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Psychrometer
Brief description
A psychrometer is a specialized hygrometer designed to measure relative humidity in the atmosphere. By comparing the temperatures of two liquid-in-glass thermometers—one dry and one wrapped in a wet fabric wick—it measures the cooling effect of water evaporation, which is directly linked to atmospheric moisture content.
Use / Function
- Meteorological Forecasting: Assessing atmospheric moisture levels to predict humidity trends, fog formation, and rainfall.
- Grain & Food Storage: Monitoring ambient moisture in granaries, cold storage, and silos to prevent mold growth and crop decay.
- Industrial Drying Operations: Regulating humidity in textile mills, timber drying kilns, and paper manufacturing plants.
Operating principle
- Evaporative Cooling: Water evaporating from a surface absorbs latent heat from its surroundings, lowering the local temperature.
- Humidity Differential: Dry air causes rapid evaporation and a significant temperature drop on the wet bulb, whereas saturated humid air slows evaporation, keeping the wet bulb temperature close to the dry bulb temperature.
- Psychrometric Calculation: The difference between dry-bulb temperature ($T_d$) and wet-bulb temperature ($T_w$), known as the wet-bulb depression, allows direct lookup or calculation of relative humidity and dew point.
How to create it
- Mount Thermometers: Mount two identical liquid-in-glass thermometers side by side on a sturdy wooden or brass frame.
- Attach Wick: Cover the bulb of one thermometer with a clean, un-sized cotton muslin fabric sleeve.
- Moisten Fabric: Saturate the muslin fabric wick with pure distilled water, extending the wick into a small water reservoir if building a stationary unit.
- Ventilate: Provide airflow over the bulbs (at least 3 m/s) either by whirling the frame on a swivel handle (sling psychrometer) or using a mechanical fan.
- Read Temperatures: Allow airflow for 1–2 minutes until the wet-bulb reading stabilizes at its minimum temperature, then record both dry and wet bulb values.
Materials needed
- Essential: Two liquid-in-glass thermometers, clean cotton Fabric wick, distilled Water, Wood or metal frame, Glass.
- Tools: Scissors, mounting hardware, psychrometric lookup chart or slide rule.
- Substitutes: Porous paper or linen can substitute for cotton muslin.
Variants and improvements
- Sling Psychrometer: Mounted on a chain or hinged handle that is whirled by hand to ensure consistent air velocity over the wet bulb.
- Aspirated Psychrometer (Assmann): Uses a spring-wound or electric fan inside polished shielding tubes to shield against radiant heat while providing constant airflow.
Limits and risks
- Freezing Point: Below 0°C, water on the wick freezes; latent heat of sublimation must be accounted for in calculations.
- Impure Water: Using tap water causes mineral deposits on the fabric wick, impeding capillary action and skewing readings.
- Radiant Heat: Direct sunlight warming the thermometers will cause false high readings unless proper radiation shields are used.