**4. RFID applications under water**

Both these values are high enough to allow a reliable long range RFID communication

Moving on to higher frequencies, the second evaluation is made for the High Frequen‐ cy band. The calculation is made using the standard frequency of 13.56MHz. The pen‐

etration depth value with a conductivity of 30 *μS/cm* (3 *mS/m*) is:

*<sup>π</sup>* <sup>∙</sup> 13.56 <sup>∙</sup> <sup>10</sup><sup>6</sup> <sup>∙</sup> <sup>4</sup>*<sup>π</sup>* <sup>∙</sup> <sup>10</sup>-7 <sup>∙</sup> <sup>3</sup> <sup>∙</sup> <sup>10</sup>-3 =2.5*<sup>m</sup>*

*<sup>π</sup>* <sup>∙</sup> <sup>800</sup> <sup>∙</sup> <sup>10</sup><sup>6</sup> <sup>∙</sup> <sup>4</sup>*<sup>π</sup>* <sup>∙</sup> <sup>10</sup>-7 <sup>∙</sup> <sup>3</sup> <sup>∙</sup> <sup>10</sup>-3 <sup>≈</sup>32.5*cm*

*<sup>π</sup>* <sup>∙</sup> 2.45 <sup>∙</sup> <sup>10</sup><sup>9</sup> <sup>∙</sup> <sup>4</sup>*<sup>π</sup>* <sup>∙</sup> <sup>10</sup>-7 <sup>∙</sup> <sup>3</sup> <sup>∙</sup> <sup>10</sup>-3 <sup>≈</sup>18.6*cm*

*π* ∙ 800 ∙ 10<sup>6</sup> ∙ 4*π* ∙ 10-7 ∙ 0.2

*π* ∙ 2.45 ∙ 10<sup>9</sup> ∙ 4*π* ∙ 10-7 ∙ 0.2

*π* ∙ 13.56 ∙ 10<sup>6</sup> ∙ 4*π* ∙ 10-7 ∙ 0.2

With a conductivity value of 2000 *μS/cm* (0.2 *S/m*) the penetration depth drops to:

30*cm*

While at lower conductivity values the realization of an efficient long range RFID sys‐ tem could still be possible, when the water conductivity grows the penetration depth drops down to values that make this solution difficult to be implemented or even to‐ tally impossible. Anyway, the chance to use HF RFID in particular environments like rivers or lakes has to be carefully evaluated case-by-case. An additional remark has to be made: in terms of performances, LF and HF systems are similar. This means that, if the system doesn't present specific requirements, the use of LF technology is howev‐

At higher frequencies the value of penetration depth drops down to values that allow the use of these systems only for contact or short range applications. At 800MHz the penetration depth with a conductivity value respectively of 30 *μS/cm* (3 *mS/m*) and

≈4*cm*

≈2.3*cm*

A remark is necessary: the values obtained for the penetration depth are ideal values and represent mainly an upper bound. This means that in most cases the effective sys‐ tem will present real reading ranges notably lower and in some cases it won't work

*<sup>π</sup>fμ<sup>σ</sup>* <sup>=</sup> <sup>1</sup>

386 Radio Frequency Identification from System to Applications

*<sup>π</sup>fμ<sup>σ</sup>* <sup>=</sup> <sup>1</sup>

channel.

*<sup>δ</sup>*13.56*MHz* <sup>=</sup> <sup>1</sup>

*<sup>δ</sup>*13.56*MHz* <sup>=</sup> <sup>1</sup>

er strongly suggested.

2000 *μS/cm* (0.2 *S/m*) is:

*<sup>π</sup>fμ<sup>σ</sup>* <sup>=</sup> <sup>1</sup>

*<sup>π</sup>fμ<sup>σ</sup>* <sup>=</sup> <sup>1</sup>

For Microwaves, these values drop down to:

*<sup>π</sup>fμ<sup>σ</sup>* <sup>=</sup> <sup>1</sup>

*<sup>π</sup>fμ<sup>σ</sup>* <sup>=</sup> <sup>1</sup>

*<sup>δ</sup>*800*MHz* <sup>=</sup> <sup>1</sup>

*<sup>δ</sup>*800*MHz* <sup>=</sup> <sup>1</sup>

*<sup>δ</sup>*2.45*GHz* <sup>=</sup> <sup>1</sup>

*<sup>δ</sup>*2.45*GHz* <sup>=</sup> <sup>1</sup>

at all.

and

RFID is currently one of the most widespread technologies for the automatic identification of items. There are countless fields where RFID is used for access control, items tracking, people and animal identification and many other different applications. Anyway, few appli‐ cations exist where RFID is used under water.

The question of the transponders waterproofing is crucial for many applications and several devices providing a high protection level against the contact with water have been realized. Plastic tags are inherently waterproof devices, while items like wristbands have been cus‐ tomized to be worn also under water. Anyway, all these devices have been designed only to resist against water intrusion, and not to be read directly under water. Moreover, no reader has been realized to be used under water. Readers providing a high protection level against water can be easily found: anyway, they are designed only to be positioned on the outside, for example on building walls for access control, and then to resist against bad weather.

A step ahead is the development of transponders realized ad-hoc to be positioned on bottles or other items containing liquids. In this case the solution mainly deals with the introduc‐ tion of a dielectric layer that simply separates the transponder and the liquid allowing thus its reading.

Anyway, the number of applications where the data exchange happens totally underwater is nowadays very little: the most part of these applications deals with animal tracking and environmental monitoring, mainly in marine environment.

#### **4.1. Animal tracking**

The chance to track animals, crucial for industrial stock-breeding activities, using RFID tech‐ nology has probably raised for the first time the question whether is possible or not to read RFID tags immersed in water. The body of most part of living beings is mainly composed by water: as an example, around 65% of human body is composed by water. The necessity to guarantee the integrity of the tracking device (In this case the transponder) has encouraged its positioning in a place where it cannot be removed, i.e. inside the body of the animal to be tracked. While the body of the animal is mainly composed by water, to read the transponder from the outside it's necessary to find a technological solution avoiding the insulating effect of the water layer.

The use of RFID for animal tracking is nowadays very common, and has also led to the reali‐ zation of two ad-hoc standards, the ISO 11784 and ISO 11785 standards, that regulate the use of RFID devices, in particular implantable transponders, for the identification of ani‐ mals. Standard RFID systems for animal tracking operate at the frequency of 134.2kHz (Low Frequency band). The transponders used for this purpose are generally glass cylinder tags that are modified to be applied under the skin of the animal, to be clasped to the ear of the animal or to be ingested by the animal.

**Figure 1.** The Virginia Aquarium and Marine Science antenna identifying fishes.

pipeline, keeping a fixed distance between one tag and the other.

side a protecting case, shaped on the curvature of the pipe.

which portion of the pipeline required assistance.

Another interesting application that foresees the use of RFID technology under water focus‐ es on the monitoring of pipelines used to carry oil [9]. This solution has been currently only tested, while no information has been retrieved on possible effective applications nowadays working. In this kind of applications Low Frequency RFID tags were applied directly on the

RFID Under Water: Technical Issues and Applications

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389

The tags operated the frequency of 125kHz and they were customized to fit exactly on the pipe: in particular, standard Phillips Semiconductor Hitag transponders were introduced in‐

Enertag, which tested the system, also developed an ad-hoc underwater reader: this was a handheld waterproof device connected with a cable to a PC positioned on a boat on

This system was employed to monitor the conditions of the pipeline. In practice, the trans‐ ponders acted as milestones, used to identify the exact portion of pipeline. This was com‐ bined with the data concerning repairs that the pipeline had undergone, and suggesting

**4.2. Pipeline monitoring**

the sea surface.

Even if these applications deal with the interaction with water, they are not properly under water systems. Anyway, RFID technology has been employed also to track animals under water. In particular, Low Frequency RFID technology has been used to identify fishes in the aquariums [8]. At the Underwater World Singapore Oceanarium, at Underwater World Pat‐ taya, Thailand and at Virginia Aquarium & Marine Science Center, Low Frequency cylinder glass tags have been applied under the skin of a number of fishes.

The tagged fishes are identified when they come close to a long range antenna posi‐ tioned on the glass of the tank where the fishes are kept. When the fish passes in front of the antenna, the identification code stored inside the transponder is read and the fish is identified. Once the fish has been identified the visitors of the aquarium can receive an interactive set of information concerning the animal. In particular, an ad-hoc software provides on a screen a picture of the fish and a description: these data are kept on the screen until a new fish passes close to the antenna.

**Figure 1.** The Virginia Aquarium and Marine Science antenna identifying fishes.

#### **4.2. Pipeline monitoring**

water can be easily found: anyway, they are designed only to be positioned on the outside, for example on building walls for access control, and then to resist against bad weather.

A step ahead is the development of transponders realized ad-hoc to be positioned on bottles or other items containing liquids. In this case the solution mainly deals with the introduc‐ tion of a dielectric layer that simply separates the transponder and the liquid allowing thus

Anyway, the number of applications where the data exchange happens totally underwater is nowadays very little: the most part of these applications deals with animal tracking and

The chance to track animals, crucial for industrial stock-breeding activities, using RFID tech‐ nology has probably raised for the first time the question whether is possible or not to read RFID tags immersed in water. The body of most part of living beings is mainly composed by water: as an example, around 65% of human body is composed by water. The necessity to guarantee the integrity of the tracking device (In this case the transponder) has encouraged its positioning in a place where it cannot be removed, i.e. inside the body of the animal to be tracked. While the body of the animal is mainly composed by water, to read the transponder from the outside it's necessary to find a technological solution avoiding the insulating effect

The use of RFID for animal tracking is nowadays very common, and has also led to the reali‐ zation of two ad-hoc standards, the ISO 11784 and ISO 11785 standards, that regulate the use of RFID devices, in particular implantable transponders, for the identification of ani‐ mals. Standard RFID systems for animal tracking operate at the frequency of 134.2kHz (Low Frequency band). The transponders used for this purpose are generally glass cylinder tags that are modified to be applied under the skin of the animal, to be clasped to the ear of the

Even if these applications deal with the interaction with water, they are not properly under water systems. Anyway, RFID technology has been employed also to track animals under water. In particular, Low Frequency RFID technology has been used to identify fishes in the aquariums [8]. At the Underwater World Singapore Oceanarium, at Underwater World Pat‐ taya, Thailand and at Virginia Aquarium & Marine Science Center, Low Frequency cylinder

The tagged fishes are identified when they come close to a long range antenna posi‐ tioned on the glass of the tank where the fishes are kept. When the fish passes in front of the antenna, the identification code stored inside the transponder is read and the fish is identified. Once the fish has been identified the visitors of the aquarium can receive an interactive set of information concerning the animal. In particular, an ad-hoc software provides on a screen a picture of the fish and a description: these data are kept on the

glass tags have been applied under the skin of a number of fishes.

screen until a new fish passes close to the antenna.

environmental monitoring, mainly in marine environment.

388 Radio Frequency Identification from System to Applications

its reading.

**4.1. Animal tracking**

of the water layer.

animal or to be ingested by the animal.

Another interesting application that foresees the use of RFID technology under water focus‐ es on the monitoring of pipelines used to carry oil [9]. This solution has been currently only tested, while no information has been retrieved on possible effective applications nowadays working. In this kind of applications Low Frequency RFID tags were applied directly on the pipeline, keeping a fixed distance between one tag and the other.

The tags operated the frequency of 125kHz and they were customized to fit exactly on the pipe: in particular, standard Phillips Semiconductor Hitag transponders were introduced in‐ side a protecting case, shaped on the curvature of the pipe.

Enertag, which tested the system, also developed an ad-hoc underwater reader: this was a handheld waterproof device connected with a cable to a PC positioned on a boat on the sea surface.

This system was employed to monitor the conditions of the pipeline. In practice, the trans‐ ponders acted as milestones, used to identify the exact portion of pipeline. This was com‐ bined with the data concerning repairs that the pipeline had undergone, and suggesting which portion of the pipeline required assistance.

allow the housing of the transponder, the pebbles were drilled. The transponder was then glued on the bottom of the small hole realized in the pebble and then it was covered with

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391

Once a large set of pebbles was realized, it was positioned on the beach to be studied, fol‐ lowing a grid pattern covering both the emerged and the submerged portion of the beach. Through an ad-hoc waterproof reader realized modifying a common reader used for access control, the pebbles were then localized after a pre-defined span of time. The starting and final positions were recorded using a GPS total station: with these data the path followed by the pebble swarm was traced, allowing geologists to easily understand the dynamics of the

This application proved to be very interesting because its biggest requirement was to ach‐ ieve the largest reading range possible. This constraint forced to test different hardware sol‐ utions in order to obtain the best performances especially for salt water, which was the environment where the system had to be employed. A few tests were made with HF (13.56MHz) devices but the results achieved discouraged from using this solution. In partic‐ ular, the reading range obtained with a common desktop reader under salt water was lower than 3cm. This result is in accordance with the theoretical data and excludes the use of this

The following experimentations were carried out on LF 125kHz systems: the theoretical analy‐ sis on this technology foresaw the chance to use them for long range applications also under sea. The tests were carried out using a long range reader usually employed for access control. Several kinds of transponders were used for the tests, from plastic discs to glass tags. The tests tried to simulate as much as possible the real environmental conditions: to achieve this result a model of the sea bottom was realized using a plastic tube. The results of the laboratory tests are shown in Table 2 and demonstrate that, using Low Frequency, long range reading is possible also under sea. Note that the experimentation was carried out in two times, and the results are then divided in two sub-sets: the first three results provide an average value from the best and worst coupling value, while the second three provide these two values separately [12]. The re‐ sults are in accordance with the theoretical analysis: the achieved reading range is lower than

the small rocky cap extracted during the drilling operation.

shoreline and the erosive effects of the meteorological events.

technology for long range under sea applications.

the penetration depth, that acts then as an upper bound.

**Tag Typology Ideal Reading Range Real Conditions**

Nylon disc 55cm 41cm ABS Plastic disc 63cm 51cm PVC disc 49cm 36cm Transparent disc 50cm 28-47cm Long Glass tag (34mm) 65cm 48-63cm Short Glass tag (14mm) 42cm 30-41cm

**Table 2.** Reading ranges of different Low Frequency transponders under water

**Figure 2.** The Enertag system for the pipeline monitoring

#### **4.3. Underwater navigation**

US Navy analysed a possible use of RFID technology as a support for the navigation of au‐ tonomous underwater vehicles [10]. In this application tags are positioned directly on the sea bottom, and they contain information related to their position inside the area where the vehicle is moving.

The reader is embedded directly inside the vehicle: every time that a transponder comes in‐ side the interrogating range of the reader, the information stored inside it is read and then used by the vehicle to manage its movements.

While no data has been found about an effective application of this solution, the possible uses of such a kind of system are many. Even if this solution has been proposed by the US Navy, it could be employed also in many civil applications, from the environmental moni‐ toring to the harbour management.

#### **4.4. Environmental monitoring**

RFID technology has been used for the monitoring of coastal dynamics. The University of Siena and the University of Pisa, in Italy, have realized the so-called "Smart Pebble" system, where Low Frequency transponders are used to trace the movements of a set of pebbles along a pre-defined span of time, in order to study the dynamics of the shoreline [11].

In this system different typologies of 125kHz transponders have been employed in the last 4 years, from plastic disc tags to cylinder glass tags. These tags were inserted inside real peb‐ bles picked up directly on the beaches where the system had to be employed: in order to allow the housing of the transponder, the pebbles were drilled. The transponder was then glued on the bottom of the small hole realized in the pebble and then it was covered with the small rocky cap extracted during the drilling operation.

Once a large set of pebbles was realized, it was positioned on the beach to be studied, fol‐ lowing a grid pattern covering both the emerged and the submerged portion of the beach. Through an ad-hoc waterproof reader realized modifying a common reader used for access control, the pebbles were then localized after a pre-defined span of time. The starting and final positions were recorded using a GPS total station: with these data the path followed by the pebble swarm was traced, allowing geologists to easily understand the dynamics of the shoreline and the erosive effects of the meteorological events.

This application proved to be very interesting because its biggest requirement was to ach‐ ieve the largest reading range possible. This constraint forced to test different hardware sol‐ utions in order to obtain the best performances especially for salt water, which was the environment where the system had to be employed. A few tests were made with HF (13.56MHz) devices but the results achieved discouraged from using this solution. In partic‐ ular, the reading range obtained with a common desktop reader under salt water was lower than 3cm. This result is in accordance with the theoretical data and excludes the use of this technology for long range under sea applications.

The following experimentations were carried out on LF 125kHz systems: the theoretical analy‐ sis on this technology foresaw the chance to use them for long range applications also under sea. The tests were carried out using a long range reader usually employed for access control. Several kinds of transponders were used for the tests, from plastic discs to glass tags. The tests tried to simulate as much as possible the real environmental conditions: to achieve this result a model of the sea bottom was realized using a plastic tube. The results of the laboratory tests are shown in Table 2 and demonstrate that, using Low Frequency, long range reading is possible also under sea. Note that the experimentation was carried out in two times, and the results are then divided in two sub-sets: the first three results provide an average value from the best and worst coupling value, while the second three provide these two values separately [12]. The re‐ sults are in accordance with the theoretical analysis: the achieved reading range is lower than the penetration depth, that acts then as an upper bound.


**Table 2.** Reading ranges of different Low Frequency transponders under water

**Figure 2.** The Enertag system for the pipeline monitoring

390 Radio Frequency Identification from System to Applications

used by the vehicle to manage its movements.

toring to the harbour management.

**4.4. Environmental monitoring**

US Navy analysed a possible use of RFID technology as a support for the navigation of au‐ tonomous underwater vehicles [10]. In this application tags are positioned directly on the sea bottom, and they contain information related to their position inside the area where the

The reader is embedded directly inside the vehicle: every time that a transponder comes in‐ side the interrogating range of the reader, the information stored inside it is read and then

While no data has been found about an effective application of this solution, the possible uses of such a kind of system are many. Even if this solution has been proposed by the US Navy, it could be employed also in many civil applications, from the environmental moni‐

RFID technology has been used for the monitoring of coastal dynamics. The University of Siena and the University of Pisa, in Italy, have realized the so-called "Smart Pebble" system, where Low Frequency transponders are used to trace the movements of a set of pebbles along a pre-defined span of time, in order to study the dynamics of the shoreline [11].

In this system different typologies of 125kHz transponders have been employed in the last 4 years, from plastic disc tags to cylinder glass tags. These tags were inserted inside real peb‐ bles picked up directly on the beaches where the system had to be employed: in order to

**4.3. Underwater navigation**

vehicle is moving.

The first experimentations on the Smart Pebble system were carried out in 2009 and this sol‐ ution has been since then employed in several on-site applications on different beaches in Italy. The effective use of the system has roughly confirmed the results recorded in the labo‐ ratory tests: during the localization process, the transponders embedded inside the pebbles were localized even from distances higher than 50cm.

While this application is interesting because sea is probably the most complex environment for the underwater use RFID, this technology has also been employed several times for the

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393

All these solutions are based on the use of Low Frequency technology. 125kHz or 134.2kHz transponders are introduced inside pebbles that act as tracers in the same way

Anyway, differences occur in the way transponders are detected. In some applications, a reader carried by hand is employed: this means that in most cases the reader is kept outside water and used as a sort of metal detector along portions of the river where the depth is very low. Other interesting solutions are based on the deployment of an array of antennas directly on the river bed. In this case, the tagged pebbles are detected only when they pass

The systems described in the previous sections represent a good starting point for the development of many other possible applications, in the same applicative fields but also

Starting from the animal tracking application, the extension of this solution to other scenar‐ ios is limited mainly by the reading range, which forces the fish to come close to the reader antenna to be identified. Anyway, the chance to track animals also under water suggests a possible use of RFID technology also in the sector of fish breeding. In this case, the use of such a solution could be used to trace the production process and to guarantee the quality of the final product. On the opposite side, the use of RFID technology to trace the movements of wild fishes is notably more difficult. The RFID reading range makes the possibility to trace fishes in the sea (or even in a lake) virtually impossible because the chances that a fish will come close to some antenna positioned elsewhere are close to zero. On the other hand RFID could be used to monitor the movements of fishes along a river. In this case, antenna arrays could be structured as a sort of RFID barrier in locations where the river depth is low enough to allow the detection of every transponder passing over it. In this case, such a sys‐ tem could be for example useful to study the migration processes of fishes like salmons.

The technique set up for the pipeline monitoring could be easily extended to other typolo‐ gies of industrial monitoring. In particular, it could be applied to monitor the state of har‐ bour infrastructures, ship hulls, oil platforms and all the other offshore industrial plants. In all these scenarios, RFID could be useful to keep trace of the maintenance interventions per‐ formed in specific locations. The operators could use RFID transponders as a sort of elec‐ tronic note where the state of the site could be read and then updated every time that any

The underwater navigation application could be a good starting point to develop appli‐ cations where RFID is used to manage the movements of boats inside the harbours. In

study of sediment transportation in rivers [13-14].

as the marine application.

over one of the antennas.

**5. Future applications**

sort of intervention is performed.

in totally new ones.

**Figure 3.** A Smart Pebble. On its surface is possible to notice the hole housing the transponder

**Figure 4.** A moment of the localization operations

While this application is interesting because sea is probably the most complex environment for the underwater use RFID, this technology has also been employed several times for the study of sediment transportation in rivers [13-14].

All these solutions are based on the use of Low Frequency technology. 125kHz or 134.2kHz transponders are introduced inside pebbles that act as tracers in the same way as the marine application.

Anyway, differences occur in the way transponders are detected. In some applications, a reader carried by hand is employed: this means that in most cases the reader is kept outside water and used as a sort of metal detector along portions of the river where the depth is very low. Other interesting solutions are based on the deployment of an array of antennas directly on the river bed. In this case, the tagged pebbles are detected only when they pass over one of the antennas.
