Sensing humidity using advanced technology

dry bulb wet bulb hygrometerAn approaching thunderstorm creates a very stuffy environment with oppressively heavy moisture in the air. The presence of water in the air is termed as humidity and this largely affects human comfort. The amount of water vapor influences many physical, chemical and biological processes. In industries, measuring and controlling humidity is critical since it can affect not only the health and safety of personnel, it can affect the business cost of the product as well.

Sleep apnea leads to repeated cessation of breathing during sleep. People, who suffer from sleep apnea, have to wear a mask to prevent nasal collapse. The mask is connected to a Positive Airway Pressure machine that sends pressurized air through the nasal passage of the patient, to prevent it from collapsing. It is important to monitor the humidity of the air the patient receives, keeping it at the appropriate level of comfort to allow the patient to sleep comfortably.

polymeric humdity-sensing filmTraditionally, humidity or relative humidity was measured with the wet and dry bulb hygrometers. This method is neither accurate nor convenient in the industrial environment. With advancement in technology, solid-state devices are now available, which measure humidity with very high accuracy, repeatability and interchangeability. Solid-state humidity sensors are generally of two types, capacitive and resistive.

In resistive type humidity sensors, the resistance of the element changes responding to variations in humidity in the environment. The construction is in the form of two intermeshed printed combs, made of a thick film conductor of a precious metal such as gold or ruthenium oxide. The two combs form two electrodes, the space between them being filled with a polymeric film. This film has movable ions whose movement is governed by humidity. The film thus acts like a sensing film whose resistance changes with change in humidity.

Polymer humidity-sensing filmThe capacitive type of humidity sensor has an Alumina substrate on which the lower electrode is formed using either gold or platinum. A dielectric polymer layer such as thermoset polymer is then deposited on the lower electrode. This layer is sensitive to humidity. On top of this polymer layer, a top electrode is placed, and this is also made of gold or platinum. The top layer is porous and allows water vapor to pass through into the sensitive PVA layer. Moisture enters or leaves the sensing layer until the vapor content is in equilibrium with the environment. This sensor is therefore a type of capacitor whose capacitance changes with the change in humidity.

humidity sensorsThe arrangement of a hygroscopic dielectric material sandwiched between two pairs of electrodes, forms a capacitor whose value is governed by the dielectric constant of the hygroscopic material and the sensor geometry. At normal room temperatures, the value of the dielectric constant of water vapor is about 80, which is much larger than the constant of the sensor dielectric material. Therefore, as the sensor absorbs water vapor from the environment, it results in an increase in the capacitance of the sensor.

Both the resistive type and capacitive type of humidity sensors are available in the form of small surface mount SMD packages, and pre-calibrated to simplify, speedup manufacturing and reduce the cost for Original Equipment Manufacturers.