Amateur design report by Johan
Liljencrants
A thermal anemometer uses a heated probe element that is
inserted into an airstream. Air speed can then be inferred from the
heating power necessary to maintain the probe at a temperature
elevation. This power should be some way proportional
to air speed.
In my efforts to monitor airflow in
my
organ trunks without disturbing its musical operations I have
experimented with a number of thermal anemometers. The purpose
was to
hint at their relative merits regarding ease of fabrication, probe
size, and time required for a measurement.
A number of experimental circuits are described, using internally
and externally heated sensors, being diodes or NTC resistors to
monitor
temperature. With internal heating
you vary the sensor operating voltage and current such that it
is heated by its internal dissipation. This implies fast response, but
also an issue of whether sensing is disturbed by the heating.
Alternatively, with
external heating the sensor is only thermally connected to a separate
heater
such that there is no such electrical interference. Then instead there
is a delay for heat to be conducted from heater to sensor. This makes
the device substantially slower and puts restrictions on the control
circuit in order to maintain stability.
The thermal circuit of a probe is modeled. Essential parameters in
this model are derived for several experimental probes of the various
types. It was found that the classical King's law, saying power
dissipation is proportional to the square root of air speed, does not
hold well for the large size sensors used in the actual case. Here a
direct proportionality to air speed makes for a better model fit to
measured data. The model data suggests two derived parameters to
characterize the probe quality and its suitable speed range. Quality
depends on the probe design and material properties. Mid-range speed
is
critically dependent on probe element size, the higher speeds one
wants
to measure, the smaller the probe has to be.
1. Different circuits and sensors
1.0 Background: Internally heated transistor -
'Tranemometer'
ZebVance once suggested to me a link to an anemometer design, see
reference below. Here is my somewhat
modified
version
of that circuit: The temperature sensing elements are the base-emitter
junctions of two probe transistors
Q1, Q2. The base-emitter junction voltage is typically 0.7 Volts with a
temperature
coefficient near -2 mV per deg C. The lower transistor Q2 has its
collector wired to its base. This one acts as a passive diode,
only there to sense ambient temperature. These transistors form the
left side of a
bridge, the right side is resistors R1, R2, and the trimmer R3.
Amplifier A1 senses the balance of the bridge. If the voltage over the
Q1 junction is too high, then A1 will drive the Q1 base up. More
current will pass through both transistors but Q2 is fully conducting
and does not change its temperature appreciably with change in
current.
Having a high collector voltage, Q1 will be heated while Q2
remains essentially at ambient temperature. That heating lowers the Q1
base-emitter voltage until balance is restored. The heater and the
temperature detection are inherent
in the transistor itself. So A1 keeps Q1 a certain number of degrees
hotter than Q2. How many depends on the trimmer setting, with this
circuit typically around 5 degrees centigrade. Resistor R4 senses how
much current is flowing through Q1-Q2. The (small)
voltage developed over R4 by this current is amplified by A2 into the
output pin 7. A2 has an offset input but otherwise simply translates
the additional current needed to maintain the temperature difference
between the two B-E junctions. The more current, the more heat is
being
removed from hot Q2. Actually A2 is not simple at all. If R9 and R10
are trimmers, you can go nuts trying to adjust them. The reason is that
“input offset” in the front. As the bias changes, the
gain is affected.
The original article mentions a problem with this circuit. The sensor
transistors may latch up in a current rush
mode, with the top Q1 fully on and current limited essentially only by
the small sensing resistor R4. Then Q1 can no more hold its
temperature
and the
bridge balancing fails. This mode is easily evoked by a minimal
disturbance, e.g. like putting a scope probe in contact with the
circuit. The remedy is the threshold feedback from A2 via two diodes
(a
transistor in the original article). If the output at
A2 goes too high, essentially over some half the supply, then the
feedback diodes open and A1 is
quenched such that probe current is cut off again. While this safety
device is in operation, the output of the circuit is in error (output
no longer goes up with airspeed). Without it, however, it goes up and
stays up until you turn off the circuit. The capacitor C1 is
not commented in the original article, but apparently slows operations
to be in the tens of milliseconds range, preventing oscillation. Still
this is much faster than the thermal time constants in Q1-Q2.
Power is supplied from a single 9V battery. The power-on indicator
LED
is used to offset the nominal ground and form a negative supply for
the
op-amps. Otherwise their
inputs come too close to the negative supply, such that they do not
operate.
P.S. СПАСИБО, ЧТО ПОСЕТИЛИ МОЮ СТРАНИЧКУ, УДАЧНОГО ВАМ ДНЯ!!! |