**2. Underwater radio signals**

#### **2.1. Water electric and magnetic properties**

Water molecule is composed by two oxygen atoms and one hydrogen atom bonded to‐ gether by a covalent bond. Oxygen has a negative charge, while the two hydrogen atoms have a positive charge: this means that the vertex of the molecule has a partial negative charge, while the two ends have a partial positive charge. A molecule with such a charge equilibrium is called electric dipole, and is characterized by its dipolar momentum µ, de‐ fined as the product between the absolute value of one of the two charges and the dis‐ tance between them. This value indicates the tendency of a dipole to orientate under the effect of a uniform electric field.

While still water has a very low electrical conductivity, this value increases in presence of ionized molecules, in proportion to their concentration. When a salt is melt in still water, the single molecules are equally perfused in the whole liquid so that each single volume portion of the solution dissociates, creating many positive and negative ions that remain in the solution together with all the other molecules that aren't dissociated. This phenom‐ enon is called electrolytic dissociation, and the so created solutions are called electrolytic solutions. These solutions can be crossed by an electrical current, in contraposition with still water that acts as a pure insulator.

#### **2.2. Marine water**

Some of these considerations can be applied also to RFID systems. First of all the use of ac‐ tive technologies is discouraged by many factors: at lower frequencies only passive systems can be found; moreover, the use of active systems is also impeded by the required dimen‐ sions of the antennas. Due to these limitations, only two RFID technologies can be employed for underwater applications: the High Frequency systems, operating at 13.56MHz and the Low Frequency systems, operating in the 125-134kHz band. The first solution (13.56MHz) still presents some severe limitations due to the reduction of the reading range: with com‐ mon desktop antennas the reduction in the range is up to 80%, forcing to bring the trans‐ ponder practically in contact with reader antenna. For the second solution (125-134kHz) the reduction is lower (around 30%) and the reading at a distance is still achievable. Laboratory

tests proved that, with long-range antennas, a 50cm reading range is still achievable.

analyse the impact of coastal erosion during sea storms. The chapter will be subdivided in four main sections.

380 Radio Frequency Identification from System to Applications

plications working in the VLF and ELF bands will be provided.

agreement of experimental data with the theoretical analysis.

described in detail.

**2. Underwater radio signals**

**2.1. Water electric and magnetic properties**

proposed.

Both these two solutions can be anyway employed to set up RFID systems working in under water environments. Some solutions can already be found in some parts of the world [1]. USS Navy is testing the use of RFID technology for their applications based on the use of Unmanned Underwater Vehicles. Other applications foresee the use of RFID for the moni‐ toring of underwater pipelines, with RFID transponders employed as markers to guarantee the integrity of the pipes. RFID has also been employed in aquariums to identify fishes, in the same way as Low Frequency RFID capsules are employed in cattle breeding. Finally RFID has been employed as a way to track the movement of pebbles on beaches, in order to

In the first section, the transmission of radio signals in water will be analysed. Details will be given on how the presence of water affects the electromagnetic fields, and examples of ap‐

The second section will focus only on RFID. Technical data will be provided concerning the signal attenuation due to the presence of water. Some results will be given to prove the

In the third section the state of the art concerning under water RFID applications already existing all around the world will be provided. The few already tested applications will be

Finally, in the fourth section some future applications based on this technology will be

Water molecule is composed by two oxygen atoms and one hydrogen atom bonded to‐ gether by a covalent bond. Oxygen has a negative charge, while the two hydrogen atoms have a positive charge: this means that the vertex of the molecule has a partial negative The chemical composition of marine water is influenced by several biological, chemical and physical factors: one simple example is the presence of rivers that add every day new chemi‐ cal materials to the water. On the other side, other materials are removed by the action of organisms and due to erosion. Anyway, the most part of the salts dissolved in marine water remains almost constant due this continuous interchange phenomenon. The most important factors that influence the chemical composition of the marine water are the following:


The elements that can be found is marine water are around 70, but only 6 of them represent the 99% of the total. These predominant salts are:


The symbol (wt%) stands for the mass fraction, and represents the concentration of a solu‐ tion or the entity of the presence of an element in a solution. The quantity of these ions is proportional to the salinity of water, a parameter describing the concentration of dissolved salts in water. Due to the evaporation, this value is lower at the poles (around 3.1%) and higher at the tropics (around 3.8%), with the highest value for an open sea reached by the Red Sea (4%, with a peak of 4.1% in the Northern parts). Moreover, salinity is lower close to the coasts due to the inflow of fresh water by the rivers. Salinity affects the conductivity of water: while this parameter also depends from the water temperature and pressure, it rang‐ es from around 2 S/m to around 6 S/m. Anyway, in most cases it can be considered constant, with a value of 4 S/m. Water is then a conductor.

good wildlife usually range from 150 to 500*μS/cm*. This value is notably lower than the aver‐ age one for marine water. The main consequence of this fact is that for fresh water the pene‐

RFID Under Water: Technical Issues and Applications

http://dx.doi.org/10.5772/53934

383

Some easy calculations prove that the electromagnetic fields can be used to transmit radio signals under water (Especially under the sea) only when their frequency is very low. As an example we can calculate the penetration depth for an electromagnetic wave traveling

This value allows a short range communication, while long range communication requires

Looking at fresh water the situations is a little bit better. The previous calculation can be

Anyway, while this value is higher, long range communication is not allowed when the op‐

As a consequence of the previous analysis, the only bands that have been used for underwa‐ ter radio communication have been the ELF (Extremely Low Frequency) band, ranging form 3 to 300 Hz, with the sub-band ranging from 30 to 300 Hz called SLF (Super Low Frequency)

The ELF band was used for the communication with submarines both by the US and the Russian Navies. The US system, called Seafarer, operated at the frequency of 78Hz, while the Russian one, called ZEVS, operated at the frequency of 82Hz. These systems had a pene‐ tration depth in the order of 10km, allowing thus a communication from a fixed station on the sea surface with a submarine traveling close to the ocean floor. Anyway, the realization of a communication channel at these frequencies presents several technical limitations that are extremely difficult to be overcome. One of the biggest problems to be solved is the size of the antenna: its dimension has in fact to be a substantial fraction of the wavelength, but at these frequencies the dimension of the wavelength is in the order of the thousands of kilo‐ metres. The solutions found by the US and Russian Navies were complex and expensive,

The VLF band ranges from 3kHz to 30kHz: this means that the penetration depth of electro‐ magnetic waves at these frequencies is in the order of ten meters. This value allows a com‐ munication with submarines positioned few meters below the sea surface. The limitations on the antenna dimensions, deriving from the big wavelength, have to be taken in account also in this case. Moreover, due to the limited bandwidth, this communication channel can‐

through salt water at frequency of 10kHz, using the average values for µ and σ:

≈2.5*m*

made, using a very low conductivity value of 30 *μ*S/cm (3mS/m):

*<sup>π</sup>* <sup>∙</sup> <sup>10</sup><sup>4</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 =92*<sup>m</sup>*

tration depth is higher and the attenuation is lower.

*π* ∙ 10<sup>4</sup> ∙ 4*π* ∙ 10-7 ∙ 4

**2.4. Underwater radio communication**

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

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

erative frequency is higher than some kHz.

band, and the VLF (Very Low Frequency band).

making prohibitive their use for civil applications.

not be used to transmit audio signals, but only text messages.

even lower frequencies.

*<sup>δ</sup>*10*kHz* <sup>=</sup> <sup>1</sup>

*<sup>δ</sup>*10*kHz* <sup>=</sup> <sup>1</sup>

Once the value of water conductivity is known, it can be used to calculate the values of the penetration depth and of the attenuation.

The penetration depth δ is the distance where the electrical and magnetic fields are reduced of a *1/e* factor, and it can be calculated using the following formula:

$$\delta = \frac{1}{\sqrt{\pi \sharp \mu \circ}} \text{ } m$$

where *f* is the frequency of the electromagnetic wave, *μ* is the absolute magnetic permeabili‐ ty of the conductor and *σ* is the conductivity. While water is a diamagnetic material, their absolute magnetic permeability can be considered the same as the vacuum magnetic perme‐ ability, i.e. *μ0* = 4π\*10 H/m. This means that, with the conductivity considered constant, the penetration depth only depends on the frequency: the higher is the frequency, the lower is the penetration depth.

The attenuation α can be calculated using the following formula [2]:

$$\alpha = 0.0173 \sqrt{f \sigma} \text{ dB} / m$$

where *f* is the frequency of the electromagnetic wave and *σ* is the water conductivity that, as said before, can be considered constant. Attenuation is then in inverse proportion with the frequency and then obviously also with the penetration depth.

#### **2.3. Fresh water**

Around 97% of the water of the world is found in seas and oceans, while two thirds of the remaining 3% of fresh water is retained as ice in glaciers and at the poles. This means that the most part of studies that can be found concerning the chance to communicate under wa‐ ter using the electromagnetic fields focuses on the marine environment.

Anyway, similar considerations as the ones made for salt water apply to fresh water. The biggest difference derives from the different values of salinity that are detected in fresh wa‐ ter. While the salinity of salt water is around 3.5% (See section 2.2), in fresh water this value decreases down to 0.05%. Anyway, unlike marine water, a general analysis concerning the quantity and typology of salts that can be found in fresh water is impossible to carry out due to the single peculiarities of rivers, lakes, and the chemical and geological composition of the territories that they pass through and where they are located.

A different value in salinity also means a different value in conductivity. In particular, con‐ ductivity of fresh water ranges from 30 to 2000*μS/*cm: these are nevertheless extreme values; river water conductivity usually ranges from 50 to 1500*μS/cm*, while rivers supporting a good wildlife usually range from 150 to 500*μS/cm*. This value is notably lower than the aver‐ age one for marine water. The main consequence of this fact is that for fresh water the pene‐ tration depth is higher and the attenuation is lower.

#### **2.4. Underwater radio communication**

higher at the tropics (around 3.8%), with the highest value for an open sea reached by the Red Sea (4%, with a peak of 4.1% in the Northern parts). Moreover, salinity is lower close to the coasts due to the inflow of fresh water by the rivers. Salinity affects the conductivity of water: while this parameter also depends from the water temperature and pressure, it rang‐ es from around 2 S/m to around 6 S/m. Anyway, in most cases it can be considered constant,

Once the value of water conductivity is known, it can be used to calculate the values of the

The penetration depth δ is the distance where the electrical and magnetic fields are reduced

where *f* is the frequency of the electromagnetic wave, *μ* is the absolute magnetic permeabili‐ ty of the conductor and *σ* is the conductivity. While water is a diamagnetic material, their absolute magnetic permeability can be considered the same as the vacuum magnetic perme‐ ability, i.e. *μ0* = 4π\*10 H/m. This means that, with the conductivity considered constant, the penetration depth only depends on the frequency: the higher is the frequency, the lower is

where *f* is the frequency of the electromagnetic wave and *σ* is the water conductivity that, as said before, can be considered constant. Attenuation is then in inverse proportion with the

Around 97% of the water of the world is found in seas and oceans, while two thirds of the remaining 3% of fresh water is retained as ice in glaciers and at the poles. This means that the most part of studies that can be found concerning the chance to communicate under wa‐

Anyway, similar considerations as the ones made for salt water apply to fresh water. The biggest difference derives from the different values of salinity that are detected in fresh wa‐ ter. While the salinity of salt water is around 3.5% (See section 2.2), in fresh water this value decreases down to 0.05%. Anyway, unlike marine water, a general analysis concerning the quantity and typology of salts that can be found in fresh water is impossible to carry out due to the single peculiarities of rivers, lakes, and the chemical and geological composition of the

A different value in salinity also means a different value in conductivity. In particular, con‐ ductivity of fresh water ranges from 30 to 2000*μS/*cm: these are nevertheless extreme values; river water conductivity usually ranges from 50 to 1500*μS/cm*, while rivers supporting a

with a value of 4 S/m. Water is then a conductor.

of a *1/e* factor, and it can be calculated using the following formula:

The attenuation α can be calculated using the following formula [2]:

frequency and then obviously also with the penetration depth.

territories that they pass through and where they are located.

ter using the electromagnetic fields focuses on the marine environment.

penetration depth and of the attenuation.

382 Radio Frequency Identification from System to Applications

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

*<sup>π</sup>fμ<sup>σ</sup> <sup>m</sup>*

the penetration depth.

*α* =0.0173 *fσ dB* / *m*

**2.3. Fresh water**

Some easy calculations prove that the electromagnetic fields can be used to transmit radio signals under water (Especially under the sea) only when their frequency is very low. As an example we can calculate the penetration depth for an electromagnetic wave traveling through salt water at frequency of 10kHz, using the average values for µ and σ:

$$\delta\_{10kHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 10^4 \cdot 4 \pi \cdot 10^{-7} \cdot 4}} \approx 2.5mv$$

This value allows a short range communication, while long range communication requires even lower frequencies.

Looking at fresh water the situations is a little bit better. The previous calculation can be made, using a very low conductivity value of 30 *μ*S/cm (3mS/m):

$$\delta\_{10kHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 10^4 \bullet 4 \pi \cdot 10^{-7} \bullet 3 \bullet 10^{-3}}} = 92mv$$

Anyway, while this value is higher, long range communication is not allowed when the op‐ erative frequency is higher than some kHz.

As a consequence of the previous analysis, the only bands that have been used for underwa‐ ter radio communication have been the ELF (Extremely Low Frequency) band, ranging form 3 to 300 Hz, with the sub-band ranging from 30 to 300 Hz called SLF (Super Low Frequency) band, and the VLF (Very Low Frequency band).

The ELF band was used for the communication with submarines both by the US and the Russian Navies. The US system, called Seafarer, operated at the frequency of 78Hz, while the Russian one, called ZEVS, operated at the frequency of 82Hz. These systems had a pene‐ tration depth in the order of 10km, allowing thus a communication from a fixed station on the sea surface with a submarine traveling close to the ocean floor. Anyway, the realization of a communication channel at these frequencies presents several technical limitations that are extremely difficult to be overcome. One of the biggest problems to be solved is the size of the antenna: its dimension has in fact to be a substantial fraction of the wavelength, but at these frequencies the dimension of the wavelength is in the order of the thousands of kilo‐ metres. The solutions found by the US and Russian Navies were complex and expensive, making prohibitive their use for civil applications.

The VLF band ranges from 3kHz to 30kHz: this means that the penetration depth of electro‐ magnetic waves at these frequencies is in the order of ten meters. This value allows a com‐ munication with submarines positioned few meters below the sea surface. The limitations on the antenna dimensions, deriving from the big wavelength, have to be taken in account also in this case. Moreover, due to the limited bandwidth, this communication channel can‐ not be used to transmit audio signals, but only text messages.

### **3. Underwater RFID**

RFID, being a radio technology, suffers from the same limitations of the standard communi‐ cation channels. This means that the higher is the frequency, the lower are the chances to have a reliable communication {3-7}.

quency value of 800MHz (varying this value from 433MHz to 930MHz the order of

RFID Under Water: Technical Issues and Applications

http://dx.doi.org/10.5772/53934

385

This value is obviously too short to use this technical solution for other than contact applica‐ tions. Only bringing a transponder in contact with the antenna of the reader, the reading be‐ comes possible. While this fact strongly limits the possible uses of these systems, in some

Finally, the Microwave band is obviously the one that provides the worst results. The value of the penetration depth is provided only for completeness, even if currently no application

Before moving to the next section a clarification has to be made. In the previous analysis no differentiation has been done on the powering method of the transponders. In fact, while ac‐ tive transponders usually provide higher reading ranges, they are generally used only at higher frequencies (UHF and Microwave bands): anyway, at these frequencies the penetra‐ tion depth is so short that even with the most powerful active transponder no improvement in the performances of the systems would be noticeable. Moreover, even at lower frequen‐ cies, the value of the penetration depth is anyhow lower than the reading range achievable using passive transponders: therefore, a study for the use of active transponders also at

The analysis for fresh water is similar to the one carried out for salt water. The main differ‐ ence derives from the fact that, while the range of the conductivity values of salt water is very short, it becomes wider in the case of fresh water. As anticipated is section 2.3, fresh water conductivity roughly varies from 30 *μS/cm* to 2000 *μS/cm*. While both these values are notably lower than the conductivity of salt water, the differences between the obtained val‐ ues for penetration depth are less distant. In order to provide an accurate set of data, the penetration depth value will be calculated both for the best (30 *μS/cm*) and the worst (2000

As in the case of salt water, the analysis will begin from the Low Frequency band. In this case, at the frequency of 125kHz, with a conductivity value of 30 *μS/cm* (3 *mS/m*), the value

≈5*mm*

these frequencies would be useless and wouldn't provide any improvement.

≈9*mm*

magnitude remains quite constant), provides the following result:

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

cases UHF systems can still become a good choice.

can be found worldwide using this technical solution:

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

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

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

**3.2. Fresh water**

*μS/cm*) case.

*<sup>δ</sup>*125*kHz* <sup>=</sup> <sup>1</sup>

*<sup>δ</sup>*125*kHz* <sup>=</sup> <sup>1</sup>

of penetration depth 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>

*<sup>π</sup>* <sup>∙</sup> 1.25 <sup>∙</sup> <sup>10</sup><sup>5</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 =26*<sup>m</sup>*

*π* ∙ 1.25 ∙ 10<sup>5</sup> ∙ 4*π* ∙ 10-7 ∙ 0.2

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

=3.2*m*

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

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

RFID systems are usually subdivided in the following bands:


#### **3.1. Salt water**

As underlined in section 2, significant differences occur according as the RFID system has to be used in salt or fresh water. Starting from salt water, some calculations show that only LF RFID can be used for systems requiring a long reading distance (over 50cm). In particular at a frequency of 125kHz, the average value (Using the salinity value of 4S/m) for the penetra‐ tion depth is:

$$\delta\_{125kHz} = \frac{1}{\sqrt{\tau\mathfrak{f}\mu\sigma}} = \frac{1}{\sqrt{\pi \bullet 1.25 \bullet 10^5 \bullet 4\pi \bullet 10^{\circ7} \bullet 4}} \approx 71 cm^{\circ}$$

This value is just lower than the maximum achievable reading range for a Low Frequency system, which is usually lower than 1m. This means that Low Frequency RFID can be theo‐ retically used for the underwater identification of items.

Moving at higher frequencies, the use of these systems for long range identification becomes virtually impossible. The calculation for the penetration depth provides an extremely low value. Starting from the High Frequency band, where all RFID systems work at the standard frequency of 13.56MHz, with the same conditions as in the previous case, the obtained value for the penetration depth is:

$$\delta\_{13.56kHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 13.56 \cdot 10^6 \bullet 4\pi \bullet 10^{-7} \bullet 4}} \approx 68mm$$

This result proves that High Frequency RFID can be used under water only for short range solutions. In particular, due to the fact that the effectiveness of every RFID system is notably influenced by the performances of the hardware devices employed, it's possible to affirm that the chance to use High Frequency systems is limited to the applications where the tag is in close contact with the reader.

The UHF band is currently employed in many different systems and probably represents the best solution for many applications due to its good performances in terms of reading range, costs and bitrate. Anyway, its frequency is too high to allow its use also for underwa‐ ter contactless applications. The calculation of the penetration depth, using an average fre‐ quency value of 800MHz (varying this value from 433MHz to 930MHz the order of magnitude remains quite constant), provides the following result:

$$\delta\_{800MHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 800 \bullet 10^6 \bullet 4 \pi \bullet 10^{\circ7} \bullet 4}} \approx 9mmm$$

This value is obviously too short to use this technical solution for other than contact applica‐ tions. Only bringing a transponder in contact with the antenna of the reader, the reading be‐ comes possible. While this fact strongly limits the possible uses of these systems, in some cases UHF systems can still become a good choice.

Finally, the Microwave band is obviously the one that provides the worst results. The value of the penetration depth is provided only for completeness, even if currently no application can be found worldwide using this technical solution:

$$\delta\_{2.45GHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 2.45 \bullet 10^{\circ} \bullet 4\pi \bullet 10^{\circ} \bullet 4}} \approx 5 mm$$

Before moving to the next section a clarification has to be made. In the previous analysis no differentiation has been done on the powering method of the transponders. In fact, while ac‐ tive transponders usually provide higher reading ranges, they are generally used only at higher frequencies (UHF and Microwave bands): anyway, at these frequencies the penetra‐ tion depth is so short that even with the most powerful active transponder no improvement in the performances of the systems would be noticeable. Moreover, even at lower frequen‐ cies, the value of the penetration depth is anyhow lower than the reading range achievable using passive transponders: therefore, a study for the use of active transponders also at these frequencies would be useless and wouldn't provide any improvement.

#### **3.2. Fresh water**

**3. Underwater RFID**

have a reliable communication {3-7}.

384 Radio Frequency Identification from System to Applications

**•** Low Frequency (LF) – 120-150kHz; **•** High Frequency (HF) – 13.56MHz;

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

for the penetration depth is:

in close contact with the reader.

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

**•** Microwave – 2.45-5.8GHz.

**3.1. Salt water**

tion depth is:

*<sup>δ</sup>*125*kHz* <sup>=</sup> <sup>1</sup>

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

RFID systems are usually subdivided in the following bands:

**•** Ultra High Frequency (UHF) – 433MHz, 868-928MHz;

*π* ∙ 1.25 ∙ 10<sup>5</sup> ∙ 4*π* ∙ 10-7 ∙ 4

retically used for the underwater identification of items.

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

RFID, being a radio technology, suffers from the same limitations of the standard communi‐ cation channels. This means that the higher is the frequency, the lower are the chances to

As underlined in section 2, significant differences occur according as the RFID system has to be used in salt or fresh water. Starting from salt water, some calculations show that only LF RFID can be used for systems requiring a long reading distance (over 50cm). In particular at a frequency of 125kHz, the average value (Using the salinity value of 4S/m) for the penetra‐

This value is just lower than the maximum achievable reading range for a Low Frequency system, which is usually lower than 1m. This means that Low Frequency RFID can be theo‐

Moving at higher frequencies, the use of these systems for long range identification becomes virtually impossible. The calculation for the penetration depth provides an extremely low value. Starting from the High Frequency band, where all RFID systems work at the standard frequency of 13.56MHz, with the same conditions as in the previous case, the obtained value

≈68*mm*

This result proves that High Frequency RFID can be used under water only for short range solutions. In particular, due to the fact that the effectiveness of every RFID system is notably influenced by the performances of the hardware devices employed, it's possible to affirm that the chance to use High Frequency systems is limited to the applications where the tag is

The UHF band is currently employed in many different systems and probably represents the best solution for many applications due to its good performances in terms of reading range, costs and bitrate. Anyway, its frequency is too high to allow its use also for underwa‐ ter contactless applications. The calculation of the penetration depth, using an average fre‐

≈71*cm*

The analysis for fresh water is similar to the one carried out for salt water. The main differ‐ ence derives from the fact that, while the range of the conductivity values of salt water is very short, it becomes wider in the case of fresh water. As anticipated is section 2.3, fresh water conductivity roughly varies from 30 *μS/cm* to 2000 *μS/cm*. While both these values are notably lower than the conductivity of salt water, the differences between the obtained val‐ ues for penetration depth are less distant. In order to provide an accurate set of data, the penetration depth value will be calculated both for the best (30 *μS/cm*) and the worst (2000 *μS/cm*) case.

As in the case of salt water, the analysis will begin from the Low Frequency band. In this case, at the frequency of 125kHz, with a conductivity value of 30 *μS/cm* (3 *mS/m*), the value of penetration depth is:

$$\delta\_{125kHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 1.25 \cdot 10^5 \cdot 4\pi \cdot 10^{-7} \cdot 3 \cdot 10^{-3}}} = 2.6\,\mathrm{m}$$

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

$$\delta\_{125kHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 1.25 \bullet 10^5 \bullet 4\pi \bullet 10^{-7} \bullet 0.2}} = 3.2m$$

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

**Low Frequency 125kHz**

Salt Water 4*S/m*

Fresh Water 30μS/cm

Fresh Water 2000μS/cm

reliability;

UHF frequencies.

**High Frequency 13.56MHz**

**Table 1.** The penetration depths for the considered frequencies for both salt and fresh water

the operative frequency as to be the lowest possible. In particular:

at frequencies higher than 13.56MHz;

**4. RFID applications under water**

cations exist where RFID is used under water.

71*cm* 68*mm* 9*mm* 5*mm*

26*m* 2.5*m* 32.5*cm* 18.6*cm*

3.2*m* 30*cm* 4*cm* 2.3*cm*

In conclusion, while theoretical data suggest that several solutions are possible when RFID is required for under water applications, it's possible to affirm that to obtain reliable results

**•** In salt water, short range or contact reading could be possible also at higher frequencies. Anyway, also in these cases a reliable reading level could be very difficult to be achieved

**•** For fresh water long range reading could be obtained not only with Low Frequency sys‐ tems, but also with the use of High Frequency devices operating at 13.56MHz. Anyway, also in this case the use of Low Frequency is strongly recommended due to their higher

**•** When short range or contact reading is required in fresh water, quite all the frequen‐ cies could be efficient, even if there is a lack of studies proving the effectiveness of

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‐

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

**•** For salt water long range reading is obtainable only using Low Frequency systems;

**Ultra High Frequency**

**Microwaves 2.45GHz**

http://dx.doi.org/10.5772/53934

387

RFID Under Water: Technical Issues and Applications

**800MHz**

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:

$$\delta\_{13.56MHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 13.56 \bullet 10^6 \bullet 4\pi \bullet 10^{-7} \bullet 3 \bullet 10^{-3}}} = 2.5m$$

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

$$\delta\_{13.56MHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 13.56 \bullet 10^6 \bullet 4\pi \bullet 10^{\gamma} \bullet 0.2}} \text{ } \Re 0 cm$$

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‐ er strongly suggested.

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 2000 *μS/cm* (0.2 *S/m*) is:

$$\delta\_{800MHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 800 \bullet 10^6 \bullet 4 \pi \cdot 10^{-\mathcal{I}} \bullet 3 \bullet 10^3}} \approx 32.5 cm$$

and

$$\delta\_{800MHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 800 \bullet 10^6 \bullet 4 \pi \bullet 10^{\circ} \bullet 0.2}} \approx 4 cm$$

For Microwaves, these values drop down to:

$$\delta\delta\_{2.45GHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 2.45 \cdot 10^{\circ} \cdot 4\pi \cdot 10^{\circ} \cdot 3 \bullet 10^{\circ}}} \approx 18.6 cm$$

$$\delta\_{2.45GHz} = \frac{1}{\sqrt{\pi f \mu \sigma}} = \frac{1}{\sqrt{\pi \cdot 2.45 \bullet 10^{\circ} \cdot 4\pi \cdot 10^{\circ} \cdot 0.2}} \approx 2.3 cm$$

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 at all.


**Table 1.** The penetration depths for the considered frequencies for both salt and fresh water

In conclusion, while theoretical data suggest that several solutions are possible when RFID is required for under water applications, it's possible to affirm that to obtain reliable results the operative frequency as to be the lowest possible. In particular:

