Hygrometers Prepared by Ben Oman, ECE 5320, March 2012

Outline What is a hygrometer Typical applications Types of Hygrometers How each type works References

What is a Hygrometer? A hygrometer is a sensor that senses the amount of evaporated water in air by a mechanical or electronic method.. Image Source:

Hygrometer Applications Meteorology and weather prediction. Guitar Shops Hardwood floor services Various manufacturing processes.

Types of Hygrometers Metal/pulp coil Hair Tension Electronic o Capacitive o Resistive o Thermal Conductive o Chilled Mirror Dewpoint Gravimetric Psychrometer

1) Metal/pulp coil sensor Image Source: Composed of metal or paper coil. Coil expands/contracts as humidity changes. Amount of expansion is calibrated to correspond with actual amount of humidity. Used to show humidity on a dial display. Quite inaccurate, differences between similar coils are 10% to 20%

2) Hair Tension Image Source: The hair on a violin bow and the hair on my head expands and contracts according to the humidity. The more humidity that is the air the longer the hair is. The less the humidity the tighter and shorter the hair gets. This transduces the amount of humidity into a force which can then be converted into an electronic or other mechanical signal.

3a) Electronic Capacitive Sensors A capacitor is like a short term battery where static charge can be built up and stored between two metal plates. Depending on the moisture between the two plates, more or less charge can be built up between them. This amount of charge is referred to as the Dielectric Constant Image Source:

For a electronic capacitive hygromotor, this relation between energy storage and humidity can be calibrated to an accuracy of +/- 2% between 5 and 95% humidity ( ). Electronic Capacitive Sensors are low- cost, small, and durable. They perform well in environments where temperatures fluctuate frequently. 3a) Electronic Capacitive Sensors Continued...

3b) Electronic Resistive Sensors Image Source: /10/29 /what-happens-if-it-gets-wet/ The famous concept where salt water conducts electricity to turn a light on is utilized in a electronic resistive sensor. As humidity decreases, the salt concentration increases causing the resistivity of the circuit to decrease. Thus, resistance is directly proportional to humidity.

Since voltage changes are much smaller when dealing with small humidity differences, precise measurement equipment must be used making these sensors less practical. Temperature variation also produces varied results during constant humidity and must be taken into account when measuring from this sensor. 3b) Electronic Resistive Sensors Continued...

3c) Thermal Conductive Sensors Image Source: 1/23/ heat-conduction-with-water-balloon-and- candle/ As air's humidity changes, so does it's ability to conduct heat. An electronic device that produces heat and and another device that precisely measures heat can be calibrated to produce results corresponding to actual humidity.

3d) Chilled Mirror Dewpoint At a constant pressure, the temperature at which water vapor turns to liquid is positively correlated and proportional to the humidity. A Chilled Mirror Dewpoint hygrometer uses an optoelectronic sensor to detect whether there is dew on a glass mirror. Using a feedback control loop, the temperature of the mirror is raised and lowered until condensation occurs. This temperature is calibrated with a measurement of humidity. This type of humidity sensor is one of the most accurate sensors available. Image Source: htm

3d) Chilled Mirror Dewpoint Continued... Image Source:

4) Gravimetric Sensors Image Source: ssWeb /PressGasLaws.html A gravimetric hygrometer uses gravity to calculate humidity. By measuring the weight of a column of air, a proportion can be determined when comparing that weight with the weight of dry air.

4) Gravimetric Sensors Continued... "[Gravimetric hygrometers are] considered the most accurate primary method to determine the moisture content of air." - Gravimetric hygrometers are typically large and cumbersome to use and are thus typically used to calibrate less precise hygrometers.

5) Psychrometer Image Source: /ice-melting-in-water-with-thermometer-at-zero- degrees-Celsius-AJHD.jpg.html A psychrometer is a device that is made up of two thermometers that are placed in two different mediums. One thermometer is placed in a wet location and is entitled the "wet-bulb" thermometer. When above freezing this is the colder thermometer. When below freezing this is the warmer.

5) Psychrometer Continued Image Source: uatic&sc=Aquarium+Equipment&ssc=Thermometers Another thermometer is placed in a dry environment in a similar location and is called the "dry- bulb" thermometer. This termometer is usually warmer above 0 degrees C and colder below 0 degrees C.

5) Psychrometer Continued Image Source: The readings from each thermometer are plotted on a "psychrometric chart" and the corresponding humidity can be found based on the altitude that the chart corresponds to.

References/Inspirations "Hygrometer" - "Psychrometrics" -