Flow sensors Flashcards
Hot wire
The hot wire is the “classical” approach where we measure the heat loss, through convective heat transfer, from a hot body exposed to fluid flow. It is composed of a hot wire in the gas stream and a constant heating power.
Two configurations:
-open loop: measure T of the wire as a function of the flow;
-closed loop: keep the wire at a constant T and measure the power required.
The response is in part governed by the temperature coefficient of the sensing material. The sensors are placed in a half bridge for temperature compensation and it has a non-linear response at high flows.
Calorimetric/Anemometric
It is composed by a central heating wire and symmetrically placed T sensors. The flow cools the upstream detector while the gas is heated by the hot wire transferring heat to the downstream detector.
There is no zero-point offset due to Tenv or pressure, it is easily miniaturizable and integrable. It is the best sensitivity technique but at high flows the maximum temperature difference between the sensor pair is reached and the response starts to decrease.
Time of flight/ Thermal tracing technique
It is composed by a heat pulse generated at an upstream heater and detected downstream. The item of interest is the time where the maximum response of the detected signal occurs. It tends to require a large applied heat pulse and suffers from low resolution but is useful over large ranges, has little dependence on the ambient temperature and on the properties of the fluid and shows good long-term stability.
How to reduce power consumption
Power consumption can be reduced drastically by thermally isolating the heating element from the substrate so that most power passes into the gas and as little as possible is lost in the substrate.
- One possibility is to place the heater and the temperature sensors on free-standing silicon bridges (or cantilevers) but the thermal conductivity of silicon is high and an appreciable part of the heating power is still lost in the substrate.
- For better thermal insulation the heating and temperature-sensing elements can be placed on free standing silicon oxide or silicon nitride bridges (or cantilever): the result is a high thermal decoupling and heating efficiency
The honeywell flow sensor
It is the most succesful and it consists of two thermally isolated silicon nitride microbridges with a central heating, serpentine-resistor-element divided equally between the two bridges. In addition, two identical thin-film serpentine resistors with relatively large TCRs serve as temperature sensors, placed symmetrically with respect to the heater on each microbridge.
Qualities:
-Excellent thermal isolation from the silicon substrate resulted in an efficient heater and a
very sensitive transducer.
-The very low thermal conductivity of the Si3N4 film microstructures also allowed for the design of a very compact sensor, where the sensing resistor grids could be placed immediately
adjacent to the heater resistor grid.
-small heat capacity so small time constant for the sensor response
-The ambient gas stream temperature can be measured on-chip by an additional thin-film
resistor on the silicon substrate
The honeywell flow sensor steps
1 (100) Si wafers with LPCVD Si3N4
2 Base and Acid clean the wafers
3 DC planar magnetron sputter-deposit a platinum layer and RF planar magnetrn sputtered chromium adhesion layer
4 DC planar magnetron spuuter-deposit gold metallization
5 Delineate gold metallization for pads and traces using KI solution
6 Delineate platinum resistor metallization in heated qua regia
7 Etch the chromium adhesion layer in chrome etch solution
8 RF planar magnetron sputter-deposit a thin silicon nitride passivation layer
9 Sputter-deposit the top RIE/KOH masking layer metals: chromium adhesion layer and gold top layer
10 Delineate top gold and chromium adhesion layers
11 Dice apart the die into “quad” and “duo”blocks
12 Dice apart quad-die-sized microscope glass spacers
13 RIE through all exposed silicon nitride layers
14 Bulk-micromachine individual quad/dual die in KOH to release/suspend sensor mcirobridge and cantilever structures
15 Remove golld and chromium masking layers from individual quad/dual die and then chrome etch solutions
Honeywell’s Wheatstone bridge sense-resistor configuration
It included an optional external differential
instrumentation amplifier, for the flow sensor,
where “Rup” is the upstream and “Rdown” is the
downstream sensing resistor on the suspended
microbridge.
Resistors R3 and R4 are either laser-trimmed
and located on-chip, or external with
potentiometer P1, which is used to balance or
null the Wheatstone bridge voltage at no flow.
Flow and direction sensors
(up till now we were measuring the flow along one axis, now two!!)
It is a directional gas flow sensor with 4 heating resistors on a suspended plate linked to the substrate by supporting arms suspending each a thermopile (Al/Si). Each termopile is made of a number of thermocouples connected in series.
The gas flow changes the symmetry of the thermal equilibrium on the suspended plate. The temperature gradient sensed by the thermopiles provides a measure of the intensisty and direction of the gas flow.
FLOW-VELOCITY MICROSENSORS BASED ON
SEMICONDUCTOR FIELD-EFFECT STRUCTURES
It is a time-of-flight-type flow-velocity sensor employing the in-situ electrochemically generation of ion-tracers. The sensor consists of an ion generator and two downstream-placed pH-sensitive Ta2O5-gate ISFETs (ion-sensitive field-effect transistor) that detect generated H+ or OH-ions (ion generation is due to electrolysis of water) . The flow velocity can be accurately evaluated using the shift of the response curve of the respective ISFETs in the array along the time scale. This shift, i.e. the time of
flight, will be inversely proportional to the flow
velocity.
