Brooks Instrument Mfi Series - Manual (Page 29)

3-1
Brooks ® Mf Series
Section 3 Operation Installation and Operation Manual
X-TMF-Mfi-Mfx-MFC-eng
Part Number: 541B074AAG
August, 2009
3-1 Theory of Operation
The thermal mass flow sensing technique used in the Mf Series works as
follows:
A precision power supply provides a constant power heat input (P) to the
heater which is located at the midpoint of the sensor tube. Refer to Figure
3-1. At zero or no flow conditions, the heat reaching each temperature
sensor is equal. Therefore, the temperatures T1 and T2 are equal. When
gas flows through the tube, the upstream sensor is cooled and the
downstream sensor is heated, producing a temperature difference. The
temperature difference T2-T1 is directly proportional to the gas mass flow.
The equation is: ΔT = A * P * C
P
* m
Where,
ΔT = temperature difference T2 - T1 (°k)
C
P
= specific heat of the gas at constant pressure (kJ/kg-°K)
P = heater power (kJ/s)
m = mass flow (kg/s)
A = constant of proportionality (S
2 -K 2 /kJ 2 )
A bridge circuit interprets the temperature difference and a differential
amplifier generates a linear 0-5 Vdc signal directly proportional to the gas
mass flow rate. The flow restrictor shown in Figure 3-1 performs a ranging
function similar to a shunt resistor in an electrical ammeter. The restrictor
provides a pressure drop that is linear with flow rate. The sensor tube has
the same linear pressure drop/flow relationship. The ratio of the restrictor
flow to the sensor tube flow remains constant over the range of the meter.
Different restrictors have different pressure drops and produce controllers
with different full scale flow rates. The span adjustment in the electronics
affects the fine adjustment of the controllers full scale flow.
In addition to the mass flow sensor, Mf Series mass flow controllers have
an integral control valve and control circuit, as shown in Figure 3-2. The
control circuit senses any difference between setpoint and flow sensor
signal and adjusts the current in the modulating solenoid valve to increase
or decrease flow.
The Mf Series has the following features incorporated in the integral control
circuit:
• Fast Response adjusted by the anticipate potentiometer. This circuit,
when properly adjusted, allows the high frequency information
contained in the sensor signal to be amplified to provide a faster
responding flow signal for remote indication and use by the control
valve.
• Soft Start enabled by moving a jumper on the PC Board. This circuit
provides a slow injection of the gas as a protection to the process,
particularly those using a volatile or reactive gas. Full gas flow is
achieved in approximately 20 seconds. Refer to Section 2-7.