On June 4, 2014, U.S. Customs and Border Protection (CBP) issued ruling # H251366 stating that monitoring devices placed in reefer containers to track temperature can be considered accessories of an Instrument of International Traffic (IIT) and are exempt from duty if the container is of foreign production, and the accessory is imported separately from the IIT.

According to 19 CFR 10.41, IIT accessories may be entered duty free if:
The container is of foreign production
The container is an IIT pursuant to 19 C.F.R. § 10.41a(a)(1
The accessory is imported into the U.S. separately from the IIT

Pursuant to the regulations, CBP decided that, “monitoring devices may be imported into the U.S. as accessories to IITs without payment of duty.  However, the port director must be satisfied that the importer of the subject accessory had the declared intention of using the accessory with the subject container of foreign production which is an instrument of international traffic, at the time of importation.”

The ruling can be viewed here