ANALYTICAL & LABORATORY EQUIPMENT 21


evaporates from the lithium chloride solution at a rate that is controlled by the vapour pressure of water in the surrounding air. When the reel begins to dry, the resistance of the lithium chloride solution increases, and less current flows through the winding. This allows the reel to cool. This heating and cooling of the reel reaches an equilibrium point where it neither takes on nor gives off water, and the equilibrium temperature is directly proportional to the dewpoint of the surrounding air.


Table 1. Typical calibration intervals for measurement equipment.


Humidity and dewpoint sensors include:


1. Psychrometer 2. Mechanical hygrometer 3. Lithium chloride dewpoint indicator 4. Resistance type hygrometer 5. Capacitance type hygrometer (dew indicator) 6. Mirror dewpoint indicator Sensors 1–6 can measure general humidity levels.


Sensors 5 and 6 are also used for low dewpoint measurement. The principle of each technology is described briefly in the following.


bulb and the other a bulb that is kept wet to measure wet-bulb temperature. The wet bulb cools by evaporation of the water. The amount of evaporation, as well as cooling of the thermometer, depends on the humidity of the atmosphere. This data, together with humidity tables or calculations, is used to determine the vapour pressure of water in the surrounding air, and relative humidity. This is a method often used in laboratories and humidity and temperature test chambers.


1


that expands and contracts with humidity changes, such as human hair. This type of measurement has been used for a long time. The accuracy of the method is not very good.


2


the hygroscopic characteristic (ability of a substance to attract water molecules) of lithium chloride. The sensor consists of a reel covered with an absorbent fabric and a bifilar winding (two insulated wires, with currents travelling through them in opposite directions) of inert electrodes. The reel is coated with lithium chloride. An alternating current is passed through the winding and the lithium chloride solution, causing resistive heating. As the reel heats, water


3


A lithium chloride dewpoint indicator is a measurement principle based on


A mechanical hygrometer measures and records humidity using an instrument


A psychrometer is a simple form of a hygrometer, which consists of two thermometers. One has a dry


absorbs moisture. Special sensors are used to measure the resistance to a current passing between wires. This type of sensor is suitable for mass production and seems to be most used for home appliances and consumer products. However, it may not measure accurately in very low or very high humidity environments.


4


polymer film. This type of sensor can easily achieve sufficient accuracy, and is mostly used in industry. The patented HUMICAP humidity sensors manufactured by Vaisala use this technology.


5


containing water vapour is cooled. A mirror is cooled until it reaches the dewpoint of the gas in question. As dew condensation forms, it changes the light reflected from the mirror. When the mirror surface reaches an equilibrium state whereby evaporation and condensation are occurring at the same rate, the temperature of the mirror is equal to the dewpoint temperature of the tested gas.


6 Sensors mostly used in cleanrooms include the


resistance type hygrometer, capacitance type hygrometer (dew indicator) and mirror dewpoint indicator. When selecting a suitable instrument, it is important not only to pay attention to the price and product specifications, but also to consider the measurement accuracy,


International level


National Measurement Standards Laboratory


In-house Laboratory Sl-Units Pressure National Standard Primary Standard


A mirror dewpoint indicator utilises the occurrence of dew at dewpoint temperature when air


A capacitance type hygrometer measures humidity by detecting the change in capacitance of a thin


A resistance type hygrometer utilises the principle that electrical resistance varies in a material that


➠ Sl-Units Temperature National Standard Primary Standard Primary Humidity Generator Working Standard Customer Working Standard Calibration for Humidity and Temperature Instruments


Table 2. Example of a traceability chain for installed humidity and temperature measurement units.


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