The difference between a psychrometer and a hygrometer

This is something that confuses a lot of people and leaves them unsure which one to buy when looking to buy one. There is actually no difference between them, a psychrometer is actually a type of hygrometer.

A hygrometer is a device that is used to measure the moisture content of the air, basically this means the relative humidity and the dew point. There are several different ways to do it. The oldest and most common is to use a dry bulb thermometer and a wet bulb thermometer, this is what is known as a psychrometer.

To determine relative humidity and dew point with a psychrometer, you should have a dry-bulb thermometer and a wet-bulb thermometer so that you can compare the difference. A wet bulb is simply a thermometer that has water-soaked muslin over the part of the bulb. This works because latent heat from evaporation will make the wet bulb temperature lower than the dry bulb, this difference can be used to find relative humidity and dew point by looking for them on a graph.

Professional weather stations will use a psychrometer to determine dew point and relative humidity, as they are the most accurate option available. They typically do this by having the two thermometers permanently mounted to a Stevenson display. It is also possible to do this with a sling psychrometer, which is a portable device that holds the two thermometers, when you want to measure the moisture content of the air, you turn the psychrometer to allow the thermometers to get an accurate temperature reading.

There are other ways to measure the moisture content of the air, including changes in capacitance, resistance, or thermal conductivity. These are measured with electronic devices that are generally labeled hygrometers, although you will often see them listed as dew point meters and, confusingly, sometimes even psychrometers. The simple answer to the difference between a hygrometer and a psychrometer is that the electronic versions are hygrometers, the ones with liquid thermometers are psychrometers.

The next question is obviously what is the best option for measuring the moisture content of the air. This will depend on your needs. If you are a meteorologist or someone for whom accuracy is the most important consideration, you will need a psychrometer. There are others who need to determine the moisture content of the air such as HVAC technicians, and in some cases painters will value convenience over overall accuracy. For them, an electronic hygrometer is usually the best option, as it is easy to read and works quickly. For most users, you don’t need an accuracy of a tenth of a degree, so the electronic version is fine.

The best option if you are someone who is simply interested in the weather is probably neither a hygrometer nor a psychrometer. Most home weather stations have a hygrometer function and since they can measure all other weather parameters, they are a good choice for most home users.