Preparation: Learn thermo-anemometric principle of mass airflow measurement
1.
Explain why thermo-anemometric flowmeter in tube/air-duct of given dimension measures mass flow rate and not volume flow rate. It can be used also outside of tube/air-duct - which quantity is then measured?
2.
Measure electric current through platinum resistor Pt100 with bridge supply voltage 6.5V and zero flow rate. (The current is measured as voltage drop across 10 Ω resistor). From measured values determine resistance of Pt100 and calculate its temperature. Record the bridge output voltage.
3.
Determine the response of bridge output voltage and current through Pt100 for various air flow rates. Modify the ventilator supply voltage in 3-8 V range in 1 V steps.
4.
For one setting (e.g. U vent
=5.5 V) try manually balancing the bridge back to zero by modifying the bridge supply voltage Vcc (so that you achieve the same bridge output as in step 2 for zero flow rate). After settling, determine Pt100 current and temperature.
5.
In the control software, activate feedback loop control and try properly set PID controller parameters for reasonable behavior.
6.
For ventilator settings in 3-8 V range in 1 V steps measure Pt100 current and bridge Vcc.
7.
Explain what is the flowmeter output quantity? Why the response to change has different dynamics than before?
8.
For individual ventilator voltage settings, calculate the mass flow rate and draw graph of dependence of squared Pt100 current on flow rate and on square-root of flow rate.
I 2   
I 2  f
 
(1)
(2)
9.
The dependence of squared current on square-root of mass flow rate should be linear.
Determine its slope and ofset -- i.e. determine a and b constants in eq. (5).
Heated element is exposed in space with flowing medium (gas or liquid). The medium cools the hot element in dependence on the medium flow rate, density, temperature and other parameters. As the combined heated element and temperature sensor is often used platinum resistor/temperature sensor (or thermistor in cheap, less precise variants).
Platinum temperature sensor is heated to temperature higher than ambient (say t
A
+100°C) by electrical current. In response to mass flow rate we observe cooling of the element and consequently change of resistance. This simple approach does not take into account temperature of medium. For compensation, another temperature sensor is used, with the same sensitivity, but its measuring current is small enough to cause only negligible self-heating.
Thus it responds only to medium temperature. These two sensors are then put in two corresponding branches of bridge as shown in Fig.1.
In this simple circuit, we just need to set initial (e.g. for zero flow) bridge balance using R
2
.
Then we measure bridge disbalance as response to flow rate. Or, in feedback mode, the bridge is rebalanced by modification of heating current using change of U
1
supply voltage so that a constant temperature difference between medium and heated element is maintained. The supply current is then proportional to flow rate.
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A
U
1
R
1
1000

B
D
R
2
100-200

U
BD
R
Pt1000 t
R
Pt100 t
C
Fig. 1 Compensated bridge
The changes in supply voltage U
1 also cause changes of the current in compensation branch of the bridge and thus changes in self-heating of the passive (compensation) temperature sensor.
Therefore it is appropriate to choose large ratio of resistances (and thus of currents) in both branches of the bridge. Recommended ratio is 1:100 or higher.
In our setup is used for compensation instead of the Pt resistor a passive NTC thermistor with approximately same sensitivity but of opposite sign. The passive resistor is then placed in opposite branch of the bridge and large resistance ratios can be used - see Fig. 2 with schematics of setup with current ration 1:100. This circuit is only appropriate for small range of medium temperatures. For large medium temperature variations the nonlinearity of NTC thermistor would cause significant errors.
Fig. 2 Setup circuitry
The setup demonstrates principle of mass flow rate measurement. It contains fan creating flow, and air channel with two temperature sensors - heated Pt100 and passive NTC thermistor (see Fig. 3 and detail in Fig. 4). Electrical circuit corresponds to Fig. 2, where
ADC and DAC are realized using analog inputs and outputs of multifunction board NI
USB-6211. Heating current is sensed by voltmeter across R
4
resistor 10 Ω.
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Pt100 thermistor
Fig. 2 Setup for mass air flow
The heated element is platinum resistor Pt100, with thermal response in 0 °C do 600 °C that can be approximated as

R
0

100
 
:
R t

R
0

1 3,90802 10

3 t
 
6  t
2
 
; , C

(3)
Compensating sensor is NTC thermistor with similar sensitivity and room temperature resistance of about 1000

, which measures the air temperature. The volume flow rate can be estimated from fan supply voltage:
Qv  2 .
43  10  4  U fan
 4 .
81  10  4

3

1

(4)
The equation is valid for voltage higher than 3V, for smaller voltage the fan is stopped and flow is zero.
NTC termistor
Pt100
Fig. 3 Detail of temperature sensors
When constant temperature difference is maintained between heated element and ambient
(bridge is balanced), then dependence of heating current on mass flow rate Q m
can be approximated by empirical equation:
I
2   m
(5) where constant a represents power losses by heat convection and by radiation. Constant b represents geometry and medium properties (heat conductivity, viscosity, heat capacity, etc.)
For calculation of mass flow rate we need density of air, for standard temperature and pressure in lab it is
ρ
= 1.293 kg.m
-3
.
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1.
Activate setup power supply, and in Start menu run the program (see Fig. 5). Wait for initial self calibration of bridge balance by the program (indicator Bridge short is lit).
2.
Set bridge supply voltage (slider Bridge VCC Set ) so that measured real bridge supply voltage was 6.5V (shown in blue indicator and graph VCC ).
3.
Stop the ventilator (push button Zastav/suspend) and wait for settling of bridge output (red indicator and graph Bridge Out).
4.
Using voltmeter measure heating current I as voltage drop across resistor R
 
.
4
10
5.
From known Vcc and current calculate resistance of Pt100 and its temperature - eq. (3)
(the quadratic term can be neglected).
6.
For fan setting 5.5V manually modify bridge supply voltage and try rebalance the bridge to zero output by change of heating current. What is now the Pt100 temperature? Why?
7.
Try operation without feedback. Record bridge output response to flow rate (fan voltage
3-8V, constant Vcc = 6.5V).
8.
Close the feedback compensation loop by Feedback button in software. Test feedback control using step change in fan voltage. Observe bridge voltage in settled state. Compare with measurement in step 7.
9.
In feedback mode operation increase the fan voltage in 3-8V range and record Pt100 heating current.
1.
Determine (exactly) temperature of Pt100 for zero flow and nonzero flow - step 2 and
4 of the procedure. Explain differences.
2.
Draw in graph dependence of bridge output on flow rate - step 3 of the procedure. The fan flow rate calculate from its voltage and eq. (4).
3.
Put into one graph reponse of feedback mode flowmeter, i.e. eq. (1) and (2). Use dual y-axes, watch for units.
4.
For linear dependence (2) determine a and b a , b constants - eq.
(5).
5.
Answer question from points 1 and 7 of objectives.
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PID controller params bridge output
(measuring diagonal) bridge supply voltage Vcc set bridge
Vcc ventilator voltage feedback controller actions
(only in feedback mode) approximate ventilator response feedback activation
Fig. 4 Control software
[1] Bosch Mass Air flow sensor brochure
[2] Handbook of Modern Sensors, Jacob Fraden, Springer 2010
[3] Airmass Assignment, Automotive Transducer Kits, Manual, Feedback Instruments Ltd.
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