issn 2249-6343 volume 3, issue 3, june 2013 … chemical media, the gas will start to burn or ignite...

ISSN 2249-6343

International Journal of Computer Technology and Electronics Engineering (IJCTEE)

Volume 3, Issue 3, June 2013

9

Abstract-Underground mining requires equipment and

manpower to operate under the earth surface. Subsurface

atmosphere may be contaminated with poisonous gases that

displace the necessary oxygen to support life or flammable gases

that may cause explosion. Therefore, it is necessary to develop

technologies and find ways to accurately measure concentration

levels of toxic and flammable gases levels in subsurface

atmosphere for safety of underground coal mines. Each sensor

has its own advantages and constraints, like some sensors are

better for sensing toxic gases and some are better for

combustible gas detection. The paper enumerates operating

principle, working procedure and application of different types

of sensors for monitoring toxic and flammable gases in

hazardous areas.

Index Terms- Gas sensors, MEMS, Nanotechnology, TLVs

I. INTRODUCTION

Underground coal mine is characterized by tough working

condition and hazardous environment. Many accidents occur

in underground coal mine which leads to fatal accidents and

huge loss of properties. These accidents have variety of

causes, including sudden rise in toxicants such as carbon

monoxide (CO), dangerous flammable gases especially

methane (CH4) or firedamp and insufficient oxygen for mine

workers to breathe. Therefore, for sustainable growth of coal

mining industry and safety of miners, it is necessary to

develop technologies and find ways to make mines hazard

free [1].

To keep atmosphere just right in underground coal mine,

the primary requirement is to regularly monitor the levels of

gases, like oxygen, methane, carbon dioxide, carbon

monoxide etc. This gives miners short and long term trending

information in the subsurface atmosphere and allows early

warning against explosive and toxic atmospheres at every

place where miners normally work or travel. No mineworker

should enter any underground work place, in particular those

places with poor air circulation (e.g. blind headings), unless

the air has been checked therein to ensure a safe breathable

atmosphere free from levels of hazardous gases.

II. HAZARDOUS AREA

Fire, toxic atmospheric contaminant and dust or gas

explosion are some critical hazards specifically linked to

underground mining. It is necessary to figure out which area

needs to be defined as hazardous area so that miners should

be alerted in advance (Table I).

A. Combustible Gases

A hazardous area is defined based on three criteria, namely

(i) depending upon type of gas, (ii) ignition temperature of

the gas, and (iii) likelihood of gas being present in flammable

concentrations. Flammability limit, thus defined, gives the

proportion of combustible gases in a mixture, between which

limits mixture is flammable.

Lower Explosive Limit (LEL): The minimum

concentration of gas or vapour mixed with air

(percentage by volume, at room temperature) that will

cause the propagation of flames when it comes in contact

with a source of ignition. In common terminology,

mixtures below the LEL are too lean to ignite.

Upper Explosive Limit (UEL): The maximum

concentration of gas or vapour mixed with air (percent

by volume, at room temperature) that will cause the

propagation of flames when it comes in contact with an

ignition source. In common terminology, mixtures above

the UEL are too rich to support combustion.

B. Toxic Gases

As toxic gases can cause harm in low levels over a long

period of time (chronic exposure) or in higher concentrations

over a short period of time (acute exposure), different

countries have established threshold limit values (TLVs) for

poisonous gases in order to advance worker protection by

providing timely scientific information to occupational and

environmental health professionals.

TLVs of airborne substances refer to those concentrations

within which personnel may be exposed without known

adverse effects to their health or safety. Followings are the

three types of TLVs:

(i) Time Weighted Average (TWA) is the average

concentration to which nearly all workers may be

exposed over given hours of work shift/week without

known adverse effects. However, many substances are

sufficiently toxic that short-term exposures at higher

concentrations may prove harmful or even fatal.

(ii) Short-Term Exposure Limit (STEL) is a time-weighted

average concentration occurring over a period of not

more than few minutes. It is also recommended that such

circumstances should not occur many times.

(iii) Ceiling Limit (CL) is the concentration that should not

be exceeded at any time. This is relevant for the most

toxic substances or those that produce an immediate

irritant effect.

Application of Gas Monitoring Sensors in

Underground Coal Mines and Hazardous Areas

A. Kumar*, T.M.G. Kingson, R.P. Verma, A. Kumar, R. Mandal, S. Dutta, S.K. Chaulya and G.M. Prasad

CSIR-Central Institute of Mining and Fuel Research, Dhanbad, India

*Corresponding Author, Email: [email protected]

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III. GAS DETECTING SENSORS

Gas sensors detect presence of various gases within an

area, usually as a part of safety system. Sensors give a

proportional electrical response depending upon the

concentration of gas to be detected. If the concentration

exceeds threshold concentration limit, the instrument

containing it will provide an alarm to nearby personnel, or it

may activate other remedial actions, such as increasing the

ventilation, switching off the power supply etc. [3]. Different

methods for detecting above gases are given in Table II and

their operating principles are summarized in Table III.

Table II: Different methods of gas detection [4]

Name Methods of detection

Oxygen Electrochemical, paramagnetic, flame

lamp

Methane Catalytic oxidation, thermal

conductivity, optical, acoustic, flame

lamp

Carbon dioxide Optical, infrared

Carbon monoxide Electrochemical, catalytic oxidation,

semiconductor, infra-red

Sulphur dioxide Electrochemical, infra-red

Nitric oxide,

Nitrous oxide,

Nitrogen dioxide

Electrochemical

Hydrogen

sulphide

Electrochemical, semiconductor

Hydrogen Catalytic oxidation

A. Catalytic Bead Sensors

Theory: Combustible gas mixtures will not burn until they

reach an ignition temperature. However, in the presence of

certain chemical media, the gas will start to burn or ignite at

lower temperatures. This phenomenon is known as a catalytic

combustion.

A coil of wire is coated with glass or ceramic material

which is coated with a catalyst. The coil is electrically heated

to a temperature that will allow it to burn (catalyze)

combustible hydrocarbon (CHC) gas being monitored.

Pellistors are miniature calorimeters used to measure the

energy liberated by burning of a combustible (flammable)

gas or vapor [6]. A pellistor consists of a coil of

small-diameter platinum wire supported in a refractory bead

coated with a layer of catalytic material (Fig. 1), on which the

gas is burnt. The coil serves two purposes: firstly, it is used to

heat the bead electrically to its operating temperature of

around 500°C, and secondly it is used to detect changes in

temperature produced by oxidation of flammable gas.

Working procedure: A Catalytic bead sensor is used in

Wheatstone bridge (Fig. 2), a circuit for measuring an

unknown resistance by comparing it with known resistances.

A balanced bridge has no output signal. R1 is trim resistor that

keeps the bridge balanced.

Resistor value RB and trim pot R1 are selected with

relatively large resistance values to ensure proper function of

the circuit. During its operation, a current is passed through

the coil, which heats up the bead to a high temperature. The

gas is burned when a flammable gas molecule comes into

contact with the catalyst layer. The reaction occurs without a

flame since the level is below the Lower Explosive Limit (or

LEL) of the gas. However, during burning reaction, heat is

released which increases the temperature of bead. This

increase in temperature causes rise in electrical resistance of

the coil.

Fig.1: Catalytic bead [7]

There is another bead in the circuit which is identical to the

detector bead, but does not contain any catalyst. This bead

will react to changes in humidity, ambient temperature etc.,

but will not react to flammable gas. All that is required is a

comparison of the resistance of one bead against another in a

Wheatstone bridge type circuit in order to obtain a

meaningful signal.

Fig. 2: A catalytic bead sensor Wheatstone bridge [8]

Reaction: Reaction takes place on the surface of catalytic

bead is given as

CH4 + 2O2 + 8N2 CO2 + 2H2O + 8N2 (1)

From the above reaction, one part of methane requires ten

parts of air for complete combustion. For a sensor to detect

methane, the signal output will respond linearly from 0–5%

of methane. As the concentration reaches close to 9%, the

signal increases very rapidly & peaks at around 10% (Fig. 3).

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The signal starts to drop slowly as the concentration of gas

passes approximately 20%; after 20% it drops sharply and at

100% sensor signal is zero.

Fig. 3: Sensor output vs. methane concentration [8]

Factors affecting the operations of catalytic sensors are as

Factors affecting the operations of catalytic sensors are as

follows:

(i) Catalyst poisoning: Poisoning compounds cause a

permanent reduction of the sensor sensitivity. The exact

cause of poisoning is very difficult to identify. Tetraethyl

lead, silicon compounds and sulphur compounds are

among the most common poisons.

(ii) Sensor inhibitors: Inhibitors cause a temporary loss of

sensitivity to sensor and may be partially or totally

recovered after a short exposure to fresh air. The most

common inhibitors are H2S, chlorine, chlorinated

hydrocarbons and halogen compounds.

(iii) Sensor cracking: The sensor, when exposed to high

concentration of gases, excessive heat and various

oxidation processes that take place on the sensor surface,

may eventually deteriorate its performance.

Table I: List of gases and their hazardous limits in underground mines [2]

Table III: Classification of sensors by transducer operating principle [5]

Sl.

No.

Types of devices Physical change Source of signal

1 Catalytic gas sensors

(pellistors), thermal

sensors

Temperature or

heat

Changes in resistance in wheat stone bridge

2 Optical sensors

(infrared, laser, optical

fiber)

Absorbance

Luminescence

Refractive index

Scattering

Either caused by gas itself, or due to reaction with certain indicator.

Emission, caused by chemical reaction

For example, caused by change in solution composition.

Caused by particles of definite sizes present in the sample.

3 Semiconductor (solid Electrical Changes in work function

Name of gas Flammability

limits in air (%)

Guideline for TLVs Hazards

Oxygen >19.5% Oxygen deficiency, may cause

explosive mixtures with reactive gases

Nitrogen CL = 81,000 ppm Inert

Methane 5 to15 At 1% isolate electricity, at 2%

remove personnel.

Explosion

Carbon dioxide TWA = 0.5%, STEL = 3.0%, CL =

1.5%

Promotes increased rate of respiration

Carbon monoxide 12.5 to 74.5 TWA = 0.005%, STEL = 0.04%,

CL = 200 ppm

Highly toxic; explosive

Sulphur dioxide TWA = 2 ppm, STEL = 5 ppm, CL

= 10 ppm

Very toxic; irritant to eyes throat and

lungs

Nitric oxide TWA = 50 ppm Oxidizes rapidly to NO2

Nitrous oxide TWA = 50 ppm Narcotic (laughing gas)

Nitrogen dioxide TWA =3 ppm, CL= 5 ppm Very toxic; throat and lung irritant;

pulmonary infections

Hydrogen sulphide 4.3 to 45.5 TWA = 10 ppm, STEL = 15ppm,

CL = 15ppm

Highly toxic; irritant to eyes and

respiratory tracts; explosive

Hydrogen 4 to 74.2 Highly explosive

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state) gas sensors conductivity

4 Electrochemical gas

sensors

(potentiometric or

amperometric )

Voltametric

Changes in current between electrodes is measured

5 Piezo-electric sensors

(quartz crystal

microbalance)

Mass Changes of resonant frequency of quartz oscillator plate due to

adsorption of a gas on its chemically modified surface

6 Flame ionization

detector, photo

ionization detector

Ionisation Amount of ionization

B. Infrared Gas Sensor

Theory: Gas molecules are made up of number of atoms

bonded to one another. These interatomic bonds are similar to

springs, connecting atoms of various masses together. This

bonding vibrates with a fixed frequency called the natural

frequency. When infrared radiation interacts with gas

molecules as shown in Fig. 4, part of energy has the same

frequency as the gas molecule‟s natural frequency and it is

absorbed while rest of the radiation is transmitted. As the gas

molecules absorb this radiation, the molecules gain energy

and vibrate more vigorously.

Fig. 4: Effect of infrared radiation on bonding of molecule

This vibration results in temperature rise of gas molecules.

Rise in temperature is detected by the detector. On the other

hand, the radiation absorbed by the gas molecules at the

particular wavelength will cause a decrease in the original

wavelength. This radiation energy decrease can be detected

as a signal also.

Working of Non-Dispersive Infrared (NDIR): It is based

on infrared absorption property of some gases which consists

of a single IR source, a beam splitter and two detectors as

shown in Fig. 5. One detector is used to monitor the

characteristic hydrocarbon wavelength. The other is a

reference that monitors an atmospheric “window” where no

IR active gases are normally present. Infrared energy

(2-5microns, where micron is a common unit to express

wavelength in infrared range) is emitted from the source,

passes through the gas cell, and is reflected back to the

detectors. If no hydrocarbons are present within the gas

sample, then energy reaching the detector is the same.

However, if some combustible hydrocarbons are present,

they will absorb some IR energy at that wavelength, thus

reducing the amount received by the analytic detector. Gas

concentration is determined by comparing the relative values

between the two wavelengths (Fig. 6). This is called dual

beam infrared detector [9]-[10].

Fig. 5: Infrared gas sensor [11]

It is well known that almost all CHCs absorb IR at

approximately 3.4 μm wave length, and at this region H2O

and CO2 are not absorbed, making the system immune to

humidity and atmospheric changes. Beer's Law describes the

exact relationship between IR light intensity and gas

concentration [12]:

I = I0 ekP

(2)

Where:

I — the intensity of light striking the detector,

I0 — the measured intensity of an empty sample chamber,

k — a system dependent constant, and

P — the concentration of the gas to be measured.

Fig. 6: Spectral absorbance by gases [13]

Infrared detectors convert electromagnetic radiation

energy or temperature changes into electrical signals. Some

of the detectors (Fig. 7) types are:

(i) Pyroelectric detector: Pyroelectric materials are crystals,

such as lithium tantalite [14], which exhibit spontaneous

polarization, or a concentrated electric charge that is

temperature dependent. As infrared radiation strikes the

detector surface, the change in temperature causes a

current to flow. This current is proportional to the

intensity of radiation.

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This detector exhibits good sensitivity and good

response to a wide range of wavelengths, and does not

require cooling of the detector. It is the most commonly

used detector for gas monitors.

(a)

(b)

(c)

Fig. 7: Different detectors used for converting into

electrical signals: (a) Pyroelectric, (b) Luft and (c)

Photoacoustic detector [13]

(ii) Luft detector: A luft detector consists of two chambers,

either linked by a micro flow sensor or divided by a

diaphragm [15]. The chambers are sealed with a target

gas at a low pressure. IR transparent windows are fitted

to seal the chambers and the same intensity of pulsed

infrared radiation is received by both chambers when no

target gas is present. When a sample containing target

gas flows through the sample cell, reduced radiation

energy is received by the detector chamber. This causes

temperature and pressure to drop in the detector

chamber. The amount of temperature or pressure drop is

in direct proportion to the gas concentration. In the case

of linked chambers, the pressure difference between the

two chambers causes a detectable flow, which is

measured as a signal. In the case where a diaphragm

separates the two chambers, a movement of the

diaphragm causes a measurable change in capacitance.

(iii) Photoacoustic detector: This detector is similar to luft

detector except that the pressure change is measured by a

condenser microphone. The sample gas is passed

through a chamber at a preset time interval and the

chamber is sealed with a fixed volume of sample gas

trapped inside. A specific wavelength of infrared

radiation is pulsed into the chamber via an infrared

transparent window. The pulsating pressure change is

measured by the microphone as a frequency change

which produces the signal [16]-[17].

Fourier Transform Infrared (FTIR): FTIR measuring

principle is used for multi-gas detection (Figs. 8). The

principle of FTIR is that the gas to be analyzed is led through

a cuvette with an IR light source at one end (i.e. sending out

scattered IR light), and a modulator that “cuts” the infrared

light into different wavelength. At other end of the cuvette, a

detector is measuring the amount of IR light to pass through

the cuvette. By data processing, Fourier Transformation

mathematics is used to turn the measured absorption values

into gas concentration for the analyzed gases. As the light is

modulated into many different wavelengths, it is possible to

analyze many different gases in the same instrument, such as

CO, H2O, SO2, NO, HF, NH3 etc. [18].

Fig. 8: FTIR infrared spectrum [19]

Infrared provides high accuracy, resistance to

contamination and reliable measurements. Unlike catalytic

bead sensor, gas sample enters and leaves the cell unchanged.

Nothing is transformed, substituted or removed from it. As

the IR source ages, its energy level decreases. But there is

only one source. Therefore, the energy level reduction will

equally affect both sensor tubes (reference and detection) and

no imbalance is detected.

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There is no need of extreme temperature for detection,

resulting in less stress on construction materials. As there is

no combustion, corrosive combustion by-products are not

produced. All electronics and active components are sealed

away from the combustible gas environment. So, there is no

inhibitor, which helps in providing improved gas response

[20].

However, close coupling of electronics to IR sensor limits

operation at high temperature. Exceeding the operational

temperature limit can cause IR sensor drift or failure. Due to

component precision and assembly, IR sensors have higher

initial cost than catalytic detectors. IR sensors do not detect

all combustible gases (e.g. hydrogen). Humidity and water

may affect the sensor. Dust and dirt can coat the optics and

impair sensor response.

C. Electrochemical Sensors

Theory: Electrochemical sensors are fuel cell-like devices

consisting of an anode, cathode, and electrolyte. The

components of the cell are selected such that target gas is

allowed to diffuse into the cell, which causes chemical

reactions and generates current.

Working procedure: It (Fig. 9) consists of a sensing

electrode (or working electrode) and counter electrode

separated by a thin layer of electrolyte/catalysts. Gas that

comes in contact with the sensor first passes through a small

capillary-type opening and then diffuses through a

hydrophobic barrier, and eventually reaches the electrode

surface. This approach is adopted to allow the proper amount

of gas to react at the sensing electrode to produce sufficient

electrical signal while preventing the electrolyte leaking out

of the sensor. The gas that diffuses through the barrier reacts

at the surface of the sensing electrode involving either an

oxidation (CO, H2S, NO, SO2 etc.) or reduction (NO2 and

Cl2) mechanism. These reactions are catalyzed by electrode

materials specifically developed for the gas of interest [21].

Fig. 9: Electrochemical gas sensor [22]

The counter electrode balances the reaction of sensing

electrode – if the sensing electrode oxidizes the gas, then the

counter electrode must reduce some other molecule to

generate an equivalent current.

Reactions: The reaction kinematics is explained by the

following reactions:

For working electrode: CO + H2O → CO2 + 2H+ + 2e

- (3)

For counter electrode: O2 + 4H+

+ 4e- → 2H2O (4)

Overall cell reaction: 2CO + O2 → 2CO2 (5)

The main responsibility of the potentiostat circuit is to

ensure that adequate current is fed to the counter electrode

and the counter electrode can operate at its preferred

potential.

For a sensor requiring an external driving voltage, the

sensing electrode potential does not remain constant due to

the continuous electrochemical reaction taking place on the

surface of the electrode. It causes deterioration of

performance of the sensor over extended periods of time. To

improve the performance of the sensor, a reference electrode

is introduced. The reference electrode maintains the value of

this fixed voltage at the sensing electrode. No current flows to

or from the reference electrode. Sometimes a scrubber filter

is installed in front of the sensor to filter out unwanted gases.

[23].

Gas specific electrochemical sensors can be used to detect

the majority of common toxic gases, namely CO, H2S, Cl2,

SO2 etc. in a wide variety of safety applications. They can be

specific to a particular gas or vapour. They are typically very

accurate. They do not get poisoned easily.

As disadvantages, electrochemical gas sensors have a

narrow temperature range and a short shelf life; they are

subject to several interfering gases such as hydrogen. Sensor

lifetime will be shortened in very dry and hot areas.

D. Semiconductor Sensor

Theory: These sensors (also called solid state sensor) are

primarily used for toxic gas measurements and limited use in

CHC gas measurement. A semi conducting material is

applied to a non-conducting substrate between two electrodes

(Fig. 10).

Fig. 10: Schematic diagram of modern resistive

semiconductor gas sensor [24]

The substrate is heated to a temperature such that the gas

being monitored can cause a reversible change in the

conductivity of the semi-conducting material. The target gas

interacts with the surface of the metal oxide film (generally

through surface adsorbed oxygen ions), which results in

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change in charge carrier concentration of the material.

This leads to alter the conductivity of the material. An

n-type semiconductor is one where the majority charge

carriers are electrons, and upon interaction with a reducing

gas an increase in conductivity occurs. Conversely, an

oxidizing gas serves to deplete the sensing layer of charge

carrying electrons, resulting in a decrease in conductivity.

Opposite effects are observed with p-type semiconductor

material (Table IV).

Table IV: Response of semiconductor for different types

of gases [25]

Classification Oxidizing gases Reducing gases

n-type Resistance

increase

Resistance

decrease

p-type Resistance

decrease

Resistance

increase

Working procedure: The gas sensing film is located on a

micro machined hotplate which operates at 100-400 °C.

Under zero gas condition, it is assumed that O2 molecules tie

up free electrons in the metal oxide semiconductor material

by absorbing on its surface, thereby inhibiting electrical flow

[26]. The depleted layers are responsible for a high contact

resistance. For conduction, electrons must cross over the

surface barriers.

As reducing gases (CO, methane, H2S or H2 gases) are

introduced, they replace the O2, release free electrons and

decrease resistance between the electrodes (Fig. 11). This

change in resistance is measured electrically. It is

proportional to the concentration of gas being measured [28].

The sensitivity of metal-oxide gas sensors can be

substantially improved by dispersing a low concentration of

additives, such as Pd, Pt, Au etc.

Reactions: Atmospheric oxygen molecules are adsorbed

on the surface of semiconductor oxides in the form of O−, O

2-

or O2−. The reaction kinematics is explained by the following

reactions:

O2 (gas) + e− → O2 (ads)

− (6)

O2 (gas)− + e

− → 2O (ads)

− (7)

The presence of chemically adsorbed oxygen causes

electron depletion in the thin film surface and building up of

Schottky surface barrier. Consequently, the electrical

conductance of thin film decreases to a minimum. The SnO2

thin film interacts with oxygen by transferring the electron

from the conduction band to adsorb oxygen atoms. The

response to H2S can be explained as a reaction of gas with the

O2 (ads)−:

H2S + 3O (ads)− → H2O(g) + SO2 (g) + 3e

− (8)

With this reaction, many electrons are released to thin film

surface. This decreases the Schottky surface barrier and

makes depletion layer thinner; consequently, the electrical

conductance of the thin film increases. Metal oxide

semiconductor (MOS) sensors may be used for toxic as well

as combustible gas monitoring.

Tin oxide semiconductor is small, mechanically rugged,

sensitive to parts per million (ppm) levels, flood-proof and

less expensive. The major disadvantage of the sensor is that

the sensor is sensitive to humidity and temperature, but not

specific to gases and vapors.

Fig.11: Conductivity model of SnO2: (a) in air; (b) during

exposure to CO (Debye length lD<< grain diameter); and (c)

conduction path after strong sintering [27]

Tungsten oxide semiconductor is somewhat rugged,

sensitive to parts per million level, can be made gas specific

and have wide temperature range. The disadvantages of this

sensor is that its need linearizing output, some sensors have

tendency to “go to sleep” or spike, the response may slow on

older sensors and more expensive than other sensors [28].

E. Laser Sensor

1) Tunable diode laser absorption spectroscopy

Theory: Tunable diode laser absorption spectroscopy

(TDLAS) is a technique used for measuring concentration of

gases (methane, water vapour etc.) in a gaseous mixture

based on the characteristic of the distributed feedback (DFB)

diode laser with narrow line width and tenability. This makes

it possible to obtain the spectroscopy of single molecule

absorption line in the characteristic absorption spectrum

region [29].

The focus here is on a single absorption line in the

absorption spectrum of a particular species of interest [30].

TDLAS sensors commonly exploit the laser‟s fast tuning

capability to modulate the wavelength, causing it to sweep

back and forth across an absorption feature at a precise

modulation frequency. When narrow-band light passes a gas

cell, Beer's Law describes the exact relationship between IR

light intensity and gas concentration same as Infrared gas

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sensor.

Working procedure: Most of gaseous materials show

characteristic optical absorptions, especially in the

mid-infrared (2–25µm) band. The optical fingerprints of

different gases have made the spectral absorption a unique

method of gas analysis [31]. In TDLAS, a diode laser emits

light at a well-defined but tunable wavelength over the

characteristic absorption lines of a target gas in the path of the

laser beam. This causes a reduction of the measured signal

intensity, which can be detected by a photodiode, and then

used to determine the gas concentration (Fig. 12).

Fig. 12: TDLAS gas detector system [32]

Different diode laser are used based on application and the

range of tuning required. Typical examples of laser diode are

InGaAsP/ InP (tunable over 900 nm to 1.6 µm), InGaAsP/

InAsP (tunable over 1.6 µm to 2.2 µm) [33]. Varying the

quantum cascade laser (QCL) temperature and/or laser

injection current provides wavelength tuning of the emitted

laser radiation from DFB devices within a limited spectral

range (~10 cm-1). The external cavity configuration,

EC-QCL approach allows tuning over a range of >200 cm-1

[34]. Wavelength modulation spectroscopy (WMS) is based

on the modulation of the light emitted by a laser that is slowly

tuned through an absorption feature of the species to be

detected. The signal of second harmonic can be measured

with lock-in amplifier and it is proportional to the

concentration [35]. In different temperature conditions, for

the same concentration of gas, the amplitude of second

harmonic detected by lock in amplifier fluctuates with the

variation of ambient temperature. WMS measures faster

response and can provide parts per million (ppm) to parts per

billion (ppb) chemical detection limits, depending on the

spectroscopic properties of the target gas and the sampling

path length. WMS is thus a highly sensitive and gas-specific

form of spectroscopic gas analysis [32].

Advantages:

a. TDLAS has been widely employed in detecting

atmospheric trace gases due to its high sensitivity, high

selectivity, and fast time.

b. TDLAS method uses a compact single mode diode laser

tuning over an interested wavelength range swiftly to

fulfill the function of spectroscopy scan, may overcome

the drawback of FTIR, especially in field on-line

applications where fast response needed [29].

c. A further advantage of near infrared TDLAS is

compatible with optical fibers for optical

communication, which makes it easy to realize

multipoint remote sensing.

d. The laser emission line-width is narrower than gas

absorption line-widths. This may be contrasted with

near-IR (NIR) absorption techniques that sample with

broadband sources and measure absorption from

multiple lines across a fairly broad range of frequencies.

TDLAS thus offers the advantage of selectivity for a

target trace gas absent spectral interferences from other

background gases.

Disadvantages:

a. In different temperature conditions, for the same

concentration of gas, the amplitude of second harmonic

detected by lock in amplifier fluctuates with the variation

of ambient temperature. For the retrieval of the trace gas

concentration, the influence of temperature fluctuations

must be taken into account.

b. Any noise introduced by the light source or the optical

system will deteriorate the detectability of the technique.

2) Differential Absorption Light Detection and Ranging

Theory: Simple TDLAS measures the presence/

concentration of a particular gas is based on receiver

measuring wavelength not absorbed by the target gas

whereas differential absorption light detection and ranging is

based on returning backscattered signal intensity absorbed by

target gas as shown in Fig. 13. This system can detect and

measure the presence of CO2, CO, CH4 etc. LIDAR (Light

Detection And Ranging) operates in the ultraviolet, visible

and infrared portion of the spectrum [36].

Fig. 13: Differential absorption LIDAR concept [36]

Working procedure: The laser source emits a laser beam in

tune with molecular absorption line of a target gas and

receives a reflected signal affected by gas absorption of the

target gas in the atmosphere [37]. Near the optimum

wavelength for the particular gas to be measured, the amount

of absorption of the transmitted light varies strongly

according to the wavelength for each particular molecule, and

this creates unique molecular “signatures” for these gases.

Therefore, the method called “Differential Absorption

LIDAR” (DIAL) can be used to determine the concentration.

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The large dust particles also backscatter radiation, but their

scattering is much less wavelength dependent [38].

F. Fiber Optic Sensor

Theory: Two distinct approaches used in fiber optic

sensors are: Firstly, by directly probing the spectrochemical

changes, when interrogating wavelength coincides with the

absorption band of the analyte. Such direct spectroscopic

measurement is observed in near infrared TDLAS. Secondly,

by utilizing an analyte specific reagent transducer converts

analyte concentration into an optical signal. Majority of this

involve an intermediate sensor element, which undergoes

chemical changes initiated in the presence of the specific

analyte [39].

In second method, sensor works based on modified

cladding approach [40]. The cladding of the optical fiber is

removed and the gas sensitive material (the conducting

polymer film) is coated on a small section of the fiber

(sensing probe). The sensing probe length, source intensity

and source wavelength, indicates influence on the sensor

response.

Working procedure: Optical fiber sensor uses a light

modulation, i.e. one of the light parameter changes according

to the analytes presence. Organic conducting polymer such as

polypyrrole, polyaniline, polythiophene shows a reversible

change in their resistance when exposed to certain vapors

[41]. The change in conductivity changes permittivity, which

leads to change in the refractive index. The analytes reacts

with the coating to change the optical properties i.e. refractive

index, absorbance, and fluorescence which is coupled to core

to change the transmission properties through the optical

fiber. An extrinsic fiber-optic evanescent wave chemical

sensor is developed by replacing a certain portion of the

original cladding with chemically sensitive material [42].

The sensor structure in which small section of cladding is

replaced with chemically sensitive layer is shown in Fig. 14.

Fig. 14: Configuration of fiber optic sensor and geometric

optics ray through the sensing region [42]

Any change in the refractive index of the modified

cladding due to the analyte can change the condition of total

internal reflection between the modified cladding and air, and

results in an intensity change.

Advantages of fiber optic sensor are geometrical

flexibility, remote sensing capability, small size and

lightweight. It offers complete electrical isolation.

G. Thermal Conductivity

The principle of thermal conductivity is very similar to that

of the pellistor. Two platinum coils are arranged in a

Wheatstone bridge circuit. One coil is in contact with the gas

stream and other is sealed into a separate chamber, used for

temperature compensation (Fig. 15). All the gases have

different thermal conductivity‟s and so will conduct heat

away from the coil in the gas stream at different rates. The

change in temperature of the coil is directly proportional to

the change in thermal conductivity of the gas mixture flowing

past it [43].

The major drawback of the sensor is that it is less selective

and sensitive. This technique for detecting gas is suitable for

the measurement of high (%V/V) concentrations of binary

gas mixes

Fig.15: Thermal conductivity [44]

H. Ionization Detector

Photo ionization (Fig. 16) and flame ionization (Fig. 17)

are common detection techniques used for gas

chromatographic (GC) systems in laboratory environments.

Both have very good sensitivity and large linear dynamic

range.

Fig. 16: Photo ionization detector [47]

Both techniques measure the current generated by the

ionized species sensed by ion collector. The photoionization

technique ionizes molecules using a high energy photon

source, such as ultraviolet (UV) radiation [45], while flame

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ionization technique burns organic molecules in a hydrogen

flame [46]. The ions generations are detected similarly in

both PIDs and FIDs. When electric field gradient is applied

across the ionization region to drive the ions to the electrodes,

the ions release their charges to produce signals that can be

processed.

Fig. 17: Flame ionization detector [48]

The main differences between these two techniques are

their ionization source and mechanism. The photoionization

technique is a nondestructive method whereas the gas sample

is destroyed in flame ionization method.

I. MEMS Technology

Micro-electro-mechanical systems (MEMS) are

three-dimensional, electro-mechanical devices, which are

made by micromachining silicon wafers using standard

microelectronic fabrication and post-process techniques [49].

MEMS have been used to reduce the size, power

consumption and cost of gas sensors while improving overall

performance [50]. MEMS can be used in different innovative

ways by combining the existing proven technologies as

described herewith.

MEMS based array of sensor (work function based):

Micromachining processes have enabled designers to place

arrays of sensors on a single chip that can be mass-produced

at reduced cost. SnO2/Pt, WO3/Au and ZnO/Pd sensing films

were found sensitive to the target gases NH3, H2S, and CH4,

respectively. Sensing pastes are deposited on the outside of a

ceramic tube with a heater on the inside (Fig. 18). But these

sensors have some cross-sensitivity. Further, this technique

suffers from baseline drift, and high resistance and recovery

time. However, sensor array response to the gases is unique,

which allows selectivity by pattern recognition [51].

Fig. 18: CMOS based micro hotplate design of gas sensor

array [50]

The resistance of a metal oxide film varies depending on

the type of gas exposure. Combustible gases such as CH4,

H2S and NH3 act as reducing agents thereby adding electrons

to the metal oxide, which increases conductivity (decreases

resistance). Metal oxide sensing films have been doped with

noble metals to increase sensitivity and reduce response time.

Dopants such as Pt or Pd are catalysts that promote chemical

reactions by reducing the activation energy between the film

and test gas without being consumed by them. This allows the

reaction to occur at a faster rate and lower gas concentrations.

Cantilever based sensor (piezoelectric based):

Piezoelectric power generator made by MEMS technology

can scavenge power from low-level ambient vibration

sources. The developed MEMS power generators are

featured with fixed/narrow operation frequency and power

output in microwatt level [52]. These devices can be used as

sensing platforms. Molecular adsorption of target gas onto

Fig. 19: Cantilever based hydrogen gas sensor [54]

the sensing element, a cantilever, shifts its resonance

frequency and changes its surface forces (surface stress).

Adsorption onto the sensing element (composed of two

chemically different surfaces) produces a differential stress

between the two surfaces and induces bending. The analyte

that induces the mechanical response is chemisorbed onto the

cantilever in a reversible or irreversible process. Devices that

respond to chemical stimuli in this manner are referred as

micro-cantilever sensors [53].

Cantilever based hydrogen sensor uses piezoelectric

principle for gas sensing. This sensor uses stress-sensitive,

palladium coated micro-cantilevers to detect hydrogen (Fig.

19). Absorption of hydrogen into palladium is fully

reversible, at any given moment the cantilever capacitance

indicates the current hydrogen concentration [54].

J. Nanotechnology

Applications of nanotechnology in gas sensors are still in

the preliminary stage of development. Presently, metal oxide

1-dimensional (1D) nanostructure as gas sensors are most

promising area for nanotechnology. Some other 1D

nanostructure gas sensors operation is based on changes in

the photoluminescence spectroscopy and the mass of sensing

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19

element, which accurately uses a quartz crystal microbalance

[55].

1D nanostructure has advantages over conventional

metal-oxide-based sensors in terms of power consumption,

sensitivity, miniaturization and large surface area to volume

ratio [56]. The change in the electrical conductivity caused by

chemisorption of gas molecules on the surface of 1D

nanostructure metal oxides is measured by

electro-transducers. The main structures of the

electro-transducers are field effect transistors (FET), resistive

gas micro-sensors, and resistive gas sensors [57]. 1D

nanostructure FET is fabricated using a single nanowire

bridge between two metal electrodes on a heavily doped

silicon substrate covered with SiO2 acting as insulating layer

between the nanowire/electrode combinations and the

conducting silicon (Fig. 20).

Fig. 20: The schematic of the single nanowire FET [57]

Resistive gas micro-sensors are manufactured by MEMS

technology in which a film composed of nanowires is

contacted by pairs of metal electrodes on a substrate (Fig.

21). The measurement is performed by monitoring the

changes in current or resistance of the device. The other one

is resistive gas sensor, the channel length between the two

electrodes for resistive sensor is usually in the millimeter

scale within a ceramic tube.

Fig. 21: Resistive nano-sensors: (a) MEMS structures with

interdigitated electrode; and (b) nanowire gas sensors on

ceramic tube [57].

For photoluminescence measurement, nanowire samples

are mounted in a small vacuum chamber with ports to pass

gasses and provide optical access. Photo-desorption and

photo-adsorption of gases are found to affect the intensity,

indicating the participation of free carriers. The time

dependence of the changes in the spectral peak intensity is

compared for measurement of gas concentration.

Quartz crystal microbalance (QCM) is an extremely

sensitive measurement device. The principle of measurement

is based on mass change as the gas adsorbs on the surface of

the sensing material. It is quantitatively measured based on

change in frequency rate (Sf) given by:

Sf= |∆f/∆t| = | (af2∆m/A)/ ∆t | (9)

Where,

∆ƒ — response frequency shift within the time interval of

∆t,

a — constant,

ƒ— fundamental frequency of the unloaded piezoelectric

crystal,

∆m — mass loading on the surface of the crystal, and

A — surface area of the electrode.

Ceramics such as zinc oxide (ZnO), tin oxide (SnO2),

indium oxide (In2O3), and titanium dioxide or titania (TiO2)

are used as nanowires/nanobelts for gas sensing. Carbon

nanotubes (CNTs) have many distinct properties that may be

exploited to develop next generation sensors. Single walled

nanotubes (SWNTs) have been highlighted as gas-sensing

elements due to the 1D structure with all the atoms residing

only on the surface.

CNTs can be designed to have specific properties by

changing the ratio of diameters of linearly joined CNTs,

which in turn can be used in fabrication of FETs. The

untreated SWNT-FET typically shows a p-type behavior,

with threshold voltages being shifted upon gas exposure [58].

The inlet and outlet valves allow gases to flow over the

carbon nanotubes of FET. Gas flow increases conductance of

the carbon nanotubes. This can be obtained using

current-voltage relationship before and after exposure to the

gases. The effect occurs because when the gas molecules

bond to the carbon nanotubes, charge is transferred from the

nanotubes to the gas molecules, which increases

hole-concentration in the carbon nanotubes and enhances the

conductance (Fig. 22). The carbon nanotubes are covered

with an insulating protection film [59]. CNTs show almost

threefold sensitivity and efficiency compare to other metal

oxide gas sensors available in the market, such as tin oxide

sensor [60].

Fig. 22: Carbon nanotubes based gas sensor using MEMS

platform [61]

IV. CHARACTERISTICS OF GAS SENSORS

Performance of sensors is characterized by the following

parameters [62]:

Sensitivity is a change of measured signal per analyte

concentration unit, i.e. the slope of a calibration graph.

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20

This parameter is sometimes confused with the detection

limit.

Selectivity refers to characteristic that determines

whether a sensor can respond selectively to a group of

analytes or even specifically to a single analyte.

Stability is the ability of a sensor to provide reproducible

results for a certain period of time. This includes

retaining the sensitivity, selectivity, response and

recovery time.

Detection limit is the lowest concentration of the analyte

that can be detected by the sensor under given

conditions, particularly at a given temperature.

Dynamic range is the analyte concentration range

between the detection limit and the highest limiting

concentration.

Linearity is the relative deviation of an experimentally

determined calibration graph from an ideal straight line.

Resolution is the lowest concentration difference that

can be distinguished by sensor.

Response time is the time required for sensor to respond

to a step concentration change from zero to a certain

concentration value.

Recovery time is the time it takes for the sensor signal to

return to its initial value after a step concentration

change from a certain value to zero.

Working temperature is usually the temperature that

corresponds to maximum sensitivity.

Hysteresis is the maximum difference in output when the

value is approached with an increasing and a decreasing

analyte concentration range.

Life cycle is the period of time over which the sensor

will continuously operate.

V. COMPARISON OF VARIOUS SENSORS

An ideal sensor would possess: (i) high sensitivity,

dynamic range, selectivity and stability; (ii) low detection

limit; (iii) good linearity; (iv) small hysteresis and response

time; and (v) long life cycle. However, no single gas sensor is

perfect in all the above parameters. Combination of

appropriate detectors and sampling techniques help in

reliable measurement of the target gas based on ambient

conditions of the measuring area. Selection of right sensor is

important for mitigating the impending hazard.

One sensor can be evaluated and compared against other

sensors. Selection of sensors depends on various parameters,

namely physical properties of target gases, ambient

conditions, required sensitivity, maintenance cycle, method

of operations etc. Given the large number of variables, it is

tempting to either oversimplify the selection process to a few

rules. Three qualities are essentially required in a sensor for

gas monitoring, i.e. simple, reliable and easy to maintain.

Simple means not much complicated, adapted to user‟s

requirement and backed by strong technology. Reliable

indicates always “alarm” when required and “never” when

should not. Easy to maintain specifies calibration is not

required often, lifespan is long and troubleshooting is simple.

Advantages and disadvantages of different types of sensor

are summarized in Table V.

Table V: Comparison of various gas detection techniques [63] - [65] Sensor Advantages Disadvantages

Catalytic Simple, robust and inexpensive. Sensor poisoning, sensor inhibitors and sensor cracking

Infrared Immune to poisons and contamination, fail-safe

operation, no routine calibration is required, and

ability to operate in continuous presence of gas.

Flammability detection is only in %LEL range, high to

medium power consumption, and the gas must be infrared

active.

Electrochemical Reliable, fast response, low power, measures toxic

gases in relatively low concentration, and wide

range of gases can be detected.

Limited to operate at low ambient temperature and narrow

pressure range, unsuitable for use in dry atmosphere and not

fail-safe.

Semiconductor

(solid state)

Robust, long operating life, wide operating

temperature range, detect wide range of gases.

Commonly poor selectivity causing „false‟ alarm, high

power consumption and response drift problem.

Fiber optic (clad

modified)

High sensitivity, short response time and operate at

room-temperature.

Not stable for long time, irreversibility, and low selectivity.

Laser No interference from other gases, high speed, high

selectivity, low maintenance and operating cost,

and self-calibration.

Only one gas can be measured with each instrument; heavy

dust, steam or fog blocks laser beam.

Flame ionization

detector

Best for CHC compounds. Not used in explosive areas, destroy the sample, responds

poorly to halogenated hydrocarbons, and non-specific

response.

Photo ionization

detector

Good method for organic compound detection at

low level.

Non-specific response, and responds only to ionizable

gases.

Thermal

Conductivity

General applicability, large linear range, simplicity

and non-destructive.

Low sensitivity

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21

Nanotechnology Ultra-sensitive, large surface area to volume ratio,

fast response time and light weight.

Fabrication and repeatability is difficult, and expensive.

VI. CONCLUSIONS

Sensors help in maintaining safe and healthy environment

in the underground mines by continuous monitoring of

ambient air and taking appropriate control measures. Several

promising new sensors have been developed, but no single

sensor is reliable enough to detect every gas accurately.

Catalytic bead sensor is simple but always not reliable and

often calibration is required. However, IR sensor is reliable

and easy to maintain, but it is expensive.

When toxic gas sensors are compared, there are apparent

design weaknesses in the semiconductor sensors as compared

to the electrochemical sensors. The biggest drawback is

response time. Traditional electrochemical sensors too have

disadvantages, but new developments in electrochemical

technology have removed many of those shortcomings.

Among many developments, MEMS is an emerging

technology which uses the tools and techniques that were

developed for the integrated circuit industry to build

microscopic sensors. These sensors are built on standard

silicon wafers. However, one of the disadvantages of the

traditional semiconducting metal oxide gas sensors is low gas

sensitivity due to the limited surface-to-volume ratio. In

addition, many of the ceramic and thin film gas sensors must

be operated at temperatures exceeding 500 ˚C in order to

improve the sensitivity.

Nano-materials offer new possibility for improving solid

state gas detection. Nanostructure sensing materials such as

nanowires, nanotubes and quantum dots offer an inherently

high surface area. In fact, carbon nanotubes have one of the

best available surface-to-volume ratios. The increased

surface area leads to high sensitivity, fast response and often

allows lower operating temperatures. In addition, different

types of nano-materials have different sensitivity. By

applying different nano-materials on the same MEMS

platform more gas mixtures can be detected.

The future of gas sensor is determined to a great extent by

new detection principles, new or enhanced sensors,

sophisticated evaluation algorithms and latest

communication technologies. Equally, new demands arising

from unconventional applications have stimulated new

solutions, combining proven technologies in novel and

innovative ways.

ACKNOWLEDGMENTS

Authors are grateful to Dr. Amalendu Sinha, Director,

CSIR-Central Institute of Mining and Fuel Research,

Dhanbad for giving permission to publish the paper.

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AUTHORS PROFILE

Mr. Anish Kumar, B.Tech. (ECE)

Junior Research Fellow, CSIR-CIMFR, Dhanbad

Research Area: Sensors and Wireless Sensor Network.

[email protected]

Mr. T M Giri Kingson, B.Tech. (AEI)

Project Assistant, CSIR-CIMFR, Dhanbad

Research Area: Sensors, Underground Coal Gasification.

[email protected]

Mr. Rajkumar Prasad Verma, B.Tech. (CSE)

Project Assistant, CSIR-CIMFR, Dhanbad

Research Area: Wireless Sensor Network and Cloud

Computing. [email protected]

Mr. Abhishek Kumar, B.Tech. (EIE) Project Assistant,

CSIR-CIMFR, Dhanbad

Research Area: Underground Coal Gasification, Gas Sensors.

[email protected]

Mr. Subhadip Dutta, B.Tech. (CSE)

Junior Research Fellow,

CSIR-CIMFR, Dhanbad Research Area: Algorithm, Wireless Sensor Network.

[email protected]

Mr. Ranjeet Mandal, B.Tech. (EIE)

Project Fellow,

CSIR-CIMFR, Dhanbad Research Area: Sensors, Underground Coal gasification,

Automation.

[email protected]

Dr. Swades kumar Chaulya, B. Tech., M. Tech., Ph.D. (IIT, BHU)

http://en.wikipedia.org/wiki/Tunable_diode_laser_absorption_spectroscopy

http://en.wikipedia.org/wiki/Tunable_diode_laser_absorption_spectroscopy

http://www.met.tamu.edu/class/metr451/Ch11folder/sld029.htm

http://www.delphian.com/sensor-tech.htm

mailto:[email protected]

ISSN 2249-6343



23

Scientist

CSIR-CIMFR, Dhanbad Research Area: Wireless Communication, Control

Monitoring Automation in Underground Mines.

[email protected]

Dr. Girendra Mohan Prasad, M.Sc., Ph.D. (ISM,

Dhanbad) Sr. Principal Scientist

CSIR-CIMFR, Dhanbad

Research Area: Mine Instrumentation and Communication.

[email protected]



issn 2249-6343 volume 3, issue 3, june 2013 … chemical media, the gas will start to burn or ignite...

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