Hobbies And Interests
Homemade Hygrometer
An amateur meteorologist can get a general idea of the relative humidity by securing three thoroughly washed pieces of human hair (all cut to the same length) to a plastic panel. When the hairs are dry, a mark is made on the plastic to identify the zero humidity mark. The hairs are then soaked in water for four hours. They soak up the water and grow to their maximum length. Another mark is made representing the one hundred percent mark. Three evenly spaced marks are made between the zero and 100 percent markings. These represent 25 percent, 50 percent and 75 percent humidity. By monitoring the length of the hairs, an amateur meteorologist can get a general reading of relative humidity.
Sling and Aspirating Psychrometers
A sling psychrometer is a hygrometer that uses two thermometers to measure the humidity in an area. The instrument places two identical thermometers next to each other, a dry bulb which is unmodified and will give the actual temperature, and a wet bulb thermometer that employs a wet cotton wick to cause evaporative cooling. When the sling psychrometer is swung around, the evaporative effect of the wet bulb thermometer causes a temperature difference between the two thermometers. By calculating the difference and referring to a standards chart, the meteorologist can determine relative humidity.
An aspirating psychrometer works in the same way, but is cooled by an electrically powered fan, so there is no need for the meteorologist to swing it around. These are much more compact than the sling versions.
The major disadvantage to psychrometers is that they're not calibrated for multiple elevations, so readings may not be accurate if they're used more than 1000 feet higher or lower than the region in which they were calibrated.
Capacitive Hygrometer
A meteorologist can use a capacitive hygrometer to measure humidity by letting it read the air temperature and then adjusting the surface temperature of the sensor unit until condensation begins to form on its surface. The condensation helps the surface hold an electrical charge. The sensor box reads the amount of electricity that is being held and once it reaches a full charge (as defined by the sensor board), a surface temperature reading is taken. The difference in the temperatures is referred to on a reference chart to determine the humidity.
Resistive Hygrometer
A resistive hygrometer works in a similar manner as the capacitive version, only it measures the amount of resistance that the sensor has to electrical current. Once enough condensation has accumulated on the surface so that it can begin to conduct electricity, the sensor box reads the electrical conductivity of the surface and the temperature at which it has allowed a completed current. The difference in temperature between the initial ambient temperature and the temperature at which condensation occurs is then compared to a reference chart to determine the humidity.