Mass flowmeter using thermal dispersion technology
A mass flowmeter employing thermal dispersion technology, and method for determining mass flow of a fluid throughout a range beyond that which possible for a constant ΔT instrument at increasing power, and below that with which a constant power instrument can provide rapid ΔT readings.
1. A mass flowmeter for fluid media, the mass flowmeter comprising:
a reference, unheated sensor having an electrical excitation input and an output, said reference sensor detecting the temperature of the fluid media;
an active, heated sensor having an electrical power input and an output, the thermal difference ΔT between the output from the reference sensor and the active sensor being continuously determined;
a source of electrical power coupled to the heater of said active sensor;
a controller which controls the power applied to the heater of said active sensor to selectively maintain ΔT constant, the power applied to the active sensor heater being below a predetermined level at start up and increasing in relationship to mass flow rate increases until the power applied as mass flow rate increases reaches the predetermined level, and maintains that power level as mass flow of the media continues to increase while ΔT diminishes, the power applied to the heater of the active sensor being continuously determined, the controller further controls the power applied to the heater of said active sensor to maintain the power level constant at the predetermined level as mass flow decreases until the power required to the heater of the active sensor drops below the predetermined level at which time the controller switches to the constant ΔT mode:
and
digital circuitry for determining the mass flow of the media as the electrical power is varied in relation to mass flow changes and for determining mass flow of the media as the ΔT changes in relation to mass flow changes when the electrical power applied to the heater of said active sensor is maintained at the predetermined level.
2. The mass flowmeter according to claim 1 , wherein said controller uses a hardware-based closed-loop control circuit to control the power applied to the heater of said active sensor.
3. A method for measuring mass flow of a fluid using an apparatus comprising a reference, unheated sensor having an excitation input and an output, an active, heated sensor having an electrical power input and output, a source of electrical excitation to the reference sensor and a source of electrical power to the heater of the active sensor, the method comprising:
applying power to the active sensor heater below a predetermined level at start up;
maintaining the temperature differential (ΔT) constant between the reference sensor heater and the active sensor as the fluid mass flow rate increases, by increasing the power to the active sensor heater up to the predetermined power level;
measuring the electrical power level as it increases in relation to increase in mass flow rate of the fluid;
determining the mass flow rate in relation to the power level applied to the heater of the heated sensor;
continuing to apply electrical power at the predetermined level to the heater of the active sensor;
measuring the ΔT as mass flow rate increases and the power level applied to the heater of the active sensor remains constant;
determining the mass flow in relation to ΔT changes as mass flow changes; and
maintaining the power applied to the heater of the active sensor at the predetermined level until the mass flow rate of the fluid decreases until the power required to maintain ΔT constant drops below the predetermined level at which time the controller switches to the constant ΔT mode.
4. The method according to claim 3 , and further comprising continuously measuring ΔT and the electrical power applied to the heater of the active sensor.