Sierra C100L-L-DD-LE-5-OV1-SV1-PV1C-V1-S1, Инструкция по эксплуатации

Страница: 9 / 55

Sierra Instruction Manual MaxTrak 180
9
1.2 Definitions Used In This Manual
The following terms are used frequently in this manual. They are presented here with their definitions for your
information.
Setpoint —The command or control signal supplied to a flow controller is called its setpoint. The controller will
maintain the flow at this value.
Full scale —The highest flow that an instrument will meter within its specified accuracy. It is often possible for
an instrument to measure a flow beyond its full scale value, but the accuracy of this measurement may be outside
of published specifications.
Purge —The MaxTrak Mass Flow Controller is supplied with the ability to open the valve far beyond the full scale
position to allow them to be cleaned. This is usually accomplished by blowing clean, dry nitrogen through the
instrument. When the valve is opened to this cleaning position, it is said to be in the Purge mode.
LFE —Laminar Flow Element (LFE) or bypass generates pressure drop forcing a small fraction of the total flow to
pass through the sensor capillary tube.
1.3 The MaxTrak Flow Sensing Principle
The operating principle of the MaxTrak instruments is based on heat transfer and the first law of thermodynamics.
During operation process gas enters the instrument’s flow body and divides into two flow paths, one through the
sensor tube, the other through the laminar flow bypass. The laminar flow bypass (often called LFE which stands for
“laminar flow element”) generates a pressure drop, P1–P2, forcing a small fraction of the total flow to pass through
the sensor tube (m1).
Figure 1. Flow Paths Through the Instrument.
Two resistance temperature detector (RTD) coils around the sensor tube direct a constant amount of heat (H) into
the gas stream. During operation, the gas mass flow carries heat from the upstream coil to the downstream coil. The
resulting temperature difference (∆T) is measured by the MaxTrak microprocessor. From this, MaxTrak calculates
the output signal. Since the molecules of the gas carry away the heat, the output signal is linearly proportional to
gas mass flow.