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Series 830/840/860 Instruction Manual Chapter 1 Introduction
IM-83/84/86-H 1-5
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 pro-
portion to the difference in the coils’ resistance; the result is the tem-
perature difference (∆T). Two other parameters, heat input (H) and
coefficient of specific heat (Cp) are both constant. The resulting out-
put is nearly linear over the transducer’s normal operating range.
Figure 1-4. Linear Range of the Transducer’s Output Signal
For mass flow controllers, once the gas flows through the monitor-
ing section, it is then controlled by the built-in servo-control valve.
For flows up to 500 slpm, Sierra’s proprietary high-efficiency Fast-
Trak Electromagnetic Valve is used. The normally closed Fast-
Trak valve is similar to an on/off solenoid valve, except that the
current to the valve coil, and hence the magnetic field, is modu-
lated so that the ferromagnetic valve armature, or valve plug, as-
sumes the exact height above the valve’s orifice required to main-
tain the valve’s command flow. The result is nearly infinite resolu-
tion. All controllers with the Fast-Trak valves are available with a
“Soft-Start” option for those processes that require it.
For higher flows of 50 to 200 scfm, controllers are equipped with Si-
erra’s microprocessor-controlled, motor-operated rotary valve. While
the rotary valve does not provide a completely positive shut-off, it
does offer accurate control for higher flow installations.