Main Difference – Hydrometer vs. Hygrometer
Hygrometers and hydrometers are both measuring devices. The main difference between hydrometer and hygrometer is that a hydrometer measures the specific gravity (and therefore, the density) of a liquid whereas a hygrometer measures the humidity in the atmosphere.
What is a Hydrometer
A hydrometer is a device which measures the specific gravity of a liquid. Hydrometers are often used to measure the density of alcohol in breweries and to measure the level of dilution in milk. Specific gravity is a measurement that tells how dense a liquid is, compared to the density of water. If the density of a liquid is 
The hydrometer works due to the principle of flotation. Whenever an object is immersed in a fluid, it displaces a volume of fluid that is equal to the volume of that object that is inside the fluid. According o the principle of flotation, the fluid exerts an upwards force on the object, called the upthrust, which is equal to the weight of the fluid which has been displaced by the object. An object floats whenever the upthrust is equal to its own weight. In other words, when an object is floating, it displaces an amount of fluid which has a weight equal to the object’s own weight.
A hydrometer consists 0f a glass tube, with some weights at the bottom. When a liquid is very dense, the hydrometer will only need to sink by a little amount before the volume of the displaced liquid is equal to its weight. At this stage, the hydrometer will start to float. We can calibrate a hydrometer using liquids of known densities, so that by looking at how much of the hydrometer has been submerged, we can measure the density of the liquid.
A hydrometer used to measure the specific gravity of an object
What is a Hygrometer
A hygrometer measures the humidity. There are many different types of hygrometers, which use different mechanisms to measure the humidity.
A hair tension hygrometer consists of a piece of hair attached to a needle. When the humidity is high, the hair can lengthen and shorten again when the humidity is low. The needle in the hair tension hygrometer can then move along a scale to indicate the humidity.
A hair tension hygrometer
A psychrometer is another type of hygrometer. This type of hygrometer consists of two thermometers: one a wet-bulb thermometer (its bulb is wrapped with a damp cloth) and the other a dry-bulb thermometer (its bulb is exposed to air). Due to evaporation of water from the damp cloth, the wet-bulb thermometer measures a cooler temperature. The rate of evaporation of water depends on the humidity. Therefore, the two different temperature readings can be used to calculate the humidity.
A psychrometer
An electrical hygrometer measures humidity by measuring the change in resistivity in a circuit component as the humidity changes.
Difference Between Hydrometer and Hygrometer
Measurement
A hydrometer measures the specific gravity of a liquid, which is an indication of the liquid’s density.
A hygrometer measures humidity, i.e. the amount of water vapour in the atmosphere.
Working Principle
Most of the time, a hydrometer works by measuring how much an object sinks in a liquid.
There are many different types of hygrometers. Each type makes use of a different principle to measure the humidity.
Image Courtesy
“Hydrometer for must: 0 … +130 °Oe (degrees Oechsle) at +20 °C” by Sönke Kraft aka Arnulf zu Linden (Own work) [CC BY-SA 3.0], via Wikimedia Commons
“Haar-Hygrometer, Made in GDR” by PDaniel FR (german wikipedia, original upload 20. Oktober 2004 by Daniel FR) [Public Domain], via Wikimedia Commons
“Psychrometer is used for measuring the atmospheric temperature and humidity.” by Arjuncm3 (Own work) [CC BY-SA 3.0], via Wikimedia Commons
