AN607

Rev. 1.7

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7. Humidity Measurement

For relative humidity measurements, it is not necessary to measure the ambient temperature unless you are using

a psychrometer. However, to determine the dew point or absolute humidity, the ambient air temperature is required.

Accurate air temperature measurements can be a significant challenge, since air is a poor thermal conductor and

the temperature at any given point can be impacted by air currents and temperature gradients. It is very important

to understand the dynamics of your measurement system and the dynamics of the environment being measured.

Before taking a measurement, you always need to wait long enough to ensure the temperature and humidity are

stable and the sensor(s) of the measurement instrument are in equilibrium with the ambient conditions to be

measured.

A wide range of techniques are employed to measure humidity. These range from simple mechanical indicators, to

highly complex and expensive analytical instruments. In general, measuring humidity (dew point, absolute

humidity, specific humidity, mixing ratio, relative humidity or equivalent wet bulb temperature) is not a trivial task.

Many of the instruments currently available have poor accuracy, narrow-bandwidth, contamination issues,

hysteresis and measurement drift over temperature and time. Regular calibration is required on some instruments,

which is both inconvenient and expensive. Some instruments are large, awkward, and expensive. Discussed

below, are different methods of humidity sensing. The new generation of humidity sensor technology used in

Silicon Labs’ solid state humidity sensors provide superior accuracy, minimum drift, low cost, low power, small size

and ease of use.

7.1. Psychrometer

A psychrometer is the oldest method for measuring humidity more commonly known as the wet bulb/dry bulb

method. A psychrometer consists of two thermometers, one with an ordinary dry bulb and the other with a moist

cloth covering the bulb (wet bulb). Evaporation from the moist cloth lowers the wet bulb thermometer's

temperature. The wet-bulb thermometer shows a lower temperature, Twb, than the dry-bulb thermometer, Tdb, if

the air is not saturated with water vapor. The temperatures are the same in saturated air. The amount of

evaporation is dependent on the relative humidity of the air (more evaporation and lower Twb with lower RH).

Given the dry bulb and wet bulb temperatures the relative humidity is looked up on a psychrometric chart. For

example, at sea level if Tdb = 25 °C and Twb = 18 °C, RH = ~50%. Looking up the %RH on a chart for every

measurement is both time-consuming and cumbersome but can be automated with a microcontroller. A

psychrometric sensor can achieve good precision with %RH resolutions of 0.01% for humidity ranges from

10–100% at temperatures from 0 to 60 °C, and accuracy of 1%. The disadvantages of a psychrometric sensor are

a slow response time, large physical size, the need to keep one thermometer bulb wet, the need to have airflow

around the wet bulb, and high cost.

7.2. Chilled Mirror Hygrometers

The chilled mirror hygrometer is considered the most accurate and reliable hygrometer. Chilled mirror hygrometers

use a cooled mirror with an optoelectronic mechanism to detect condensation on the mirror surface at an

accurately measured temperature. The system is configured to reflect LED light off a mirror at an angle of

approximately 45 degrees with a photo-transistor detecting the reflected light. The temperature of the mirror is

electronically controlled, typically with a Peltier-effect device. The system works by cooling the mirror's surface

below ambient temperature until condensation forms, causing the LED's light to scatter resulting in a sudden drop

in the output of the photo-transistor. The surface temperature of the mirror is read using an accurate temperature

sensor such as a thermistor. The temperature at which condensation forms is the dew point. All humidity values

can be calculated from the dew point. The mirror temperature can be controlled with a feedback loop to

continuously track the dew point. The chilled mirror is the most stable and accurate method to determine relative

humidity. However, it is crucial to keep the mirror clean, provide a method of clearing the condensation and to

ensure that the temperature sensor and mirror are of high quality. This method operates over the full humidity

range (0-100%RH) and can be used for numerous gases at many pressures. Chilled mirror hygrometer

instruments are bulky and very expensive.