Introduction
Hot-wire and hot-film anemometry is a technique for measuring variables occurring in turbulent flows, such as variable and mean velocity, temperature, etc. The sensors are resistive elements with a known temperature coefficient, and as such the temperature can be controlled by the resistance and vice-versa.
From the temperature (or resistance) attained by the sensor, it is then possible to deduce information on the flow. More than one sensor, or more than one value of the heating current, is often necessary to investigate thoroughly a turbulence field.
Two techniques of hot-wire anemometry are commonly used: CTA (Constant-Temperature Anemometry) and CCA (Constant-Current Anemometry). Alliantech’s hot-wire CTA conditioner uses CTA as it has some advantages over CCA:
- Lower thermal inertia
- Higher passband
- Lower risk of burning the probe
The diagram below shows the simplified circuit schematic of a CTA conditioner for a hot-wire or hot-film probe. The CTA is based on a Wheatstone bridge with a balance resistor responsible for controlling the temperature on the probe.
In CTA, the temperature of the probe is kept constant, while the current on the bridge changes to compensate for the flow.
Figure 1 : CTA conditioner.
The following diagram shows the connection of the measurement setup to Alliantech’s conditioner.
Figure 2: Measurement setup.
Alliantech’s Hot-Wire CTA Conditioner
The AT-HW Conditioner can be used in standalone mode or in a n 8-channel rack. Each channel can be removed from the rack and used independently. Figure 3 show the rack and figure 4 shows a view of a standalone channel.
The rack is grounded and can be wall powered through a 230 V 50 Hz socket (no stabilized laboratory power supply is needed).
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Figure 3: AT-HW Conditioner in an 8-channel rack.
AT-HW differs from classical hot-wire conditioners available on the market with its simplicity and features:
Each of the channels work on a 12 V standard power supply, with a cylindrical connector.
The temperature can be set with a digital potentiometer for precision and simplicity.
Integrated square-wave generator for calibration.
Probe protection circuit.
BNC connectors for input and output.
Indicator LEDs
Figure 4 displays the front panel of a single channel along with its functions.
Figure 4: AT-HW Conditionerfront panel.
ON/OFF switch. This switch connects the a mplifier output to the bridge (see figure 1). This allows for a safe connection and a temperature setting without disconnection of the probe.
Generator button. Once pressed, the generator is activated and connected to the loop allowing for a square-wave calibration of the CTA. Once activated, a green LED lights up in the button.
Adjust potentiometer. Used for calibration alongside the generator.
RBALANCE (figure 1) potentiometer used for setting the temperature on the probe
Probe input BNC connector.
Output voltage BNC connector.
Green status LED, indicating that the bridge is on, and the sensor is connected.
Red status LED. If this LED is blinking, the bridge is off. Static LED indicates that the bridge is ON.
Characteristics of Alliantech's CTA Conditioner
4 – 3960 Ω
0 – 5 V
12 V for individual channel 230 V, 50 Hz for 8-channel rack
7, 12, 25 and 150 Ω
500 Hz
450 mA max @ 12 V: 5.3 W max
-20 °C à + 85 °C
Use Cases for CTA Conditioner
Airflow monitoring in bio-safe cabinets, chemical fume hoods and laminar flow hoods.
Test of varied materials in wind tunnels.
HVAC systems and exhaust systems.
Setting the Temperature
The temperature can be set by the value of RBALANCE which brings the bridge into balance.
First, we must determine which value of temperature we want the probe on and calculate the resistance for the given temperature:
(1)
Where:
- R is the desired resistance at a given temperature.
- RTOT is the total probe resistance.
- α20 is the temperature coefficient of resistance (TCR) of the sensors
- R20 is the resistance of the probe at 20 degrees Celsius.
- Tsensor is the desired temperature of the probe.
- T0 is the ambient temperature
The total probe resistance is the sum of the sensor resistance, leads resistance and cable resistance:
(2)
And finally, the temperature can be set by the digital potentiometer on the front panel of the conditioner, matching the closest value of RBALANCE :
(3)
Calibration and square-wave generator
The CTA is a circuit with a feedback loop, which acts as a second-degree system and can eventually oscillate. The offset voltage of the amplifier must then be regulated to provide an optimal damping of the system. This can be verified and adjusted through the square wave test, as shown in figure 5.
The offset should not be too low as to affect the bandwidth of the system.
Alliantech’s system already integrates the square-wave generator to make it more practical to calibrate the conditioner.
Figure 5 : Square-wave test
Protection circuit
Sometimes setting the temperature of the probe or the overheat ratio (Rtarget/Rinitial) can be tricky and prone to errors. If one mistakenly touches the potentiometer the temperature can be set to a higher value than the maximum supported by the probe, burning the sensor, and possibly ruining a whole test setup.
Alliantech has included a protection circuit for the probe on all of the channels a temperature monitor for the probe. In case the temperature or the resistance exceeds the maximum established by a certain probe, the protection triggers and the loop is broken as to prevent damage to the sensor; this state is then latched. The conditioner must then be reset through the ON/OFF switch as to reactivate the loop.
Figure 6: Complete diagram of Alliantech’s Hot-Wire conditioner.
The threshold is set using a DIP switch inside the package of each channel and must be set according to the type of sensor use (Hot-Wire, Hot-Film, etc.).
Indicator LEDs
The LEDs on the front panel work as such:
Both off: Conditioner is off, no power is applied.
Red LED blinking: Conditioner is powered, switch is in the OFF position
Red LED on and green LED off: Switch is in the ON position, loop is closed but the sensor is not connected (or protection triggered).
Red LED on and green LED on: Switch is in the ON position, loop is closed, the sensor is connected, and the output is valid.
