The mass flow is determined using a thermal measurement principle. A heating element is positioned at the center of a thin membrane, with a temperature sensor both upstream and downstream of the membrane in the direction of liquid flow. Every flow of liquid over the membrane causes a thermal transfer of heat from the microheater to the temperature sensor positioned downstream and, because of the resulting temperature difference, creates a precisely measurable signal. The microthermal flow sensing element is integrated on the microchip by etching the membrane into the silicon chip. The CMOSens^® Technology integrates this miniaturized sensing element together with the entire high-precision processing circuitry on a tiny CMOS microchip.

Integrated on the same chip, not even a millimeter away from the sensing element, 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 CMOS chip is an additional temperature sensor, which is used to compensate for any arising temperature effects. This enables high-precision flow rate measurement over a wide temperature range without the need for additional temperature compensation by our customers. 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 memory to store calibration data. Every sensor manufactured by Sensirion is individually calibrated during production. As a result, the signal our customers receive from the sensor 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. Therefore, the sensor’s output signal is typically a digital I²C signal and update at a max. readout frequency of 1-2 kHz. There are a few liquid flow sensors which can also provide analog output signals with the help of special sensor cables.

Sensirion’s liquid flow sensors feature a specialty: the integrated digital CMOSens^® microchip described above is media-isolated from the liquid medium by means of a straight and very thin-walled flow channel. I.e. the microchip is bonded to the outside of this flow channel and precisely measures the mass flow through its wall. Depending on the targeted flow rate range, the flow channel’s inner diameter varies in size from a few millimeters to micrometers. It has no dead volumes nor restrictions and is made from chemically inert materials like glass, stainless steel or plastics making our sensors ideal for use in medical and life sciences, diagnostics and industrial applications.