For more than 30 years, thermal mass flow measurements using coils around a steel capillary have been the standard in the precise measurement and dosing of mass flow rates. Now, new technology integrates this underlying measuring principle in a miniaturised thermal sensor with all of the high-precision signal-conditioning circuitry on a single CMOS microchip.

Combined with specially developed sensor packaging, a miniaturised thermal system can be produced at a lower cost with a 10x higher control speed (150ms) and a significantly higher accuracy (0.8 per cent of the measured value over 10 - 100 per cent FS), which represents an actual quantum leap in mass-flow measurement.
CMOSens marks the fusion of sensor and evaluation circuitry on one CMOS chip.
This guarantees that the sensitive, analog sensor signals can be amplified and digitised without noise and with high precision. CMOSens sensors also include a temperature sensor that is used for exact temperature compensation.
For the integration of a thermal flow sensor on a silicon chip, a pressure-stabilised membrane, which has a glass-passivation layer is etched into the silicon chip from below. A controllable heater element is mounted in the middle of this membrane and temperature sensors are mounted symmetrically upstream and downstream from this heater element in the direction of flow. Any flow over this membrane causes a transfer of heat and thus generates a precise measurable signal. The patented CMOS control circuitry allows highly precise amplification, evaluation, linearisation and temperature-compensation of the analog sensor signal.

Accurate and fast

A conclusive factor in performance for thermal mass-flow controllers is the control speed. For conventional mass-flow controllers (MFC), the sensor element typically has a reaction time of a few seconds. Thus, to accelerate the control time for good MFCs, the reaction of the sensor is analysed before the signal change and the possible final value is estimated in advance with the help of additional electronics. This produces faster control times on the order of almost one second at the price of higher system costs and lower control stability. Because a CMOSens mass-flow sensor reacts thermally about 1000 times faster, direct and much faster control can be realised. A typical CMOSens MFC achieves control times of less than 150ms.

The second important feature of an MFC is its accuracy and the fundamental reproducibility. Through the symmetry of the sensor element and the offset-compensated evaluation circuit, CMOSens gas-flow sensors typically achieve an offset stability of <0.01 per cent <0.01 per cent FS/y. According to demand, CMOSens MFCs can achieve an accuracy of 0.8 per cent MV or even more in the range of 10 - 100 per cent FS.
This high dynamic range changes the decision on the selection and use of a corresponding instrument. The accuracy of the controller is indicated in percentages of the setpoint (per cent SP) instead of percentages of the full scale (per cent FS). This means that the same MFC can be used for 400sccm and 40sccm, each with an accuracy of 0.8 per cent of the setpoint. Previously, MFCs of conventional technology required separate instruments calibrated correspondingly for each range.
Two weaknesses for silicon flow sensors have been pressure resistance and the tightness of the sensor housing (packaging). Therefore, a stainless steel housing with integrated flow channel and vacuum-tight glass feeding through was developed for electrical contacts. As sealing materials, only glass and gold-plated pins are used.
All previously realised CMOSens solutions were limited to a maximum operating pressure of 10bar, even though a higher operating pressure is possible. Conventional MFCs are available for over 200bar operating pressure. Another disadvantage is that the silicon with a glass-passivation layer comes into direct contact with the gas.

Applications

Due to the accuracy and measurement dynamics, CMOSens is suitable, above all, for OEM applications, for which the most important factors are performance and cost. Typical applications include analytical instruments, process control equipment, calibration systems, but also medical applications (eg anaesthesia flow meters) or even fuel cells. Examples of CMOSensMFCs are the field bus-controlled, high-end MFC from Burkert, the cost-effective Red-y SMART from Vogtlin/Insentys, and the PerformanceLine from Sensirion (see below).

Conclusion

The combination of factors of higher performance with simultaneously lower system costs gives CMOSens technology the potential to create a new generation of instruments after 30 years of thermal mass-flow measurement with steel capillaries. Steel capillaries, however, will not be replaced completely within the foreseeable future.
In applications using very high pressures orextremely aggressive gases, conventional MFCs clearly remain the first choice in the short term. However,if performance or cost is most important, then CMOSens should very soon become the newstandard.

The CMOSens PerformanceLine from Sensirion is based on the new CMOSens technology. The high degree of system integration on the sensor chip, in comparison with conventional mass-flow controllers, achieves significantly higher performances at lower cost. The sensor is sealed in a stainless steel housing and it can be used under very harsh conditions.
The most important properties of the PerformanceLine are:
* Accuracy: 0.8 per cent of setpoint (in range of 10 per cent to 100 per cent of measurement range).
* Control speed: t98 =150ms (time until new setpoint has reached 2 per cent accuracy).
* Control range: 1:1000.
* Digital calibration, linearisation and temperature compensation.
* Analog interfaces.
* Low system costs for OEMs.

For more information, visit www.sensirion.com