The CR1000 requires a 4WPB100 to measure the internal PRT. (Dataloggers such as the CR3000 and CR5000 have the necessary PRT bridge module built in to measure the PRT.) Note that the CNR4-L also includes an internal thermistor, which can be directly measured by the CR1000. Because of this, when using a CR1000, Campbell Scientific typically recommends monitoring the CNR4-L’s internal temperature using its internal thermistor instead of the PRT.