Four questions on using differential pressure sensors in HVAC systems

Patrick Niedenführ
Patrick Niedenführ, February 2019

In practice, the measurement of flow data in HVAC (heating, ventilation, air-conditioning, cooling) systems asks a lot of the measurement technology used. In addition to temperature fluctuations, sensors are also exposed to a lot of dust.

Conventional differential pressure sensors often fail in long-term use due to blockages. By using micro-flow channels, this risk can be almost completely eliminated. In this article, you will learn how First Sensor’s LMI series ensures the highest level of precision and durability, whatever the application.

Essential requirements for ultra-low differential pressure sensors

The most important questions at a glance:

  • What aspects are relevant when using sensors in HVAC systems?
  • How does First Sensor’s micro-flow technology meet these requirements?
  • What are the differences between a micro-flow channel and conventional solutions?
  • How do you improve long-term stability?

You can read more about the requirements regarding differential pressure sensors in air-conditioning technology in the blog article “What requirements must pressure sensors meet in HVAC?”

Volumetric flow measurement as an application in air-conditioning technology

Differential pressure sensors measure the pressure drop of flow elements such as an orifice or a measuring cross in order to determine the volume flow. The sensor is located in a bypass to the main flow.


In variable air volume (VAV) controllers, pressure measurement is used to control the position of the air valve within a closed control circuit and thus to regulate ventilation as required.


Other HVAC applications:

  • Filter monitoring
  • Monitoring of indoor air pressure
  • Intelligent measuring instruments
  • Burner control
  • Security systems

HVAC systems often require highly sensitive pressure sensors for the following reasons:

1. Only sensors that use the principle of thermal mass flow are sensitive enough to determine values in the pascal range.


2. Easy installation, no interference with the main current and a compact size are the absolutely basic requirements.

Sensors are exposed to air contaminated with dust particles. This may result in...

  • dust particles being deposited in the inner flow channel of the sensor;
  • an increase in the sensor’s pneumatic impedance;
  • a decrease in the sensor’s output signal;
  • the sensitivity of the sensor being disturbed;
  • decalibration;
  • the dust particles blocking the internal flow channel of the sensor and thus leading to total failure.

Excellent dust immunity thanks to advanced micro-flow technology

Micro air flows are guided through the sensor’s flow channel, thus reducing the speed of the incoming air. As a result, dust may be deposited before it reaches the sensor input. The goal is microscopically small flow channels in the sensor in the µ range.

Conventional technology vs. micro-flow channels

With both methods, the measurement takes place in a separate bypass channel. In the LMI sensor, this is a micro-flow channel in the sensor chip, in conventional solutions simply a much larger channel in the plastic housing.


In conventional measuring systems, the air flow is guided in parallel past the sensor chip. This involves a certain risk of particle deposits on the sensor and incorrect measurements.


In the LMI sensor, the micro-flow channel leads directly through the silicon sensor chip. The risk of particle deposition is virtually eliminated.


LME-LMI_Volumetric flow measurement_reversed_E

LMI sensors in a dust test

“Standard” dust (ASHREA Test Dust #2) was used for the test. The test conditions corresponded to those of real HVAC applications. In a closed circuit, very dusty air was guided through a pipe system. The tests were carried out with different pressures at standard air speeds. To adequately test the special requirements, a very high dust concentration was used, simulating years of exposure under the worst conditions.

LMI sensors are suitable for a wide range of applications

The space-saving design of LMI sensors cannot be beaten. The height – including the inputs and outputs – is just 9 mm. The design also ensures a high degree of compatibility with brackets and applications of all kinds. Complete modules can also be developed and manufactured if required.


Depending on the requirements, all parameters can be adjusted to always ensure the highest possible accuracy. We are also happy to carry out test series for you and take your special requirements into account.

The advantages of dust-proof sensor technology

  • LMI sensors can tolerate 1,000 times more dust than comparable sensors.
  • LMI sensors are significantly more accurate over long periods of time, permanently increasing the reliability of the entire HVAC system.
  • LMI sensors therefore need to be replaced much less frequently than other products.
  • This results in considerably lower inspection, maintenance and repair costs.

Webinar on the use of differential pressure sensors in HVAC systems 

LMI sensors are consistently precise over long periods of time, even under very dusty conditions, and are extremely low-maintenance. Find out how First Sensor’s LMI series performed in the test in this webinar.


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