AIr flow meter and mass airflow 1

[yoshimitsuspeed]

It seems to be pretty common terminology to call an AFM a MAF especially in the automotive world. It's my understanding that on it's own there is no way for an AFM to calculate mass airflow.
An AFM could be used with other components like a temp sensor and MAP sensor to calculate mass airflow but on it's own it cannot. It seems to me akin to calling a hotwire a MAF. A hotwire MAF cannot calculate mass airflow without the other included components to calculate mass airflow and that make it a mass airflow sensor. A hotwire is nothing more without those things.
It is very hard to find information talking about technical details and technical function in regards to the airflow's effect on the vane on an AFM. I have thought in the past that a vane AFM measured volumetric airflow but in thinking about it I realize that's not entirely correct either. X CFM passing through the vane at the fringes of space would open the sensor much less than the same CFM flowing through it at sea level so the the AFM should respond to a reduction in mass airflow at the same volumetric airflow but not in a way that can calculate volumetric airflow without other sensors right?

Many AFM devices also have a temp sensor. This will help but it still seems to me you wold need a pressure sensor to calculate mass airflow.

So is the whole entire internet wrong? Or am I missing something?

 

[140Airpower]

Gruntguru, you said "compensation will be required" for the vane type MAF, but this is true of the hot wire meter and all of the transducers I know of. The hot wire anemometer was not the first choice in the early days because of non linearities and dynamic range problems. Compensation is built-into the circuit especially current control.
I am certainly not arguing that the hot-wire is not superior to the vane. Number one is that it has no moving parts. I am stating that the hot-wire has NO input from mass in its operation while the vane DIRECTLY measures mass.

 

[yoshimitsuspeed]

If an AFM directly measured mass airflow then why do some even run a temp sensor? Is the amount the door opens directly related to the mass passing by regardless of density?
Here is an extreme scenario to think about. Some people put their AFM after the turbo on boosted builds or NA motors converted to turbo. This seems like a very bad idea to me because it doesn't seem to me like the AFM will read properly. If someone were to run 44 PSIA boost through the AFM would the AFM read the same as it would before the turbo having air flowing past it at one third the pressure and 3 times the volumetric flow rate?

Another thought that is much less related but operates on the same principles.
If an AFM is able to measure mass airflow simply by the air passing by it and that airs effect on the vane then shouldn't it be very easy to make a carburetor that never needs jetting? Yet as you change altitude you need to run leaner jets because the change in mass airflow is not directly linear to the force the air applies to draw the fuel out of the jet. Wouldn't the same factors change in relation to the mass airflow and the force applied to the metering plate?

 

[BrianPetersen]

Yes, but the electronic systems to which these devices are connected nowadays are more than smart enough to deal with the difference.

 

[yoshimitsuspeed]

Sure but then why not just call an O2 sensor a MAF because it often gets incorporated into systems that can rather accurately estimate mass airflow?
I mean heck with an O2 sensor and knowing injector duty cycle alone I would bet you could estimate mass airflow more accurately than an AFM on it's own.

 

[1gibson]

The sensor reacts based on the mass of the air going through it. Sure it is a combination of density, temperature, velocity, but the dominating factor is mass flow.

Will it read the same before a turbo and after? No, because the nature of the sensor is to take a sample with a small element in a known location inside a tube, use a calibration table that assumes a certain flow profile (at or near atmospheric pressure) and guess what the flow is through the entire tube. Based on the measurement from that one location.

The majority of the error is probably due to the higher density air at a lower velocity, not matching the calibration table that is expecting higher velocity/lower density for that raw value of the sensor output.

You can't denounce an entire sensor type for being inaccurate, when it's not calibrated for the situation it's being put in. It's less than ideal, but better than no sensor at all (some may disagree, that's fine, use speed density.) If you added another dimension to the calibration table that would pick a different value based on varying pressure, then it could be just as accurate as a naturally aspirated configuration. More so in fact, because it would compensate better for atmospheric conditions.

 

[1gibson]

I am referring only to hot wire anemometers (MAF sensors) to be clear.

 

[Lou Scannon]

The sensor reacts based on the mass of the air going through it. Sure it is a combination of density, temperature, velocity, but the dominating factor is mass flow.

Will it read the same before a turbo and after? No, because the nature of the sensor is to take a sample with a small element in a known location inside a tube, use a calibration table that assumes a certain flow profile (at or near atmospheric pressure) and guess what the flow is through the entire tube. Based on the measurement from that one location.

The majority of the error is probably due to the higher density air at a lower velocity, not matching the calibration table that is expecting higher velocity/lower density for that raw value of the sensor output.

Actually, as explained by gruntguru in the second post, the hot wire concept [with a reference temperature element] is quite immune to changes in pressure or temperature, since it is functioning on a heat transfer principle, which depends on the number of air molecules passing the sensor per unit time, at a constant delta T. The reference temperature element is used to ensure that the delta T between the sensing element and the sample air flow is constant, removing the sampled air temperature as a variable.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[1gibson]

Oh absolutely the concept is bulletproof. The implementation is a different story when you try to use it outside the calibrated range.

 

[gruntguru]

1gibson.
I think the main problem with comparing a MAF sensor before and after a turbo would be a) the possibility of turbulence or swirl. Either of these will increase the local velocity in the region of the hot wire. b) The duct geometry prior to the MAF. The velocity profile across the MAF is crucial. I recall (many years ago) installing an "orifice plate" to slightly reduce the cross section to correct a lean mixture problem. A very slight reduction in area and the resulting "vena contracta" produced a marked increase in MAF output and solved the problem.

je suis charlie

 

[140Airpower]

Yoshimitsu, you asked "...If an AFM directly measured mass airflow then...?". The vane or swinging door meter is an aerodynamic device and directly measures mass. It is in the operational equation as a causal element. But, the equation often has non-linear components, like v^2. Compensation may be required. It can be built in like the side ports of a pitot tube, analogous to the compensating reference element of a hot wire meter.