Determine the psychrometric constant A, actual water vapor pressure e, relative humidity RH, and dew-point temperature of ambient air across wet-bulb depressions from 2.0 °C to 10.0 °C at dry-bulb temperature 25.0 °C using a ventilated wet-and-dry bulb psychrometer and a polished-mirror dew-point hygrometer, verifying the Ferrel-Sprung psychrometric equation Δe = e_s(t_w) − e = A · P_atm · (t_d − t_w). Explore a virtual apparatus, collect readings and analyse a graph.
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Adjust the apparatus, then take a reading.
Readings are generated by an ideal model. The diagram is schematic and is not to scale.
The teaching model assumes a constant psychrometric factor A = 6.60 × 10⁻⁴ K⁻¹ corresponding to ideal forced ventilation and pure water. In an unventilated room or with a contaminated wick, A can increase to ~8.0 × 10⁻⁴ K⁻¹, and non-ideal gas compressibility factors become slightly noticeable at high ambient temperatures. The Ferrel-Sprung psychrometric equation and the dew-point method illustrate the fundamental thermodynamic balance between sensible convective heat transfer and latent mass transfer. The experimental psychrometric constant A = 6.60 × 10⁻⁴ K⁻¹ bridges microscopic molecular diffusion with macroscopic meteorological observations of humidity and weather.
Comprises two matched, radiation-shielded secondary standard thermometers graduated in 0.1 °C divisions, equipped with a motor-driven aspiration fan delivering a uniform airflow (≥ 3.0 m s⁻¹) across the dry bulb and cotton-sleeved wet bulb.
A thin-walled silvered copper thimble containing volatile diethyl ether, equipped with an aspirator air tube to induce rapid evaporative cooling and an illuminated viewing window to detect the exact onset of moisture condensation.
Measures the prevailing ambient atmospheric pressure P_atm in mmHg and kPa to ±0.1 mmHg, required for the barometric factor in the Ferrel-Sprung psychrometric relation.
A high-purity, degreased tubular cotton sleeve fitted snugly over the wet bulb, drawing pure distilled water continuously by capillary action without introducing dissolved salts that would lower water vapor pressure.
Submerged inside the dew cup ether bath adjacent to the polished silver mirror wall to register the appearance and disappearance dew temperatures to ±0.02 °C.
Provides controlled, gentle airflow through the volatile ether bath, allowing the experimenter to regulate the rate of cooling and warming around the dew point.
Determine the psychrometric constant A, actual water vapor pressure e, relative humidity RH, and dew-point temperature of ambient air across wet-bulb depressions from 2.0 °C to 10.0 °C at dry-bulb temperature 25.0 °C using a ventilated wet-and-dry bulb psychrometer and a polished-mirror dew-point hygrometer, verifying the Ferrel-Sprung psychrometric equation Δe = e_s(t_w) − e = A · P_atm · (t_d − t_w). Suggested independent settings: 2.0, 4.0, 6.0, 8.0, 10.0 °C.
Read the laboratory Fortin mercurial barometer to 0.1 mmHg, applying temperature corrections to establish the true ambient barometric pressure P_atm (baseline P_atm = 101.325 kPa = 760.0 mmHg). Clean the wet-bulb cotton muslin sleeve thoroughly with pure distilled water, ensuring it fits snugly without wrinkles or air gaps over the thermometer bulb, and fill the wetting reservoir with fresh distilled water.
Ambient pressure directly scales the psychrometric constant term (γ = A · P_atm). Clean muslin and pure distilled water ensure unimpeded capillary wicking and prevent dissolved minerals from depressing the saturation vapor pressure.
Activate the motor-driven aspiration fan on the psychrometer to generate a steady airflow of at least 3.0 m s⁻¹ through the protective radiation shields. Allow the airflow to stabilize for three minutes, then record the steady dry-bulb temperature t_d (nominal baseline t_d = 25.0 °C). Confirm that the dry bulb reading remains constant to within ±0.05 °C.
A minimum air speed of 2.5 to 3.0 m s⁻¹ is required to saturate the convective boundary layer and render the psychrometric constant A independent of airflow fluctuations.
Observe the descending temperature of the wet-bulb thermometer under forced aspiration, recording readings every thirty seconds. Once the wet-bulb temperature stabilizes at its minimum plateau for two consecutive minutes (indicating dynamic equilibrium between convective sensible heat gain and latent heat loss), record the wet-bulb temperature t_w and compute the wet-bulb depression Δt = t_d − t_w.
The steady minimum wet-bulb reading represents true thermodynamic psychrometric equilibrium, free from transient thermal lag.
Gently squeeze the rubber aspirator bulb to bubble ambient air through the volatile ether contained within the silvered dew cup, watching the highly polished mirror surface through the optical magnifier. As evaporative cooling lowers the cup temperature, note the exact thermometer reading t_1 at the first instant a dulling film of condensed dew droplets mists the silver mirror.
The initial dulling marks the exact point where the adjacent air layer reaches 100% saturation, providing the upper bound for the dew point.
Immediately cease air aspiration through the ether, allowing the dew cup to warm slowly by heat conduction from the room. Note the exact temperature t_2 at which the moisture film completely vanishes and the mirror regains its brilliant specular reflection. Compute the true dew-point temperature as the arithmetic mean: t_dew = (t_1 + t_2) / 2.
Averaging the appearance and disappearance temperatures cancels thermal conduction lag across the metallic cup wall, yielding an accurate measure of the true thermodynamic dew point.
Using saturation vapor pressure tables (or the Tetens equation), evaluate the saturation vapor pressure at the wet-bulb temperature e_s(t_w) and the actual vapor pressure e = e_s(t_dew). Compute the vapor pressure depression Δe = e_s(t_w) − e in Pascals. Repeat the observations across multiple ambient humidity states (wet-bulb depressions Δt = 2.0, 4.0, 6.0, 8.0, 10.0 °C). Plot Δe versus Δt, determine the slope γ, and compute the psychrometric constant A = γ / P_atm.
Linear regression of Δe against Δt directly verifies the Ferrel-Sprung relation and yields the psychrometric constant A with high statistical precision.
Points are your observations. The fit uses SI x and y values before display scaling.
| # | Wet-bulb depression Δt (°C) | Vapor pressure depression Δe (Pa) | Wet-bulb depression Δt (°C) | Vapor pressure depression Δe (Pa) | Remove |
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