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22 ST/Industrial Sensing Solutions It is also possible to use two pressure sensors for measuring water level in water level monitoring systems. In the inner top (ceiling) of the tank, a pressure sensor can measure the ambient pressure, and the second pressure sensor may get employed as a probe within the water. The dif- ference between the data that are provided by the two sensors can be used as a threshold to stop the further flow of water into tank. Temperature Sensor There are different types of temperature sensors, among others, thermocouples, RTDs (Resistance Temp Detectors), thermistors, and Si Band-gap sensors. The graph illustrates various types of temperature sensors with their respective operating temperature range (Figure 9). The following will focus on Si band-gap sensors, which are offered by ST. In addition to using the temperature sensor to measure the ambient temperature values, a temperature sensor plays a crucial role in many applications such as compensation of temperature impact on various machine components, predictive maintenance (monitoring and avoiding system overheating), asset tracking, smart home & thermostat, HVAC, respiratory equipment, white goods, solar panels, and power supplies (Figure 10). Machine Condition Monitoring Temperature data are critical when it comes to monitoring the overheating of machines. The collected data via temperature sensors may get employed as part of a data set to develop a predictive model that will then be used to monitor the temperature of the equip- ment for predictive maintenance solution. Humidity Sensor MEMS-based humidity sensor measures the relative humidity that is the ratio of the amount of moisture actually in the atmosphere compared to how much moisture could be in the atmosphere at a given temperature. MEMS-based humidity sensor provides an accurate and cost-effective solution in a small form factor with digital interface making it an ideal choice for integra- tion in a multi-sensor environment for industrial applica- tions. Humidity sensors find applications in weather stations, smart home (air quality monitoring), heat index alert, smart agriculture, respiratory equipment/ humidifiers, incubators, home appliances such as dryer and fridge, and in stor- age/goods tracking solutions (Figure 11). MEMS Microphone A MEMS microphone consists of two components in a single package. The acoustic sensing structure uses a vibrating membrane and a fixed plate to convert acous- tic pressure waves into a change in the capacitance of the sensing structure. The second component is a CMOS interface IC that converts the change in capacitance into a Pulse Density Modulation (PDM) output. Figure 10: A variety of critical temperature sensor applications. Figure 9: Types of temperature sensors with their respective operating temperature range.