Flow Sensor

Flow sensor

Determine flow velocity in your sample

The flow velocity microsensor allows you to monitor e.g. muscle tissue flow and pO2 or flow in porous sediments. The novel principle of the flow sensor is based on diffusion of an inert trace gas out of the sensor tip and into the surrounding sample, which the microsensor is inserted in. Flow velocity is now measured by registering the concentration of tracer gas around the microsensor tip. The flow velocity microsensor is constructed with two inner parts, one for emitting the tracer gas and one for detecting the concentration of tracer gas at the microsensor tip. 

The sensor typically has an outer tip diameter of 50 µm and a 90% response time of 3 to 60 seconds depending on flow velocity. Sensors can be tailored to be sensitive to either very low (down to 5 microns/sec) or higher velocities (above 10 cm/s). The flow microsensor must be used together with a high-quality picoammeter e.g. Microsensor Multimeter

The principle of this flow sensor (or more accurately, flow velocity sensor) holds some advantages over existing methods:

  • it does not introduce thermal turbulence (as hot beads/films do)
  • it can detect very low velocity changes (few microns/second)
  • it can measure in places where visual methods are not working
  • it can be introduced into very small cavities
Flow theory

Precautions to be taken when using the flow velocity sensor:
The signal is very sensitive to particles adhering to the tip. Even small dust grains attached to the tip will create a significant signal increase due to the associated stagnant water. For this reason it is recommended that a fine brush is implemented into the measuring setup, so the sensor tip can be cleaned at regular intervals.
The flow sensors are always somewhat directionally sensitive. This is because it is impossible to build the sensor completely symmetrically. Thus, you either need to know the direction of flow in your system and calibrate in the same direction, or you operate with substantial uncertainties, especially at high velocities.
You need a dedicated calibration setup preferable consisting of a system with stagnant water, which can be moved at well-defined speeds. Contact Unisense about details and support on this issue.
A source of hydrogen gas must be available for a continuous flushing of the tracer reservoir (gas consumption approx. 5 ml/minute). Alternatively, the sensors can be made with a refillable but closed reservoir for measurements where a source of hydrogen gas is not available. However, the signal will decrease at a rate of typically 10%/day due to inevitable leakage, necessitating re-calibration at regular intervals.
Like all other sensors, the flow sensor is temperature sensitive.

Ordering Information

Standard Glass Sensor Outside tip diameter
FS-20 20-50 µm

Related products

Microsensor Multimeter

Brand, Andreas et al (2007), Microsensor for in situ flow measurements in benthic boundary layers at submillimeter resolution with extremely slow flow, Limnology and Oceanography: Methods, 185 - 191, vol. 5

Christensen, Bekka Ozer et al (2006), Quantitative evaluation of a novel sensor for measuring muscular tissue flow and gases, Scandinavian Cardiovascular Journal, 374 - 379, vol. 40

Nakano, Shin Ichi et al (2006), Vertical profiles of current velocity and dissolved oxygen saturation in biofilms on artificial and natural substrates, Limnology, 213 - 218, vol. 7 Read abstract

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