Wink of Knowledge: Thermal flow measurement and correction by means of gas detection
Why this test?
Calorimetric flow sensors are typically adjusted to a specific medium, as the measured value is influenced by the thermal conductivity and capacity of the gas. Thus, a specially calibrated sensor is required for each gas. In combination with a density measurement for clean gas detection or concentration determination of binary gas mixtures, calibration data and correction factors can be assigned flexibly and while the process is running. This means that the correct flow value is calculated at all times, regardless of the clean gas or mixing ratio of the mixture, and only one sensor solution is required for this measuring point.
What is a wink of knowledge?
Do you need to quickly measure, draw or do something? The speed to the result counts more than the perfect (scientific) approach. For this reason, we have introduced the wink of knowledge. Science with a wink, so to speak. We don’t want to prove anything scientifically, but quickly demonstrate something pragmatically. If you are interested, we would be happy to discuss these results in more detail with you and your project.
In Figure 1, the raw flow values are plotted in blue on the right-hand axis and the corrected volume flow in orange on the left-hand axis. The blue linear fit clearly shows how the different thermal parameters of CO2 and N2 falsify the determined measured value without density or concentration correction. However, by adding an inline density measurement, the flow can be corrected to within approx. 2% of the set 100 sccm, as shown by the orange linear fit. It has thus been demonstrated that the combination of the TrueDyne density sensor DGF-I1 and the IST flow sensor SFS01 can accurately determine the flow velocity regardless of the concentration ratios of the binary gas mixture.
In addition to binary gas mixtures of known gas components, this principle can also be extended to switching between gases with different densities. Using a clean gas detection system, it is thus possible to select the appropriate calibration data or correction factors and correct the measured value in real time. This makes it possible to realise a flexible, price-performance-optimised flow measurement with all the advantages of the thermal measuring principle:
- Long-term stable and vibration-resistant measurement
- Compact design
- Suitable for process control, thanks to particularly fast response times (<10 msec.)
- High sensitivity from the lowest flow velocities including direction detection
- Clean gas detection
- Multi-parametric measuring system (flow, density, pressure, temperature)
- Microleakage monitoring
Figure 1: Comparison between corrected and raw flow rate
Figure 2 shows the setup of the experimental station. The desired flow rate value for various pure gases and gas mixtures was set using thermal mass flow controllers (MFC 1-5) connected in parallel. The mass flow controllers are each calibrated to the corresponding clean gas, which means that the mixing ratio can be precisely controlled. The gas mixture then flows through the two density and flow sensors connected in series.
The I2C measurement signal from the flow sensor is transmitted directly to the gas density sensor, which uses the measured density and the concentration derived from it to calculate the raw value with a correction factor. The resulting flow measured value can now be compared with the set target value of the gas mixers to verify the functionality of the prototype.
Figure 2: Experimental setup
Which sensors were used?
The density measurement, on which the clean gas detection and concentration determination is based, was carried out with the DGF-I1 density sensor for gases from TrueDyne. The gas flow and its direction were determined using the SFS01 Evalkit thermal flow sensor from Innovative Sensor Technology (IST AG).
Two external thermal mass flow controllers (MFC) were used to set several concentrations of a binary gas mixture of CO2 and N2 and to constantly flow through the sensors. Using the logging function, three readings per second were recorded for the following parameters: concentration, flow rate raw value and corrected flow rate.
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