquid fills the main flask to a certain level. The deeper the second flask goes, the lower the density of the liquid. To determine the quality of the tested environment, you need to wait for the "float" to calm down.

Device types

Since liquid substances have their own density, hydrometers are calibrated for each of them separately. If the device is used for other purposes, its performance cannot be considered correct.

What is a hydrometer? How it works and what is it for

In addition to the weight of the weight, calibrated for the corresponding liquid, the device can have three types of scales:

  • To determine the density of a substance;
  • To measure the percentage of impurities in the environment;
  • To determine the percentage of an additional substance dissolved in water (or other basis), for example, the amount of sulfuric acid in a distillate for electrolyte preparation.

Outwardly, all hydrometers are similar to each other and work according to the same principle, however, each of them is calibrated for its own environment and for specific parameters.

What is a hydrometer? How it works and what is it for

Similar devices are used to measure indicators:

  • Percentage of alcohol content;
  • Concentrations of sugar or salt;
  • Density of acid solutions;
  • Fat content of milk;
  • The quality of petroleum products.

Each modification of the hydrometer has a corresponding name.

Alcohol meter

Allows you to measure the strength of an alcoholic drink. In this case, its scale will show the percentage of alcohol in the drink. It is worth considering that such devices are not universal, but are also calibrated for certain categories of drinks.

What is a hydrometer? How it works and what is it for

For example, for measuring vodka, liqueur and other spirits, hydrometers are used, the graduation of which is within 40 degrees. In the case of wine and other low alcohol drinks, more accurate flasks are used.

Hydrometer for petroleum products

This category of devices is designed to measure the quality of gasoline, kerosene, diesel fuel and other oil products. The device allows you to determine the presence of impurities that reduce the quality of the fuel.

What is a hydrometer? How it works and what is it for

They are used not only in industrial plants. An ordinary motorist can also purchase such a device to make it easier to determine at which gas station it is worth refueling his car.


What is a hydrometer? How it works and what is it for

Refractometers are used in the food industry, mainly in the production of juices. The device allows you to check the ripeness of the fruit. It measures the concentration of sugar in the test medium.

Automotive hydrometer

Motorists use hydrometers to measure the density of antifreeze and electrolyte. Less commonly used to measure brake fluid and gasoline. In the case of models for testing acidic liquids, the device is slightly modified.

In addition, it has a large hollow flask, inside which is a glass float with a corresponding scale. On the one hand, such a device is narrowed (or with a rubber tip like a pipette), and on the other, a rubber bulb is put on it to take a portion of the electrolyte.

What is a hydrometer? How it works and what is it for

This design is the safest, since contact of acidic and toxic substances with the skin is undesirable. Most models for cars are universal and are used for measuring the density of different liquids.

What is a hydrometer? How it works and what is it for

Since the float is immersed in a separate medium to its depth, the parameters corresponding to a particular liquid are plotted at different levels of the scale.

In addition to the modifications listed above, hydrometers are also used in medicine (for measuring the density of some biological materials in humans), in cooking, in the food industry (for example, a lactometer measures the fat content of milk, and a salt meter helps to determine the suitability of water for food purposes and its hardness), as well as on enterprises producing chemical products.

Design and parameters of hydrometers

The device is a flask sealed at both ends. There is a metal shot inside it. Its amount is determined by the purpose of the device (each liquid has its own density). The flask has a scale that allows you to accurately determine the required parameter. Some hydrometers additionally fit into a large hollow tube (as with the electrolyte model).

What is a hydrometer? How it works and what is it for

An additional flask is used to measure some hazardous liquids. It is designed to take a portion (for example, automobile hydrometers make it possible to accurately take a small volume of electrolyte). This design prevents electrolyte or other toxic material from entering the skin.

Depending on the design and purpose, the second flask can be made in the form of a bottle with a long neck or in the form of a thick test tube with a scale applied. Some models are made of dense transparent plastic that is resistant to aggressive acid and alkaline solutions.

What is a hydrometer? How it works and what is it for

The glass counterpart has several advantages:

  • The bulb retains its transparency regardless of the frequency of use;
  • Glass is more resistant to organic compounds.

One of the disadvantages of glass hydrometers is that they are fragile, so the collapsible model must be properly stored (in a case with separate cells for each flask). In this case, the float must be removed from the large flask and stored in special packaging so that it does not break.

What is a hydrometer? How it works and what is it for

When purchasing a hydrometer of the same type, you should pay attention to the error (it is indicated as a percentage). Most often this parameter is very important for making accurate measurements in production.

Also an important factor is the graduation of the scale. The longer it is, the more accurate the measurement will be. Cheap hydrometers most often have a small scale, so determining the exact density of an electrolyte or antifreeze becomes more difficult.

To make it easier for a motorist to determine whether the indicator is within the norm, the scale contains marks with the minimum permissible value (red mark). The optimal value is marked in green.

How to use a hydrometer

The device is very easy to use. To determine the required parameter, the float is placed in a container with a solution. He must calm down, which will give the most accurate indicator.

When working with hazardous fluids, this procedure must be carried out in a special way. Since the correct operation of the battery depends on the density and concentration of the acid in the electrolyte, it is necessary to periodically check these parameters using a hydrometer (for how to extend the battery life, read in a separate article).

What is a hydrometer? How it works and what is it for

The density indicator of the electrolyte in batteries should be in the range of 1,22-1,29 g / cm3 (depends on the climate in which the car is operated). Some battery models are equipped with an inspection window with a charge indicator. Its indicators:

  • red - the electrolyte level has dropped, it is necessary to replenish the volume (while the charge may still be enough for the starter to spin the flywheel);
  • white color - the battery is approximately 50% discharged;
  • green - the power supply is sufficiently charged.
What is a hydrometer? How it works and what is it for

These indicators will help determine whether the power source can be used to operate energy-intensive equipment, for example, an audio system (how to properly connect a car amplifier is described here).

Periodic maintenance of the power supply will help determine if distillate needs to be added or the battery needs recharging. In serviced batteries, measurements are made with a car hydrometer. Here's a quick guide on how to use it correctly.

Step-by-step instructions for taking measurements

Before measuring a service fluid, it is important to ensure that the temperature is correct for this procedure. Manufacturers recommend taking measurements at temperatures within +20 degrees (not the environment, but the tested environment). The density of the same liquid changes with different thermometer readings, therefore, to eliminate inaccuracies, you must adhere to this recommendation.

What is a hydrometer? How it works and what is it for

For ease of measurement, some modern modifications are equipped with a thermometer to determine the temperature of the liquid. in order to be able to accurately determine whether the liquid meets the required parameters, sometimes a correction is indicated on the scale (or in the technical documentation of the device) taking into account a non-standard temperature.

The procedure is performed in the following sequence:

  1. you need to make sure that at least six hours have passed since the last charge;
  2. all the battery plugs are unscrewed;
  3. the float (hydrometer) is inserted into a large flask, a pear is put on top, and on the other side - a cork with a narrowed neck;
  4. before lowering the rubber tip into the electrolyte, the pear is completely compressed;
  5. the pipette is immersed in liquid, the pear is unclenched;
  6. the volume of electrolyte should be so much that the float inside the flask floats freely and does not touch the walls of the flask;
  7. after reading the indicators, the electrolyte smoothly returns to the battery bank, the plugs are twisted.

For best preservation, the hydrometer must be washed with water. This will prevent the formation of plaque inside the flask, which can affect the accuracy of measurements in the future.

Measurement safety

What is a hydrometer? How it works and what is it for

Technical fluids in a car are often toxic and, with prolonged contact with the skin, can damage it (especially in the case of an acid solution), therefore it is important to adhere to safety measures when working with them. Here's what every motorist should remember:

  • to avoid contact of acid with the skin of the hands, rubber gloves must be used;
  • during the operation of the battery, water from it can evaporate (applies to serviced modifications), therefore, when unscrewing the plugs, you need to be careful not to inhale acid fumes;
  • when working with the battery, it is strictly forbidden to smoke and use any source of open flame;
  • it is important to take measurements in a well-ventilated area;
  • work with hazardous liquids does not tolerate haste (due to inattention, the electrolyte can get on the car body and corrode the metal).

Overview of popular hydrometer models

Finding a quality hydrometer is not difficult because it is a fairly simple instrument that can be found at any auto parts store. There are several types of such devices. They differ from each other by the parameters for which they are calibrated. Here are some popular hydrometers.

For antifreezeper:Estimated cost, cuAdvantagesdisadvantages
Jonnesway AR0300028Compact, multifunctional, easy to use, reliableDear
JTC 10405Lightweight and compact, multifunctional (freezing point and boiling point marked on the scale)Reacts poorly to prolonged contact with acids
AV Steel AV-9200974Budget price, ease of use, reliable, versatileSmall markings on the scale
For electrolyte:
Jonnesway AR0300017Versatile, lightweight, multi-colored scale, durableHigh price
Heyner Premium 925 0106Reasonable price, plastic case, small volume of the tested electrolyteStored without a cover, the pear may subside over time
Autoprofi АКБ BAT / TST-1185Easy to use, color scale, affordable priceUsed only in lead-acid battery models, the results do not always reflect the real indicator
JTC 10414Low-cost option, flask strength, resistant to acid solutions, measurement accuracy, compactThe float often sticks to the wall of the flask, there is no case
Pennant AR-02 50022Lightweight, sealed, glass, cheapRubber bulb quickly loses elasticity, no case

Before choosing a modification, you need to consult with specialists, since every year manufacturers create new models with improved characteristics. Some modifications may be ineffective in measuring certain types of liquids.

What is a hydrometer? How it works and what is it for

In stores, you can find universal models with which you can measure the quality of both coolant and electrolyte. Some of them have a dial and are calibrated with distilled water for any type of liquid. Practice shows that such expensive modifications are more suitable for professional service stations than for domestic use.

As you can see, the hydrometer is not a complicated device with which even a beginner can correctly measure the state of the electrolyte or antifreeze. Thanks to this simple procedure, the motorist will be able to significantly extend the battery life and ensure the proper operation of the engine cooling system.



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