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Principle of Operation
Thermal Mass Flow Meters have two sensors often
constructed of reference grade platinum. These two RTDs are
clad in a protective sheath and are driven by a 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 to become an In-Line Style flow
meter. As gas flows by the heated sensor (flow sensor), the
gas molecules carry heat away from the surface of the sensor,
and the sensor cools down as it loses energy. The sensor drive
circuit replenishes the lost energy by heating the flow sensor
up until it is a constant temperature differential above the
reference sensor. The rate of heat absorbed by a flow stream
and hence the electrical power required to maintain a constant
temperature differential is directly proportional to the gas
mass flow rate.
It is essential that this constant temperature differential
be maintained, even if there are wide fluctuations in gas temperature.
It is the "job" of the 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 the job of Sage Metering to properly calibrate the device
with the actual gas (or close equivalent with certain gases),
in the Sage NIST traceable calibration facility. By accomplishing
these two critical objectives, the flow meter will have an extremely
repeatable (.2% of full scale) and accurate output directly
proportional to the mass flow rate of the gas that it is set
up to measure.
As molecules of a moving gas come into contact with a heat source,
they absorb heat and thereby cool the source. At increased flow
rates, more molecules come into contact with the heat source,
absorbing even more heat. The amount of heat dissipated from
the heat source in this manner is proportional to the number
of molecules of a particular gas (its mass), the thermal characteristics
of the gas, and its flow characteristics.
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