Humidity and Moisture Sensors Panca Mudji Rahardjo, ST.MT. Electrical Engineering - UB

Concept of Humidity The water content in surrounding air is an important factor for the well-being of humans and animals. The level of comfort is determined by a combination of two factors: relative humidity and ambient temperature. Humidity is an important factor for operating certain equipment (e.g., high-impedance electronic circuits, electrostatic-sensitive components, high-voltage devices, fine mechanisms, etc.). A rule of thumb is to assure a relative humidity near50%at normal room temperature (20–25◦C). This may vary from as low as 38% for the Class-10 clean rooms to 60% in hospital operating rooms.

Humidity can be measured by instruments called hygrometers Humidity can be measured by instruments called hygrometers. The first hygrometer was invented by Sir John Leslie (1766–1832) . To detect moisture contents, a sensor in a hygrometer must be selective to water, and its internal properties should be modulated by the water concentration. Generally, sensors for moisture, humidity, and dew temperature can be capacitive, conductive, oscillating, or optical.

There are many ways to express moisture and humidity, often depending on the industry or the particular application. The moisture of gases is expressed sometimes in pounds of water vapor per million cubic feet of gas. The moisture in liquids and solids is generally given as a percentage of water per total mass (wet-weight basis), but may be given on a dry-weight basis. The moisture in liquids with low water miscibility is usually expressed as parts per million by weight (PPMw).

The term moisture generally refers to the water content of any material, but for practical reasons, it is applied only to liquids and solids, whereas the term humidity is reserved for the water vapor content in gases.

The following are some useful definitions: Moisture: the amount of water contained in a liquid or solid by absorption or adsorption which can be removed without altering its chemical properties. Mixing ratio (humidity ratio) r: the mass of water vapor per unit mass of dry gas. Absolute humidity (mass concentration or density of water vapor): the mass m of water vapor per unit volume v of wet gas: dw =m/v . In other words, absolute humidity is the density of the water vapor component. It can be measured, for example, by passing a measured quantity of air through a moisture-absorbing substance (such as silica gel) which is weighed before and after the absorption.

Relative humidity: the ratio of the actual vapor pressure of the air at any temperature to the maximum of saturation vapor pressure at the same temperature. Relative humidity in percent is defined as where Pw is the partial pressure of water vapor and Ps is the pressure of saturate water vapor at a given temperature. The value of H expresses the vapor content as a percentage of the concentration required to cause the vapor saturation, [i.e., the formation of water droplets (dew) at that temperature]. An alternative way to present RH is as a ratio of the mole fraction of water vapor in a space to the mole fraction of water vapor in the space at saturation.

Dew-point temperature: the temperature at which the partial pressure of the water vapor present would be at its maximum, or saturated vapor condition, with respect to equilibrium with a plain surface of ice. The dew point is the temperature at which relative humidity is 100%. In other words, the dew point is the temperature that the air must reach for the air to hold the maximum amount of moisture it can. When the temperature cools to the dew point, the air becomes saturated and fog, dew, or frost can occur.

Sensor Types and Technologies Capacitive RH Sensors Capacitive RH sensors are used widely in industrial, commercial, and weather telemetry applications. They dominate both atmospheric and process measurements and are the only types of full-range RH measuring devices capable of operating accurately down to 0% RH. Because of their low temperature effect, they are often used over wide temperature ranges without active temperature compensation. In a capacitive RH sensor, change in dielectric constant is almost directly proportional to relative humidity in the environment. Typical change in capacitance is 0.2–0.5 pF for 1% RH change. Bulk capacitance is between 100 and 500 pF at 50% RH at 25°C. These sensors have low temperature coefficient and can function at high temperatures up to 200°C. They are able to fully recover from condensation and resist chemical vapors. Response time ranges from 30 to 60 seconds for a 63% RH step change.

Applications for capacitive RH sensors are wide ranging, including Automotive onboard devices such as windshield defoggers Computer printers Medical devices such as ventilators and incubators Appliances such as microwave ovens, refrigerators, and clothes dryers HVAC Recorders and data loggers Leak detection Weather stations Industrial and food processing equipment Environmental test chambers

Advantages (capacitive RH sensor) Near-linear voltage output Wide RH range and condensation tolerance Interchangeable, if laser trimmed Stable over long-term use Disadvantages Distance from sensing element to signal conditioning circuitry limited

Resistive Humidity Sensors Resistive humidity sensors measure the impedance change, which usually has an inverse exponential relationship to humidity. Typically, the impedance change of a medium such as a conductive polymer, salt, or treated substrate is measured.

Selecting Resistive Humidity Sensors Resistive sensors are small, low-cost humidity sensors that provide long-term stability and are highly interchangeable. They are suitable for many industrial, commercial, and residential applications, especially control and display products.

Advantages (resistive RH sensors) No calibration standards, so highly interchangeable and field replaceable Long-term stability Usable from remote locations Small size Low cost Disadvantages Exposure to chemical vapors and contaminants may cause premature failure Values may shift when water-soluble coatings are used

Thermal Conductivity Humidity Sensors Thermal conductivity humidity sensors (also known as absolute humidity sensors) measure absolute humidity by calculating the difference between the thermal conductivity of dry air and air containing water vapor. These sensors are constructed using two negative temperature coefficient (NTC) thermistor elements in a DC bridge circuit. One of the elements is sealed in dry nitrogen, while the other is exposed to the environment. The difference in the resistance between the two thermistors is directly proportional to absolute humidity.

Selecting Thermal Conductivity Humidity Sensors Thermal conductivity humidity sensors are commonly used in appliances, including clothes dryers and microwave ovens. They are used in many industrial applications including wood-drying kilns, drying machinery, pharmaceutical production, cooking, and food dehydration.

Advantages Disadvantages Very durable Work well in corrosive and high-temperature environments up to 575°F Better resolution than capacitive and resistive sensors Disadvantages Responds to any gas with thermal properties different than dry nitrogen, which may affect measurement.

Reference JACOB FRADEN. “HANDBOOK OF MODERN SENSORS PHYSICS, DESIGNS, and APPLICATIONS”. T h i r d E d i t i o n. Springer. 2003. Jon S. Wilson. “Sensor Technology Handbook” Newnes. 2005.