Sierra Instruments Instruction Manual
Smart-Trak
®
2 Series 100
10
Figure 1-3. Sensor Temperature Distribution
Figures 1-2 and 1-3 show the mass flow through the sensor tube as inversely
proportional to the temperature difference of the coils. The coils are legs of a bridge
circuit with an output voltage in direct proportion to the difference in the coils’
resistance; the result is the temperature difference (
∆
T). Two other parameters, heat
input (H) and coefficient of specific heat (Cp) are both constant. Through careful
design and attention to these parameters, this output signal is made linear over the
transducer’s normal operating range (Figure 1-4). As a result, the measured flow
through the sensor tube is directly proportional to the gas flow in the main body.
Figure 1-4. Linear Range of the Transducer’s Output Signal
In the Smart-Trak 2 mass flow
controllers
, the gas which flows through the
monitoring section is precisely regulated by the built-in electromagnetic valve. The
normally closed valve is similar to an on/off solenoid valve, except that the current
to the valve coil, and hence the magnetic field, is modulated so that the
ferromagnetic valve armature, or valve plug, assumes the exact height above the
valve’s orifice required to maintain the valve’s command flow (set point). The
result is excellent resolution.