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).
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 (like 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?
Differential pressure is the difference in air pressure between two points. It’s the ‘push’ that drives airflow, moving air from a high-pressure to a low-pressure area.
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. E.g.: the differential pressure over a fan indicates how much “push” is moving the air.
- Differential pressure and filter monitoring
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.
Differential pressure sensors provide real-time measurements of the pressure difference across the filter, similar to how a thermometer measures temperature. Simpler solutions also exist, such as differential pressure relays with adjustable setpoints. These relays indicate whether the pressure difference is above or below a setpoint but do not provide the actual measured value. - Differential Pressure and AirflowIn nature, airflow or wind, occurs because air moves from areas of high pressure to low pressure.
Ventilation is essential for achieving good indoor air quality. It creates airflow — a movement of air. Fresh outdoor air is supplied while stale indoor air and pollutants are extracted.
In ventilation systems, a pressure difference is often 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. Fan speed also plays a role: higher speeds create greater pressure differences and stronger air movement.
You can 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.
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 AirflowAir 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.
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.
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.
In nature, airflow or wind, occurs because air moves from areas of high pressure to low pressure.
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.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.
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.
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.
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.
A Pitot tube is a small device used to measure how fast air is moving inside an air duct, a pipe, or even around an airplane. It works like a tiny “air speedometer” by sensing how strongly the air pushes into it. Based on this pressure, the differential pressure sensor can calculate the air velocity. At the top of the Pitot tube are two connection points that are linked to the sensor with transparent air hoses.
A connection set is another way to measure differential pressure. It 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.
A Pitot tube is a small device used to measure how fast air is moving inside an air duct, a pipe, or even around an airplane. It works like a tiny “air speedometer” by sensing how strongly the air pushes into it. Based on this pressure, the differential pressure sensor can calculate the air velocity. At the top of the Pitot tube are two connection points that are linked to the sensor with transparent air hoses.
A connection set is another way to measure differential pressure. It 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.
In this example, we 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.
This results in an airflow volume of 72 m³/hr.