What is the Difference between Hydrometer and Hygrometer?

A lot of people get confused between Hydrometer and Hygrometer. To help you in clearing the confusion, we have listed all the differences between the hydrometer and hygrometer. To begin with, we will start with the basic difference between the two. The hygrometer is used to the humidity in the air whereas hydrometer is used to measure the specific gravity of the liquid. Let us now look into more details of each of these.


As mentioned above, the hydrometer can be used to check the specific gravity of any liquid. This can be alcohol, oil or even a petroleum product. Most commonly, you will find the hydrometer in breweries and at the petrol station. The hydrometer is also used to check the dilution in milk. Basically, specific gravity represents the density of a liquid which can also translate into viscosity. This is a comparative measurement and it measures the specific gravity against the specific gravity of water which is 1.

The formula to calculate the specific gravity is derived by dividing the density of liquid by density of water. This device actually floats on the liquid and you can then take the reading. Talking about the typical construction, the hydrometer has a glass tube which has a specific amount of weight at the bottom. You can also find the digital hydrometers which can quickly give you the readings. If we talk about the accuracy, the manual ones are much more accurate than the digital ones but the last call depends on you.

Some of the factors that can impact the reading includes the cleanliness of the instrument, the temperature of the liquid and the immersion technique. Considering these factors, you can easily get the readings from hydrometer.


Talking about the hygrometer, as mentioned above, it is used to measure the amount of moisture in the air which is also known as humidity. This has a dial like construction and you will be surprised to know that this was invented in the 15th century whereas hydrometer was invented in the 5th century. There are many different types of hygrometer available which include hair tension hygrometer, psychrometer, and an electric hygrometer. The digital ones which are also called the electric hygrometer measure the humidity by measuring the change in resistivity of the digital circuit component. This change in sensitivity is recorded as the humidity level changes. In terms of the accuracy, psychrometer hygrometer remains to be the most accurate ones as they have a wet bulb and dry bulb thermometer

It should be noted that the hygrometers can be quite inaccurate if the temperature of the surrounding is below the freezing point of the water. Apart from this, the readings are also sensitive to temperature, pressure, electrical charge and mass.


This is the major difference between the hydrometer and the hygrometer. We hope this would help you in distinguishing the two. It should be noted that these two are not replaceable and they can’t work inter-functionally.