Brooks Instrument 5851i - Manual (Page 19)

3-1
Installation and Operation Manual
X-TMF-5851i-MFC-eng
Part Number: 541B109AAG
September, 2009
Brooks ® Model 5851 i
Section 3 Operation
3-1 Theory of Operation
The thermal mass flow sensing technique used in the 5851 i works as
follows:
A precision power supply provides a constant power heat input (P) at 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 (one upstream and one downstream of the heater) 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: Δ T = A * P * C
p
* m
Where,
Δ T = 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 controller's full scale flow.
In addition to the mass flow sensor, the Model 5851 i Mass Flow Controller
has an integral control valve and control circuit, as shown in Figure 3-2.
The control circuit senses any difference between setpoint and the flow
sensor signal and adjusts the current in the modulating solenoid valve to
increase or decrease the flow.
The Model 5851 i 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.