When placed in the middle of a pallet of canned goods, a tag cannot be read. It is the same when the tag is placed in the center of a pallet of bottled water. The reason is that the energy just cannot reach the center of the pallet, since the metal will reflect the energy away from the tag, and the water will absorb the energy.
Metal and water are probably two main problems that RFID industry experts are trying to solve. However, as the technology develops, they may no longer be major obstacles.
Metal causes two problems for RFID transponders. For one thing, it can reflect energy away from the tag. For another, it detunes the tag antenna, thereby preventing it from receiving energy from the reader.
But now, there are tags whose spacers can be placed on metal objects, such as racks and servers in a data center. In many cases, tags even utilize the metal to reflect a greater amount of energy to the tag, in this way increasing read range. There are also tags that can be placed into metal objects, such as oil pipes and medical instruments.
At the North Dakota State University Center for Nanoscale Science and Engineering, researchers have developed an antennaless RFID tag. What’s amazing about it is that the tag essentially transforms a metal object into the device’s antenna. The tag includes a chip and a small metal loop that straddles a strip of magnetic material. When a tagged metal object receives energy from an interrogator, an electric field is created. The magnetic material helps to capture the charge induced on the metal and diverts it into the loop, where it powers the chip. Although this innovation is not yet ready for commercialization, it could eliminate the need for a spacer between a tag and a metal object.
On the other hand, the problem related to water is a bit more challenging, since it absorbs energy and detunes the tag.
However, by changing the antenna impedance, the detuning issue can be resolved, so that when it is near water, it becomes tuned to the proper frequency. In this way, the tag could be read, though the range would be relatively short. You could also put metal behind the tag, and use an on-metal tag to boost the read range.
All in all, this means that there are solutions to the challenge of using RFID on or around metal and water. But given that specially-designed tags are more expensive, the question is not whether tags can be read in the presence of water and metal, but rather, whether the application will deliver a ROI or solve the problem one is trying to solve.