Sierra 860, 840, 830 instruction manual Sensor Temperature Distribution

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.