Air flow devices

[dicer]

An auto manufacture calls the induction air metering device an AIR FLOW METER, it works on the Carmen vortex principle.
An other manufacture calls the induction air metering device a MASS AIR FLOW SENSOR, it works on the hot wire principle.

Other than how they function what do they measure that is different? Is it just a nomenclature deal?
If air density is the same and calibration is the same won't they both see the same flow?

 

[Lou Scannon]

Just about every gas mass flow measuring device I can think of infers the mass flow via sensing of one or more physical phenomena; and rests on certain assumptions. Except for positive displacement devices, even volume flow devices are inferring the volume flow, again via the sensing of one or more physical phenomena. Of course volume flow device can be combined with a density calculation from sensed or assumed temperature and pressure, to yield a calculated mass flow.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[GregLocock]

The sprung loaded flap type MAF actually measures momentum so it is a true mdot device. Whatever type they use it is checked vs a table of TPS and RPM and intake temperature and MAP, so it can throw codes it can detect sensor failure etc.

Cheers

Greg Locock


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[gruntguru]

Greg - my understanding has always been that flap AFM respond (primarily) to volume flow due to pressure differences.

Hemi - although almost all sensors of any property only "infer" the value of that property, there are many MAF sensors that respond in a direct relationship to mass air flow. Hot wire MAF sensors (plus other heat transfer types) and Coriolis mass flow sensors to name a couple.

je suis charlie

 

[GregLocock]

moment about hinge =f(density, area, volumetric flow rate,flap angle). Shirley?

Cheers

Greg Locock


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[Lou Scannon]

although almost all sensors of any property only "infer" the value of that property, there are many MAF sensors that respond in a direct relationship to mass air flow. Hot wire MAF sensors (plus other heat transfer types) and Coriolis mass flow sensors to name a couple.

Certainly, their output signal is tuned to give a linear response to inferred mass flow, but in no way are they actually measuring mass flow. Same with flap meters. Last time I checked, they needed a density correction, but I'm no gas dynamics expert.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[gruntguru]

Greg, vane AFM designs use a very soft (clock) spring to maintain a near-constant hinge moment and therefore a near-constant velocity (similar to a CV carb). "If density changes due to temperature fluctuations or the altitude, the measurement changes with density^1/2. An air temperature sensor and a barometric pressure sensor must be used to compensate for this" Bosch Automotive Handbook, 9th ed, p1319

Also did some checking on hot wire devices and they do require compensation for temperature due to the viscosity and thermal conductivity dependence of air. They do not require compensation for density changes due to altitude (pressure).

Hemi, the response of HW AMM (filament current) to mass flow is extremely precise but nonlinear (approx square root).

je suis charlie

 

[GregLocock]

Wow, thanks for that. That is a very curious relationship, sqrt of mass is not a dimensional analysis I remember seeing before.

Cheers

Greg Locock


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[gruntguru]

Something to do with Bernoulli. Velocity =(2xP/rho)^1/2

je suis charlie

 

[SomptingGuy]

The infer vs measure debate...

Splitting hairs. Engineers don't normally get into that stuff.

In this discrete universe, counting is the only true measure you really have. Mass flow is possible, given good vision, enough fingers and some absolute definition of time (that's not defined in relation to something else).



Steve

 

[dicer]

Great. An now what about my original question?

 

[IRstuff]

see:

https://en.wikipedia.org/wiki/Mass_flow_sensor#K.C3.A1rm.C3.A1n_vortex_sensor

The vortex sensor is measuring the frequency of vortex oscillations, which are proportional to mass flow, while the hot wire sensor is measuring the forced convection property of the air flow. Fundamentally, they are using different properties of air to infer what the mass flow was.

TTFN
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[GregLocock]

dicer- all measurements are estimates, so these all deliver a signal related to the actual value+systematic errors +noise.

Measuring mass flow rate of an airstream without causing too much back pressure, reliably and cheaply, would appear to be trickier than a first glance indicates.

Bear in mind that in addition to the sensor check I mentioned, the O2 sensor and fuel injector pulse width also provides another check on the MAF. A significant effort is expended in the last 2 years before job 1 in replacing actual measured properties by ones that can be inferred from the multiple methods of measuring the engine.

Cheers

Greg Locock


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[Lou Scannon]

Hemi, the response of HW AMM (filament current) to mass flow is extremely precise but nonlinear (approx square root).

Quite right; I misspoke myself when I wrote "linear". The non-linear relationship is helpful, as the required precision is somewhat tuned to the magnitude of the flow.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz