Measurement of Atmospheric Moisture

Wet & Dry Bulb Hygrometer

• This is an instrument equipped with two thermometers, side by side.
• The bulb of one of the thermometers is covered with a muslin wick dipping into water.
• The water is absorbed up the muslin and evaporates, cooling.
• The difference between the wet and dry bulb temperatures is called the wet bulb depression.
• If the air is fully saturated (100% relative humidity) the water cannot evaporate, so both the wet and dry bulb temperatures are the same.

                                                            

Hair Hygrometer

• These devices use a human or animal hair under tension.
• The hair is hygroscopic (tending toward retaining moisture); its length changes with humidity, and the length change may be magnified by a mechanism and indicated on a dial or scale.

• The traditional folk art device known as a weather house works on this principle. Whale bone and other materials may be used in place of hair.
• This instrument uses strands of human or horse hair with the oils removed attached to levers that magnify a small change in hair length.

Sling psychrometer

• It gets its name from the fact that this instrument comprises a chain/handle on one end, which has to be used to swing the two thermometers mounted on its frame around in circles to generate air flow.
• This process allows the device to measure a temperature difference, which is directly proportional to the humidity in the air. Thus, the user is able to determine the value of humidity with an appreciable level of accuracy.
• Due to lack of air movement around the D&W bulb, sling Psychrometer is used in testing Lab.

The sling psychrometer works by allowing the water soaked up by the cloth wick/sock covering the wet-bulb thermometer to get evaporated into the air.

To assist in this process, the handle/chain provided at the other end is used to whirl the device around. This whirling action generates air flow, and enables the wick/sock to come in contact with higher quantity of air, thus allowing evaporation to take place more effectively.

Electrolytic hygrometer

• A formerly used type of hygrometer that measured humidity based on the function of temperature and electrical conductivity of a hygroscopic salt.
• Most LITHIUM CHLORIDE HYGROMETERS contain a bundle of glass fibers coated with a dry film of lithium chloride salt.
• The fibers are wrapped with metal wires connected to a continual AC source. When the fibers are dry they are nonconductive.
• When the fiber surface absorbs moisture, they become conductive allowing a current to pass through the wire.
• This current heats the fibers which in turn drives off the moisture from the salt. Thus the wires are kept at the temperature required to just maintain dryness. This temperature can be directly converted to humidity.
• Lithium chloride hygrometers can take 3-10 minutes to respond to environmental changes (thomson 1978). They require a continual electrical source.
• Electrolytic hygrometer. The electrodes are covered with a thin, hygroscopic film of phosphoric pentoxide (P2O5). The sample gas is passed through the glass tube.
• When voltage is applied to the electrodes, current flowing will vary with change in the percentage of R.H of the atmosphere.