Brooks Instrument 5850EM - Manual (Page 28)

3-4
Brooks ® Models 5964, 5850EM
Section 3 Operation Installation and Operation Manual
X-TMF-5964-5850EM-MFC-eng
Part Number: 541B121AHG
November, 2008
3-3 Theory of Operation
The thermal mass flow sensing technique used in the Model 5964/5850EM
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
3-3. 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 sensor 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:
ΔT = A * P * Cp * m
Where:
ΔT
= 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 3-2 Externally Accessible Adjustments
Span
Zero
Side View