1. Introduction

Prior to the development of thermometers, or indeed a scale of temperature, Renaissance Italy had already began to question the nature of relative temperatures with scientific postulations such as Bardi's problem. This problem was posed by Count Bardi di Vernio to Galileo to determine why a person felt cold upon entering a river in summer, yet grew comfortable over time. While forgoing a direct solution to Bardi's Problem, Galileo reportedly developed a thermoscope in response [1]. Although the device was capable of showing variation in sensible heat it differed from a thermometer in that it did not have a defined scale, it also suffered from barometric influences due to the nature of its construction [2]. It should be noted at this point that there remains uncertainty over the original inventor of the thermoscope, however four prime candidates have been identified, these being; Galileo, Sanctorius Sanctorius, Robert Fludd and Cornelius Drebbel [1]. The advent of precision thermometry originated with the designs of Fahrenheit in the early eighteenth century, with his sealed mercury-in-glass thermometers being a significant advancement on the then state-of-the-art [2]. While specialised liquid-in-glass thermometers have demonstrated resolutions in the region of 0.2 K [3] they are the least useful for accurate measurements in a temporal sense. This is primarily due to the inability to perform transient measurements and relatively cumbersome geometry when compared to electronic sensors.

Currently, industrial temperature sensor designs typically rely on either the thermoelectric effect or temperature dependant resistance. Owing to their wide

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*Applications of Optical Fibers for Sensing*

[2] Schneider U, Pedroni E. Proton radiography as a tool for quality control in proton therapy. Medical Physics.

[3] Gallo G, Lo Presti D, Bonanno D, Longhitano F, Bongiovanni D, Reito S, et al. QBeRT: An innovative instrument for qualification of particle beam in real-time. Journal of Instrumentation.

[4] Lo Presti D. Detector based on scintillating optical fibers for charged particle tracking with application in the realization of a residual range detector employing a read-out channels reduction and compression method. INFN Patent No. WO2013186798; 2013

[5] Cirrone GAP et al. A 62-MeV proton beam for the treatment of ocular melanoma at Laboratory Nazionali del SUD–INFN. IEEE Transactions on Nuclear Science.

[6] SGC Products. Scintillating Fiber Webpage. http://www.crystals.saintgobain.com/products/scintillating-fiber

[7] Lo Presti D, Bonanno D, Longhitano F, Pugliatti C, Aiello S, Cirrone G, et al. A real-time, large area, high space resolution particle radiography system. Journal of Instrumentation.

[8] Lo Presti D, Aiello S, Bonanno D, Cirrone GAP, Leonora E, Longhitano F, et al. OFFSET: Optical Fiber Folded Scintillating Extended Tracker. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers,

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[1] Poludniowski G, Allinson N, Evans P. Proton radiography and tomography with application to proton therapy. The British Journal of Radiology.

Detectors and Associated Equipment.

[9] Lo Presti D, Bonanno D, Longhitano F, Bongiovanni D, Russo G, Leonora E, et al. Design and characterisation of a real time proton and carbon ion radiography system based on scintillating optical fibres. Physica Medica. 2016;**32**(9):1124-1134

[10] Lo Presti D et al. An innovative proton tracking system for qualification of particle beam in real-time. IEEE Transactions on Radiation and Plasma Medical Sciences. 2017;**1**(3):268-274. DOI: 10.1109/TRPMS.2017.2690842

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operating range, thermocouples have been utilised in general engineering applications ranging from precision temperature control for tea processing factories to monitoring the cell temperatures of hepatoblastomas, with peak sensitivities in the region of 20 mK [4, 5]. However due to relatively poor response times, thermocouples are unsuited to high speed measurement. Another robust temperature sensor design is the negative temperature coefficient thermistor (NTC). A particular benefit to using NTCs is the ability to custom mould them for a desired application along with easily tunable resistance at the device and circuit levels [6]. Clow et al. [7] presented an array of NTCs used as a single sensor which was capable of 0.1 mK resolution with application in paleothermometry. While the sensor had excellent resolution it suffered from several drawbacks, the most relevant being a 7 s response time, which prevented it from recording highly transient temperatures gradients. Furthermore the sensor was sensitive to triboelectric effects from snow deposits on sensing wires and self-heating related noise, these being issues which scaled with distance. Precision resistance temperature detectors (RTDs) with milli-Kelvin (mK) resolution are readily available with a response time of 950 ms, and high-speed RTDs in the region of 400 ms [8]. While these sensors present high precision over their working range, a drawback of using RTDs is their inherent fragility. While suited for use in a laboratory, and other controlled environments, general deployment is still proving difficult at present. Hewlett & Packard pioneered the initial development of quartz thermometers in the mid-1960s, with resolution being in the range of 0.1 mK [9]. While capable of achieving sensitivity equivalent to the sensor presented by Clow et al. [7] the reported response time was 30% slower, and had a larger overall footprint.

pharmaceutical calorimetry. A review of the topic conducted by Buckton et al. [14] highlighted the need for identical and constant temperatures within the fermenter and calorimeter, along with maintaining an isothermal condition within the calorimeter itself. As the heat released in these reactions is typically of the order <sup>1</sup> � <sup>10</sup>�<sup>4</sup> mW it is readily evident that precision thermometry is beneficial in obtaining repeatable results. Rossi et al. [15] examined the relationship between intracranial and core temperatures in patients within 2–168 h post severe head injury. From this, intracranial temperatures were verified by comparison to the Delta OHM HD 9215, which had a resolution of 0.1 K. It was reported that core temperature was a poor indicator of intracranial temperature during pyrexic episodes, which was indicative of benefits to independent high resolution temperature sensing within the cranium. As magnetic resonance imaging (MRI) is common in patients post trauma, significant benefit would be provided should the thermometer be unaffected by the strong magnetic field, as it may remain in place to continue

Company General Electric Hanna Instruments SENSTECH Sensor type PRT [18] TC (K-Type) [8, 19] NTC [20] Min. temperature 283 K 73 K 218 K Max. temperature 450 K 573 K 398 K Resolution 0.003 K 0.1 K 0.044 K Response time 0.5 s 4 s 0.75 s Accuracy 0.5 K 0.5 K 0.5 K

Sampling rate is an important factor when deciding on a temperature sensor as it

δT

Further to the items outlined above, specialised considerations may be required. In automotive engine development it is commonplace to embed multiple sensors inside the cylinder head/wall. While electronic sensors operate with little issue for

As mentioned in the introduction, optical fibre sensors present many advantages over their electronic counterparts. Therefore, the following section will examine several properties of optical fibres which indicate their viability as an alternative to

before or after top dead centre. As this is a period of strong pressure and thermal gradients, EMI introduced by the spark plug creates difficulty in analysing the recorded data [17]. A small sample of commercially available high resolution tem-

<sup>δ</sup><sup>t</sup> (1)

) the spark plug activates for several degrees

has to be considered in conjunction with the sensor response time. If the heat transfer from the system is known, then a rate of temperature change may be determined by Eq. (1) [16]. Evidently as temporal resolution is increased smaller variations in heat, and as a consequence temperature, may be determined.

> δQ <sup>δ</sup><sup>t</sup> <sup>¼</sup> <sup>c</sup> �

real-time monitoring.

Table 1.

electronic sensors.

97

the majority of a four-stroke cycle (720<sup>∘</sup>

Commercially available high resolution temperature sensors.

Review of Liquid-Filled Optical Fibre-Based Temperature Sensing

DOI: http://dx.doi.org/10.5772/intechopen.80653

perature sensors is presented in Table 1.

2. Advantages of optical fibre sensors

The issues posed by the use of these traditional sensors in the presence of electromagnetic interference (EMI) and other considerations such as damage resilience and projected lifespan, has resulted in significant investment being made in the exploration of optical fibres as a means of high resolution temperature sensing. While still relatively fragile compared to thermocouples and glass bulb thermometers, optical fibre temperature sensors (OFTS) offer the possibility of a cost effective, high speed and precise sensor. The purpose of this review is to present the current state-of-the-art in a subset of optical fibre sensors that is to say, liquid-filled optical fibre temperature sensors (LiF-OFTS). Further to this, applications in which they have been utilised will be presented along with a discussion of potential future developments.

### 1.1 Temperature sensor requirements

As temperature sensors can be a mission critical piece of equipment various standards have been issued with respect to their usage. Common publishers of temperature sensor requirements for calibration and reporting include the ASTM [10] and IEEE [11]. The measurement range of a given sensor is largely determined by the nature of its operating environment. In an industrial process the range may be several hundred Kelvin, whereas a homoeostatic biomedical thermometer requires a range in the order of only 15 K [12]. Some commercially available probes made from exotic materials such as a tungsten-rhenium alloy thermocouples (Types G, C and D) have wide working ranges, approximately 273–2590 K [13]. These commonly find application in processes involving high temperatures, however require further protection if being utilised in an oxidising environment.

The overall resolution of a temperature sensor may be more important than the operating range depending on an intended application. While it is not critical that a domestic oven have precision temperature control, it can be of great assistance to


Review of Liquid-Filled Optical Fibre-Based Temperature Sensing DOI: http://dx.doi.org/10.5772/intechopen.80653

### Table 1.

operating range, thermocouples have been utilised in general engineering applications ranging from precision temperature control for tea processing factories to monitoring the cell temperatures of hepatoblastomas, with peak sensitivities in the region of 20 mK [4, 5]. However due to relatively poor response times, thermocouples are unsuited to high speed measurement. Another robust temperature sensor design is the negative temperature coefficient thermistor (NTC). A particular benefit to using NTCs is the ability to custom mould them for a desired application along with easily tunable resistance at the device and circuit levels [6]. Clow et al. [7] presented an array of NTCs used as a single sensor which was capable of 0.1 mK resolution with application in paleothermometry. While the sensor had excellent resolution it suffered from several drawbacks, the most relevant being a 7 s response time, which prevented it from recording highly transient temperatures gradients. Furthermore the sensor was sensitive to triboelectric effects from snow deposits on sensing wires and self-heating related noise, these being issues which scaled with distance. Precision resistance temperature detectors (RTDs) with milli-Kelvin (mK) resolution are readily available with a response time of 950 ms, and high-speed RTDs in the region of 400 ms [8]. While these sensors present high precision over their working range, a drawback of using RTDs is their inherent fragility. While suited for use in a laboratory, and other controlled environments, general deployment is still proving difficult at present. Hewlett & Packard pioneered the initial development of quartz thermometers in the mid-1960s, with resolution being in the range of 0.1 mK [9]. While capable of achieving sensitivity equivalent to the sensor presented by Clow et al. [7] the reported response time was

The issues posed by the use of these traditional sensors in the presence of electromagnetic interference (EMI) and other considerations such as damage resilience and projected lifespan, has resulted in significant investment being made in the exploration of optical fibres as a means of high resolution temperature sensing. While still relatively fragile compared to thermocouples and glass bulb thermometers, optical fibre temperature sensors (OFTS) offer the possibility of a cost effective, high speed and precise sensor. The purpose of this review is to present the current state-of-the-art in a subset of optical fibre sensors that is to say, liquid-filled optical fibre temperature sensors (LiF-OFTS). Further to this, applications in which they have been utilised will be presented along with a discussion of potential future

As temperature sensors can be a mission critical piece of equipment various standards have been issued with respect to their usage. Common publishers of temperature sensor requirements for calibration and reporting include the ASTM [10] and IEEE [11]. The measurement range of a given sensor is largely determined by the nature of its operating environment. In an industrial process the range may be several hundred Kelvin, whereas a homoeostatic biomedical thermometer requires a range in the order of only 15 K [12]. Some commercially available probes made from exotic materials such as a tungsten-rhenium alloy thermocouples (Types G, C and D) have wide working ranges, approximately 273–2590 K [13]. These commonly find application in processes involving high temperatures, however require further protection if being utilised in an oxidising environment.

The overall resolution of a temperature sensor may be more important than the operating range depending on an intended application. While it is not critical that a domestic oven have precision temperature control, it can be of great assistance to

30% slower, and had a larger overall footprint.

Applications of Optical Fibers for Sensing

1.1 Temperature sensor requirements

developments.

96

Commercially available high resolution temperature sensors.

pharmaceutical calorimetry. A review of the topic conducted by Buckton et al. [14] highlighted the need for identical and constant temperatures within the fermenter and calorimeter, along with maintaining an isothermal condition within the calorimeter itself. As the heat released in these reactions is typically of the order <sup>1</sup> � <sup>10</sup>�<sup>4</sup> mW it is readily evident that precision thermometry is beneficial in obtaining repeatable results. Rossi et al. [15] examined the relationship between intracranial and core temperatures in patients within 2–168 h post severe head injury. From this, intracranial temperatures were verified by comparison to the Delta OHM HD 9215, which had a resolution of 0.1 K. It was reported that core temperature was a poor indicator of intracranial temperature during pyrexic episodes, which was indicative of benefits to independent high resolution temperature sensing within the cranium. As magnetic resonance imaging (MRI) is common in patients post trauma, significant benefit would be provided should the thermometer be unaffected by the strong magnetic field, as it may remain in place to continue real-time monitoring.

Sampling rate is an important factor when deciding on a temperature sensor as it has to be considered in conjunction with the sensor response time. If the heat transfer from the system is known, then a rate of temperature change may be determined by Eq. (1) [16]. Evidently as temporal resolution is increased smaller variations in heat, and as a consequence temperature, may be determined.

$$\frac{\delta Q}{\delta t} = c \cdot \frac{\delta T}{\delta t} \tag{1}$$

Further to the items outlined above, specialised considerations may be required. In automotive engine development it is commonplace to embed multiple sensors inside the cylinder head/wall. While electronic sensors operate with little issue for the majority of a four-stroke cycle (720<sup>∘</sup> ) the spark plug activates for several degrees before or after top dead centre. As this is a period of strong pressure and thermal gradients, EMI introduced by the spark plug creates difficulty in analysing the recorded data [17]. A small sample of commercially available high resolution temperature sensors is presented in Table 1.
