Principle Of Operation - Sage SIP Operation And 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
sheath and are driven by a proprietary sensor drive
circuit. One of the sensors is self-heated (flow sen-
sor), 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 function of the
Sage hybrid-digital 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 proper-
ly 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 objec-
tives, the Sage meters provide an extremely repeat-
able (0.2% of Full Scale) and accurate output directly
proportional to the mass flow rate of the gas being
measured.
27
FLOW SENSOR
(Self Heated)
TEMPERATURE SENSOR
(Reference Sensor)
REV. 16-SIP/SRP