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Do the Flow Sensor HVAC and Get the Freshness Back...

Jens Vogt, Market Development Manager - Sensor Products - Omron Electronic Components Europe, discusses how their new MEMS flow sensor can help enhance HVAC system efficiency.

With the need to reduce energy consumption, there is a growing focus on flow sensing in HVAC systems as a key technology to help monitor and thus reduce energy use. A new alternative to the popular differential pressure sensor, the mass flow sensor, uses MEMS technology to produce much higher sensitivity and repeatability whilst being even easier to use and install. These sensors use thermopile chips together with micro machined structures to measure flows from 1mm/s to 40m/s – from the fluttering of a butterfly’s wings to the roar of a typhoon.

Selection criteria:
Well placed flow sensors and flow meters can have a huge impact on the operating cost of a HVAC system, providing that they are correctly selected and installed. In choosing the correct flow sensor, the system designer must consider a number of factors.

The most important consideration is the accuracy and repeatability of the flow sensor in the context of the gas that is being monitored. Most flow sensor manufacturers state their performance figures for a specific gas, and they may vary significantly between a flue gas environment, for example, and a clean air duct in an air conditioning system. The flow rate is also a key parameter in this context – ideally the change in the output voltage of the sensor should be very steep at the likely median flow rate, giving high resolution and high sensitivity.

Second, consider the location of the sensor. Flow measurement devices need a minimum length of unobstructed straight run pipe to achieve the specified performance figures.

Finally, consider the maintenance issues. Some devices need more frequent recalibration, especially in a dusty environment.

MEMS mass flow sensors:
Omron’s solution is based on a sensitive flow chip that is only 1.5mm2 in area and 0.4mm thick (figure 1). The chip has two thermopiles either side of a tiny heater element, and measures the deviation in heat symmetry caused by a passing air or other gas flow in either direction. A thin layer of insulating film protects the sensor chip from exposure to the surrounding gas.

When there is no flow present, temperature distribution concentrated around the heater is uniform, and the differential voltage of the two thermopiles is 0V. When even the smallest flow is present the heat symmetry collapses. The temperature on the side of the heater facing the flow cools, and the other side warms. The difference of temperature appears as a differential voltage between the two thermopiles, allowing the flow direction, velocity and the mass flow rate to be measured.

This technology offers highly repeatable accuracy at flow rates from 1lpm to 50lpm – higher if used in bypass configuration. Omron’s MEMS sensors can detect mass flows with repeatability down to +/-0.1% available on the most precise devices. Furthermore, a simple adjustment to the amplifier circuit allows the sensor to be calibrated for different gases and flow rate, allowing customers to specify their desired characteristics for each application.

The key benefit of MEMS mass flow sensors, compared with conventional differential pressure sensors, can be seen in figure 2. The graph shows that at low flow rates the output signal change in is much more rapid for a mass flow sensor, giving a clear electrical response to even relatively small changes in the flow rate – and higher accuracy and repeatability in the system as a whole.

Dust segregation system:
One of the most popular contexts for flow sensors is in the detection of clogged filters – which by definition entails installation in a dusty environment. Clearly, a sensor that is itself impaired by dust is useless in this context.

Omron’s MEMS flow sensors feature a patented dust segregation system (figure 3) which uses a 3D flow path structure that would separate and discharge the dust using centrifugal and inertial forces. Cyclonic air flows are set up inside the two centrifugal separation chambers (figure 4). The centrifugal force of the cyclone separates the dust and discharges it through the exhaust vent regardless of the installation direction of the chamber. Any dust that does accumulate is held at the bottom of the chamber by gravity, and does not enter the detection path of the flow sensor. This patented 3D Dust Segregation Chamber removes up to 99.5% of dry air-borne particles, based on Omron’s FEM analysis.

The basic principle of mass flow measurement of air or another gas using a thermopile associated with a MEMS structure can be applied in a great variety of different applications including domestic gas boilers, ventilators, medical instruments, fuel cells, pneumatic systems etc. The same basic chip is suitable for air, oxygen, city gas, LPG and other non-corrosive gases. A benefit of this design approach is that changes to the circuit constants can easily tune the chip to different environments, and can be associated with variations to the circuitry and flow path to create the best solution for each application.

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