3. Testing recommendations
It is recommended to start by testing the communication with the device, using the EK-F5x Evaluation Kit and Control
Center.
As a second step, connect the device to a gas supply (or a source of vacuum). For better accuracy, mount the Mass Flow
Controller horizontally and use the same pressure as during factory-calibration (specified in the datasheet, by default for
most products 3 bar inlet vs outlet).
Try using different setpoint steps to test the Mass Flow Controller. If the chosen setpoint can’t be reached, try increasing
the supply pressure, removing fluidic resistances like filters, or using tubes with a larger diameter. If the supply pressure
used is significantly lower than the calibration pressure, the controller can be slower than expected. On the other hand, if
the supply pressure is significantly larger than the calibration pressure, the Mass Flow Controller may become unstable. In
these cases, lowering or increasing the controller gain to adapt the controller accordingly. This can be done conveniently
with Sensirion Control Center. For detailed instructions, see the “Control Center manual”, linked in “Useful resources”
Section 7.3.1.
4. Common pitfalls
4.1. Flow appears to be off by several %
If the flow appears to be off by several % (especially around 7%) against your reference, there are good chances the flow
units of the two devices are not the same. The conversion difference between standard litres and norm litres is around 7%,
due to the different reference temperatures used for standard and norm litres. Please make sure that the Mass Flow
Controller and your reference are using the same units when testing. Further information about the flow unit conversion
can be found in a dedicated application note, “Reference and Flow Conversions between mass and volumetric flow”, in the
“Useful resources” Section 7.1.
4.2. Settling time is longer than specified
The most common cause for observing longer than expected settling time is trying to verify it from a position of fully closed
valve. Solenoid valves are commonly affected by so-called “sticking effect”. To open the valve from closed position (setpoint
0), a certain overvoltage is required to overcome the stiction effect. This can lead to a spike in the flow (especially for low
setpoints) or a longer than expected settling time. By adjusting the “Initstep” or “Controller gain” of the Mass Flow
Controller, the behaviour can be tuned to the specific requirements.
The settling time of Sensirion’s Mass Flow Controllers is specified for a step answer from 10% to 100% of full scale within
5% of setpoint. When starting from zero flow the settling time may be longer than specification.
5. Design recommendations
5.1. The effect of orientation and temperature on accuracy
To achieve best accuracy, it is recommended to use a design where the Mass Flow Controller is mounted horizontally.
The operation of the valve produces a significant amount of heat. The Mass Flow Controller should therefore not be placed
in a hermetically sealed space. It is recommended to design a cooling flow over the Mass Flow Controller in order to
counteract the heating from the valve. Large temperature gradient between the Mass Flow Controller and the gas it is used
to control, may adversely influence the accuracy of the device.
