CHEMICAL SENSORS FOR BIOMEDICAL APPLICATIONS · 357 measurements of the temperature and humidity of the analyzed air are made, with further analysis and data saving by the computer. The work presented by Fleischer and Simon (2005) was carried out in the course of the Low- Cost Gas Sensor System in Microsystems Technology for Medicine and Biotechnology (LOCMED) project. This joint university and industry research project targeted the development of innovative microelectromechanical system (MEMS) gas sensors for medical use and biological process control. Motivation for the selection of bronchial asthma as the medical target application was clear: besides the fact that it is a widespread disease with prevalence in the populations of industrialized countries of 5% to 15%, a well-defined and reasonably well-described marker gas exists. This is NO, which constitutes a general biomarker for pulmonary inflammation processes. Exhaled air is very humid, which strongly affects the signals from conventional low-cost gas sensors based on semiconducting oxides of the SnO 2 or WO 3 type, so this sensing technology was discarded for the selective detection of low NO levels. An alternative and more modern type of gas sensor was selected and further developed, based on work function changes in a sensing material and read out via field effect transistors (FETs). This hybrid construction consists of two parts, a gateless FET and a suspended gate covered with a gas-sen- sitive layer, attached using an appropriate process. A construction based on flip-chip technology allows industrial use of this technology and low-cost production. In these devices, the gas to be detected adsorbs at the gas-sensitive layer and generates a small voltage (typically 100 mV) at the interface of the sensitive layer and the ambient gas (physically speaking, this is a change in the work function). This voltage then couples via the air gap capacitance to the transistor channel and modulates the source- drain current of the device. Advantages of this device type include its operability at temperatures from 120°C down to room temperature, as well as allowing a free choice of sensitive materials. 7.6. HUMIDITY SENSING There are several special definitions relevant to humidity measurement techniques. Humidity is defined as the mass of vapor carried by a unit mass of vapor-free gas. The partial pressure of each species in the mixture is in direct proportion to its molar fraction. Thus H = (M A x A )/(M B (1 - x A )), (7.18) where M is the molecular weight and x is the molar fraction of the constituents; component A is the water vapor and component B is the air. Relative humidity (RH) is defined as the ratio of the actual partial pressure of water vapor (p) to the saturated vapor pressure ( p s ) of the water at the gas temperature: RH = p/p s . (7.19) The dew point can also be used as a characteristic parameter for the water vapor content in the air. It is defined as the temperature (T d ) at which the gas becomes saturated during cooling: p(T d ) = p s (T ), or T d = T p = ps . (7.20) The connections between the mentioned parameters are given by the laws of psychrometry (Nor- ton, 1982), and a few examples are given in Table 7.2. There are three major groups of humidity measuring methods: those that measure mechanical property changes; psychrometric measurements, which compare the latent heat of evaporation of a