RELATIVE HUMIDITY IN COOL STORES: MEASUREMENT, CONTROL AND INFLUENCE OF DISCREET FACTORS

The reliability of commercially available measuring instruments is not yet sufficient. The manufacturers of sensors are facing the challenge of measuring accurately the variations of rh in a magnitude of 1 %, which, at 0 °C, represents about 5 mg of water per m³ of air. The advantages and drawbacks of various measuring systems are reviewed.

Hair or synthetic fiber hygrometers may reach an accuracy of ± 3% if they are conveniently maintained and calibrated, which has to be done every 3 months approximately. Psychrometers provided with precision thermometers may be used as calibration devices if correctly used. The precision of electronic capacitive or electrolytic sensors is comparable to that of hair hygrometers. The advantage is their compatibility with computer control systems. Capacitive sensors have the tendency of getting saturated and concealing the drops of humidity. Electrolytic sensors do not bear water spattering. Mirror hygrometers are the most precise reference devices, but difficult to use in the actual environment of cool cells.

Optimum humidity levels may be reached and kept with humidifying equipments. Maintenance costs will be high if some factors have not been considered in the design and construction of the cooling facility. The quality and thickness of wall insulation have a straight influence on the heat flux, as will the temperature of the neighbouring rooms; the working time of the refrigerating and humidifying systems will be affected consequently. The temperature difference (Δt) and surface of the evaporators will make up for the quantity of water subtracted from the circulating atmosphere. The relative humidity will tend to stabilize at considerably different levels, depending on the ventilation system, linked to the (automatic) refrigeration, continuous or delayed.

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