Phasmophobia is all about detective work, as players will need to use what little information they have available to them in order to deduce what type of ghost they're dealing with. The thermometer item goes a long way in helping with that goal, though it isn't entirely clear how to get it at first.
One method of narrowing down the ghost type is by using the thermometer to measure the temperature in rooms. If the temperature drops below freezing, then players know for certain that the ghost is capable of manipulating temperatures. This seemingly unimpressive tool is actually very helpful in Phasmophobia for that reason.
The thermometer will quickly become a staple of the player's tools, so it's worth picking up relatively early if possible. Whenever the thermometer shows a temperature below freezing, players can check off that piece of evidence. This is a quick and dirty way to get a little intel on the ghost.
The thermometer, like many other items in Phasmophobia, can be purchased from the game's shop. Players will need to make sure they have enough money, though, as this item costs $30. That isn't a ton of money, but it's certainly a lot for a thermometer. Even so, players will find themselves using this item frequently, as it is a solid way to narrow down the list of potential options when trying to figure out what kind of ghost is in the area. It should also be said that this item can't be purchased until the player reaches a certain level.
Once the player purchases the thermometer, it's theirs to keep until they die. If the player happens to die on a mission, all of their items are lost, but the player is given a bit of insurance money so they can start building back up their stock again. Freezing temperatures in Phasmophobia on the thermometer are indicated by anything below 0°C or 32°F, but players can be fairly certain that the ghost controls temperature if the thermometer reads anything close to either of those numbers. It should also be said that the air will get colder the closer the player is to the ghost, so this can also work as a way of locating the spirit.
By default, the thermometer shows temperature in Celsius, but this can be changed in the Phasmophobia's settings. Finally, it should be said that if the temperature drops below freezing, players will start to notice that they can see their characters breath. For this reason, a thermometer isn't absolutely necessary, but it will certainly help.
Phasmophobia is available now on PC.
Source: Phasmophobia Wiki