IM-215 Rev-A.1 Se ries Innova-Switch™ Page 10 of 42
When the senso r is inserted into a liquid or gas the heated RTD is strongly affecte d
by the velocity of the medium. Flow past the heated RTD changes the h eat
transferred f rom the surface of the sensor. This cooling effect reduces th e
temperature of the sensor. The Innova-Switch™ compares this change to a preset
flow trip point to switch the output. Figure 2B shows the mo del FS4200 signal
change vs. flow rate for air, light hydrocarbon liquids, and water.
The signal change vs velocity has the sa me general shape for a ll three media but t he
change is larger for air and the sensitiv e range is different for each. For air and most
gaseous media the range is 0.1 to 50 0 feet per second ( FPS). For most liq uid media
the range is 0.01 to 5 FPS. Appendices in section 9.0 contain flow conversio n
information to facilitate conversion from various units and pipe dimensio ns into flow
velocity in feet per second.
Note: The fluid velocity and
heat absorption ability
determine the differential
between the tips. Their
combination determines the
measurable velocity. In
water velocities from 0.01 to
5 FPS are measurable,
whereas in air velocities of
0.1 to 500 FPS can be
measured.
For a no flow condition the
thermal differential between
the two tips is high because of
relatively low heat transfer.
Flow across the tips decreases
the thermal differential
because of the higher heat
transfer of flowing fluids. This
differential is compared with
the trip point.
When the lower differential
matches the customer select
flow velocity trip point (set
point) the switch relay and
red LED are tripped.
When flow is above the trip
point the differential is smaller
than at the set point and the
relay and Led remain tripped.