Frequently Asked Questions (English only)
Mass Flow Meters
What is the impact on accuracy when the operating conditions alter from the calibration conditions?
Sensirion MFCs directly measure the number of gas molecules (or gas mass) flowing past the sensor and thus show only very minor dependency on deviations in pressure and temperature from the calibration conditions. The respective temperature and pressure coefficients can be gathered from the datasheets. A change in gas temperature induces both a zero point and a span error. By contrast, a deviation in pressure causes only a span error.
What is the impact of a different reference temperature?
Changing the reference temperature has an effect on the volume at a constant pressure of 1013.25 mbar. This volume change is governed by the Gay-Lussac gas law.
If you want to convert a norm liter into standard liters, the formula is as follows:
- V/T=constant (Gay-Lussac law)
- Example: standard liter/standard temperature in K = norm liter/norm temperature in K -> standard liter = 293.15/273.15 * norm liter = 1.0732 norm liter
- Thus, a mistake with the reference temperature can result in an error of 7.3%.
How do I interpret the settling time specification listed in the datasheet?
Sensirion guarantees a settling time of <100m/sec for set point changes from 10% to 100% FS. This definition of settling time means that the flow is stable within 2% of the set point. However, the settling time depends on many factors:
- Set point changes: faster at higher set points; slower if starting from zero flow or changing set points at lower flows
- Pressure: at higher pressures, settling times typically are faster, but with a higher overshoot; at lower pressures, the settling times are typically slower, with less or no overshoot.
Which fittings does Sensirion offer for MFCs/MFMs?
The fitting is the pneumatic interface through which the MFC can be mechanically connected to the external flow channel, such as a pipe or a tube.
Sensirion’s fittings for MFCs include downmount, 6mm Swagelok, 1/4" Swagelok, 1/8" Swagelok, 1/4" VCO and 1/4" VCR, C-Seals und W-Seals (both 1.125").
Are Sensirion devices electromagnetically compatible?
As the CMOSens® technology enables signal processing directly on the sensor chip, Sensirion MFCs are not susceptible to electromagnetic interference and all Sensirion MFCs are CE-certified. In particular, Sensirion devices fulfill the following norms for electromagnetic compatibility: EU guideline 89/336/EWG, Swiss guideline SE 734.5.
What do the pressure specifications in the datasheet mean? How can they be interpreted?
Pressure drop at full flow: This figure refers to the pressure reduction that the flow experiences due to obstructions such as the valve and the orifice at the full scale flow rate. Consequently, the differential pressure between input and output needs to be greater than this value in order for the flow to reach full scale.
Maximum input pressure: This figure refers to the maximum pressure that can be applied at the upstream input. This results from the mechanical limitations of the seals. Above this value, the seals and valve may fail and the device may become less tight as a result.
Maximum differential pressure: This figure refers to the maximum pressure differential across the valve. If this value is exceeded, the pressure differential causes leakage through the valve even with zero flow and unstable control.
How does Sensirion define control range?
Control range is defined as the ratio between maximum and minimum flows that the device can control. For example, a control range of 1000:1 means that flow can be controlled from 0.1–100% FS.
What communication interfaces does Sensirion offer?
Sensirion MFC/MFMs feature the following analog and digital communication interfaces:
- Analog: 0–5V, 0–10V, 4-20mA
- Digital: RS485, DeviceNet, IO-Link
The standard Sub-D 9pin connector is the physical interface for both analog and digital communication.
What is the CMOSens® technology used by Sensirion devices?
Sensirion’s patented CMOSens® technology encompasses several aspects. A highly innovative new measuring technology – employing the symmetric arrangement of two temperature sensors around a heating element – enables a very fast and accurate gas flow measurement. Another key aspect is the patented CMOS evaluation circuitry integrated on the same chip, which allows programmable, highly precise amplification and evaluation of the generated analog sensor signal.
Most measuring technologies used by competitors rely on steel capillaries to measure the up and downstream temperature from which the mass flow can be deduced. These steel capillaries have a high thermal mass and thus can measure changes in temperature only relatively slowly, which in turn leads to slow control of the flow rate.
In contrast, Sensirion integrates both temperature sensors and heating elements onto a single chip, which has a significantly lower thermal mass and thus records changes in temperature relatively quickly. Consequently, the device is capable of controlling flow more quickly and thus achieves best-in-class settling times.
Does Sensirion offer multigas/multirange calibrations?
Sensirion can calibrate MFC/MFMs both with multiple calibrations of different gases (multigas) and with multiple full-scale flow ranges of the same gas (multirange).
The following precondition must be fulfilled to provide a multigas option with optimal calibration performance:
- N2 equivalent FS flow ranges of the specified gases must be close to each other.
Multigas/multirange calibration is available with both real gas and gas conversion calibrations.
Can I connect a 90° Legris connector directly to the sensor? Will it influence the accuracy?
It’s possible to connect the sensor in this way. Internal tests have shown that the influence is less than 1% m.v..