CSEM Scientific and Technical Report 2008

Laser Flow Meter
C. Verjus, P. Renevey, V. Neuman, J.-A. Porchet
CSEM has developed together with its industrial partner DIGMESA AG [1] an innovative, non-intrusive flow measurement sensor system. In the
designed prototype, the liquid flows through a straight glass tube of 4 mm internal diameter, well adapted for a nominal range of a few milliliters per
minute. The measurement is based on an optical measurement technique, keeping the response time in the order of a dozen milliseconds.
Technical standards of today for flow sensors require to be
non-invasive. For the handlings of liquids in the chemistry,
food or semiconductor industries, measurements are to be
performed through a straight tube to allow easy cleaning along
with preventing pollution of the fluid or sensor damage.
Large flows can easily be monitored non-invasively, for
instance with ultrasonic devices. Low flows are more critical,
particularly if the fluid does not contain particles to take
advantage of the Doppler effect. Today commercially available
solutions to measure flows of liquids in the millilitre per minute
range are based on the thermal mass measurement principle,
where the flow influences the temperature distribution up- and
downstream from a heating resistor. Drawbacks include a long
response time and media dependency.
A small-sized Swiss company, Digmesa AG [1] , developing and
manufacturing flow meters for over 25 years, entrusted CSEM
with the development of an innovative, non-intrusive sensing
technology, able to measure flows in the 1-20 ml/min range.
Subsequently, a new flow measurement principle has been
developed and patented, involving only a laser beam crossing
the straight glass tube where the media is flowing. A sensor
based on this technology has been designed. Prototypes have
been built to demonstrate the method, with glass tubes with
an inside diameter of 4 mm as well as 0.5 mm.
Figure 1: Laser Flow Meter prototype (bottom) with the sensing and
processing electronics (top)
Figure 1 shows the prototype with a 4 mm inside diameter
tube. The electronics includes a DSP development board for
the implementation of the algorithms to measure the flow.
A major difficulty is the absence of reference systems or
pumps stable enough to cope with the short response time.
Ultimately, a DC-motor coupled with calibrated syringes has
been used for calibration, testing and demonstration purposes.
Sensitivity proved to be very high: the noise accounts for
displacements of a few micrometers of the liquid in the glass
tube over dozens of milliseconds.
Figure 2: Flow (top) and volume (center) measured with the sensor
prototype and volume error compared to the reference (bottom)
Figure 2 illustrates the response of the prototype compared to
the reference. The DC-motor moves forward for about 27 s,
letting 6 millilitres of water flow out of the calibrated syringe.
Then the motor moves backward during the same duration,
the syringe regaining 6 millilitres of water. The motor
acceleration is kept constant. As a result, at the measurement
location, the liquid moves in one, then in the opposite direction,
following each time a velocity ramp corresponding to a flow
varying linearly from 0 to 30 ml/min. The measured flow has
been integrated to compute the volume. Thus the volume
error accounts for the cumulated error of the measured flow.
The achieved accuracy is very promising for the future product
and together with DIGMESA AG the sensor industrialization
has been initiated.
[1] Digmesa AG, www.digmesa.com
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