CMOSens® Technology for Gas Flow and Differential Pressure

The mass flow is determined using a thermal measurement principle. An adjustable heating element is positioned at the center of a pressure-stabilized membrane, with a temperature sensor both upstream and downstream of the membrane in the direction of gas flow. Every flow of gas over the membrane causes a thermal transfer of heat to the temperature sensor positioned downstream and, because of the resulting temperature difference, creates a precisely measurable signal. The microthermal flow sensor is integrated by etching the pressure-stabilized membrane, which has a passivation glass layer, into the silicon chip. The CMOSens® Technology integrates this miniaturized sensor together with the entire high-precision processing circuitry on a tiny CMOS microchip.

Integrated on the same chip, not even a millimetre away from the sensor component, are an analog-to-digital converter and a signal amplifier, which both contribute to the precision of the measurement. Sources of error are minimized and fault-prone solder points eliminated. This drastically improves the signal-to-noise ratio and reduces the risk of failure.

Likewise on the same chip is a temperature sensor, which emits a signal that compensates for any arising temperature effects. This enables high-precision measurement over a wide temperature range without the need for additional temperature compensation. It also removes the necessity for additional corrective sensors and, as a result, makes Sensirion technology a very low-cost, space-saving solution.

Apart from this, the chip features digital signal-processing circuitry and an EEPROM memory cell to store calibration data. Every sensor manufactured by Sensirion is individually calibrated during production. As a result, the signal received by the sensor chip is always fully calibrated, linearized and temperature-compensated.

The signal processing circuitry on the chip also includes the digital communication interface. It uses an I2C protocol, the standard for communication between the various components on the circuit board.

Sensirion's microthermal gas meter modules are equipped with integrated gas recognition. An innovative, high-tech algorithm recognizes various gas mixtures and compensates for them. Gas quality compensation, as it is known, permits accurate and reliable flow metering despite differing gas qualities.

With Differential Pressure Sensors

Because of the metering principle used, our differential pressure sensors are highly suitable for measuring gas flows in a bypass configuration. Most products intended to measure mass flow are designed with a bypass configuration in which the main flow is guided through a straight channel. A flow restrictor forces a small amount of air through a bypass channel in which the sensor is positioned.

And with Flow Sensors

The sensor chip can also be placed in the main flow channel, which eliminates the need for a bypass. The main advantage using mass flow meters is higher sensitivity for very low flow rates.

The pressure connectors in the differential pressure sensors can be connected up to two different air or gas volumes. In the case of a pressure difference, a tiny flow, limited by a small restrictor, passes through the flow channel and is measured by the sensor chip. Sensirion's differential pressure sensors replace traditional diaphragm gas meters and offer many advantages as regards precision and reliability.

When a valve with a fast control algorithm is inserted, the CMOSens® Technology is highly suitable for controlling gas flow rates. The mass flow controllers from Sensirion have an extremely fast control speed and a wide control range.

In summary, we can say that CMOSens® Technology replaces traditional measuring techniques and offers a wealth of benefits, mainly in terms of speed and precision.
Other advantages of the technology include:

• Long-term stability with no zero-point drift
• Accuracy and reproducibility (over a wide dynamic metering range)
• Fast response times
• Sensitivity
• Reliability and robustness