Difference between Hydrometer and Hygrometer

Key Difference – Hydrometer vs Hygrometer

Hydrometer and Hygrometer are two different measuring gauges and the key difference between them is that Hydrometer measures the relative density of a liquid while Hygrometer measures the humidity in a given volume.

What is a Hygrometer?

By definition, Hygrometer is an instrument that can measure humidity in a given volume. Humidity level indicates the likelihood of precipitation, dew, or fog. Hygrometers measure humidity by indirect methods. There are several types of Hygrometers. One of the oldest types of Hygrometer includes hair tension type. It uses the length of a human or animal hair as the humidity sensor. Since the change of the hair length with humidity is very small to detect, a magnifying mechanism is used to read the length easily. Dry and wet bulb Hygrometers are reliable and widely used. It comes with a chart to find relative humidity using the temperature difference between thermometers. Nowadays, electronic humidity sensors are also available. Humidity measurement plays a huge role when humidity level restrictions are specified for some sensitive electronic devices and pharmaceuticals. Humidity measurements are very important in forecasting weather.

 Hydrometer vs Hygrometer-

Dry and wet bulb Hygrometer

What is a Hydrometer?

A hydrometer is used to measure the density of a liquid. Precisely, it measures the specific gravity, in other words, the relative density of a liquid. Sometimes, solute content is extracted by the density of the solution.

A typical Hydrometer has 2 major parts; cylindrical stem and weighted bulb. It has a scale on the stem which allows to read specific gravity or some other measurement easily. Hydrometers should be handled carefully since most of them are made of glass.

The theory behind Hydrometer comes from the Archimedes age. An object which is buoyed partly or fully in a fluid experiences a force equal to the weight of the fluid displaced by the submerged part of the object. Simply, specific gravity is the ratio of the mass of liquid to that of the water for any given volume. The weighted bulb is included to lower the center of gravity of the Hydrometer, consequently bringing  down the buoyant center. Therefore, the Hydrometer floats upright in liquids. The submerged height is proportional to the fluid displaced. When the quantity of displaced fluid increases, the specific gravity of liquid decreases.Further; less the density of the fluid, more the Hydrometer will sink. Since the mass is not distributed uniformly along the Hydrometer, the submerged height is not linearly proportional to the specific gravity, hence the scale on the stem is a non-linear scale.

Depending on the place of use, Hydrometer can be called by different names. All of them uses the same principle but are used  for specified purposes. Lactometer is used to measure the density of milk, which indicates the water quantity in milk. The quality of the milk strictly depends on the water quantity inside. Alcoholmeter measures the alcohol percentage in alcohol/ water liquid while the saccharometer indicates the amount of sugar in a solution.

 One can be asked to use lactometer instead of alcoholmeter to determine the alcoholic strength of a liquid which is directly proportional to the specific gravity of the liquid. But it may not be a good practice since both have unique scales in order to measure relevant quantities.Difference between Hydrometer and Hygrometer

Fluid Temperature (°C) Specific Gravity
Acetone 25 0.787
Alcohol, ethyl (ethanol) 25 0.787
Alcohol, methyl (methanol) 25 0.791
Alcohol, propyl 25 0.802
Ammonia (aqua) 25 0.826
Aniline 25 1.022
Benzene 25 0.876
Carbon tetrachloride 25 1.589
Coconut Oil 15 0.927
Gasoline, Vehicle 15.5 0.739
Kerosene 15.5 0.82
Mercury 25 13.633
Milk  25 1.035
Octane 25 0.701
Olive Oil 15 0.703
Sea water 25 1.028
Pure Water 4 1
Sea Water 25 1.025

What is the difference between Hygrometer and Hydrometer?

It is important to note the temperature at the moment of measuring either humidity or specific gravity. Specific gravity depends on the ambient temperature. Though the absolute humidity does not depend on ambient temperature, relative temperature does. Therefore, every specific gravity or relative humidity value should be recorded with the corresponding temperature.

Definition of Hygrometer and Hydrometer

Hydrometer: Hydrometer is an instrument that is used to measure the specific gravity, or in other words, the relative density of a liquid.

Hygrometer: Hygrometer is an instrument that is used to measure the humidity in a given volume.

Characteristics of Hygrometer and Hydrometer

In-built Thermometer

Hydrometer: Most of the advanced Hydrometers have built-in thermometers to measure the temperature of the liquid easily.

Hygrometer: For a Hygrometer, an inbuilt thermometer is not a necessity, but a requirement for relative humidity calculations.


Hydrometer: Hydrometer is divided into categories concerning the specific occasion to be used (the purpose of the measurement). But the architecture is the same for everyone.

Hygrometer: Hygrometer is categorized by the method of used to measure the required humidity value.

 Image Courtesy:
“Hydrometer6455″. Licensed under (Public Domain) via Commons 
“Psychrometer1861″ by John William Draper – 1861 “A textbook on chemistry”. (Public Domain) via Commons