Ambient Water Vapor Pressure Research Articles

Critical environmental limits for exercising heat-acclimated lean and obese boys.

Environmental limits for uncompensable heat stress, above which an imbalance between heat gain and heat loss forces body core temperature upward (i.e., the upper limits of the prescriptive zone), are unknown for children. To determine these limits, 7 lean and 7 obese 9- to 12-year-old heat-acclimated boys performed four randomized trials each on separate days to determine the critical water vapor pressure (P (crit)) forcing an upward inflection of body core temperature at several ambient temperatures. Subjects walked continuously on a treadmill at 30% maximal aerobic capacity at a constant dry bulb temperature (T (db) = 34, 36, 38 or 42 degrees C). After a 30-min equilibration period at 9 torr, ambient water vapor pressure increased approximately 1 torr every 5-min until a distinct breakpoint in the core temperature versus time curve was evident. Compared to the lean subjects, obese subjects had significantly lower environmental limits (P < 0.03) in warm environments (P (crit), for lean vs. obese, respectively = 32.9 +/- 0.7 vs. 30.3 +/- 0.8 torr at T (db) = 34 degrees C; 29.6 +/- 0.6 vs. 27.2 +/- 0.9 torr at T (db) = 36 degrees C; 27.8 +/- 0.6 vs. 24.7 +/- 0.9 torr at T (db) = 38 degrees C; 25.5 +/- 0.7 vs. 24.5 +/- 1.5 torr at T (db) = 42 degrees C). These results suggest that separate critical environmental guidelines should be tailored to lean and obese children exercising in the heat.

Lowering of resting core temperature during acclimation is influenced by exercise stimulus

The decrease in resting core temperature (T (co)) and its relation to the reduced physiological strain during heat acclimation was analysed with rectal temperature data measured in three groups of eight semi-nude persons (6 males, 2 females) who were acclimated for 15 consecutive days to dry, humid and radiant heat, respectively, with equivalent WBGT (33.5 degrees C), by performing 2-h treadmill work. A fourth group followed the same protocol for 12 days in a neutral climate. After acclimation, both resting T (co), prior to heat exposure, and final T (co), measured at the end of work, were significantly reduced. The reduction in final T (co) increased with decreasing ambient water vapour pressure and was higher for the data pooled over the heat conditions (0.46 +/- 0.31 degrees C) than in the neutral climate (0.21 +/- 0.25 degrees C), whereas resting T (co) declined similarly in the heat (0.20 +/- 0.25 degrees C) and the neutral environment (0.17 +/- 0.23 degrees C). The lowering of resting and final T (co) after heat acclimation showed a significant correlation (r = 0.67) and regression analysis showed that 37% of the average reduction in final T (co) was attributable to the lowering of resting T (co). The same analysis was applied after extending the database by short-term series of clothed persons (17 females, 16 males) acclimated at 29.5 and 31.5 degrees C WBGT for 5 days. A significant correlation was found between the lowering of resting and final T (co) (r = 0.57) that did not depend on climatic conditions and gender, although the reduction in resting T (co) was significantly smaller for females (0.06 +/- 0.22 degrees C) than for males (0.21 +/- 0.23 degrees C). It is concluded that the lowering of resting core temperature contributes to the reduced physiological strain during heat acclimation. Similar effects under neutral conditions point to the exercise stimulus as a probable explanation.

石炭灰堆積層中の水分凝縮による固化現象の数値解析

In this study, hardening phenomena of coal ash beds in high-humidity condition are examined. We employ a diffusion simulation in clarifying the distribution of condensed water in coal ash beds, in which water vapor is absorbed into sulfur trioxide adhered on the surface of coal ash particles. Aqueous sulfuric acids solution formed by the absorption is in equilibrium with the ambient water vapor. The diffusion rate of water vapor is proportional to the difference between the pressure and ambient water vapor pressure. The simulation successfully describes the results of previous experiment, at a constant temperature, with the variations in the equilibrium moisture and particle diameter of coal ash. Moreover, it is clarified that the relationship between water content and hardening phenomena is very similar for various coal ashes, with a correction of particle diameter of coal ash and the volume of condensate bridging between particles.

X-ray diffraction, 133Cs and 1H NMR and thermal studies of CdZrCs 1.5(H 3O) 0.5(C 2O 4) 4· xH 2O displaying Cs and water dynamic behavior

A novel mixed cadmium zirconium cesium oxalate with an open architecture has been synthesized from precipitation methods at room pressure. It crystallizes with an hexagonal symmetry, space group P3 112 (no. 151), a=9.105(5) Å, c=23.656(5) Å, V=1698(1) Å 3 and Z=3. The structure displays a [CdZr(C 2O 4) 4] 2− helicoidal framework built from CdO 8 and ZrO 8 square-based antiprisms connected through bichelating oxalates, which generates channels along different directions. Cesium cations, hydronium ions and water molecules are located inside the voids of the anionic framework. They exhibit a dynamic disorder which has been further investigated by 1H and 133Cs solid-state NMR. Moreover a phase transition depending both upon ambient temperature and water vapor pressure was evidenced for the title compound. The thermal decomposition has been studied in situ by temperature-dependent X-ray diffraction and thermogravimetry. The final product is a mixture of cadmium oxide, zirconium oxide and cesium carbonate.

High water vapour pressure deficit influence on Quercus ilex and Pinus pinea field monoterpene emission in the central Iberian Peninsula (Spain)

The results of a field study carried out in September–October 2000 near Madrid, regarding Quercus ilex and Pinus pinea monoterpene emission and its relation to ambient and physiological parameters, are presented in this paper. The major compounds in diurnal Q. ilex emission were limonene, α-pinene and β-pinene. Emission rates during warm days fitted reasonably well to the temperature and light-dependent model of Guenther ( E S=13.4 μg g DW −1 h −1). However, during hot days at mid-day and afternoon hours, dramatic decreases of monoterpene emission, photosynthetic activity and stomatal conductance were observed. The poor soil–water availability combined with low relative humidity and high temperature is likely to be responsible for the observed emission drop. A parameterisation of emission, based on ambient atmospheric water vapour pressure deficit (WVPD), has been attempted in this study. Monoterpene diurnal emission from P. pinea was lower than that of Mediterranean oak ( E S=1.5 μg g DW −1 h −1). A reduction in the total emission during the hottest hours of the day was not observed in this monoterpene storing species. Limonene emission rates, accounting on average for nearly half of the emission, were well described by the temperature-dependent model of Tingey. The rest of the emission was comprised of several compounds (cineole, myrcene, α-pinene, linalool) and was reduced at high WVPD values.

Heat Balance Limits in Football Uniforms

ABSTRACTBACKGROUND: Scant evidence documents the physiological and environmental stresses for football players wearing partial or full uniforms, but such information would be useful for determining the ambient temperatures and humidities associated with uncompensable heat stress during practice and games.OBJECTIVE: This laboratory study used a physiological approach to determine critical heat balance limits (various combinations of ambient temperature and relative humidity) for subjects exercising in typical American football uniforms.DESIGN: Eight nonheat-acclimatized men exercised at 35% Vo2max in a programmable environmental chamber. In multiple trials, either dry-bulb temperature (Tdb) was held constant and ambient water vapor pressure (Pa) was systematically increased, or Pa was held constant and Tdb was systematically increased. The critical heat balance limits were determined as the environments at which body core (esophageal) temperatures were forced out of equilibrium, reflecting uncompensated heat storage imposed by those environments.RESULTS: Critical environmental limits are presented that define the combinations of air temperature and relative humidity above which thermal balance cannot be maintained. These zones of uncompensable heat stress result in continuously rising core temperatures. Retrospective analysis reveals that documented heatstroke deaths in football players wearing full uniforms occurred at or above these critical environments.CONCLUSION: Heat balance limits can be used in decision making and are especially relevant for preventing heat-related illness or injuries early in the football practice season. The critical limits are expanded when shorts are substituted for football pants with pads.

Effect of Number Density of CCN on Equilibrium Radius and Temperature of Cloud Droplet during Competitive Growth

Usually, for the estimation of the cloud droplet size in the equilibrium state, Kohler equation is utilized, assuming constant vapor content and constant temperature. However, the condensational growth of the cloud droplet consumes the ambient water vapor and releases the latent heat. Therefore, the ambient water vapor pressure and the temperature are changed. Taking account of the variations of the water vapor content and the temperature, a new model for the estimation of the equilibrium cloud droplet size and temperature has been developed by modifying Kohler equation. The modification of Kohler model is based on the mass and the heat conservation laws for the water (vapor and liquid) in the air parcel occupied by one cloud droplet. Variations of size and temperature with number density of CCN are simulated numerically.

Body temperatures in free-flying pigeons

We examined the relationship between body temperature (Tb) of free flying pigeons and ambient water vapor pressure and temperature. Core or near core Tb of pigeons were measured using thermistors inserted into the cloaca and connected to small transmitters mounted on the tail feathers of free flying tippler pigeons (Columba livia). Wet and dry bulb temperatures were measured using modified transmitters mounted onto free-flying pigeons. These allowed calculation of relative humidity and hence water vapor pressure at flight altitudes. Mean Tb during flight was 42.0 ± 1.3 °C (n = 16). Paired comparisons of a subset of this data indicated that average in-flight Tb increased significantly by 1.2 ± 0.7 °C (n = 7) over that of birds at rest (t = −4.22, P < 0.05, n = 7) within the first 15 min of takeoff. In addition, there was a small but significant increase in Tb with increasing ambient air (Ta) when individuals on replicate flights (n = 35) were considered. Inclusion of water vapor pressure into the regression model did not improve the correlation between body temperature and ambient conditions. Flight Tb also increased a small (0.5 °C) but significant amount (t = 2.827, P < 0.05, n = 8) from the beginning to the end of a flight. The small response of Tb to changing flight conditions presumably reflects the efficiency of convection as a heat loss mechanism during sustained regular flight. The increase in Tb on landing that occurred in some birds was a probable consequence of a sudden reduction in convective heat loss.

Effects of metabolic rate and ambient vapour pressure on heat strain in protective clothing.

Studies have shown that variations in ambient water vapour pressure from 1.7 to 3.7 kPa have little effect on heat tolerance time at a metabolic rate above 450 W while wearing protective clothing. With lighter exercise, where tolerance times exceed 60 min, variations in vapour pressure have a significant impact on evaporative heat loss and, therefore, heat tolerance. The present study has examined whether these findings extend to conditions with more extreme variations in vapour pressure. Twelve males performed light (L, 350 W) and heavy (H, 500 W) exercise at 40 degrees C in a dry (D, 1.1 kPa) and humid (H, 4.8 kPa) environment while wearing a semi-permeable nuclear, biological and chemical protective clothing ensemble (0.29 m2 x degree C-1.W-1 or 1.88 clo; Woodcock vapour permeability coefficient, im = 0.33). Partitional calorimetry was used to determine the rate of heat storage (S) with evaporative heat loss from the skin (Esk) calculated from changes in dressed mass or the physical properties of the clothing and the vapour pressure gradient between the skin and the environment. Skin vapour pressure was predicted from measurements of water vapour pressure above the skin surface and in the clothing with humidity sensors coupled with thermistors. Final mean skin temperature (Tsk) was higher for the humid trials and averaged 37.4 (0.3) degree C, 38.9 (0.4) degree C, 37.6 (0.5) degree C and 38.5 (0.4) degree C for LD, LH, HD and HH, respectively. Final rectal temperature (Tre) was higher for D with respective values for LD, LH, HD and HH of 39.0 (0.4) degree C, 38.7 (0.4) degree C, 38.8 (0.4) degree C and 38.5 (0.4) degree C. Tolerance time was significantly different among the trials and averaged 120.3 (19.3) min, 54.8 (7.3) min, 63.5 (6.9) min and 36.8 (3.1) min for LD, LH, HD and HH, respectively. Esk was overestimated and, therefore, S was underestimated when the changes in dressed mass were used to determine evaporative heat loss. When skin vapour pressure determined from the humidity sensor data was used to calculate Esk, heat storage was significantly different among the trials and averaged 15.0 (3.0), 13.0 (1.8), 14.2 (2.6) and 12.2 (1.9) kJ.kg-1 for LD, LH, HD and HH, respectively. It was concluded that while wearing the protective clothing all indices of heat strain, including tolerance time, were significantly affected by the change in ambient water vapour pressure from 1.1 to 4.8 kPa during both light and heavy exercise.

Rebreathing improves accuracy of ventilatory monitoring

Our objective was to determine if rebreathing would reduce the gradient between arterial and end-tidal CO2 tension during positive-pressure ventilation. Experimental investigation. Anesthesiology laboratory. A total of 10 dogs of either sex. Anesthesia (sodium pentobarbital) and muscle relaxation (pancuronium) were induced and animals were tracheally intubated and ventilated with a standard anesthesia ventilator and breathing circuit with CO2 absorber and then with a Mapleson D circuit with a fresh gas flow rate (VF) equal to alveolar ventilation plus the sampling flow rate of two capnometers. Rebreathing was varied by adjusting the respiratory rate (RR) so that minute ventilation (VE) to VF ratio was 1:1, 2:1, 3:1, and 4:1. CO2 production (ATPD) was determined as the product of expired concentration of CO2 and VE (BTPS). Alveolar ventilation (VA) was calculated by dividing the product of CO2 production and barometric pressure corrected for ambient temperature and water vapor pressure at body temperature by PaCO2. Tidal volume, RR, airway gas temperature, concentration of CO2 in gas at the tracheal tube and inlet/outlet of the mechanical ventilator, body temperature, arterial gas tensions and pH, heart rate, arterial blood pressure, and cardiac output were measured. Minute ventilation, mean arterial blood pressure and end-expiratory CO2 tension (PECO2) (BTPS) were calculated. During positive-pressure ventilation, concentration of inspired CO2 was zero with standard circuitry, and significantly increased with Mapleson D when VE:VF ratio was 1:1 (0.56 +/- 0.19%), 2:1 (1.97 +/- 1.30%), 3:1 (2.56 +/- 1.05%), and 4:1 (3.01 +/- 1.45%) (p < 0.05). PECO2 was 34.8 +/- 3.2 mm Hg during ventilation with the standard circuit, and significantly increased during ventilation with Mapleson D when VE:VF ratio was increased from 1:1 (35.4 +/- 2.5 mm Hg) to 2:1 (40.2 +/- 3.6 mm Hg) and was not further increased at a VE:VF ratio of 3:1 (41.8 +/- 2.7 mm Hg) or 4:1 (41.3 +/- 2.4 mm Hg). The selected fresh gas flow rate was appropriate, because PaCO2 remained unchanged regardless of VE:VF ratio, indicating PaCO2 was dependent on VF, not on VE. The gradient between PaCO2 and PECO2 during ventilation with the standard circuit was 6.6 +/- 3.0 mm Hg; during ventilation with Mapleson D, it decreased significantly when VE:VF ratio was increased from 1:1 (6.5 +/- 3.6 mm Hg) to 2:1 (2.9 +/- 1.5 mm Hg), but was not significantly reduced further at 3:1 (1.7 +/- 1.1 mm Hg) or 4:1 (1.8 +/- 0.5 mm Hg) (p < 0.05). Rebreathing with a Mapleson D circuit and a VF equal to VA permitted normal CO2 elimination. Arterial PCO2 to PECO2 gradient decreased significantly during rebreathing, thus improving the reliability of capnography for estimating arterial PCO2. Consideration should be given to using the Mapleson D as a rebreathing circuit.

Protonic defects in oxides and their possible role in high temperature oxidation

Protons from hydrogen or water vapour may dissolve in oxides and other corrosion products, and may affect, and in some cases, dominate their defect structures. Defect-dependent properties like scale growth and plastic deformation then become dependent on the ambient hydrogen or water vapour pressure. Small concentrations of protons may enhance the transport of oxygen in the form of hydroxide ion diffusion. Furthermore, the transport of protons through a scale (dissolving as hydrogen in the metal), can replace the outward transport of electrons during scale growth. Recent literature results indicate that protons can be dominant defects in Y 2 O 3 , Al 2 O 3 and Cr 2 O 3 under certain conditions

Computer analysis, design and simulation of horizontal ground heat exchangers

This paper presents a new computerized procedure for dealing with the design of horizontal ground heat exchangers (HGHE). The computer program is based on the transient model of coupled nonlinear partial differential equations governing heat and mass flow in soils. The model is two-dimensional and delineates the operation of ground heat storage with the HGHE and such phenomena as freezing/thawing and drying/rewetting of soil moisture. Comprehensive climatological data, such as ambient temperature, solar radiation, wind velocity, rainfall, snowfall, snow characterstics, and water vapour pressure is used to simulate conditions at the ground surface over any required length of time. The package can be applied to any geographical location by changing climatic and soil data input. The designer has the possibility of selecting any of 12 types of soils from sand to clay, 12 commercial heat pumps, nine different configurations of the HGHE, 16 plastic pipes for ground coils, and 13 ground coil fluids. The program, however, does not calculate the length of the HGHE but it evaluates the thermodynamic performance of a ground heat pump system and provides comprehensive data on thermal and hydraulic conditions in ground heat storage. The length of the ground heat exchanger is obtained from a line source theory model or from site dimensions and pipe spacing. Computed results for ground heat exchanger operation correlate fairly well with experimental data. Simulation of temperature and moisture content in the ground for natural conditions (no heat extraction/deposition) showed a fair agreement with field data. The entire computer program is user-friendly, interactive, menu-driven, and written in FORTRAN 77.

Simultaneous derivation of clothing-specific heat exchange coefficients

Clothing adds resistance to heat exchange between the wearer and the environment. If clothing-specific heat exchange coefficients are known, a combined rational/empirical approach can be used to describe thermal exchange between clothed humans and the environment. However, during exercise these coefficients--typically calculated using thermal manikins--change, primarily due to wetting of the fabric during intense sweating and body movement. A procedure is described that allows for the simultaneous determination of both total insulation (IT) and resistance to water vapor permeation (Re) on exercising clothed subjects without the need to directly measure skin water vapor pressure or continuously weigh the subjects. Two tests are performed by each subject in each clothing ensemble. In one test, ambient water vapor pressure (Pa) is systematically increased in stepwise fashion while dry-bulb temperature (Tdb) is held constant; in the second test protocol Pa is held constant while Tdb is increased. Heat exchange data are collected at the time at which core temperature is forced out of equilibrium by the environment (according to the assumption that heat production is balanced by heat loss immediately prior to this critical environmental limit). Previous studies using similar approaches have typically estimated IT a priori and used this value in the subsequent derivation of Re for each clothing ensemble or condition tested. In the proposed method, IT and Re are derived from the solution of two simultaneous equations based on heat balance data from both tests. This paper describes and critiques this methodology via an error analysis, and compares the coefficients obtained with those from similar trials using other physiological and nonphysiological approaches.

Mean skin temperature in warm humid climates.

Mean skin temperature (Tsk) was measured in 24 subjects during experiments in a climatic chamber. Three conditions of ambient temperature (Ta = 25.6 degrees, 28.9 degrees and 32.2 degrees C), and three of humidity (relative humidity = 50%, 70% and 90%) were studied. A relationship was established by a linear regression technique. It is valid in the 24 degree-34 degree C range, for air velocity = 0.2 m.s-1, clothing insulation = 0.077 degrees C.m2.w-1 (0.5 clo), metabolic rate = 64 w.m-2 (1.1 met) and radiant temperature = air temperature. In these conditions Tsk = 28.125 + 0.021 Pw + 0.210 Ta (Pw: ambient water vapour pressure in mb). It shows a small humidity influence. The influences of sex, transition from one condition to the next, and air velocity were also studied. Measurements in Africa confirmed the small influence of humidity. Ethnic life-style differences indicated that a high precision in Tsk determination is difficult to achieve.

Simplified model for the behaviour of a roof-spray cooling system

Roof-spray cooling systems are used to reduce the heat flux entering through the roof of a building by spraying a fine mist of water on the roof surface. This paper presents a simple periodic thermal model to predict the performance of a roof-spray evaporative cooling system. The governing one-dimensional transient equation of heat conduction is solved along with appropriate periodic boundary conditions, and expressions for the temperatures of the roof surface and ceiling have been obtained. The present study also includes the effect of wind velocity and ambient water-vapour pressure on the system's performance.

Psychrometric limits to prolonged work in protective clothing ensembles.

When work is performed by workers in protective clothing, sweat evaporation is limited and body temperature rises. In an attempt to quantify the limits such ensembles place on safe work, 6 acclimated men and women walked at 30% VO2max (150-200 W/m2) in 2 protocols involving environmental transients. In one, ambient water vapor pressure (Pw) was fixed at 10 torr, and after rectal temperature (Tre) plateaued, ambient dry-bulb temperature (Tdb) was raised 2 degrees C every 10 min. In the second, Tdb was constant and Pw was increased 2 torr every 10 min. Critical temperature (Tcrit) and pressure (Pcrit) were defined as the Tdb or Pw at which thermal balance could no longer be maintained and Tre rose sharply. Each test was performed in various clothing ensembles ranging from light cotton work clothes to "impermeable" suits. Lines connecting mean Tcrit and mean Pcrit define a limit for safe prolonged exposure/exercise for approximately 50% of the population in each ensemble. Similar lines, drawn to represent values 2 standard deviations below the mean, should provide critical environmental limits for 95% of the population.

Psychrometric Limits to Prolonged Work in Protective Clothing Ensembles

When work is performed by workers in protective clothing, sweat evaporation is limited and body temperature rises. In an attempt to quantify the limits such ensembles place on safe work, 6 acclimated men and women walked at 30% [vdot]O2max (150–200 W/m2) in 2 protocols involving environmental transients. In one, ambient water vapor pressure (Pw) was fixed at 10 torr, and after rectal temperature (Tre) plateaued, ambient dry-bulb temperature (Tdb) was raised 2°C every 10 min. In the second, Tdb was constant and Pw was increased 2 torr every 10 min. Critical temperature (Tcrit) and pressure (Pcrit) were defined as the Tdb or Pw at which thermal balance could no longer be maintained and Tre rose sharply. Each test was performed in various clothing ensembles ranging from light cotton work clothes to “impermeable” suits. Lines connecting mean Tcrit and mean Pcrit define a limit for safe prolonged exposure/exercise for approximately 50% of the population in each ensemble. Similar lines, drawn to represent values 2 standard deviations below the mean, should provide critical environmental limits for 95% of the population.

Dielectric properties of montmorillonite clay films: Effects of water and layer charge reduction

An initial survey is made of the dielectric properties of a series of oriented smectite clay films including Na +-montmorillonite, Li +-montmorillonite, and charge-reduced montmorillonite. The properties of a structurally-related mica silicate, muscovite, was included for comparison purposes. In contrast with previous measurements on aqueous clay suspensions in which the clay content was a few wt.%, the measurements reported here are for comparatively dry clays where the water content is a few wt.%. The dielectric properties in the direction perpendicular to the clay sheets depend strongly on the amount of water takeup, which in turn depends strongly on the type and concentration of the cations in the gallery. Mica does not absorb water and thus its dielectric constant and conductivity are independent of ambient water vapor pressure. On the other hand for the swelling clays, Na + and Li +-montmorillonite, the dielectric constant and conductivity are highly dependent on the ambient water vapor pressure. These quantities are greatly diminished for charge reduced montmorillonite in which the Li cations are diffused into the silicate layers. Dielectric constants much greater than that for pure water are observed in the hydrated clays. These are ascribed to a Maxwell-Wagner effect.

Physiologically derived critical evaporative coefficients for protective clothing ensembles.

When work is performed in heavy clothing, evaporation of sweat from the skin to the environment is limited by layers of wet clothing and air. The magnitude of decrement in evaporative cooling is a function of the clothing's resistance to permeation of water vapor. A physiological approach has been used to derive effective evaporative coefficients (he) which define this ability to evaporate sweat. We refined this approach by correcting the critical effective evaporative coefficient (K for sweating efficiency (Ke,eta') since only a portion of the sweat produced under such conditions is evaporated through the clothing. Six acclimated men and women walked at 30% maximal O2 consumption (150-200 W.m-2) at a constant dry bulb temperature as ambient water vapor pressure was systematically increased 1 Torr every 10 min. Critical pressure was defined as the partial pressure of water vapor (Pw) at which thermal balance could no longer be maintained and rectal temperature rose sharply. Each test was performed in various clothing ensembles ranging from cotton shirt and pants to "impermeable" suits. This approach was used to derive he by solving the general heat balance equation, M - W +/- (R + C) = w.he.(Psk - Pw), where M is metabolic heat production, W is external work, R is radiant heat exchange, C is convective heat transfer, w is skin wettedness, and Psk is water vapor pressure of fully wet skin.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of water vapour on the properties of A-Si:C thin films prepared by evaporation

A quantitative relationship has been identified between the deposition parameters and the electrical and optical properties of a-Si:C thin films. The major effects of the deposition parameters derive from their influence on the incorporation of water vapour into the films. Even very small changes in the amount of water vapour, as an ambient during deposition, produce very large changes in the electrical and optical properties of the films. An increase in the ambient water vapour pressure from 1 × 10 −8 to 4 × 10 −8 Torr decreases the room temperature conductivity of the resulting films by as much as three orders of magnitude. The important deposition parameters are expressed in terms of a quantity N H 2 O , which represents the actual amount of water which remains in the film.