Brooks Instrument SLA7840 - Manual (Page 19)

3-1
Model SLA7840
Section 3 Operation Installation and Operation Manual
X-PR-SLA7800-RT-eng
Part Number: 541B048AAG
August, 2009
3-1 Overview
This section contains the following information:
• Theory of Operation
• Functional Description
3-2 Theory of Operation for Flow Measurement
The thermal mass flow measurement system consists of two components:
the restrictor and the flow sensor. Figure 3-4 contains a diagram of the flow
stream through the RT with an enlarged view of the flow sensor. Gas flow
entering the RT is separated into two paths; one straight through the
restrictor and the other through the flow sensor. This is represented in
Figure 3-1 where the total flow A+B enters the RT and is separated into
streams A and B. The streams are joined again at the far side of the
restrictor.
The separation of the flow streams is caused by the restrictor. During flow
conditions there will be a pressure differential across the restrictor which
forces gas to flow in the sensor. The pressure difference caused by the
restrictor varies linearly with total flow rate. The sensor has the same linear
pressure difference versus flow relationship. The ratio of sensor flow to the
flow through the restrictor remains constant over the range of the RT
(A/B = constant). The full scale flow rate of the RT is established by
selecting a restrictor with the correct pressure differential for the desired
flow.
The flow sensor is a very narrow, thin-walled stainless steel tube. Onto this
tube are built upstream and downstream temperature sensing elements on
either side of a heating element. Constant power is applied to the heater
element, which is located at the midpoint of the sensor tube. During no-
flow conditions, the amount of heat reaching each temperature sensor is
equal, so temperatures T1 and T2 (Fig. 3-1) are equal. Gas flowing
through the tube carries heat away from the upstream temperature sensor
and toward the downstream sensor. The temperature difference, T2 - T1,
is directly proportional to the gas mass flow. The equation is:
DT = A x P x Cp x m
Where,
DT = Temperature difference T2 - T1 (°K)
A = Constant of proportionality (S
2 -°K 2 /kJ 2 )
P = Heater Power
Cp = specific heat of the gas at constant pressure (kJ/kg - °K)
m = Mass Flow (kg/s)
A bridge circuit and a differential amplifier interpret the temperature
difference and generate an electrical signal directly proportional to the gas
mass flow rate.