A soil moisture sensor, also called a hygrometer, measures the amount of moisture, or water, in the soil. Therefore, we can tell whether the soil has enough moisture or not. So it is a good diagnostic tool for caring for plants of all types.
The DHT22 goes by several names. The other common name is the RHT22. The data sheet says that it will run from 3.3V to 6V which makes it ideal for Arduino application. Temperature range is -40°C to 80°C with 0% to 100% relative humidity.
Liquid level sensors are used for all types of applications. They are extensively used within automobiles, which rely on a substantial amount of different fluids in order to operate to check for how much gas in the car, windshield washer fluid, oil levles.