Frequently Asked Questions
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