Brooks Instrument 5850E - Manual (Page 21)

Brooks ® Model 5850E
3-1
Section 3 Operation Installation and Operation Manual
X-TMF-5850E-MFC-eng
Part Number: 541B102AHG
September, 2009
3-1 Theory of Operation
The thermal mass flow sensing technique used in the Model 5850E 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 is equal. The temperatures T1 and T2, therefore, are equal. When
gas flows through the tube, the upstream sensor is cooled and the
downstream sensor is heated which produces a temperature difference.
The temperature difference T2-T1 is directly proportional to the gas mass
flow. The equation is:
DT = A * P * Cp * m
Where:
DT = Temperature difference T2 - T1 (°K)
Cp = 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 5850E Mass Flow Controller
has an integral control valve and control circuit as shown in Figure 3-2. The
control circuit senses any difference between the flow sensor signal and
adjusts the current in the modulating solenoid valve to increase or
decrease the flow. The Model 5850E 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. This provides a faster responding flow signal for
remote indication and use by the control valve.
Soft Start
This is enabled by moving a jumper on the printed circuit 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 15 seconds. Refer to Section 2-6.