Operational Theory of Thermal Mass Flowmeter
The rate of heat absorbed by a flow stream is directly proportional
to its mass flow. 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.
EPI’s proprietary thermal mass flow sensors use two ratiometrically-matched,
reference-grade platinum Resistance Temperature Detectors (RTDs).
The platinum sensing element wire is wound on a ceramic base, given
a thin protective glass coating, and encapsulated in a 316 stainless
steel sheath. As a result, the only materials exposed to the gas
stream are 316 SS.
A forced null Wheatstone Bridge preferentially heats one RTD.
The second RTD acts as a temperature reference by taking on the
temperature of the flowing gas. The resistance ratios are maintained
through the Wheatstone Bridge to compensate for the dynamic changes
in process temperature. By maintaining a constant temperature difference
between the RTDs, EPI can measure the amount of heat dissipated
by the flowing gas. As heat is dissipated, more power is needed
to maintain the constant temperature. The power demand is directly
proportional to the gas mass flow rate, allowing our sensors to
measure the gas molecular rate of flow without further compensation
for outside effects. EPI’s standard flow sensors can respond to flow velocities as low as 15 feet per minute and as high as 45,000 feet per minute for most gases. Consult our factory or a local sales representative for details.
A Brief Overview of Signal Processing
The flow calibration process records the raw voltage generated by the sensor at a series of known flow rates. This is a non-linear correlation. An example of the non-correlation of the bridge voltage to flow rate is shown below. In this example, the voltage at no flow, or Zero (0 SCFM), is 3.241 VDC; the voltage at the sample maximum flow, or Full Scale (140 SCFM), is 7.585 VDC.
The bridge circuitry normalizes the raw bridge voltage to a scale
of 0 to 5 VDC. This results in a one-to-one non-linear correspondence
between the sensor's input voltage and the bridge circuit's output
voltage. The continuous polynomial curve fit uses coefficients
stored in the microprocessor to convert the non-linear curve to
a linear curve, resulting in a one-to-one correspondence between
the flow rate and the output voltage.