Step Changes In Relative Humidity Research Articles

Interlaboratory study of the operational stability of automated sorption balances

AbstractAutomated sorption balances are widely used for characterizing the interaction of water vapor with hygroscopic materials. These instruments provide an efficient way to collect sorption isotherm data and kinetic data. A typical method for defining equilibrium after a step change in relative humidity (RH) is using a particular threshold value for the rate of change in mass with time. Recent studies indicate that commonly used threshold values yield substantial errors and that further measurements are needed at extended hold times as a basis to assess the accuracy of abbreviated equilibration criteria. However, the mass measurement accuracy at extended times depends on the operational stability of the instrument. Published data on the stability of automated sorption balances are rare. An interlaboratory study was undertaken to investigate equilibration criteria for automated sorption balances. This paper focuses on the mass, temperature, and RH stability and includes data from 25 laboratories throughout the world. An initial target for instrument mass stability was met on the first attempt in many cases, but several instruments were found to have unexpectedly large instabilities. The sources of these instabilities were investigated and greatly reduced. This paper highlights the importance of verifying operational mass stability of automated sorption balances, gives a method to perform stability checks, and provides guidance on identifying and correcting common sources of mass instability.

Spectroscopy for the Analysis of Nanoporous Silicon Gas and Humidity Sensors

In this work, a Raman spectroscopic study of nanoporous silicon sensor samples demonstrated its use as a method of gauging the sensor potential via quantitative data it provides on the sensor nanostructure dimensions. This special property of the Raman spectroscopy technique also showed its potential to determine mechanical stability of the samples over 3 months. This work also shows that the Raman spectroscopy technique is sensitive to step changes in relative humidity in all the sensor samples via its measurement of the strain-free crystalline silicon (c-Si) Raman peak. Since the Raman technique is non-destructive and senses remotely on the fragile nanoporous sensor samples it will be the ideal replacement of the presently used electrical capacitance techniques as the primary determination of relative humidity.

Hygromorphic response dynamics of oak: towards accelerated material characterization

When a wood board is exposed to a change in relative humidity on only one of its surfaces, e.g. in case of flooring or a panel painting, the resulting asymmetric moisture content profile induces differential expansion over the thickness. Consequently a bending moment causes the board to curve. A theory is presented to describe the bending of a wood board due to a step change in relative humidity. The board is assumed to be homogeneous, isotropic, and linearly elastic. Moisture transport is presumed to obey the diffusion equation with constant coefficients, such that moisture transport can be directly related to the bending of the board. It is shown that the transient deflective behavior provides the diffusion coefficient and the final length change yields the linear hygroscopic expansion coefficient. Derived diffusion coefficients are in good agreement with values in literature. Furthermore, a scaling law for the deflection of the board is proposed, which is seen to be followed qualitatively but not quantitatively by experiments. Finally, by assuming the deflection of the board to be the response of a linear system, the deflective frequency response of the board can be predicted from its step response. The results allow upscaling of deflection and expansion, such that behavior of thick boards can be determined from an experiment using a thin board.

A rapid scan vacuum FTIR method for determining diffusion coefficients in viscous and glassy aerosol particles.

We report a new method to investigate water transport kinetics in aerosol particles by using rapid scan FTIR spectroscopy combined with a custom-built pulse relative humidity (RH) control system. From real time in situ measurements of RH and composition using high time resolution infrared spectroscopy (0.12 s for one spectrum), and through achieving a high rate of RH change (as fast as 60% per second), we are able to investigate the competition between the gas and condensed phase diffusive transport limits of water for particles with mean diameter ∼3 μm and varying phase and viscosity. The characteristic time (τ) for equilibration in particle composition following a step change in RH is measured to quantify dissolution timescales for crystalline particles and to probe the kinetics of water evaporation and condensation in amorphous particles. We show that the dissolution kinetics are prompt for crystalline inorganic salt particles following an increase in RH from below to above the deliquescence RH, occurring on a timescale comparable to the timescale of the RH change (<1 s). For aqueous sucrose particles, we show that the timescales for both the drying and condensation processes can be delayed by many orders of magnitude, depending on the viscosity of the particles in the range 101 to 109 Pa s considered here. For amorphous particles, these kinetics are shown to be consistent with previous measurements of mass transfer rates in larger single particles. More specifically, the consistency suggests that fully understanding and modelling the complex microphysical processes and heterogeneities that form in viscous particles may not be necessary for estimating timescales for particle equilibration. A comparison of the kinetics for crystalline and amorphous particles illustrates the interplay of the rates of gas and condensed phase diffusion in determining the mass transport rates of water in aerosols.

Short hold times in dynamic vapor sorption measurements mischaracterize the equilibrium moisture content of wood

Recently, the dynamic vapor sorption (DVS) technique has been used to measure sorption isotherms and develop moisture-mechanics models for wood and cellulosic materials. This method typically involves measuring the time-dependent mass response of a sample following step changes in relative humidity (RH), fitting a kinetic model to the data, and extrapolating the asymptotic mass. A series of steps covering the full RH range is used to generate the sorption isotherm. The majority of prior DVS data were taken with hold times of either 60 min or until the change in moisture content was <0.002% per minute over a 10-min period. Here, DVS measurements on wood and isolated wood polymers are presented where the hold times at certain relative humidity steps were much longer, ranging between 24 and 50 h. The data clearly show that the criteria for hold time in previous DVS measurements result in significant errors in prediction of the asymptotic mass. Although the data at short times are consistent with previous measurements, the data exhibit slow sorption behavior with characteristic times on the order of 500–2000 min that cannot be identified with shorter hold times. The results suggest that new hold time criteria need to be developed for dynamic vapor sorption measurements in wood.

Hot-air Drying Kinetics of Yam Slices under Step Change in Relative Humidity

Abstract The drying kinetics and mathematical modeling of hot-air drying of yam slices were investigated under two-stage relative humidity (RH) control strategy with 60 °C and 1.5 m/s as its constant drying temperature and air velocity, respectively. Results indicate high RH in the initial stage results in high sample’s temperature that enhances water diffusion in the falling rate drying period. Within the scope of current work, change in RH in the later drying period has insignificant influence on sample’s temperature rise while low RH can accelerate the drying rate. Compared to drying at constant 20 % RH at the same drying air temperature, the drying strategy of using 40 % RH over the first 15 min and then lowing to 20 % RH for the remainder time yields a shorter drying time. Weibull model adequately described the moisture content variation with time for all experiments with the scale parameter ranging from 105.02 to 122.38 min and the values of shape parameters from 0.988 to 1.183. The effective moisture diffusivity determined from the Weibull model varied from 2.032 to 2.610×10−8 m2/s. The rehydration ratio increased as the overall drying time was reduced. Microstructure examination shows that higher RH in the initial drying stage can lead to a more porous microstructure which enhances drying, while fast drying rate in the initial drying period generates a crust layer which hinders drying.

In Situ Observation on the Dynamic Process of Evaporation and Crystallization of Sodium Nitrate Droplets on a ZnSe Substrate by FTIR-ATR

Sodium nitrate is a main component of aging sea salt aerosol, and its phase behavior has been studied repeatedly with wide ranges observed in the efflorescence relative humidity (RH) in particular. Studies of the efflorescence dynamics of NaNO3 droplets deposited on a ZnSe substrate are reported, using an in situ Fourier transform infrared attenuated total reflection (FTIR-ATR) technique. The time-dependence of the infrared spectra of NaNO3 aerosols accompanying step changes in RH have been measured with high signal-to-noise ratio. From the IR difference spectra recorded, changes of the time-dependent absorption peak area of the O-H stretching band (ν-OH, ∼3400 cm(-1)) and the nitrate out-of-plane bending band (ν2-NO3(-), ∼836 cm(-1)) are obtained. From these measurements, changes in the IR signatures can be attributed to crystalline and solution phase nitrate ions, allowing the volume fraction of the solution droplets that have crystallized to be determined. Then, using these clear signatures of the volume fraction of droplets that have yet to crystallize, the homogeneous and heterogeneous nucleation kinetics can be studied from conventional measurements using a steady decline in RH. The nucleation rate measurements confirm that the rate of crystallization in sodium nitrate droplets is considerably less than in ammonium sulfate droplets at any particular degree of solute supersaturation, explaining the wide range of efflorescence RHs observed for sodium nitrate in previous studies. We demonstrate that studying nucleation kinetics using the FTIR-ATR approach has many advantages over brightfield imaging studies on smaller numbers of larger droplets or measurements made on single levitated particles.

MEASUREMENT OF DYNAMIC SORPTION BEHAVIOUR OF SMALL SPECIMENS OF PINUS RADIATA - INFLUENCE OF WOOD TYPE AND MOISTURE CONTENT ON DIFFUSION RATE

Sorption behaviour of radiata pine has been investigated by weighing small specimens continuously during isothermal step changes in relative humidity. The use of wood specimens shorter in the longitudinal direction than the average tracheid length of radiata pine ensures that all tracheids in a specimen are exposed, reducing the effect of wood structure on bound water transport. The small size also allows the specimens to be prepared from a single band of earlywood or latewood. Using the dynamic sorption platform developed at Scion, a number of sorption experiments were undertaken comparing dynamic sorption behaviour of individual bands of earlywood and latewood, which had been heat treated to mimic the chemical changes that occur during high temperature drying. Diffusion coefficients and surface emission coefficients have been calculated from the sorption data, and are presented here. Earlywood and latewood had different sorption behaviour, but no measurable changes in sorption behaviour were seen with the different heat treatments. Diffusion coefficients were strongly dependent on moisture content.

Elastoplastic damage modeling of desaturation and resaturation in argillites

AbstractThe development of a coupled elastoplastic damage constitutive model for argillites is presented. Emphasis is put on the description of post‐failure and coupled hydromechanical behavior of argillites during the desaturation/resaturation processes. A short summary of experimental investigations is given in the first part, which shows an important plastic deformation coupled with damage and a significant influence of water content on the mechanical behavior of argillites. Based on the framework of poroplasticity and continuum damage mechanics, a general constitutive model is proposed for the poromechanical behavior of argillites in both saturated and unsaturated conditions. Main features observed in experiments are taken into account, in particular the elastic degradation due to microcracks, residual strength, coupling between plastic deformation and induced damage, influence of water content on plastic flow, variation of permeability induced by microcracks as well as the deformation generated in the desaturation/resaturation processes. The performance of the model is examined by comparing numerical simulations with test data in representative load paths. Finally, the model is applied to hydromechanical coupling study of four time‐dependent tests subjected to a step change in relative humidity. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Water Uptake Characteristics and Backbone Flexibility of Novel Polymers for Proton Exchange Membranes

Gravimetric analysis was performed during a step change in relative humidity for a series of sulfonated polymers to determine their dynamic water uptake characteristics. The mass change was recorded as a function of time for both stiff-backboned and flexible-backboned polymers when the humidity was instantaneously increased from 40 to 50 %. The observed water uptake profiles were distinctly different based on the inferred stiffness of the sulfonated polymer backbone. A sulfonated poly(sulfone)-based membrane had a slower mass uptake than more flexible Nafion with time constants of 307 s and 150 s for the sulfonated poly(sulfone) and NRE212, respectively. The dynamic water absorption properties were measured for a series of sulfonated poly(styrene)-b-poly(vinylidene fluoride)-b-sulfonated poly(styrene) membranes. The backbone flexibility of these polymers was intermediate to that of the poly(sulfone) and Nafion membranes and could be tuned based on the molecular weight of the poly(styrene) blocks and sulfonation level.

The Transient Response of Capacitive Thin-Film Polymer Humidity Sensors

The transient response of capacitive thin-film polymer humidity sensors to step changes in humidity and temperature as a function of air velocity was investigated. Two duct-mounted humidity sensors (designated Candidate X and Candidate Y) from two manufacturers, each with thin-film capacitive humidity sensing elements and temperature output, were used in the study. The transient performance of each test sensor was characterized by measuring the time constants of the relative humidity (RH) and temperature outputs in response to step changes in RH and temperature over a range of air velocities. Duct air velocities of 0 (still air), 200, 500, 800, and 1100 ft/min (0, 1.0, 2.5, 4.1, and 5.6 m/s) were considered along with step changes in RH of 20%–40% and step changes in air temperature of 0°F (isothermal), 9°F, and 18°F (0°C, 5°C, and 10°C). The effects of a positive or negative RH step change direction (lower to higher RH and vice versa) were also considered. Results showed that the RH time constants of both sensors decreased asymptotically with increasing duct air velocity for all testing conditions. For the isothermal tests, the average RH time constants of the Candidate X and Y sensors were approximately 4 and 18 s, respectively. When a step change in temperature was introduced, the RH time constants increased by factors of 100 and 12 over the isothermal values, respectively. With the exception of the still-air tests, the time constant of the Candidate X sensor for negative RH step changes was, on average, about 65% less than compared to positive RH step changes. For the Candidate Y sensor, a time-constant dependency on RH step-change direction was not observed.

Studies on hygroscopic moisture transport in Norway spruce (Picea abies) Part 2: Modelling of transient moisture transport and hysteresis in wood

This article presents a transient model for moisture transport in wood. The model is based on Fick's law with water vapour pressure and temperature as driving potentials. A model for hysteresis has been proposed and included in the model. Comparisons between experimental results presented in Part 1: Sorption measurements of spruce exposed to cyclic step changes in relative humidity (Time 2002) and calculations have been performed. The model only deals with moisture transport in the hygroscopic region, liquid water is not included.

Studies on hygroscopic moisture transport in Norway spruce ( Picea abies )

) have been exposed to repetitive daily and weekly step changes in relative humidity with the surrounding temperature at an approximate constant level. The results show that the moisture sorption is a two-step sorption process with the major change in moisture content in the first fast initial part. The major part of the moisture change happens within a relatively short period of time. No significant phase lag has been observed for neither transverse nor longitudinal specimens up to 10 mm thickness. The moisture change in absorption is slightly different from the moisture change in desorption. Curves in absorption and desorption for specimens with both transverse and longitudinal direction are rather similar in shape.

A Test Method to Determine the Relative Humidity Dependence of the Air Permeability of Woven Textile Fabrics

Abstract This paper describes a test method used to determine the relative humidity dependence of the air permeability of hygroscopic woven textile fabrics. Changes in fabric structure as hygroscopic fibers swell at high humidities can have a large influence on the measured air permeability of materials such as cotton, wool, silk, and nylon fabrics. The method is sensitive enough to show sorption hysteresis effects in the relative humidity versus air permeability curve. The instrumentation also permits dynamic measurements during a step change in relative humidity. Typical results are shown for seven fabrics, covering a range of fiber hygroscopic properties and air permeabilities.

Study of the effect of humidity on AlO— x thin-film resistors

The effect of humidity on the d.c. resistance of AlO— x thin-film planar resistors on Teflon is reported. The AlO— x films were obtained by immersing Al thin films in distilled water at 58 °C. The porosity variation and the thickness dependence of the porosity of the AlO— x films were obtained by the method of packing fractions using refractive index data. Results indicate that the porosity decreases with thickness from 67% at 120 nm to 48% at 450 nm with a porosity scatter of ±6%, and that films thicker than 220 nm may be more moisture sensitive. A change in the d.c. resistance of three orders of magnitude (10 9 Ω to 10 12 Ω) was observed between 20 and 100% R.H. The AlO— x resistor response (on a log scale) to changes of relative humidity from 20 to 100% is similar to the nature of the adsorption isotherm curve. The observed porosity scatter mainly leads to variations in hysteresis between films and also affects the reproducibility to some extent. Aging in the ambient atmosphere over a five month period resulted in an increased d.c. resistance in the 40 to 90% R.H. range, apparently due to a partial phase transformation to Al 2O 3. The high sensitivity of the d.c. resistance to relative humidity and its fast response to step changes in relative humidity indicate a potential use of these resistors as humidity sensors.