Instruction manual

Instruction Manual Smart-Trak
®
50 Series
IM-50 Rev. A.2 8
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.
In Smart-Trak
®
50 Series 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.
View an animated tutorial on the concepts discussed in this section at:
http://sierratechsupport.com/video/flow_control.html
Figure 1-3. Sensor Temperature Distribution
Figure 1-4. Linear Range of the Transducer’s Output Signal