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Home , Spectroscopy

Detection in the UV Region Using an Sensor

Hadi Manap, Gerald Dooly, Sinead O'Keeffe, Elfed Lewis Department of Electronic & Computer Engineering University of Limerick Ireland e-mail: [email protected]

Abstract—This paper describes an optical fiber sensor for the 1.1 Current ammonia sensors and their working principle monitoring of ammonia gas. An open path optical technique is used to analyze the absorption lines of ammonia within the There are many types of NH3 sensors that have been ultra violet region. Experimental results describing the previously reported. They can be categorized into five main operation of the sensor are presented and are compared with types and each of them has their own advantages and theory. A comparison between a commercial sensor has been disadvantages. carried out and cross sensitivity testing with regard to and gas is reported. 1.1.1. Metal oxide

The first type of NH3 gas sensor is metal oxides 1. INTRODUCTION semiconductor. Usually this type of sensor is based from tin dioxide (SnO ) [3]. This gas sensor operates on the principle Increased ammonia (NH ) emissions are one of the 2 3 of conductance change due to chemisorption of gas concerns of the European Union, especially NH that is 3 to the sensing layer. Much research has been done naturally produced by livestock farming. According to the on this type of sensor demonstrating promising results using Official Journal of the European Communities, a directive a sensor that is rugged and inexpensive [4-8]. However this setting national emission ceilings (NECs) has been set in type of sensor can have some drawbacks, the primary one 2001 to combat NH emission. The aim of the directive is to 3 being that it has a low level of selectivity. There are different gradually improve, through a stepwise reduction of the approaches to cater to this selectivity problem such as neural pollutants, the protection both of human health and the network method [9-11], mathematical modelling [12] or by environment throughout the EU countries. By 2010, EU using metals or additives that enhance the chemisorption of member states must have limited annual national emissions so specific gasses [13,14]. that they do not exceed the emission ceilings [1]. They must also ensure that these emission levels do not exceed the 1.1.2. Catalytic ammonia sensor emission ceilings in any year after 2010. According to European Environmental Agency, EEA report 2003, 93% of The working principle of catalytic NH3 sensor is based on the charge particle in the catalytic metal. When the EU15 NH3 emission in 2001 was caused by agriculture sector as shown in Fig. 1. Agriculture sector is the main contributor of the test gas introduced to the sensing layer changes, it affects the concentration of the charge carrier in to NH3 emission since 1990. Hence, a new NH3 sensor that suits the agriculture environment need to be developed as an the catalytic metal. This change in charge carriers can be alternative to current ammonia sensor. quantified using a field effect device, like a capacitor or a transistor [15,16]. The main disadvantage of this sensor system is that they are not immune to electrical interference since the reading is based on the charge carrier, which is sensitive to electrical field.

1.1.3. Conducting polymer gas sensors

This is another type of NH3 gas sensor, usually based on polyaniline [17,18] as the conducting polymer. In other research [19,20], polypyrrole is used as polymer film which causes a change in the conductivity of the , making it a suitable material for measurement in resistance [21] or electrical current for NH3 detection [19]. This kind of sensor has a good detection level down to 1 ppm as reported in this

paper [22]. However it suffers from irreversible reaction Figure 1. Sector split of NH emissions in 2001 (%), EEA report 2003. 3 978-1-4244-5335-1/09/$26.00 ©2009 IEEE 140 IEEE SENSORS 2009 Conference which makes this sensor less sensitive over time when gas, σ (cm2/) is the absorption cross section and N 3 exposed to NH3 [20]. (Molecules/cm ) is the gas concentration. 1.1.4. Optical gas sensing 2.1 Unit Conversion

NH3 optical gas sensing can be divided into two types. The concentration of test gas, N in the Beer Lambert Law 3 The first is based on optical absorption when NH3 is exposed in equation (1) is given in unit Molecules/cm , so we need to to the source such as . It has very low detection change the unit to ppm as the pollution gas concentration is limit, 1 ppb and a response time of 1 s has been reported normally read in this unit. [23]. It also shows a good selectivity from other interfering The ideal gas law (PV = nRT ) is also used in this unit gases such as CO2 and vapor [24]. However this type of sensor can cost thousands of dollars and is not suitable to conversion, where P = pressure (atm), V = volume (L), n = detect NH at small volumes. Another type of optical gas of substance (moles), R = ideal gas constant (0.082 atm 3 L mol-1K-1), T = absolute (ºC + 273) in degrees sensing is based on a change in colour when NH3 reacts with a reagent. The best example is using a pH paper. The Kelvin. of the detected gas will change based on the atm ⋅ L coloration reaction. There are different coloration reactions (0.082 ) (273 K) V RT mol ⋅ K in use for dissolved NH3 such as the Nessler reaction [25]. = = (2) This NH3 detection method is commercially available and n P 1 atm can be used for determining the total NH3 concentration in L water. However this reagent is toxic and therefore is not = 22.4 suitable for frequent use. mol Standard temperature and pressure are defined as 1 atm 1.1.5. Indirect gas analyzers and 0 ºC (273 K). Under these conditions, a mole of an ideal Most of the available gas sensors suffer from poor gas occupies a volume of 22.4 L. selectivity towards NH3 [2]. Hence, the gas sensor system should comprise a selection mechanism that allows only the At times, gaseous are expressed using mixed units of mass per unit volume such as mg/m3. The gas of interest to be introduced to the detector of the system. 3 A few techniques such as gas diffusion separation with gas relationship between ppm and mg/m depends on the density permeable membranes have been done [26–28]. This sensor of the gas which depends on its pressure, temperature and molecular weight as shown in equation (3). is sensitive enough to detect NH3 below 1 ppm [28]. Since this sensor is using membranes for gas separation, it will mg ppm × ω 273 P(atm) suffer from a low lifespan. ρ( ) = × × (3) 3 L T (K) 1 atm m 22.4 All of NH3 sensors discussed above have their own mol drawbacks. Hence a new NH3 sensor needs to be developed as an alternative for the current detection method. where ρ(mg/m3) is the density of the gas, ω is the 2. THEORY molecular weight, P(atm) is the measured pressure and T(K) Different gas species absorb light at different is the measured temperature of the test gas. characteristic and for NH3 gas, it has its own Equation (3) is commonly used in many text books [44- specific gas absorption spectrum. A comprehensive 47] and can be simplified as shown in (4) with assumption collection of absorption cross sections for gaseous molecules the test gas temperature is 298 K and at 1 atm pressure. can be accessed from the Institute, MPI Mainz database including NH gas [29]. The data vary from source mg ppm × ω 3 ρ( ) = (4) to source and they depend on temperature and m3 L range. Only one that suits the experimental condition 24.4 parameter is selected and compared with the measured mol results. Before we can use (4) and read the concentration in ppm For NH absorption comparison with the theory, the unit, we need to find the relation between the density of the 3 gas, ρ(mg/m3) and the concentration of test gas, Beer-Lambert Law has been utilised. The Beer-Lambert law 3 described the linear relationship between and N(Molecules/cm ). concentration of an absorbing species and its general form is Since N(Molecules/cm3) * (1000 mg/1g) * (1 cm3/10-6m3) shown in (1). * (1 mol/6.022x1023 Molecules) * (ω g/mol) = ρ(mg/m3) 3 I (−σ .N.l) Hence the relation between N(Molecules/cm ) and = e (1) ρ(mg/m3) is given by, I o ρ -9 Where I is the transmitted intensity, Io is the incident N = N A x 10 (5) intensity, l(cm) is the distance that light travels through the ω

141 where N A is Avogadro’s constant. In order to calculate the gas concentration, N in parts per million, ppm, (4) and (5) are plugged in the Beer Lambert Law equation (1) above; Hence I − [ln ][24.4] = I o ppm − (6) σ × N × l ×10 9 A

Figure 2. spectrum for ammonia and water vapour [43]. Using (6), we can accurately calculate the gas concentration with σ known. This σ value varies depend on the wavelength measured. This method of gas concentration Based on UV spectrum shown in Fig. 3, the water vapour calculation was similarly described in [30-32]. absorption lines are relatively too small compared to NH3. Theoretical gas concentrations can give a strong Hence it will not affect the reading for NH3 absorption. Since comparison to measured concentrations from the sensing our measurement region is from 200-230 nm, we can just system and are usually referred to in the development stages ignore the presence of in the wavelength range of new sensing technologies. The theoretical gas measured. In fact, the highest wavelength for water vapor concentrations can be calculated if the flow rate of the gas is spectrum found in the MPI Mainz database is 199 nm whilst known during the testing stages. Since mass flow controllers the NH3 spectrum exceed 230 nm. The data for water 1 and are used to mix the gases, so the concentration of test gases water 2 in Fig. 3 are taken from [40] and [41] respectively. can be calculated precisely. Each gas has its own flow rate Absorption coefficients for water vapor from 200-230 nm and mix with different conversion factor. The gas conversion can be assumed to be close to zero as expressed in Fig. 3. factor for all test gases is provided by the mass flow Although research [38] has been done to determine the controller supplier, Bronkhurst High-Tech and can be spectrum for water molecule in the region 190-250 nm, it is assessed on their webpage. These conversion factors vary only for high temperature from 1000 to 3700 K. H. Okabe et depending on the gas condition such as pressure, temperature al has reported [37], that, water vapor absorption cross and flow rate of the gas. section becomes appreciable only in the UV region below than 190 nm. It is also reported in the UVACS database, the 2.1. Cross sensitivity water vapor spectrum only exists in the range 120-189 nm The air consists of different gases which contribute to the and 260-330 nm [39]. This will justify our interpolated earth’s atmosphere. In general the air contains 78% , spectra in Fig.3 that water doesn’t absorb the UV light 21% oxygen, 0.038% carbon dioxide and 0.93% . It (relative to NH3) within 200-230 nm region. also consists of small amount of other gases such as , nitrogen oxides, sulphur oxides and water vapour. Other atmosphere gases, carbon dioxide (CO2) and In gas detection, cross sensitivity is one of the constraints oxygen (O2) must also be considered within the respective since the air consists of variety of gases. A few techniques UV region. Tests were conducted for both gases to prove that have been introduced such as using gas separation in practice there were no discernible cross sensitivity effects techniques [26-28] or a ratio calculation [32] to overcome on ammonia measurements. the problem.

For NH3 gas sensing, it has been reported [33-36] that the cross sensitivity with humidity is the main problem and occurs in certain wavelength ranges. Cross sensitivity with humidity happens across many types of NH3 gas sensors including optical gas sensors. The use of pH paper in optical gas sensing for coloration detection method can have a cross sensitivity problem in the presence of water vapour [36]. It produces noticeable distortions of the short-wavelength wing of the band profile and also at its peak. The cross sensitivity also occur in light absorption technique if infrared is used as the light source. Based on the spectrum shown in Fig. 2, NH3 and water vapour absorb light at the same wavelength. Therefore UV light is used in this setup to avoid cross sensitivity. Figure 3. Water vapor absorptions lines relative to ammonia gas.

142 However, it is not possible to test the cross sensitivity for The maximum flow rate of each MFC is 1 l/min. Each of every atmosphere gas as there are too many of them and the gas has their own flow rate and the gas mixing outside CO2 and O2, the concentrations of those gases are calculations are based on the flow constant provided by small, thus their impact can be ignored. However, in the Bronkhorst. laboratory, a reference measurement is performed prior to every ammonia test cycle in order to establish the correct 4. RESULTS AND ANALYSIS calibration and thus yield the correct values of ammonia Initially, nitrogen gas was released into the test gas . concentration in the subsequent tests. The integration time in the SpectraSuite software was set to 3000 ms. The data recorded was taken as incident intensity, 3. EXPERIMENTAL SETUP Io. Following this, 100 ppm of NH3 gas was released into the The experimental arrangement is shown in Fig. 4. A test gas cell. From the SpectraSuite software, we can see - lamp (DH-2000 from Ocean ) clearly a small drop on the spectrum intensity. This data was was used as a light source. The light was transmitted through then recorded as transmitted intensity, I. a UVNS fibre (Ultra Violet Non Solarising) from CeramOptec Inc. with 644.4 nm core size. Two collimating The was repeated but the integration time was changed to 4500 ms in order to check whether the lens were placed at both ends of the gas cell and were used to focus the incident and transmitted light. The transmitted light integration time has significant effect on the absorption then travelled through another optical fibre at the other end process. The recorded data sets were plugged into (6) in of the test gas cell to the light detector. The light detector order to get the absorption cross section of the NH3 gas. The that was used in this experiment was an Ocean Optics absorptions spectrum was then plotted versus wavelength in HR2000 . The spectrometer has a range from the region of 200-230 nm and compared with the theory as 200 to 650 nm and it provides resolution to 0.65 nm shown in Fig. 5. (FWHM). Although the measured NH3 has pattern similar to the The spectrometer interfaced with computer using theoretical spectrum, clearly some errors exist especially at SpectraSuite software. SpectraSuite is a specifically designed the lower wavelength range of the x-axis. This is due to high program provided by Ocean Optics in order to acquire the caused by optical fibre at the lower wavelength data from the spectrometer in real time. The transmitted range. intensity, I and the incident intensity, Io were recorded and In order to evaluate cross sensitivity with CO2, the (6) was used to get absorption cross section. These values experiment was repeated for 100 ppm NH3 gas mixed with were plotted against the wavelength and compared with the one percent of CO2. One percent was chosen to create the theory. similar atmosphere environment as this was a much higher Two mass flow controllers, (MFC) model F-201CV by value than the atmospheric concentration (380 ppm) and thus Bronkhorst High-Tech were used to control the flowrate of represents a much more stringent test. The absorption cross the test gas. The MFCs were powered by 15 V power supply section for the mixing gas was calculated and the resulting and were operated at the gas pressure above 1 bar. It can be mixed gas absorption spectrum is shown in Fig. 6. It shows controlled by software provided by Bronkhorst called that the present of CO2 has no effect on NH3 measurement. FlowDDE V4.58.

Figure 5. Ammonia absorption cross section spectrum. Figure 4. Experimental setup.

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Figure 8. Comparison of concentration measurement with Tetra.

Figure 6. Ammonia spectrum with the presence of 1% of CO2. The result shows that optical fiber sensor has very close measurement to Tetra’s reading as shown in Fig. 8 and it The above method was repeated to check the cross also has better response time which is less than 4 s. sensitivity with 21 percent of O2. The absorption spectrum was monitored and the result is shown in Fig. 7. It also 5. CONCLUSION AND FUTURE WORK shows that the shape for NH3 mixed with 21 percent of O2 A novel optical fibre sensor for NH3 gas has been spectrum is almost the same as NH3 spectrum. This indicates described and reported. The cross sensitivity with CO2 and that O2 also has no significant effect on NH3 measurements in the region of 200-230 nm. O2 gas has also been tested and it clearly shows that these two gases have no impact on the NH3 measurement at the Tests to measure NH3 at various concentrations were region of 200-230 nm. Thus, it will be a good point to conducted previously and were published [42]. However, measure NH3 in UV range. Initial tests using a commercial only calculated concentration from flow-rate was used to sensor as a second measurement was also reported and it justify the reading. In this report, a measurement comparison shows very close result. Future work will focus on how to with commercial NH3 sensor will be done to validate the conduct a full set of experimental tests in-situ in sensing system. order to fully characterise the sensing system. Initial test was carried out with ammonia sensor called Tetra from Crowcon Detection Instruments Ltd. The ACKNOWLEDGMENT comparison was done for NH3 concentration less than 20 The author would like to thank the staff of Electronic and ppm to get an optimum result. This is because Tetra sensor Computer Engineering Department of the University of works best at low flow-rate. In order to maintain low flow- Limerick, for their assistance and input during the research. rate through Tetra sensor, MFCs was set to certain limit ,thus The authors would like to acknowledge resources made resulting in low concentration of NH3. available through the Higher Education Authority, PRTLI cycle 4 for Environment and Climate Change. Also the authors would like to acknowledge the support of the University of Malaysia, Pahang (UMP) and the Ministry of Higher Education, Malaysia in providing scholarship for the research studies.

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