J. A. P. Lima

Measurement of the thermal properties of liquids using a thermal wave interferometer

In this paper we discuss the use of an alternative photothermal technique for measurements of thermal properties of liquids. The proposed technique is based upon the concept of thermal wave interferometry. The liquid sample is confined between two thin pyroelectric detectors. One of these detectors acts as a modulated absorber of light while the other is used for sensing the temperature fluctuations transmitted through the liquid layer. The good agreement between the values of the thermal properties we got with the present technique and those reported in the literature demonstrates the capability of the technique for full characterization of the thermal properties of liquids.

Optical and thermal properties of liquids measured by means of an open photoacoustic cell

A double-purpose open photoacoustic cell, suitable for optical and thermal characterization of liquid samples is described. An experimental method for the determination of very low concentrations of contaminants in liquid substances is proposed. The proposed technique was used to determine the concentration of Chromium (VI) in water via the measurement of its optical absorption coefficient. To test the suitability of the proposed technique for experimental determination of thermal properties of liquids, studies on ethanol were performed. The agreement between experimentally obtained thermal properties and values reported in the literature is good.

On the use of the thermal wave resonator cavity sensor for monitoring hydrocarbon vapors

A gas sensing device based on a thermal wave resonator cavity is outlined. It is experimentally tested by monitoring the presence of several hydrocarbon vapors in air via the measurement of the thermal diffusivity. It is also shown that its time-dependent response may be used to follow the vapor diffusion. It is shown that its characteristic response time is linearly correlated to the thermal diffusivity value of the mixture. The steps toward the development of a practical sensing device are further discussed.

Application of the thermal wave resonator to the measurement of the thermal diffusivity of gas mixtures

A gas analyzer device based on thermal wave interference in a cavity is presented. The thermal diffusivity of CO2:air mixtures as a function of the relative concentration is measured. It is demonstrated that different concentrations of CO2 in air can be detected with accuracy using the described experimental device. The results presented here open the possibility to perform routine measurements of thermal diffusivity of binary gas mixtures and using this parameter to monitor the relative gas concentration.

Characterization of the thermal properties of gases using a thermal wave interferometer

A method suitable for the thermal characterization of gases is described. It is based on thermal wave interference (TWI) in a cavity. A TWI device was constructed and the thermal diffusivity of different binary gas mixtures as a function of their relative concentration was measured. From these values, using a logarithm mixing model for the thermal conductivity of a two-phase gas system, the thermal properties of the corresponding pure gases were calculated. The data obtained for several test gases show good agreement with values reported in the literature.

Measurement of mass diffusivity in air using thermal wave interference detection

A discussion on the use of the thermal wave interference (TWI) for the monitoring of the transient of hydrocarbon in air is presented. The thermal wave signal was modeled using the logarithm-mixing model for the thermal diffusivity of a two-phase gas system in which the hydrocarbon vapor concentration in the air-filled TWI cell is a varying function of time. The time varying hydrocarbon vapor concentration was described assuming the simple Fick’s model for mass diffusion of the hydrocarbon vapor in the stagnant air column of the TWI cell. The transient TWI signal amplitude data fitting yielded two parameters, namely, the saturation concentration and the characteristic diffusion time. From the corresponding values of the diffusion time the hydrocarbon mass diffusivities were straightforwardly obtained.

Monitoring of hydrocarbon vapor diffusion in air using a thermal wave interferometer

A discussion on the use of thermal wave interference (TWI) for the monitoring of the transients of hydrocarbon in air is presented. The thermal wave signal was modeled using the logarithm-mixing model for the thermal diffusivity of a two-phase gas system in which the hydrocarbon vapor concentration in the air-filled TWI cell is a varying function of time. The time varying hydrocarbon vapor concentration was described assuming the simple Fick’s model for mass diffusion of the hydrocarbon vapor in the stagnant air column of the TWI cell. The transient TWI signal amplitude data fitting yielded two parameters, namely, the saturation concentration and the characteristic diffusion time. From the corresponding values of the diffusion time the hydrocarbon mass diffusivities were straightforwardly obtained. The obtained values for the hydrocarbon mass diffusivities were found to be in good agreement with the ones reported in the literature.

On the Use of the Optothermal Window Technique for the Determination of Low Concentrations of Chromium (VI) and Phosphorus in Water

Abstract In this work we report about the use of the Optothermal Window (OW) technique, actually a variant of Photoacoustic Spectroscopy, combined with well-proven colorimetric methods to the determination of low concentrations of pollutants in water. As a first approach, chromium (VI) and phosphorus were determined in distilled water samples. The determination of Cr (VI) and P species in environmental and biological systems is currently of considerable interest due to the toxicity of their compounds to live organisms. Their maximum allowed values in drinking water were well discriminated in our experiments as well as the limits of optical spectrophotometric measurements. The detection limit in our measurements was 0.1 μmol/L P-PO4 3- for phosphorus at 632.8 nm and 0.2 μmol/L for chromium (VI) at 514 nm.

On the use of the optothermal window technique for the determination of iron (II) content in fortified commercial milk

This work reports on the use of the optothermal window and a well-proven phenanthroline colorimetry method for determination of iron (II) content in a commercial fortified milk. Initially, iron (II) in distilled water was determined using a series of calibration samples with ferrous sulfate acting as the source of iron (II). In the following phase, this calibration methodology was applied to commercial milk as the sample matrix. The phenanthroline colorimetry [American Public Health Association, Washington, DC (1998)] was chosen in an attempt to achieve proper selectivity (i.e., to obtain the absorption band, the wavelength of which is centered near the radiation wavelength available for our experiments: Excitation wavelength at a 514-nm line of a 20-mW tunable Ar ion laser). Finally, samples of commercially available fortified milk were analyzed in an attempt to access Fe (II) content.

Correlation of PVR, Octane Numbers and Distillation Curve of Gasoline with Data from a Thermal Wave Interferometer

Abstract The ASTM standard methods for experimental determination of gasoline properties, such as octane numbers and Reid vapour pressure (PVR) are, in general, expensive, time consuming and cumbersome. Therefore, the study and development of faster and cheaper methods is of great industrial and scientific interest, particularly when one thinks of large numbers of gasoline samples to be tested. However, gasoline is a mixture of hydrocarbons and presents very complex chemical properties that require the use of several analytical techniques, and so the development of any new method will need to be validated through a series of statistical techniques. This contribution is aimed at the validation of a Thermal Wave Interferometer (IOT), conceived and developed by Vargas et al. at the Norte Fluminense State University — UENF, for the determination of octane numbers (MON and RON), distillation curve and PVR of gasoline and gasohol. The IOT determines these properties through a mathematical correlation of the thermal diffusivity of the components present in the gasoline, or gasohol, sample. This IOT validation study involved the use of a multivariate regression technique, namely SIMCA (Soft Independent Modelling of Class Analogy) for the classification of sample data and identification of outliers and the development of new correlation models for the prediction of gasoline octane numbers. In this study 97 samples of various types of commercial gasoline and gasohol, the latter containing up to 30% v/v of ethanol, were prepared and had their corresponding octane numbers (MON and RON) determined through the CFR motor method and their thermal diffusivities determined through the IOT. These experimental data were then correlated and a PLS (partial least squares) model, based on 89 thermal diffusivity variables, was developed for the prediction of the AKI, a parameter which is the mean value of the octane numbers (MON and RON) of a sample, i.e. AKI = (MON+RON)/2. Better results were obtained when the model was built for distillation curve points (Initial boiling point, 10% evaporated) and PVR data, and the thermal diffusivity data (30-870s) were centred at the mean value and smoothed, giving an RMSEC =1.832 and an RMSEV=2.270. These results indicate that the IOT is a promising, fast and cheap method for the prediction of AKI, provided the user takes into account the inherent errors of the experimental method. Thus, the IOT may be very useful as a tool for undertaking screening analysis, i.e., selection of gasoline, or gasohol, samples that must be submitted to the more expensive and time consuming CFR motor method.