The amount of water vapour (vapour pressure) in the air is referred to as relative humidity. It’s a proportion of the amount of moisture that the air can store. The real water content of the air, the temperature, and the barometric pressure all contribute to relative humidity. Temperature increases the level of vapour that can be held in the air. The greater the relative humidity percentage, the more humid (wet) the air feels, and the lesser the percentage, the drying it feels. When air holds the maximum quantity of water vapour possible at the current pressure and temperature, it is said to be saturated. Saturation occurs when the relative humidity is equal to 100 percent, resulting in precipitation.
The relative humidity (RH) of a water-air mixture is a measurement of just how much water vapour is present compared to the maximum quantity attainable. RH is a comparison of a water-air mixture’s humidity ratio to the saturated humidity ratio at a given temperature (dry-bulb). It’s vital to remember that having both the dry-bulb temperature and the RH is required to apply relative humidity to a specific application. For example, the quantity of moisture in a water-air combination at 80 percent relative humidity at 40 degrees Celsius differs from the amount of water vapour in a water-air mixture at 80 percent relative humidity at 10 degrees Celsius. This is why, for example, postharvest storage recommendations have included relative humidity and temperature.
Even though relative humidity is a calculated value, it can be evaluated. The most common method for measuring RH is to utilise absorptive polymers, which change their conductive or capacitive properties according to just how much moisture they acquire from the air. The rate of absorption is relative to the quantity of moisture in the air. This method can be difficult for postharvest uses, especially those involving extremely high humidity and low temperature. Many of these sensors have poor reliability in the extreme RH range, making them unsuitable for condensing settings like coolers with high RH and low temperature. The sling psychrometer, which is detailed below as part of the discussion on wet-bulb temperature, is a conventional way of RH determination.
Relative humidity sensor
Together moisture and air temperature are sensed, measured, and reported by a humidity sensor (or hygrometer). The relative humidity is defined as the ratio of moisture in the air to the highest quantity of moisture at a given air temperature. Whenever it comes to finding comfort, relative humidity plays a big role.
Humidity sensors analyse fluctuations in the electrical currents but rather temperature in the air to determine humidity levels.
Humidity sensors are divided into three categories:
- Capacitive
- Resistive
- Thermal
To compute the humidity in that air, all 3 forms of sensors observe minute changes in the atmosphere. Let’s take a closer look at each of these types:
Capacitive:-
The relative humidity of a capacitive humidity sensor is measured by inserting a thin strip of metal oxide between two electrodes. The electrical capacity of metal oxide varies depending on the relative humidity of the environment. The main usage fields are weather, commercial, and industry.
The linear capacitive type sensors can measure the relative humidity in the range of 0% to 100%. The catch is that the circuit is complicated and that calibration is required regularly. However, for designers, this is a less arduous task than accurate measurement, therefore atmospheric and process measurements are dominated by these. These are the only full-range relative humidity measuring instruments that can measure relative humidity down to 0% relative humidity. Because of the low-temperature effect, they are frequently utilised throughout a large temperature range without intentional temperature correction.
Resistive:-
The electrical impedance of atoms is measured by resistive humidity sensors, which use ions in salts. The resistance of the electrodes along either side of the salt medium changes with humidity changes.
Thermal:-
The humidity of the surrounding atmosphere causes two heat sensors to conduct electricity. One sensor monitors ambient air while another is sealed in dry nitrogen. The humidity is measured by the difference between the two.
Condensation of moisture at the dew point:
Water vapour condenses when cold surfaces are accompanied by wet air and the temperature falls below the dew point. The surface appears to be sweating. Dehumidifiers are used to lower the humidity in the air, which helps to prevent condensation.
Dehumidifiers are necessary for cooling tunnels for wafer book cooling or chocolate enrobing coolers to prevent condensation on the object surfaces, even when the cooling temperature is set low. The item can then be chilled more quickly, eliminating the need for a larger, more costly cooler.
The relative humidity of the atmosphere at a specific temperature when a substance does neither gain nor lose moisture is known as equilibrium relative humidity (ERH). Hygroscopic materials achieve the ERH by absorbing or desorbing moisture at a specific relative humidity of ambient air. The concept of ERH calculation for ready-to-eat waffles and cakes from a recipe is used to determine the mould-free shelf-life. After a substance has attained its equilibrium relative humidity, the term ‘Equilibrium Moisture Content (EMC)’ is used to indicate the amount of moisture it holds.
Conclusion:-
The ratio of air’s vapour pressure to its saturation vapour pressure is known as relative humidity. The relative humidity of the air surrounding the food that is in balance with its surroundings is defined as the equilibrium relative humidity (ERH) of the food product. When equilibrium is achieved, the ERH (percent) equals the water activity multiplied by 100, i.e. ERH (percent) = aw x 100. When food is subjected to continual humidity, it will gain or lose moisture until it reaches the ERH. As previously stated, moisture migration has a substantial impact on the physical and chemical qualities of the food.