Difference Between Similar Terms and Objects

Hydrometer versus Hygrometer

Hydrometer versus Hygrometer


In the name of science, how would we measure the moisture and sugar content in a steaming mug of cocoa? Well for starters we can measure the humidity levels in the rising steam of the cocoa by using a hygrometer. Thereafter, we can measure the relative density of the cold cup of cocoa (in comparison to water) by using a hydrometer! For the love of science!

Physical attributes

A hydrometer / areometer, measures the density (weight per unit volume) and specific gravity, that is density of said liquid in relation to density water. Based upon Archimedes’ principle of floatation i.e. a solid body will displace its weight in the immersion liquid. This instrument is a cylindrical glass stem with a bottom bulb containing mercury or lead shot, and a scale inside the stem. We gradually insert the hydrometer into the cool liquid, until it is able to float freely in the liquid. Once the hydrometer levels out, a reading can be taken. This reading is taken where the surface of liquid being measured, meets the hydrometers’ scale. There are many different scales available for different conditions or contexts. There are hydrometers for high and low density liquids, with a specific gravity range from  1.0 to 0.95, .0.95 to 0.9 and so on[i].

Whereas, the hygrometer uses a graduated scale in two degree increments, in order to measure degree of the moisture content / humidity / water vapour, in the atmosphere. Hygrometers come in a variety of designs, i.e. from simple hair hygrometers to much more detailed, complicated instruments. The simple hair hygrometer, utilises human or animal hair under tension, and measures humidity by measuring the change in hair length in moisture rich atmospheres.


The first hydrometers are said to have been invented in the early 5th century, by a Greek scholar named Hypatia. Since then, instruments have been improved upon, with advancements in science and technology. Giving rise to many different types of hydrometers, used in various scenarios or industries. The most common or widely used hydrometers are listed below:

  • Lactometer: measures the purity of cows’ milk
  • Alcoholmeter: measures the alcoholic strength of the liquid
  • Saccharometer: measures the sugar content in a solution
  • Thermohydrometer: consists of a thermometer inside the hydrometer float portion and is used in measuring the density of petroleum products.
  • Urinometer: used in medical science to conduct a urine analysis
  • Barkometer: measures the strength of tanning liquors
  • Battery hydrometer: used to measure the charge of the battery
  • Antifreeze tester: measures the quality of antifreeze solution used in cooling engines
  • Acidometer: measures the specific gravity of an acid
  • Salinometer: measures salt content in water solution


The great Leonardo da Vinci invented the first hygrometer in the 15th century. This initial concept was improved on to create a more modern version in 1755. The many different types of hygrometers now include:

  • Metal-paper coil hygrometers: water vapour is absorbed by a paper strip indicating moisture on a dial
  • Psychrometer: consists of both a wet and dry bulb, where the wet bulb is connected to the water tank
  • Chilled mirror dew point hygrometer: these instruments are more precise and are used to detect condensation on the surface of a mirror
  • Capacitative hygrometers: these are suitable for various applications
  • Resistive hygrometers: measures the changes in electrical resistance due to humidity
  • Thermal hygrometers: measures the change in thermal conductivity due to humidity
  • Gravimetric: this is the most accurate of hygrometers, testing humidity levels[ii].

Ranges and Scales

Hydrometers generally utilise the following scales:

  • Baume
  • Brix
  • Plato

While, most basic hygrometers use a graduated scale.


Hydrometers can be used in the following contexts:

  • Used in soil analysis
  • Winemaking and brewing
  • Tests the quality of milk
  • Used in sugar content analysis
  • Used for measurement of density in petroleum products
  • Measures electrolyte temperature and specific gravity as part of a battery test
  • Used to conduct urine analysis

Whereas, Hygrometers are used in various contexts namely

  • Incubators
  • Industrial spaces
  • Greenhouses
  • Saunas
  • Humidors
  • Museums
  • To care for wood musical instruments such as guitars, harps, pianos, etc.
  • Residential uses to monitor humidity levels
  • Coating industry e.g. paints can be sensitive to humidity.

Accuracy of measurement

Hydrometers’ accuracy is dependant on three factors; cleanliness, temperature, and proper immersion. Both hydrometer and the tank or cylinder housing the liquid to be tested, needs to be clean. So that the liquid can rise uniformly, giving an accurate reading. In addition, the temperature should be similar in the liquid and surrounding atmosphere. Thus, preventing changes in density. Accordingly the jar containing the liquid should be large enough to allow for immersion of hydrometer[iii].

Whereas, with hygrometers it is difficult to maintain accuracy. Factors such as temperature , pressure, electrical change, and mass need to be accounted for. Conventional hygrometers are inaccurate below freezing point.

Comparison between Hydrometer and Hygrometer

Hydrometer Hygrometer
Measures a liquids specific gravity Measures the amount of water vapour / moisture in the air
Initially  invented by Greek scholar Hypatia Initially invented by Leonardo da Vinci


Invented n early 5th century Invented in early 15th century
Hydrometers are categorized by the instruments purpose or context of use Hygrometers are categorized the different  methods used to measure humidity
Easier to ensure accuracy Difficult to maintain accuracy. Inaccurate below freezing point
Factors such as cleanliness, temperature, and proper immersion need to be accounted for to maintain accuracy Factors such as temperature, pressure, mass, and electrical charge need to standardised to ensure accuracy


Hydrometer are needed to prove accuracy in specific gravity and density of liquids, while hygrometers are important to measure humidity levels. If humidity levels are to high this leads to, lethargy, tiredness, respiratory infections. On the other hand if humidity levels are to low, this promotes deterioration of wood furnishings, musical instruments, etc. Thus helping humanity maintain balance. However, both instruments require calibration and maintenance of temperature in order to ensure accurate readings. There will come a time where advancements in technology, provide us with increased accuracy of measurements. Till then both Hypates and Da Vinci would have been proud to see their prototypes eminent evolution.

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References :

[0][i] Hydrometers: Introduction, Function and Use. (n.d.). Retrieved April 08, 2017, from http://www.benmeadows.com/refinfo/techfacts/hydrometers-introduction-function-use-117.htm

[1][ii] Hygrometer. (2017, April 07). Retrieved April 09, 2017, from https://en.m.wikipedia.org/wiki/Hygrometer

[2][iii] Hydrometer. (2017, March 23). Retrieved April 07, 2017, from https://en.m.wikipedia.org/wiki/Hydrometer


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