Frequently Asked Questions (English only)
Liquid Flow Meters
I don’t get any pulses on Dout of the SCC1 analog cable. What might be the reason?
Since the digital output is realized as an open collector circuit, a pull-up resistor has to be connected between the digital output and an external voltage Vhigh, which serves as the high-level voltage. The value of this high-level voltage may be chosen independently or be identical to the supply voltage in order to match the logic levels of your control system. See the SCC1 analog sensor cable datasheet.
I need an analog liquid flow meter in the range of 80ml/min for water. Can I use the SLQ-HC60?
No, SLQ-HC60 is calibrated for hydrocarbon (IPA) only and is not suitable for measuring water-based media. Depending on the required flow rate, the SLS-1500 or SLQ-QT500 cover the flow range from 3 to 80 ml/min for water. In combination with the SCC1 analog sensor cable, these sensors provide a 0–10V analog output.
In the datasheet, you specify the accuracy as '% of m.v'. What does this stand for?
It stands for % (percentage) of the measured value (m.v.) or sensor reading.
Is the sensing element in direct contact with the liquid?
No, it is not. The sensing element is not wetted due to the specific design of Sensirion’s liquid flow sensors where the sensor measures the liquid flow rate through the wall of a straight capillary. The wetted material list can be found in the Mechanical Specification section on the datasheet for your sensor.
I need fluidic connectors. Can I buy them from Sensirion?
A basic set of fluidic connectors is included in your Flow Meter Kit. If other connectors are needed, we recommend purchasing them through fluidic connector manufacturers, such as Idex, Vici or Nordson Value Plastics. The sensor’s datasheet contains details on the fluidic connectors suitable for your specific model. For more details, see the Sensor Ports and Tubing Connections application note in the Download Center.
Can the liquid flow meter be used to monitor liquids it has not originally been calibrated for by Sensirion?
Yes, this is in principle possible. Select the standard calibration field that best matches your fluid's chemical composition. For example, the water calibration can be used for saline solutions or the hydrocarbon calibration can be used for lubricant oils. A matching calibration field increases the possibility of a linear response from the sensor. A good rule of thumb is to use the IPA calibration for all liquids that do not contain any water.
Make sure you mention the planned media when talking to your Sensirion contact. Further solutions exist for use of Sensirion's liquid flow sensors, even when using liquids other than water or IPA. Please contact us for further assistance on this topic.
Regardless of the fluid, your liquid flow meter is capable of a very high repeatability. Typical values for repeatability range from 0.8% to 1.5% for all media, depending on the liquid flow meter at hand. This allows you to use the sensor as a very precise relative gauge when liquid flow is required to stay within an acceptable range. In such a case, the sensor's output can be compared to set maximum and minimum flow rates and used as upper and lower thresholds for reference. The liquid flow meter can then monitor process repeatability within verified acceptable limits. Since absolute values may vary from sensor to sensor, this tolerance band must be newly set for each individual flow meter.
Are Sensirion’s liquid flow meters calibrated when shipped?
Yes, Sensirion’s liquid flow meters are 100% factory calibrated with at least one standard liquid (typically H2O or IPA). See the datasheet for your sensor for further information.
What could be the reason for a high signal noise on the sensor signal?
Make sure the fluidic system is not influenced by mechanical disturbances. Vibration or movement of the tubing or the sensor itself can have an impact on the liquid inside the fluidic system, which is visualized on the sensor's output signal. The sensor is highly sensitive and fast. It may show effects that you did not observe have previously. Due to the low mass, microfluidic systems tend to be highly dynamic. The sensor itself is not sensitive to movement or (reasonable) vibration, but it senses the real flow caused by the vibration.
Why does the flow signal change even though static conditions apply?
There may be several causes for observing such an effect, some linked to the environmental conditions of your set-up. These can be, for example, a change in the physical properties of the medium used or a strong temperature change, a proceeding blockage of the fluidic path or the flow source (e.g. pump not properly working), or even a system leakage. Make sure the environmental conditions and physical properties of the medium do not change. Check the fluidic path and fluid supply for possible errors.
What could be the root cause if the flow signal shows unexpected behavior (e.g. fluctuations, increasing/decreasing flow, although the pump is working steadily)?
Analyze each part of the fluidic system in order to understand the effects monitored. In most cases, the sensor represents reality. The sensor is highly sensitive and fast. It may show effects that you did not previously observe. Due to the low mass, microfluidic systems tend to be highly dynamic. At truly constant flow, the sensor’s noise signal should be almost the same as at zero flow. To see this for yourself, use a pressure difference to generate a constant flow. Contact Sensirion for additional support if the observed behavior is still not understood. Please provide flow data, graphs and a description of your fluidic system set-up.
What is the smallest measurable flow rate?
There is no strict limit below which the liquid flow meters cannot measure a flow rate. To answer the question, you have to consider the sensor's repeatability and the required accuracy of the measurement at a given low flow rate. The relative accuracy is either specified in % m.v. (of measured value) or % of full scale (whichever error is larger). The repeatability is basically the resolution of the sensor. Looking at the relative accuracy, there is a particular flow rate at around 5%-10% of full scale where the absolute measurement accuracy of 'm.v.' and 'full scale' are equal. For flow rates below this value, the error of the full scale is valid and remains valid down to zero flow.
Example: The specification for the SLI-2000 is 'Accuracy below full scale 5.0 % m.v. or 0.2 % of full scale' (whichever error is larger). The full scale flow rate is 5000 ul/min. At a flow rate of 200 ul/min, both the calculated absolute 'accuracy of full scale' and 'accuracy of m.v.' result in 10 ul/min. Thus, the possible measurement error below 200 ul/min remains constant at ±10 ul/min.
Looking at the repeatability, we consider the sensor's resolution. As explained before, at a flow rate of 200 ul/min the absolute measurement accuracy is ±10 ul/min. But the sensor has a repeatability of 1 ul/min (0.02% of full scale) and even at much lower flow rates than 200 ul/min e.g. at 50 ul/min, the repeatability remains at 1 ul/min.
In process control applications, the repeatability is very often more important than the accuracy of the actual flow rate. In such a case, you can basically rely on a repeatability 'accuracy' of 1ul/min (in the case of the SLI-2000).
I am certain no liquid flow is occurring in my system, but the liquid flow meter is giving a small positive or negative output. Is this normal?
Yes, this is normal. The measurement accuracy also applies at zero flow. For flows around zero, the accuracy specification in '% of full scale' applies. Below is an example with the liquid flow sensor SLI-2000.
Example: The specification for the SLI-2000 around zero flow is 'Accuracy 0.2 % of full scale'. The full scale flow rate is 5000 ul/min. At and below a flow rate of 200 ul/min, the calculated absolute 'accuracy of full scale' results in ±10 ul/min.
For details about the different liquid flow meters, please consult the relevant datasheets.
Liquid Flow Meter Kit
What can I do if there is no reaction at all after connecting the sensor with the PC, or if the software cannot find the sensor?
- Check that the USB driver is providing a virtual COM port and you are using the correct number. Ensure that the sensor cable is properly plugged in.
- Ensure all other programs using COM ports are closed. It may be necessary to reboot the computer.
- Ensure the operating system of your PC corresponds to the requirements specified in the operating guidelines. When plugging in the USB cable, the Virtual Com-Port (VCP) driver should be installed automatically. After the driver has been successfully installed, the device appears in the device manager as USB serial port.
- If this does not happen, please install the necessary VCP driver from the following link: Virtual Com Port Driver from FTDI