Differential pressure devices in ventilation systems

09/09/2025 Yves Vinck

Fresh, clean air is essential for human health and comfort, yet many people rarely think about the invisible forces that make proper ventilation possible. Behind every breath indoors lies a balance of gases, pressures, and flows that determine whether the air feels crisp and energizing—or stale and suffocating. One of the key players in this process is air pressure and, more specifically, differential pressure, the driving force that makes air move through ventilation systems. By understanding how air pressure works, and how differences in pressure affect airflow, we can better appreciate the importance of well-maintained ventilation systems and the sensors that keep them running efficiently.

What is air pressure?

Human beings cannot live without fresh air. Our bodies need oxygen (O₂) to produce energy. Without sufficient oxygen, we feel tired or dizzy and can even suffocate. While breathing, we inhale oxygen and exhale carbon dioxide (CO₂).

When many people are in a closed space, CO₂ levels will rise. Ventilation or the supply of fresh air can reduce CO₂ build up. Without the supply of fresh air, CO₂ builds up indoors and can cause headaches, drowsiness, … or worse. A ventilation system guarantees sufficient fresh air supply to wash away CO₂ and other pollutants.

Fresh air is the clean, outdoor air that has a natural balance of gases and is free from harmful levels of pollutants, smoke, or accumulation of stale air (like in closed rooms). Nearby sea level, fresh air is composed of:

Nitrogen (N₂): ~ 78 %
Oxygen (O₂): ~ 21 %
Argon and other noble gases: ~ 0,9 %
Carbon dioxide (CO₂): ~ 0,04 %
Water vapor: variable (0–4 %, depending on the humidity)

Air consists of tiny particles called 'molecules'. Billions of molecules are moving around with every breath you take. Although they are very light, they have an own weight. The molecules move around at high speed and collide with everything, including you. Each collision is a small push. The push from a single molecule is small, but there are so many of them colliding with surfaces from all directions that together they create a noticeable force, air pressure.

At sea level, the weight of the air column above you presses down with about 1 bar (100.000 Pascal) of pressure. That’s roughly 1 kilogram of force on every square centimeter of your skin. Comparable to the weight of a bag of 5-6 apples pressing on every cm² of your body! You don’t feel crushed because the fluids inside your body are under the same pressure, balancing it out.

Air pressure is simply how hard the air pushes on a surface. It works just like water pressure when you dive: the deeper you go the stronger the push. Air is like an invisible ocean of gas, always pressing on everything around us.

For the techies among us: Air pressure is measured in Pascals (Pa), which represent the amount of force acting on a surface. Specifically, 1 Pa = 1 Newton per square meter (N/m²). The official SI (International System of Units) uses the Pascal as the standard unit of pressure, but in practice pressure is often also expressed in bar, where 1 bar = 100,000 Pa. The SI itself is the modern, internationally agreed version of the metric system.

What is differential pressure?

The air we live in exerts a certain pressure on all objects. This pressure isn't the same everywhere. In some places, the air pressure is higher than in others. Differential pressure is the difference in air pressure between two points.

This differential pressure also causes air to move, pushing it from areas of higher pressure to areas of lower pressure. Differential pressure is the ‘push’ that drives airflow, moving air from a high-pressure to a low-pressure area.

But you can also look at it from another perspective: When you walk outside in stormy weather with an umbrella, it will block a lot of wind. This will cause air to accumulate on one side of the umbrella, creating positive pressure or overpressure. On the other side, negative pressure or underpressure will be created. Thus: by placing an object in an airflow, you create a differential pressure.

In ventilation systems, differential pressure is typically measured across a component like a fan or air filter. Monitoring differential pressure helps assess filter condition, airflow, air velocity and system performance.

Differential pressure and filter monitoring
Imagine a narrow corridor with a gate in the middle to count people passing by. This gate will cause a queue (overpressure). Behind the gate, the flow of people will flow smoothly again. An air filter can be seen as the gate positioned in an airflow. The airflow collides with the filter, creating overpressure. Particles are retained, while air molecules can pass through the filter. A differential pressure will therefore develop across the air filter. The harder it is for air to pass through the filter, the higher the differential pressure over the filter will be.

A rising pressure difference across a filter signals it may be clogged and needs cleaning or replacement. A ventilation system can only provide clean air if its filters are properly maintained. Clogged or poorly maintained filters restrict airflow and lose their effectiveness at capturing particles. Timely cleaning or replacement is essential to ensure the system functions correctly.

When a filter is clean, it only slightly restricts airflow, resulting in a minimal differential pressure across the filter. As the filter accumulates dust and particles, airflow becomes increasingly obstructed, causing the differential pressure to rise. Monitoring this pressure difference provides a clear indication of the filter’s condition over time.

In systems like SenteraWeb cloud, thresholds can be set for each sensor. When differential pressure reaches the alert zone, maintenance should be scheduled. If it reaches the out-of-range zone, urgent replacement is required to prevent compromised indoor air quality.

To measure air filter contamination, the following solutions can be used:
- Differential pressure sensors provide real-time measurements of the pressure difference across the filter (similar to how a thermometer measures temperature). This differential pressure measurement provides an indication of the filter's condition. The higher the differential pressure, the more contaminated the air filter is.
- Simpler solutions also exist, such as a differential pressure relay. A pressure relay switches when the switching point is exceeded. Pressure relays indicate whether the pressure difference is above or below a setpoint but do not provide the actual measured value. It only signals when the air filter needs to be replaced.
- Sentera has combined the advantages of differential pressure sensors and differential pressure relays in a turnkey solution for monitoring air filters: the FIM series. The differential pressure measurements are continuously stored in the cloud. When the threshold is exceeded, a notification is sent via email or text message.
Differential Pressure and Airflow
Airflow is the movement of air particles. Air particles are pushed from a zone of high air pressure to a place of lower air pressure. This movement of air particles is called airflow. So airflow flows from an area of high air pressure to an area of lower air pressure. In nature, weather phenomena create these pressure differences and cause wind. In a building we want to create an air flow to supply sufficient fresh air. Fresh outdoor air is supplied while stale indoor air and pollutants are extracted. In ventilation systems, a pressure difference is created by a fan. The fan increases the pressure on its outlet side (overpressure) and decreases it on its inlet side (underpressure). This imbalance produces airflow. The larger the pressure difference across the fan, the stronger the airflow.

Simply summarized: higher fan speed creates greater pressure differences and stronger air movement.

Now imagine airflow like people moving through a street: a wider street allows more people to pass, and if the people move faster, even more pass through in the same time. Similarly, in an air duct, a larger cross-section allows more air to flow, and the faster the air moves, the more air passes through per hour. Mathematically, the airflow volume is calculated by multiplying the air velocity by the cross-sectional area of the duct.

Differential pressure sensors measure the pressure difference before and after a fan (or filter). From this difference, the sensor can calculate the airflow, making it an easy way to check if the fan is delivering the correct amount of air. If the exact amount of airflow is less important, and only an indication of airflow is needed, a pressure relay can be used.

Airflow can be calculated either based on the cross-section of the air duct or the K-factor of the fan. The K-factor is a constant that links the airflow through a fan to the pressure it produces — essentially describing how much air a specific fan moves for a given pressure difference. Each fan has its own K-factor, which can usually be obtained from the supplier.

To measure airflow using a fan with a known K-factor, a differential pressure sensor is combined with a simple connection set. The measurement points should be placed far enough from the fan’s inlet and outlet to avoid being placed in the turbulent zone of the airflow. The inlet side (lower pressure) connects to the sensor’s “–” nozzle, and the outlet side (higher pressure) connects to the “+” nozzle. For a simpler approach, the “–” nozzle can remain open to ambient pressure, which serves as a reference and provides a reasonably accurate measurement of the airflow volume.

For the techies among us: Air volume flow is measured in cubic meters per hour (m³/h) and indicates the amount of fresh air supplied or extracted over a given period. The airflow can be determined by measuring the differential pressure.

This is an example of calculating airflow volume using a differential pressure measurement. Suppose a fan has a K-factor of 150, and while it is running, the differential pressure across the fan is 100 Pa. This pressure is measured with a differential pressure sensor using a standard connection set. The calculation proceeds as follows:

In this example, the fan generates an airflow of 1.500 cubic meters per hour.
Air Velocity and Airflow
Air velocity describes how fast air is moving, much like how a car has a certain speed. It is usually determined from velocity pressure, which can be measured using a Pitot tube. A Pitot tube is a small instrument that can be placed inside an air duct, a pipe, or even around an aircraft, and it measures the pressure created by moving air. In a way, it works like a tiny “air speedometer.” From the pressure it detects, the sensor can calculate the speed of the airflow. At the top of the Pitot tube are two connection points that are linked to the sensor with transparent air hoses.

To measure air velocity, the Pitot tube is connected to a differential pressure sensor. The tube has two openings: one facing directly into the airflow, which captures the total (impact) pressure, and one on the side, which senses the static pressure of the air. The difference between these two pressures is called velocity pressure, and it provides a measure of how fast the air is moving.

Once the air velocity is known, the airflow volume can be calculated if the size of the air duct is known.

By combining a differential pressure sensor with a Pitot tube, it is possible to accurately measure both air velocity and airflow volume, providing essential information for ventilation system performance and efficiency.

How do differential pressure sensors work?

A differential pressure sensor always has two connection points, called 'nozzles'. These nozzles allow the air to flow over the electronic sensor element. Therefore, it is very important that the measured air is clean and free from corrosive elements.

The nozzle that is indicated with a '+' must be connected to the point with the highest pressure (over pressure side).
This is before the air filter or at the fan's output side.
The nozzle that is indicated with a '-' must be connected to the point with the lowest pressure (under pressure side or ambient pressure). In some applications this nozzle may not be connected in order to measure the ambient pressure.
This is after the air filter or at the fan inlet side.

The nozzles can be connected either to a normal connection set (set of plastic tubes) or either to a Pitot tube.

When a pitot tube is connected to the differential pressure sensor, the air velocity can be calculated. The sensor uses the measured differential pressure and the diameter of the air duct to calculate the air velocity.

A connection set connected to the differential pressure sensor can be used to measure either the differential pressure or the airflow volume. A connection set consists of two plastic fittings that are easily mounted in an air duct. These fittings are also connected to the differential pressure sensor using a transparent air hose.

If the fan's K-factor is unknown, the airflow volume can be calculated in another way. The differential pressure sensor can then be calculated using a pitot tube (air velocity) and the air duct diameter. In this example, we will calculate the airflow volume. Let's assume that the duct cross section is 0,02 m² (circular duct with D160 mm) and that the air velocity is 1 m/s.

This results in an airflow volume of 72 m³/hr.

Differential pressure plays a central role in understanding and controlling ventilation systems. By monitoring the pressure differences across fans, filters, and ducts, facility managers can ensure that fresh air is delivered efficiently, filters are maintained in time, and energy is not wasted. Whether through advanced sensors with cloud-based monitoring or simpler mechanical relays, measuring differential pressure provides reliable insight into airflow volume, air velocity, and overall system performance.

In practice, this means healthier indoor air quality, optimized system efficiency, and reduced operational costs. Just as a thermometer is indispensable for temperature control, differential pressure measurement is an essential tool for ensuring that ventilation systems work as intended—quietly, continuously, and effectively safeguarding the comfort and well-being of building occupants.

Sentera Differential Pressure Devices - The Product Range

Sentera's product range of differential pressure devices is divided into pressure relays, differential pressure sensors and differential pressure controllers. Pressure relays and sensors measure differential pressure, while controllers maintain the differential pressure constant at the desired setpoint. They control devices like a fan or a damper.

Pressure relay: above or below the switching point?

A pressure relay is a very simple device that detects whether the differential pressure is higher or lower than a certain value. It does not provide an accurate measurement of the differential pressure, it only indicates whether the differential pressure exceeds the switching point or not. It operates mechanically and therefore requires no power supply to function. The switching point can be adjusted using a screwdriver.

PSW series are pressure relays that are typically used to verify whether the air filter needs to be cleaned (or replaced). Another typical application is to verify whether the fan is functioning normally (whether there is a minimum airflow). PSW series are available for a certain pressure range (20-200 Pa or 50-500 Pa). They can be purchased individually or as a package with a corresponding connection set.

Sensors measure the differential pressure

A differential pressure sensor measures the differential pressure and transmits it via the analogue output signal (typically 0-10 volts or 0-20 mA) and via Modbus RTU communication (if available). The differential pressure measurement is accurate, and the full range is translated into a 0-10 volt (or 0-20 mA or PWM) signal, where 0 volt corresponds to the minimum differential pressure and 10 volts to the maximum differential pressure. The minimum and maximum values can be modified within the sensor's operating range. The measured differential pressure can also be read via the Modbus input register. Sentera offers differential pressure sensors to monitor fans and air filters and differential pressure sensors optimised for Air Handling Units.

Measure air flow and air filter pollution:

HPS series are available in following pressure ranges: -125 to +125 Pa | 0-1000 Pa | 0-2000 Pa | 0-4000 Pa | 0-10.000 Pa. For each pressure range, we offer the F version and the G version. The F version needs 24 VDC supply and features separate (isolated) GND connections for supply and analogue output. Therefore it is suited for 4-wire connection. The G version can be supplied with 24 VDC or 24 VAC. It has only one common GND for supply and analogue output. Therefore it is suited for 3-wire connection.
DPS series are identical to the HPS series, but additionally offer a display. They are also available in the same pressure ranges: -125 to +125 Pa | 0-1000 Pa | 0-2000 Pa | 0-4000 Pa | 0-10.000 Pa. For each pressure range, we offer the F version and the G version.
FIM18 series monitor differential pressure over an air filter (or fan). They do not have an analogue output. The differential pressure is logged in the SenteraWeb cloud. The evolution of the differential pressure can be visualised. It sends warnings and alert messages via email or SMS in case a threshold is exceeded and filter replacement is required. FIM series require a 24 VDC supply and a local internet connection via Wi-Fi or LAN ethernet cable.

Monitor air filters in Air Handling Units

HPD series are differential pressure sensors were specially developed to monitor both air filters in Air Handling Units (AHUs). A single sensor allows for differential pressure measurements at two different locations. Therefore, this sensor features two analogue outputs. It is available for following differential pressure ranges: 0-1000 Pa | 0-2000 Pa | 0-4000 Pa | 0-10.000 Pa. For each pressure range, we offer the F version and the G version. The F version needs 24 VDC supply and features separate (isolated) GND connections for supply and analogue output. Therefore it is suited for 4-wire connection. The G version can be supplied with 24 VDC or 24 VAC. It has only one common GND for supply and analogue output. Therefore it is suited for 3-wire connection.
DPD series are identical to the HPD series, but additionally offer a display. They are also available in the same pressure ranges: 0-1000 Pa | 0-2000 Pa | 0-4000 Pa | 0-10.000 Pa. For each pressure range, we offer the F version and the G version.
FIM28 series monitor the differential pressure over both air filters of the Air Handling Unit. They do not have an analogue output. The differential pressure is logged in the SenteraWeb cloud. The evolution of the differential pressure can be visualised. It sends warnings and alert messages via email or SMS in case a threshold is exceeded. FIM series require a 24 VDC supply and a local internet connection via Wi-Fi or LAN ethernet cable.

Controllers regulate fan speed or dampers

A differential pressure controller works differently than a sensor. It allows you to define a differential pressure setpoint – you can see it as the desired differential pressure or the desired amount of airflow. The setpoint can be adjusted via Modbus RTU communication. The Modbus master device can write the differential setpoint in the corresponding Holding register of the differential pressure controller.

Sometimes they are also referred to as CAV controllers or Constant Air Volume controllers. A constant amount of air is supplied, regardless of the demand or need for ventilation.

The differential pressure controller generates an analogue output signal (typically 0-10 volts or 0-20 mA) to keep the differential pressure equal to this setpoint. To achieve this, the sensor uses PI control. PI control combines proportional and integrated actions. Thanks to PI control, the differential pressure can be kept as close as possible to the desired value in a decisive but non-aggressive manner. We distinguish differential pressure controllers for fans and for dampers. In both cases, PI control guarantees optimal control of the fan or damper.

Control fan speed to maintain constant pressure

The differential pressure controller regulates fan speed (this means: creating more or less airflow) to maintain the desired differential pressure. If the differential pressure is too low, the fan speed must be increased to build up more pressure (difference).

HPSP series are available in following pressure ranges: -125 to +125 Pa | 0-1000 Pa | 0-2000 Pa | 0-4000 Pa | 0-10.000 Pa. For each pressure range, we offer the F version and the G version. The F version needs 24 VDC supply and features separate (isolated) GND connections for supply and analogue output. Therefore it is suited for 4-wire connection. The G version can be supplied with 24 VDC or 24 VAC. It has only one common GND for supply and analogue output. Therefore it is suited for 3-wire connection.
DPSP series: These are identical to the HPSP series, but additionally offer a display. They are also available in the same pressure ranges: -125 to +125 Pa | 0-1000 Pa | 0-2000 Pa | 0-4000 Pa | 0-10.000 Pa. For each pressure range, we offer the F version and the G version.
SPS2 series: Sometimes you need to switch between a high and low air volume. SPS2G series are designed for applications that sometimes require a constant low airflow and sometimes a constant high airflow. For this purpose, they feature two setpoints. One of both setpoints can be selected via the dry contact input. SPS2G series are available in following pressure ranges: 0-2000 Pa | 0-6000 Pa. For each pressure range, we offer the F version and the G version. The F version needs 24 VDC supply and features separate (isolated) GND connections for supply and analogue output. Therefore it is suited for 4-wire connection. The G version can be supplied with 24 VDC or 24 VAC. It has only one common GND for supply and analogue output. Therefore it is suited for 3-wire connection.

Control a damper to maintain constant pressure

The differential pressure controller regulates the position of the damper blade (this means: more or less air can pass) to maintain the desired differential pressure. If the differential pressure is too low, the damper must close to build up more pressure and to let less air pass through.

HPSA series are available in following pressure ranges: 0-1000 Pa | 0-2000 Pa. For each pressure range, we offer the F version and the G version. The F version needs 24 VDC supply and features separate (isolated) GND connections for supply and analogue output. Therefore it is suited for 4-wire connection. The G version can be supplied with 24 VDC or 24 VAC. It has only one common GND for supply and analogue output. Therefore it is suited for 3-wire connection.
DPSA series: These are identical to the HPSA series, but additionally offer a display. They are also available in the same pressure ranges: 0-1000 Pa | 0-2000 Pa. For each pressure range, we offer the F version and the G version.