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Last edited 27 Jun 2022
Types of building sensors
A sensor is a device that detects the change in an environment and responds with an output. In terms of contemporary building design this is normally a digital signal or a measurable analog voltage. This output can then be processed digitally and formatted into a form that can be read, understood, displayed, recorded and monitored over time. Sensors have for a long time been key components in building services equipment and are often crucial to the functioning of modern buildings. The growing trend for the Internet of things (IoT) and its use in smart buildings has increased the use of smart sensors and their capability to track, remotely-control and regulate various aspects of the internal and external environment. Sensors have also been, for some years, a key tool in the field of post occupancy evaluation (POE) and building performance evaluation (BPE).
There is a wide range of sensors that might be found and employed in the built environment, some perform similar tasks in different ways, some different tasks in similar ways, their technology as well as type is continually expanding but some examples are given below.
Motion, occupancy or vacancy sensors are used in a variety of situations within and around buildings, for security and lighting control as well as more continual in-use monitoring and assessment. A key specification of sensors that monitor use on a more continual basis is that they need to do so whilst maintaining privacy and and comply with the General Data Protection Regulations (GDPR). Below a variety of examples are given.
Passive infrared (PIR) sensors- are small, low powered and cost effective, they sense a change in temperature between the background and a warm heat emitting body. Everything emits low-level radiation, the pyroelectric sensor detects the differing levels of infrared radiation and thus senses movement or presence. Also called presence detectors these might be used within rooms such as offices, cubicles such as in washrooms or on the underside or desks or tables to detect occupancy.
Microwave sensors have a wider range but are more expensive, vulnerable to electrical interference and with the ability to penetrate surfaces which can cause false alarms. Continuous waves of microwave radiation or a high radio frequency are sent outwards and the reflections off an object are measured by noting the shift in frequency as the waves return.
 Dual tech/hybrid
Dual tech/hybrid sensors are a combination of the above, so that both sensors need to sense changes to cause a trigger which reduces the number of false alarms, from wind movement or temperature changes etc.
 People counters
Time-of flight sensors are similar to people counters often using the reflection of an infrared beam on a sensor, to determine whether occupants are moving towards or away from a point, for example as an entry/exit sensor.
Infrared arrays use the same technology as described but have greater capacity to detect directional movement within spaces and as such are able to assess how occupants might move around a building or room.
People flow sensors, is a more generic term for sensors that can detect more accurately movement in a space in the same way infrared arrays can. These can monitor the real-time movement or flow of people around a space. They only detect movement, not faces, and as such are not cameras which makes them privacy and GDPR compliant.
An image sensor detects light waves and converts these into digital information used to convey and often record an image or series of moving images via a photographic camera. In general within buildings the installation and use of digital cameras and the use of associated software is complex as a result of privacy and data protection regulations (GDPR). However they are an increasingly common technology employed in the built environment and used for a variety of reasons many of which fall under privacy legislation.
Closed-circuit television, commonly known as CCTV, is a video monitoring system in which all of the circuits are closed and all of the elements are directly connected. This is unlike broadcast television where any receiver that is correctly tuned can pick up the signal. CCTV may employ point to point (P2P), point to multipoint, or wireless links.
Night vision (IR) surveillance cameras are used for surveillance in poor lighting conditions such as fog or during the night, using infrared technology. Some of the major applications of these infrared night vision cameras include; continuous monitoring of critical assets, face recognition, threat detection, and traffic management.
 Medical imaging
 Facial recognition
Biometric sensors are a relatively new breed of sensor that combine a variety of technologies with the aim of identifying individuals. They often work across different types to enable the cross checking of identities, for example facial recognition being cross checked with fingerprints.
 Facial recognition
A facial recognition system uses biometrics to map facial features from a digital photograph or video, often in real time. It compares the information in the image with a database of known faces to find a match and can help in verifying a person's identity. This kind of use is controversial and much debated as it raises a number of privacy issues.
Fingerprint sensors might be used in security entrance systems in buildings connected to locking devices. They might include capacitive sensors, which use an array of tiny capacitor circuits to collect data from the surface of a scanner, optical sensors which use light to illuminate the finger, and detect and create a fingerprint image by its ridges or multispectral which captures tissue features below the skin surface as well as on it.
Iris sensors are used but not commonly as they need darkness to optimise the infrared image contrast and facilitate analysis. A special camera is needed to illuminate the iris with infrared light and then filter out other wavelengths.
A sound sensor has a thin material diaphragm that vibrates when hit by sound waves. This vibration is converted by the sensor into an electrical signal for interpretation by recognition systems. Voice recognition sensors as part of an IoT approach are increasingly common in buildings from electronic voice activated home assistants to control lighting, heating, television and radio to automated service systems in hotels. As with cameras where sensors or microphones become recording devices a number of privacy issues arise and are much debated. Whilst sensors that recognise human commands are none the less quite common, sensors that recognise specific voices for identification are less common.
Keystroke sensors are essentially keyboard sensors that recognise, interpret and assess when certain keys on a keypad are pressed. The way a key pad works is more to do with the type of sensor - there are 5 basic systems with a number of subcategories. Conductive systems are either with metal or leaf contacts as well as membrane and reed systems. Electromagnetic keys are capacitive, magnetic movement or valves and inductive. Optoelectric with a light source and detector and acoustic keys sense pressing through vibration detection.
 Electrical current sensors
Electrical current sensors can measure the real-time energy consumption of a circuit, of a whole building or particular zones within a building or independently for particular machines or pieces of equipment. This can be used to track energy costs but can also help identify specific areas where usage is high or energy is being wasted, allowing adjustments and potential savings to be made.
Sensors can detect direct or indirect electrical currents, and do so with open loop systems, where the current is measured without electrical contact to the primary circuit that is being measured ie with isolation or in closed loop systems that to varying degrees make contact with the primary system being measured.
Depending on the system installed they generate an analog voltage, a current (which can be measured with a ammeter) or produce a digital output, all of which will be relative to the current that is passing through the system being monitored. This is turn gives an indication of energy demand made on the electrical supply or the consumption. These sensors work in a variety of ways
 Current transformers (CT)
A transformer might be used to step down or reduce a current to a more usable level, particularly if an electric current is too powerful, for a specific piece of equipment. CTs are also used where the primary current is being measured on the basis of Faraday's Law of induction, which predicts how a magnetic field will interact with an electrical circuit, producing an electromotive force, known as electromagnetic induction. Transformers work with two sets of coils, one which take in the higher input current and the other that outputs the lower current that is stepped down by a certain ratio to make it useable and safe for an appliance. In the case of measuring electrical current, measuring the lower AC current will give an indication of the higher AC current as there is a ratio between the two. These cannot measure DC current and are often called current clamp meters and are generally only used temporarily.
 Current transducer split core
Current transducers convert electrical energy into other types of energy, such as power, voltage, or current and are used in applications such as microphones, car horns, and photoelectric cells. Current transducers, in particular, convert alternating current (AC) to direct current (DC) signals for use in control systems that monitor electricity usage. A current is sensed and then converted or transduced into a proportional and measurable output. The split in the core allows the CTs to open and be placed around the conductor without disconnection or disruption to the wiring. They are convenient but in general more expensive.
 Current transducer solid core
Current transducer convert electrical energy into other types of energy, such as power, voltage, or current and are used in applications such as microphones, car horns, and photoelectric cells. Current transducers, in particular, convert alternating current (AC) to direct current (DC) signals for use in control systems that monitor electricity usage. A current is sensed and then converted or transduced into a proportional and measurable output. These are generally less expensive, more accurate and used in new installations.
Hall effect current sensors use the principle of converting the magnetic field that is produced around the current being measured to a voltage, that can be read. The output voltage of a hall sensor is directly proportional to the strength of the field so gives an accurate measure for both. This relationship to a magnetic field is known as the Hall effect and is also the basis of other types of sensor used to measure speed, proximity and positioning.
 Rogowski coil
A rogowski coil is an inexpensive way measure alternating current using special coreless, non-magnetic helical coils, which can measure high currents. They have low inductance which is suited to measuring high-speed current pulses they are immune to DC so can withstand large overloads and are used to find small AC currents in the presence of large DC currents. They can be used for very high voltage systems such as for arc furnaces, in large motors, high-end welding systems, and for short-circuits.
Temperature sensors measure changes in temperature, used for years in heating and air conditioning systems, but also in manufacturing and computing for protected from overheating. IoT now allows for automation control of heating, ventilation and air conditioning to maintain ideal conditions and automatically detect failure.
There are four types :
 Resistance temperature detector
 Negative temperature coefficient thermistor
Humidity or relative humidity (RH) is often important for machinery as well as building fabric and occupants. Sensors might be used to control heating, ventilating, and air conditioning systems in homes, hospitals, manufacturing plants, museums, greenhouses and weather stations.
Contact sensor or position, status and building monitor sensors are a very simple on off sensor to tell whether a door, window or something else is opened or closed. Two parts use magnetic fields to detect when they are in contact or touching and thus the window for example is closed. They contain a reed switch which is released to transmit a signal, in real time either via a wired or wireless connection. These are an inexpensive way to monitor aspects of a building to aid with security and energy efficiency, automatically detecting unlocked doors and open or broken windows, based on live occupancy data.
These devices are used to detect contaminants in the air, including particulates, pollutants and noxious or unwanted gases that may be harmful to human health or impact performance. These sensors work by measuring the attenuation of infrared radiation radiation (of a specific wavelength) in the air or through a porous membrane to chemically oxidize or reduce on an electrode amount of current produced determines the type and concentration of gas. In general there are a few specific substances monitored in buildings and also more specialist monitoring tools that may also include particlulate levels in and around the built environment, with the development of IoT many of these can also be monitored remotely or set up with warning messages directly to a smart phone.
Smoke sensors or smoke detectors are some of the most common sensors, now often legally required in buildings and detect different levels of airborne particulates and gases to trigger sound alarms in the case of localised fire.
Oxygen electrochemical sensors can detect any gas that can be oxidized or reduced electrochemically. Chemical sensors detect the presence of chemicals in water or air. They’re used to track air and water quality in cities, to monitor industrial processes and to detect harmful chemicals, explosives and radioactive materials.
Carbon Monoxide electrochemical sensors, function similarly to oxygen sensors: These detect levels of carbon monoxide which can cause serious harm and may leak from localised boiler systems and are often required by law.
Carbon Dioxide sensors related to ventilation levels in buildings are increasingly being used, as evidence show's that increased levels of carbon dioxide can lead to tiredness and poor concentration levels as well as being an indicator of poor ventilation. The sensor transmits an infrared beam through a light tube, the remainder of the beam’s energy detected on the other side of the air can be converted in to the level of carbon dioxide in the air. These sensors may also be fitted with warning lights or alarms that act as a trigger or prompt for windows to be either manually or automatically opened to keep CO2 levels down, for example in schools that have window actuators installed or just for teachers to manually open windows so as not to impact students performance.
Chemical sensors detect the presence of chemicals in water or air, used across cities but also in buildings to detect harmful chemicals, explosives and even radioactive materials. There are many different types of chemical sensors to be found; electrochemical, mass, optical, magnetic, and thermal sensors, often sensitive enough to detect single chemical vapours or biological molecules. Specialist examples where these might be used in buildings are to detect the levels of Volatile Organic Compounds (VOC's) from materials and equipment and Persistent Organic Pollutants (POP's) which might arise from the burning of fuels, both are various toxic chemicals that adversely affect human health and the environment.
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