Inventory control for long-term storage items has become an increasingly important field of endeavor. One particularly noteworthy aspect of inventory control is environmental control of the storage location, whether the commodities are stored in a depot, warehouse or retail establishment. One parameter that should be monitored during long-term storage is humidity, because such changes in humidity can have drastic and long-reaching impact on many different commodities. If the proper humidity range for the stored object is not maintained during storage, the useful lifetime, capability, safety, and other characteristics of the stored object can be deleteriously degraded or lost, which can lead to a number of undesirable, and possibly dangerous, results.
For monitoring a long-term storage condition, it is a disadvantage for any humidity monitor to require a battery power source, because any battery has a finite useful life over a certain period of time. Therefore, it is essential that a long-term humidity monitor must operate without battery power.
One type of prior art humidity monitor that does not require electrical power is the dial hygrometer. FIG. 1 is a perspective view of a prior art dial hygrometer with a needle that moves in response to changes in humidity, which is set against a clock-type face. While the prior art dial hygrometer is useful for indicating current humidity through movement of the needle, it does not record the history of humidity changes during storage. The inability of the prior art dial hygrometer to record the history of humidity changes during storage is a key disadvantage of this device. Thus, there has been a long-felt need for a real-time humidity monitor that can also record humidity change history.
Radio Frequency Identification (RFID) is an emerging area of technology with numerous applications, particularly in the field of inventory control. RFID is an automatic identification method, relying on storing and remotely retrieving data using devices that are called RFID tags or transponders. An RFID tag is a small object that can be attached to, or incorporated into, a product, animal, person or other stored object and contains silicon chips and antennas to enable it to receive and respond to radio-frequency queries from an RFID transceiver. When an RFID tag is combined with one or more sensors, the RFID tag can report the information that is measured by the sensors. A passive RFID tag requires no internal power source, while active RFID tags do require a power source. A passive RFID tag is better for inventory control for long-term storage items because the use of battery power to operate the RFID tag is ordinarily unacceptable. In the RFID arrangement, the device receives electrical power from the RF when it is being read, therefore for such applications, a passive RFID tag combined with a sensor that also does not need power for its operation is critical. However, there is no currently available RFID tag that can effectively monitor changes in humidity. Thus there has been a long-felt need for a humidity monitor using RFID technology that not only responds to changes in humidity but can also provide a real-time record of those humidity changes. Up until now, the long-felt need for an all-humidity-sensitive RFID tag has not been met.