3-1
Brooks
®
Models 5965, 5851EM
Section 3 Operation
Installation and Operation Manual
X-TMF-5965-5851EM-MFC-eng
Part Number: 541B122AAG
September, 2009
3-1 Theory of Operation
The thermal mass flow sensing technique used in the Model 5851EM
works as follows:
Constant power (p) is provided to the heater winding which is located at
the midpoint of the sensor tube. Refer to Figure 3-1. During 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:
Δ
T = A * P * Cp * m
Where:
Δ
T
=
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.
Figure 3-1 Flow Sensor Operational Diagram
