# The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings
## 2 Sensors and Monitoring Technologies
### Temperature Sensors  
Temperature sensors are devices designed to measure the temperature of a flowing media such as air, water, etc., and are one of the most essential and commonly employed sensors for monitoring and control in buildings today. The four main kinds of temperature sensors currently available on the market include (1) Thermocouples; thermoelectric devices which operate based on the Seebeck effect (a phenomenon in which a voltage difference is produced between two dissimilar electrodes or semiconductor wires proportional to the temperature difference between them [23]), and are easily identified by a thermocouple probe consisting primarily of two color-coded wires, (2) Resistance temperature detectors (RTDs); devices which measure temperature based on a change in resistance in a metal resistor inside them, with Pt100 sensors (using platinum as the metal, having a resistance of 100 \(\Omega\) and a typical temperature coefficient of 4 \(\Omega\)/\({}^{\circ}\)C [24]) being the most widely adopted RTDs, (3) Thermistors; devices similar to RTDs but containing a ceramic or polymer rather than a metal inside them, and typically featuring two wires, (i.e., red for excitation and black for ground), and (4) Integrated Circuit (IC) sensors; two terminal integrated circuit temperature transducers that produce a current or voltage signal (either analogue or digital) proportional to the absolute temperature and are readily available on the market in small IC packages. Although each of the aforementioned temperature sensors will provide the end user with the required results in most cases, published literature and information available online [4; 24; 25; 26; 27; 28; 29; 30] suggest that their respective features and properties, such as simplicity of operation, accuracy, temperature measurement range, response time, and inevitably cost can vary significantly (see Table 1). Thermoouples are generally the most commonly used sensors in the temperature measurement world, which is attributed to their good blend of wide measurement range (\(-\)100-500 \({}^{\circ}\)C), fast response times (5-80 s), self-powering capability, and low cost (SS6-SS50). However, these devices are not as accurate (\(\pm\)1-4 \({}^{\circ}\)C) or stable (due to chemical changes such as oxidation within the sensor) as some of the other types of temperature sensors, and typically, do not possess wireless communication capabilities. RTDs, on the other hand, offer good accuracy (\(\pm\)0.2-1 \({}^{\circ}\)C), both wired and wireless capability, repeatability, higher immunity to electrical noise, and linearity (i.e., the temperature-resistance relation in RTDs is linear, whereas, in thermocouples, it tends to be an 'S'-type plot). However, RTDs have a narrower temperature measurement range (\(-\)50-250 \({}^{\circ}\)C), slower response times (1-8 min), are larger in size and are relatively expensive (SS30-SS100). In comparison, thermistors offer exceptional accuracy (\(\pm\)0.05-0.5 \({}^{\circ}\)C), faster response times (0.2-10 s), high sensitivity, both wired and wireless capability, availability in small sizes, and a lower cost in comparison to RTDs (SS20-SS70). Despite these features, thermistors can only measure temperatures over a fairly limited range (\(-\)50-130 \({}^{\circ}\)C), are non-linear  
Figure 2: Prominent sensors and monitoring technologies for buildings applications. (i.e., the large non-linear effect needs to be calibrated before using it in a specified measurable range), suffer from self-heating (at higher temperatures where their resistances are lower), and can be somewhat complex to use. Finally, IC sensors are the latest of the four temperature sensors currently available and are typically available in two main types; analogue output, and digital interface devices.