By Benjamin Brits

When thinking about sensors, their technology, or the function behind these varied devices, one would not particularly realise that they have in fact become a part of everyday living.

While clients are generally more concerned about aesthetics, function and how much they have to spend, intricacies in the systems of the HVAC&R sector are not top of mind even though it’s the little things, like sensors, that play a significant role in short- and long-term savings. Their use in new plants today is a given, and their addition to old plants with a little adjustment can generate immediate results in savings.

Sensors have, in some instances, been referred to as pervasive in the human context. They are embedded within so many devices and tallying these up could open your mind to the reasoning for statements such as this. They are in the computers or laptops we use on a daily basis, in our smartphones, in our watches, electronic appliances, cars, and especially required in our air conditioning and refrigeration systems, to name but a few.

Sensors come in a variety of forms and capacities. This image illustrates the types and connectivity. Image credit: Carel

Now although these devices are found extensively across so many applications, one should also consider the fact that without sensors we would not enjoy many of the products, processes, or conveniences we consider critical to our modern-day existence, not to mention that automation could not exist. Improvement in system efficiency is also made possible through sensors, their continual development and improvement.


Humidity control has been linked to various wellness aspects for offices and plays a further role in a number of other applications where the correct humidity is essential for stores goods for example. Image credit: Testo

It would be safe to say that for any engineering concept, a sensor already exists. For the HVAC&R environment, these would include sensors for temperature, pressure, humidity, indoor air quality, smoke or gas detection, flow rates, motion, consumption and even occupancy. Further, sensors in this field would be broken down into categories such as sensitivity, range of measurement, degree of accuracy, response time and importantly output that would include, in simple terms, a ‘monitor only’ or ‘read and perform action’.

Any sensor works by acquiring a physical input and converting this into a signal suitable for a measuring or processing task through detecting differences in quantity. Today, this ‘signal’ is more commonly output in an electrical form and could also be wired to a controller, be part of a network, or even enabled to deliver that signal via wireless or Bluetooth options. Each type of sensor would also have a different means of ‘translating’ the input to an output – for example internally you would see components that react under pressure or particular heat range, simple different metal strips/plates that react differently to the input, resistive materials, or particular chemical reaction.

Looking back to where sensors originated, this spans many decades and was directly linked to the progress observed in each particular industry sector. Pressure sensors were invented in the mid-1800s owing to a number of accidents involving steam units, thermal sensors are shown to have been developed on a large-scale in the mid-to-late 1800s, smoke detection was patented at the start of the 1900s, infrared technology came online in the 1940s and motion sensors were first used in the 1950s.

Sensors3Temperature sensors and temperature probes are used in all HVAC&R applications – some of which require extreme sensitivity. Iamge credit: Danfoss

The sensors of old, were of course far different from the sensors we see today; which come in nano-technology form, such as what is seen in the latest HVAC window applications that aim to contribute to managing a buildings heat loads or energy conversion techniques. Compared to 20 years ago, there is a far broader requirement for digital connected technology today, because with that comes data, and data can be translated or converted into many more functional aspects today that holistically improve all technologies and also opens up integration opportunities that have become far more important to clients than before, and tie into their digitising strategy.

Sensors4Many commercial applications make use of in-duct CO₂ sensors. Iamge credit: Distech Controls

HVACR systems have become reliant on these clever devices that perform the function of verifying operational conditions and improve performance. Through sensors in a building the occupant health, safety, comfort, and productivity can be monitored and improved, or, in a refrigeration application for example, are involved in several measurement points to ensure that system function is in order and that produce is kept at the right temperature. Data gathered from sensors further offer the ability to see historic events and play a role in predictive future events.


Given their obvious importance, sensors that are faulty or out of calibration can affect the ability of any system, process, or action to be completed as designed and with the holistic drive around the world to improve every aspect of our living, comfort and how we do and make things, ensuring that sensors are running efficiently, and smoothly is critical. The slightest variations could ruin a batch of processed foods in the cold chain or inaccurate readings could lead to the transmission of airborne pathogens or even death due to gas leaks or smoke detection failing. Sensors therefore are said to also be key in avoiding mishaps.

Common sensors in HVACR

Given the supposed endless range of sensors and metering devices available, what follows are some of the most common, or critical sensors used in the sector, where they could be used and some important highlights of their requirements:

  • Pressure sensors
    Pressure sensors are used in compressors, boilers, coolers, heat recovery systems, air handling units and variable air volume systems. They typically monitor components such as filters for drops in pressure which may indicate that there is a fault or that the system requires maintenance through some type of alert mechanism. The ability to monitor pressure is also useful for optimising air flow, heating, or cooling, and so on.

    These sensors can measure extremely high and low pressures in air and fluid applications offering precise measurement of pressure, differential pressure, and velocity.

    Monitoring the pressure drop across filters is known to be an essential approach to reducing energy costs and preventing unnecessary load on air-moving equipment. Various case studies exist, illustrating the use of sensors versus not, produces significant energy savings.

    In large buildings, the fan/air circulation portion of the HVAC system may include a device to regulate the static pressure developed by the fan.

    In addition to being able to be permanently installed in a system, some pressure sensors may be used by service technicians for maintenance or trouble shooting that can identify ‘leaks’ and save the building owner money.

  • Temperature sensors
    Temperature sensors measure air and fluids and can signal adjustment requirements to the system (controller) based on the programmed setpoint and therefore preventing wasted energy. Once could also use the sensor’s produced data to learn about a particular room’s conditions over time.

    Incorporating temperature sensors would also prevent a system from running harder than required. Accurate and reliable temperature readings are necessary for producing optimal conditions in comfort, processing, storage, or industrial applications.

    Sensors of this type can also be fixed or mobile/wireless to achieve tasks such as:

    • Spot checking of foods
    • Temperature monitoring of food transportation
    • Measuring the temperature in the centre of flue gas flow in chimneys/flue gas ducts
    • Surface measurements
    • Immersion measurement in chemical solutions
    • Measurement of any mechanical components
  • Humidity sensors
    Controlling humidity in buildings is critical for occupant comfort, safety and protecting building infrastructure, production processes, stored goods, and even museum artwork. Combined temperature and humidity sensors provide a flexible and cost-effective solution to achieve this. Humidity control typically adds clean steam to the airstream to raise space humidity.

    Although sensors for humidification are of course as important as the humidification equipment itself, one cannot really work without the other. For regular HVAC applications, the normal run of the mill, capacitive sensors are generally suitable as long as they are accurate within the units’ tolerances and the sensors are importantly placed in the right position, which has been shown to sometimes be incorrect. If these minimum criteria are met these sensors becomes a forgotten component.

    There are, of course, certain applications where extreme accuracy in specialised applications is demanded which in turn requires specialist devices. This would be relevant for example is a pharmaceutical production line, or in applications where particular chemicals are developed. The wrong conditions during transportation of produce as another example can result in major losses in quality, right through to a complete loss of value.

  • Indoor air quality sensors
    Sensors that measure indoor air quality (IAQ) have become increasingly important, particularly over concerns by the general public over health and safety aspects and heightened awareness around airborne pathogens. The basic IAQ sensor is a carbon sensor, which detects carbon levels in the air. Carbon levels are an indicator of air circulation (or poor air circulation) that increases the chances that other contaminants could be present.

    In an occupied office for example both carbon dioxide and carbon monoxide levels must be kept to certain maximum levels according to building standards applicable around the world. The carbon dioxide concentration and the IAQ are deemed to be the most important indicators that the quality of the indoor air is ‘good enough’, when people themselves are the main source of this emission. Many studies have shown that poor indoor air quality leads to tiredness, lack of concentration and can even bring about illnesses. The CO₂ concentrations in an indoor environment should therefore not exceed 1000 parts per million (ppm) as a rule.

    Sensors5Monitoring of refrigerant gas is particularly required in confined zones such as plant rooms and where occupants may be exposed such as hotel rooms. Image credit: Bacharach

    Carbon monoxide is a combustion gas (harmful gas) that is released when combustion processes are incomplete. The most frequent day-to-day sources of CO are petrol engines, gas ovens, heating systems and solid fuels, such as wood and charcoal that are commonly used in and around South African cities. Carbon monoxide is colourless and odourless and prevents the blood from taking up oxygen when too great a concentration is breathed in. Breathing in a CO concentration of 700 ppm in an enclosed space leads to death after around 3 hours in adults.

    A dangerous gas like carbon monoxide requires measuring instruments which are particularly reliable. To be on the safe side, you have to know which CO meter detects flue gas absolutely reliably. The highest permissible concentration of carbon monoxide in a workplace is 30 ppm.

    For many of these factors, facility managers are increasingly adapting their strategies such as increased ventilation, improved filtration, and air cleaning aimed to enhance occupant safety, comfort, and ensure productivity levels are not compromised.

  • Occupancy/room sensors
    In their most basic model, occupancy sensors identify the presence of a person in a room. Occupancy sensors can help building owners or tenants improve their overall energy efficiency, particularly when used in combination with zoned HVAC systems.

    There are two main types of occupancy sensors, PIR and ultrasonic. PIR or passive infrared, sensors measure heat and motion. They work via line of sight, so the sensors have to be aimed at where inhabitants will be and are best used in open rooms. Ultrasonic sensors send out a high-frequency sound wave. If it bounces back with a change, the system is triggered. Ultrasonic waves can pass through solid objects, so they can pick up smaller movements and are more sensitive than PIR sensors.

    Integration to the building management system (BMS) allows multiple savings through controlling any zone’s temperature to the ‘unoccupied’ setpoint. Occupancy sensors can also detect ambient brightness levels, enabling the BMS to control powered light levels in the space. These sensors are typically in high-traffic or isolated areas, such as the building entrance, hallways, lavatories, floor, office rooms, etc.

    Room sensors monitor motion and share data on occupancy activity patterns and environmental changes in temperature, humidity, and CO₂. This data is invaluable in running an environmentally friendly building.

  • Gas and smoke detectors
    Gas and smoke risks are some of the most dangerous aspects to have to deal with in any building and therefore require sensors that are extremely reliable in order to avoid accidents and potential damage.

    When system pipes are getting on in years and there is increased wear over time, the risk of leaks increases. Leaks of any kind can lead to major losses, such as water damage or life-threatening gas/refrigerant accidents. A leak flow meter is indispensable in terms of preventing consequences of this kind due to dilapidated pipes or pipes that have been damaged through renovation or maintenance.
    Sensors6Sensors today can be wired or wirelessly linked to a system and building management system. Image credit: Testo

    Ducts can move toxic gasses, smoke, and even flames from one area to another. Duct smoke detectors are used primarily to prevent the HVAC system from spreading smoke through a building. Duct smoke detectors are also required by regulations and building codes in many countries. For example, some regulations call for smoke detectors to be installed in return ducts for air conditioning units larger than a certain capacity.

    Sensors also detect volatile organic compounds (VOCs). VOC sensors use a process called photoionization. Ionization is the changing of an atom’s charge. VOC sensors can ionize particles with ultraviolet light and measure electron levels. These measures allow them to detect toxic or combustible gases.


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