Principle Of Operation - Sage RIO SIX Instruction Manual

O p e r a t i o n s a n d I n s t r u c t i o n M a n u a l
Principle of Operation of the Thermal Mass Flow Meter
Sage Thermal Mass Flow Meters have two sensors
constructed of reference grade platinum windings
(RTDs). The two RTDs are clad in a protective 316SS
or Hastelloy C sheath and are driven by a proprietary
sensor drive circuit. One of the sensors is self-heated
(flow sensor), and the other sensor (temperature/
reference sensor) measures the gas temperature.
The pair is referred to as the sensing element, and
is either installed in a probe as an Insertion style,
or inserted into a pipe section as an In-Line style
flow meter.
As gas flows by the flow sensor, the gas molecules
carry heat away from the surface, and the sensor
cools down as it loses energy. The sensor drive
circuit replenishes the lost energy by heating the
flow sensor until it is a constant temperature differ-
ential above the reference sensor. The electrical
power required to maintain a constant temperature
differential is directly proportional to the gas mass
flow rate and is linearized to be the output signal
of the meter.
S A G E M E T E R I N G , I N C .
It is essential that this constant temperature differ -
ential be maintained, even if there are wide fluctua-
tions in gas temperature. It is the "job" of the Sage
proprietary sensor drive circuit to maintain the
differential, whether or not the gas temperature
changes, or however quickly molecules cool off the
flow sensor. It is also necessary to properly calibrate
the device with the actual gas (or close equivalent
with certain gases), in the Sage National Institute
of Standards certified (NIST) calibration facility. By
accomplishing these two critical objectives, the Sage
meters provide an extremely repeatable (0.2% of Full
Scale) and accurate output directly proportional to
the mass flow rate of the gas being measured.
27
REV. 10-SIX/SRX