A genuine Maxim sourced DS18B20+ One Wire Digital Temperature Sensor. The DS18B20+ is the perfect low-cost solution for a range of Raspberry Pi and Arduino temperature control and data-logging projects! The DS18B20+ measures temperature in degrees Celsius with 9 to 12-bit precision and includes an alarm function with nonvolatile user-programmable upper and lower trigger points. Sensing range is -55C to 125C (accurate to ±0.5°C over the range of -10°C to +85°C), and each sensor has a unique 64-bit serial number hard-programmed enabling the use of a number of sensors on a single data bus.
The DS18B20 communicates over a 1-Wire bus that by definition requires only one data line (and ground) for communication with a central microprocessor, and can be powered on the Raspberry Pi's 3.3V power line.
The DS18B20+ Features:
Unique 1-Wire Interface Requires Only One Port Pin for Communication
Each Device has a Unique 64-Bit Serial Code Stored in an On-Board ROM
Requires No External Components
Can Be Powered from Data Line; Power Supply Range is 3.0V to 5.5V
Measures Temperatures from -55°C to +125°C (-67°F to +257°F)
±0.5°C Accuracy from -10°C to +85°C
Thermometer Resolution is User Selectable from 9 to 12 Bits
Converts Temperature to 12-Bit Digital Word in 750ms (Max)
User-Definable Nonvolatile (NV) Alarm Settings
Alarm Search Command Identifies and Addresses Devices Whose Temperature is Outside Programmed Limits (Temperature Alarm Condition)
What is the max. Cable length the sensor can reasonably be expected to work with using the pi's power supply
There's not a simple answer for this really as there are many things that will have an affect. Your best bet is to have a search on the internet and see how other people have managed to get the sensor working over long distances.