Search Results

You are looking at 71 - 80 of 227 items for :

  • "relative humidity" x
  • All content x
Clear All
Journal of Thermal Analysis and Calorimetry
Authors: E.T. Stepkowska, J. Perez-Rodriguez, M. Aviles, M. Jimenez de Haro, and M. Sayagues

Abstract  

Specific surface, S, of CSH-gel particles of disordered layered structure, was studied by water sorption/retention in two cement pastes differing in strength, i.e. C-33 (weaker) and C-43 (stronger), w/c=0.4. Hydration time in liquid phase was t h=1 and 6 months, followed by hydration in water vapour either on increasing stepwise the relative humidity, RH=0.5→0.95→1.0 (WS) or on its lowering in an inverse order (WR). Specific surface was estimated from evaporable (sorbed) water content, EV (110C), assuming a bi- and three-molecular sorbed water layer at RH=0.5 or 0.95, respectively (WS). On WR it was three- and three- to four-molecular (50 to 75%), respectively, causing a hysteresis of sorption isotherm. At RH=0.5 the S increased with cement strength from 146 m2 g-1 (C-33, 1 m) to 166 m2 g-1 (C-43, 1 m) and with hydration time to 163 (C-33, 6 m) and to 204 m2 g-1 (C-43, 6 m). At RH=1.0 (and 0.95), higher S-value were measured but these differences were smaller: S amounted to 190-200 m2 g-1 in C-33 (1 and 6 m) and 198-210 m2 g-1 in C-43 (1 and 6 m). Thus no collapse occurred on air drying of paste C-43 (6 m).

Restricted access

Abstract  

Aerosol particles smaller than 1.8 m were size-fractionated with micro-orifice impactors at two urban sites near Washington, DC, and analyzed for 44 elements including, As, Se, Sb, and Zn, i.e., elements strongly associated with coal combustion, incineration, and regionally transported secondary aerosol, by Instrumental Neutron Activation. Size distribution parameters were determined nonparametrically and with a least-squares peak-fitting method using impactor calibration data. Geometric and fitted mass mean aerodynamic diameters typically differed by <10% and increased continuously with increasing relative humidity (RH) in the range 56 to 79%, but along different curves for samples influenced by local and distant sources. The geometric mass mean diameters for samples influenced by winds from the direction of local sources were uniformly smaller than those influenced by westerly winds bearing aerosol from distant, regional, sources. At 60% RH, gmmads were As, 0.30±0.03 and 0.46±0.04; Se, 0.33±0.06 and 0.54±0.04; Sb, 0.39±0.03 and 0.53±0.04; and Zn, 0.39±0.06 and 0.53±0.08; respectively.

Restricted access

Abstract

These days, the investigations of the physical properties of the insulating materials are very important. In this paper we report the measured water uptaking capabilities of a thermal insulating paint. They can be found in liquid phase, but we measured dried solid samples. The thin thermal insulators (insulator coatings) are starting to spread on the market of the building materials. The proper understanding of sorption behavior of the materials is important from the applied building technology point of view. Moisture sorption and desorption measurements were carried out on two samples with different geometry by using climatic chamber method. After drying the samples at 90 °C for one hour in the Venticell 111 type drying equipment they were wetted with a Climacell 111 type climate chamber, where the relative humidity (RH) was varied from 25% to 90% at 293 K for 2 hours. The samples were wetted for 2 and 4 hours as well. Sorption and desorption as well as kinetic curves are presented in this paper.

Restricted access

Abstract  

The seasonal variation of the 7Be activities in air and the size distribution of the 7Be aerosols were studied by using a continuous air sampler and a five stage cascade impactor. The mean monthly activity level of 7Be at the Korea Atomic Energy Research Institute (KAERI) site varied from 0.5 to 4.8 Bq·m−3 and revealed a seasonal variation, in which the 7Be activity levels were high in winter and low in summer. The mass size distribution showed a bi-modal distribution with a higher peak around 0.49 μm and a smaller peak between 3 μm and 7.2 μm. The activity median aerodynamic diameter (AMAD) decreased with increasing ambient 7Be concentrations. The mean residence times by using a mean growth rate of 0.004 μm·h−1 were estimated to be 2.5–6.4 days. The AMAD has an increasing tendency with higher relative humidity. It seemed that the high humidity condition facilitated the growth of the aerosol, resulting in increased deposition rates of the aerosols and the low 7Be content in the surface air. The AMAD of the 7Be aerosols increased with an increasing temperature, but the temperature dependency of AMAD should be explained with geological and meteorological conditions.

Restricted access

Abstract  

The chemical stability of a propellant and its influence on the ballistic properties during aging is a subject of interest. The effect of aging on ballistic properties, viz., ignition delay, burning rate, and heat of combustion for an aluminised ammonium perchlorate–hydroxyl-terminated polybutadiene (AP/HTPB) composite propellant during accelerated aging were investigated. Samples of composite propellants were aged at 60 and 70 °C at relative humidity of 50% in a climatic chamber. The propellant samples were tested with pressurized nitrogen gas environment for ignition delay measurement. Test results indicate that aging does not have any appreciable effect on ignition delay. The change in ignition delay time is less than 3% within the scatter of the data. Experiment results indicate that burn rate do affect with pressure but aging does not have much effect on burn rate. It was also observed that the burning rate at low pressures did not undergo significant changes during the aging period. The most significant of all the ballistic properties of this propellant is the burning rate exponent which increased by about 10% during the aging period.

Restricted access

Abstract  

7Be activity concentrations were measured in the lower atmosphere at Thessaloniki, Northern Greece (40°38′N, 22°58′E) over the year 2009, a year of a particular minimum of solar activity. The mean annual activity concentration of 7Be at that year was 6.01 mBq m−3. The variability of 7Be surface concentrations related to the solar cycle appeared to be deviated about 40% between the maximum and the minimum values. A positive correlation (R = 0.97) was revealed between the activity concentrations of 7Be and the temperature, confirming that the increased rates of vertical transport within the troposphere, especially during the warmer months, resulted in carrying down to the surface layer air masses enriched in 7Be. Relatively high values of 7Be activity concentrations were observed by increasing of the tropopause height. A negative correlation (R = −0.65) between the 7Be activity concentrations and the relative humidity was due to the condensation process in the lower atmosphere which resulted in increased aerosol particle sizes with higher scavenging rates of aerosols and low activity concentrations of 7Be in the atmosphere. Influence of precipitation on the changes of 7Be activity concentrations was also observed.

Restricted access

Abstract  

The interaction between samples of metallic zinc and water vapour was studied gravimetrically, both in the absence and in the presence of oxygen. The experimental total mass gain vs. time curves exhibited two plateaus, whose heights increased with, elevations both of relative humidity and of temperature. The amount of product retained on the surface after desorption was also determined as a function of time. The product was identified as hydrated zinc oxide. In the runs conducted without oxygen, the retained product curves displayed a time delay with respect to the total mass gain curves. In the presence of oxygen, however, there was practically only one chronogravimetric curve. This behaviour is interpreted on the basis of a common mechanism involving the formation of an intermediate precursor oxide, which is more readily formed in the presence of oxygen than in its absence. A set of mathematical equations was derived, from which the rate constants for both processes were obtained. The second step was ascribed to a further weak adsorption of water.

Restricted access

In food science, colour is a fundamental property for the evaluation of freshness, quality and conformity. One of the most important attributes indicating sweet cherry freshness is stem colour and shape. Fruit post-harvest cool storage retards respiration and colour ripening changes. The common technology of refrigeration is based on active cooling determining high evapotranspiration for the passage of air over the products surface. An innovative preservation system, such as the Passive Refrigeration System (PRS™), could guarantee perfect shelf-life preservation maintaining optimal temperature and relative humidity close to 100%, minimizing colour changes in medium-long storage range. There is the need to numerically quantify cherry stem thickness and colour changes to compare fruit postharvest conditions under the two systems. The use of the Thin-Plate Spline 3D warping (TPS3D) in the 3-dimensional-RGB colour space allowed an efficient colour calibration. Sweet cherry stem images belonging to the two different storage systems (active and passive refrigeration) were acquired before and after 7 days to preservation through a professional high resolution scanner. Thickness and colour (after calibration) were measured. Results indicate, in terms of sweet cherry quality of preservation, that the ones preserved in passive refrigerator after 7 days appears similar to the fruits at the beginning of the experiment.

Restricted access

Sensory and physicochemical changes of three apple cultivars (‘Golden Delicious’, ‘Idared’ and ‘Gloster’) were monitored during 14 weeks of storage at 6 °C and 70% relative humidity in three following years. The aim was to investigate physicochemical parameters that were best correlated with sensory evaluation of apples during their storage. Sensory quality was assessed using a category point scale and the sensory profile was evaluated using a graphical unstructured scale. Physicochemical changes, such as weight, soluble solids, titratable acidity, and dry matter content, were monitored. All the examined parameters changed during the observed period, with the greatest changes occurring in weight, texture, and taste. There were also differences between the tested cultivars. Statistically significant correlations between sensory and physicochemical data were observed between texture characteristics and weight changes. In case of sweet taste, only a weak correlation between the intensity of sweet flavour and soluble solid content values was found. No statistically significant correlation between the intensity of sour taste and titratable acidity was observed.

Restricted access

The effect of sprouting of onions on total phenolic content, antioxidant activity, and anthocyanin profile of freeze dried powders produced from Indian onion cultivars was examined. Sprouting was carried out at an average light intensity of 64 Lumen, average relative humidity of 80.2%, and average temperature of 15.22 °C for three weeks. Two way analyses of data revealed significant (P<0.05) effect of sprouting and cultivar on functional composition of freeze dried powders. In all sprouted powder samples, there were significant (P<0.05) enhancements in total phenolic content, total flavonoid content, vitamin C content, antioxidant activity, and colour due to increase in anthocyanin contents. The anthocyanin profile depicted an increase in total anthocyanin concentration in powders developed from all four cultivars with the highest increase in powder from PRO-6 (23.87–27.15 mg kg−1), followed by Punjab Naroya (9.97–12.92 mg kg−1), commercial (7.15–8.43 mg kg−1), and Punjab White cultivar (3.45–4.09 mg kg−1).

Restricted access