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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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