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Laser Calorimetry Spectroscopy for Ppm-Level Dissolved Gas Detection and Analysis Received: 01 September 2016 Nagapriya K

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Laser Calorimetry Spectroscopy for Ppm-Level Dissolved Gas Detection and Analysis Received: 01 September 2016 Nagapriya K www.nature.com/scientificreports OPEN Laser Calorimetry Spectroscopy for ppm-level Dissolved Gas Detection and Analysis Received: 01 September 2016 Nagapriya K. S.1,*, Shashank Sinha1,†, Prashanth R.1,‡, Samhitha Poonacha1, Accepted: 10 January 2017 Gunaranjan Chaudhry1, Anandaroop Bhattacharya1,$, Niloy Choudhury1,#, Published: 20 February 2017 Saroj Mahalik1,@ & Sandip Maity1,* In this paper we report a newly developed technique – laser calorimetry spectroscopy (LCS), which is a combination of laser absorption spectroscopy and calorimetry - for the detection of gases dissolved in liquids. The technique involves determination of concentration of a dissolved gas by irradiating the liquid with light of a wavelength where the gas absorbs, and measuring the temperature change caused by the absorbance. Conventionally, detection of dissolved gases with sufficient sensitivity and specificity was done by first extracting the gases from the liquid and then analyzing the gases using techniques such as gas chromatography. Using LCS, we have been able to detect ppm levels of dissolved gases without extracting them from the liquid. In this paper, we show the detection of dissolved acetylene in transformer oil in the mid infrared (MIR) wavelength (3021 nm) region. Analysis of dissolved gases in liquids is of prime importance in a wide array of fields like transformer health monitoring1,2, food and beverage industries (eg measuring amount of ethanol in wine), oil and gas industry (measuring dissolved methane in crude etc.), etc. In many of these fields chemical and electrical analysis cannot be used. Electrical analysis is ruled out due to harsh environmental conditions and due to safety requirements, while in many cases solid-state gas sensing and chemical methods (like electrochemical methods) do not have enough specificity3,4. In most cases, gas chromatography (GC) - both offline and exsitu - has been used as the best possible solution. Some form of optical analysis is preferred due to its high specificity and its non-invasive nature5,6. However, optical spectroscopies suffer from need for line of sight, and if insitu, the liquid should be transparent or substantially transparent to the radiation (wavelengths) used. For liquids like water or oil, finding a wavelength where the gas absorbs strongly while the liquids do not absorb is very difficult. Usually the absorbance of the liquid is much stronger than the absorbance of the gas (at the concentrations that need to be detected). Therefore conventionally, the gases are extracted from the liquids and then analyzed, either using an optical method or using GC. Transformer dissolved gas analysis (DGA) is the measurement and study of composition and concentration of dissolved gases in insulating transformer oil to determine the type and severity of electrical (or thermal) fault occurrence in a transformer. Case studies have shown that DGA and timely intervention can prevent substantial transformer damage. Usually, DGA involves sampling the oil, extracting the gas and analyzing the composition and concentration of the extracted gas. The gas extraction is usually done using extraction techniques such as vac- uum extraction or head space extraction. The analysis is typically done using GC7,8. The key gases measured are acetylene, methane, ethane, ethylene, carbon monoxide, carbon dioxide, hydrogen and moisture. Extraction of gas leads to certain uncertainties and ambiguities in gas concentration determination9–12. An in-oil measurement without gas extraction will help get rid of these uncertainties. Here a method of detecting and analyzing dissolved gases in liquids without extracting them is presented. The method, though optical (and therefore is non-invasive and has a high specificity), does not require a line-of-sight 1GE Global Research, GE India Technology Centre Pvt. Ltd., Bangalore, 560066, India. †Present address: RGBSi, Bangalore, 560027, India. ‡Present address: Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore, 560012, India. $Present address: Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, 721302, India. #Present address: GE Global Research, Niskayuna, New York 12309, United States. @Present address: European Space Research and Technology Centre, 2201 AZ Noordwijk ZH, Netherlands. *These authors contributed equally to this work. Correspondence and requests for materials should be addressed to N.K.S. (email: [email protected]) SCIENTIFIC REPORTS | 7:42917 | DOI: 10.1038/srep42917 1 www.nature.com/scientificreports/ Figure 1. Schematic of the principle of (a) LCS (b) Differential LCS - used to eliminate the effect of room temperature (c) Dual-beam differential LCS that gives the ability to quantify gas concentrations even when the liquid absorbance is significant compared to that of the gas. In differential LCS, the laser beam is incident only on the sample cell, while in dual-beam differential LCS, laser beams are incident on both the sample and reference cells. and can detect dissolved gases in liquids even when the liquid is substantially non-transparent to the incident radiation. Theory of LCS Principle. Laser calorimetry spectroscopy is a combination of laser-based optical absorption spectroscopy and calorimetry. Laser-based absorption spectroscopies function on the principle that gases preferentially absorb wavelengths that correspond to transitions between electronic, vibrational, or ro-vibrational levels13,14. Each gas, therefore, has an absorption fingerprint. When light of a wavelength corresponding to a transition between quantum states of a gas is incident on the gas, the radiation is absorbed. The amount of radiation that is absorbed depends on the line strength of the transition, the concentration of the gas and the length of the path the light traverses in the gas (according to the well-known Beer-Lambert law). −β gcL PPabsorbed =−0(1 e ) (1) where, P0 =​ Incident laser power Pabsorbed =​ Power absorbed βg =​ absorption coefficient of dissolved gas c =​ concentration of dissolved gas in oil L =​ interaction path length. Most hydrocarbons, CO and CO2, have their fundamental ro-vibrational modes in the MIR region and there- fore the detection sensitivity is maximum in the MIR region. The availability of quantum cascade lasers15 and MIR detectors has thus enabled very sensitive (sub ppm) detection of these gases. In LCS, the rise in temperature due to absorption of radiation is measured. When a sample cell containing the transformer oil with dissolved gases is irradiated with light of a wavelength where a dissolved gas absorbs, the gas absorbs the laser energy and causes heating of the oil (see Fig. 1(a)). If the sample cell of heat capacity C is SCIENTIFIC REPORTS | 7:42917 | DOI: 10.1038/srep42917 2 www.nature.com/scientificreports/ connected thermally to the outside world (or a thermal base) by a link of thermal resistance Rth, then the temper- ature change of the sample due to the gas absorbing the radiation is given by −tR/ thC ∆=Tt() Peabsorbed(1 − )Rth (2) At equilibrium, ∆=TPabsorbedRth (3) or, −β gcL ∆=TP0(1 − eR) th (4) For βgcL ≪ 1, ∆=TP0()β gtcLR h (5) Therefore, the temperature change of the oil is directly proportionate to the concentration of the dissolved gas. Eliminating effect of room temperature (Differential LCS). The temperature that is measured in the experiment is ΔT + T0 (where T0 is room temperature or the base temperature). The foremost challenge in meas- urement of gas concentration using LCS is that the temperature change ΔT for ppm levels of gas is very small. For −5 example, for acetylene, the highest βgcL ≈ 8 × 10 for 20 ppm concentration (c) and 3 mm path length (L). (This is about the maximum path length that can be used. The reason for this is explained in the experimental design section). If P0 =​ 1 mW and Rth ≈​ 5000 K/W, Δ T ≈​ 0.4 mK. Detection of 0.4 mK when room (base) temperature T0 is ≈300 K is experimentally challenging and can lead to large errors in the measurement of ΔT , thereby leading to erroneous gas concentration estimates. Accurate measurement of Δ T (even when the base temperature is 300 K) can be achieved by using a Wheatstone bridge-based differential measurement so that the common mode signal due to room temperature is cancelled out. Figure 1(b) shows a schematic of the differential arrangement used. Here, Rvar is a variable resistor used to balance the bridge such that the output of the bridge is zero when the laser is off. When the laser beam is turned on, the output of the bridge Δ Tmeasured is the difference between the temperature of the sample (Tsample) and the temperature of the reference cell (Tref). ∆=TTmeasured sample − T ref (6) TTsample ≈+00Pc()β gtLR h (7) TTref = 0 (8) Therefore, ∆≈TPmeasured 0()β gtcLR h (9) Dealing with non-transparent liquids (Dual beam differential LCS). The above equation is valid only when the liquid is transparent to the incident radiation. If the liquid also absorbs, TTsample ≈+00Pc()ββgl+ LRth (10) where βl is the absorption coefficient of the liquid (transformer oil in our case). Therefore, ∆≈TPmeasured 0()ββglcL+ Rth (11) In such a case, gas concentration can be quantified only if βl is ≪ βgc, (then the Δ Tmeasured reduces to that in Equation 9). To be able to quantify gas concentration even when βl is significant when compared to βgc, the dual beam differential method has been developed. Here, the incident laser beam is split using a beam splitting arrangement such that equal laser powers are incident on both the sample and reference cells (see Figs 1(c) and 3(a)). Therefore, TTsample ≈+00Pc()ββgl+ LRth (12) TTrefl≈+00PL()β Rth (13) and, ∆≈TPmeasured 0()β gtcLR h (14) Once again, the measured temperature change is proportionate to the gas concentration. If the laser powers incident in the two cells are not exactly the same, or if the reference liquid is not the same as the sample liquid, the above equation is no longer valid. Then the experiment is carried out twice - once with the laser wavelength at the absorbance peak of the gas to be detected and the second experiment with the laser SCIENTIFIC REPORTS | 7:42917 | DOI: 10.1038/srep42917 3 www.nature.com/scientificreports/ Figure 2.
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