Brooks Instrument 5860i - Manual (Page 17)

3-1
Brooks ® Model 5860 i
Section 3 Operation
Installation and Operation Manual
X-TMF-5860i-MFM-eng
Part Number: 541B110AAG
November, 2008
3-1 Theory of Operation
The thermal mass flow sensing technique used in the 5860i 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 ( O K)
C
p
= specific heat of the gas at constant pressure
(kJ/kg-
O K)
P = heater power (kJ/s)
m = mass flow (kg/s)
A = constant of proportionality (S 2 - O 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 meters with different
full scale flow rates. The span adjustment in the electronics affects the fine
adjustment of the meter's full scale flow.
The Model 5860 i has the following features incorporated in the integral
signaling conditioning 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.
Removable cleanable sensor permits the user to clean or replace the
sensor. Refer to Section 4-4.
Output Limiting prevents possible damage to delicate data acquisition
devices by limiting the output to +6.8 Vdc and -.7 Vdc, on the voltage
signal output and 0-26 mA on the current signal output.