Humidity, Airflow and Psychrometric Measurement

Relative humidity

Relative humidity is the ratio of the actual water-vapour pressure to the saturation vapour pressure at the same temperature, expressed as a percentage. It does not state the total amount of water vapour independently of temperature. If air is cooled without removing moisture, relative humidity rises; if it is heated, relative humidity falls. Every RH result should therefore be reported with temperature, location and time. An RH value copied without those conditions is incomplete.

Dew point and wet-bulb temperature

Dew-point temperature is the temperature at which the present water vapour would reach saturation and condensation would begin during cooling. It is a useful measure of the moisture content of the air and helps explain condensation on cold walls, coils or product. Wet-bulb temperature is inferred from evaporative cooling around a wetted sensor and can be used with dry-bulb temperature and pressure to calculate humidity. Sling or aspirated psychrometers require correct wick wetting, ventilation and stabilisation; they are not reliable when used as two ordinary thermometers in still air.

Humidity sensor types

Electronic curing-chamber instruments usually use capacitive polymer sensors, while other systems use resistive sensors, chilled-mirror dew-point measurement or wet- and dry-bulb methods. Capacitive sensors are practical for continuous monitoring but can drift after condensation, chemical exposure or contamination. Chilled-mirror instruments determine dew point directly and are used where higher metrological performance is needed, but they require clean optics and controlled operation. Sensor specifications must cover the chamber’s temperature, humidity and condensation exposure.

Probe location

A sensor near a humidifier outlet, evaporator coil, wall, door or warm control box may not represent the product zone. Dense hanging loads create sheltered microclimates, while open spaces can experience greater air exchange. Place routine sensors where mapping shows that they represent or conservatively control the process. Protect the probe from direct droplets and product contact without enclosing it so tightly that response becomes slow. In a small chamber, even a few centimetres from a cold wall can change the reading.

Temperature equilibrium

Humidity measurement is especially sensitive to temperature difference between the probe and the air. A probe moved from a warm room into a cold humid chamber can temporarily report a large error and may condense. Allow the sensor, reference and chamber to equilibrate before comparison or adjustment. Avoid holding a handheld probe in a way that warms the sensing head. During calibration, the reference environment must be stable and the probe must reach the same temperature before results are accepted.

Humidity verification

Humidity probes can be compared with a calibrated reference instrument or in a controlled humidity generator. Saturated salt solutions can establish known humidity points only when the specified chemical, concentration, temperature, sealed volume and equilibration conditions are maintained. The assigned RH varies with salt and temperature and carries uncertainty. A household container with damp salt is useful only as an indicative check unless its preparation and conditions follow a recognised method or manufacturer system.

Air-velocity measurement

Air movement can be measured with vane, hot-wire or thermal anemometers, pitot systems and other velocity probes. Vane instruments need enough flow to turn the rotor and are directional. Hot-wire or thermal probes can measure lower velocities but are sensitive to orientation, temperature, contamination and turbulence. The instrument’s lower measuring limit and stated uncertainty must cover the expected product-zone velocity. A fluctuating display below the sensor’s useful range is not a precise airflow result.

Airflow pattern and mapping

A single velocity number does not describe airflow distribution. Measure at defined positions and orientations, including different heights, load densities and distances from fans or ducts. Smoke visualisation, ribbons or other qualitative methods can reveal direction and short-circuiting but do not quantify product-surface velocity. Mapping should distinguish supply-air movement, room circulation and air moving directly across product. Repeat the assessment after changes to fans, shelving, hanging density, door practice or humidification hardware.

Interpretation in curing

RH and airflow influence the difference in vapour pressure between the product and surrounding air, but they do not alone determine drying. Product water activity, surface condition, casing, diameter, temperature, load and time also matter. Sensor data should be interpreted with product weight loss, surface condition, condensation and internal-to-surface moisture movement. A high fan setting can create local drying even when the chamber RH display appears acceptable, while still air in a dense load can create wet pockets and unwanted surface growth.

Records and faults

Record temperature with RH, instrument identity, location, stabilisation time and whether the result is routine monitoring, a spot check or calibrated measurement. Airflow records must also state probe type, orientation and averaging period. Common faults include locating the sensor beside a moisture source, calibrating before equilibrium, allowing condensation on the probe, using the wrong salt value, measuring airflow below the instrument range, and treating one point as representative of a loaded chamber.