# AirFlow Calculations/ Anemometer anyone?

• CstmIndctn
In summary, an anemometer is helpful in calculating CFM, but you have to take into account the area of the measuring device and the velocity of the air supply.
CstmIndctn
AirFlow Calculations/ Anemometer anyone??

Hello all,
I am trying to solve a problem with my anemometer. I am not sure if there is anyone here that has used one but here goes. My purpose is to calculate CFM of a particular AIR INTAKE of my truck. The problems lies in that CFM= Air Velocity Ft/Min X Area ft^2...with Area= PIE * (r^2)...however my intake is 8" diameter at the entrance, then the filter, then a 4" exit...The air supply I am using is (shopvac) 2" diameter hose...what do I use as the area?

I have a chart that says @ 4500 feet per minute on a 4" diameter is 390cfm

2nd question-- What am I doing wrong if I do 3.14 (2^2)= 12.56 Area

CFM= 4500 * 12.56= crazy big number which is not 390cfm =0(

The last problem first...you need to convert your area value to square feet. You can not use square inches. You're a factor of 144 off.

As far as the first problem, you can choose any section. The idea being that the mass flow is going to be the same through all of the parts. That being said, the velocities will change when there is a change in area. So, if you're interested in the volumetric flow rate, select the section of the shop vac. Take your measurements in the shop vac tube. Then you can calculate the volumetic flow rate in that section. From there you know you'll have the same flow rate at other locations. Just adjust the velocity based on the new area. The filter housing will probably not be a great spot for measurements.

Yea wow I am retarded...as far as measuring at the filter housing goes. I am tryin to determine the difference in my intake vs the stock intake. I figure the only was is to hold the air supply a certain distance away (approx 9 inches away gives a 50mph wind) and then measure the flow rate threw the air filter and housing, etc to see how much the actual engine is getting. I know the calculations to determine the Volumetric Efficiency and how much CFM the engine needs at a specific RPM (Volumetric FlowRate)...how else would I determine which intake flows better other than this method?

Last edited:
Amateur input here. (Sorry, Fred, but you know that I can't help chirping up when something bugs me.)
Cst(etc.), just make sure that you take the flow rate of the shop vac when it's hooked up as opposed to free-flowing. With a filter in that housing, you'll get some resistance that will probably slow it down a bit. (Or not... Fred?)

Danger...Yes the air filter gives resistance, but that is the whole point of creating a intake is to make one that supports more air and supports the CFM of a better filter (K&N) therefore, I would have to compare the air flow through the stock housing and filter to the new air filter and housing setup to see how much more CFM it will flow over stock or if it doesn't create one that will, etc...thats why I was asking what to calculate the area of since in essence I am measuring the airflow through the bigger housing and not the air supply source...

I have determined that the air supply (shop vac) has a velocity of approx 50mph @ 9inches from the point of measurement (where I hold the anemometer) therefore, I hold the air supply 9 inches from the entrance of the airfilter/housing (simulating 50mph wind speed) and hold the anemometer on the exit side of the airfilter/housing (after the actual filter) and then I would get the CFM that is capable of flowing threw that setup @ 50mph...correct? (Fred) only problem is now is trying to calculate the actual CFM...anyone ever use a anemometer? Its a lil tricky

At this point, I must turn everything over to Fred. The closest that I ever got to using an anemometer was reading the airspeed indicator on a Cessna, and that won't be of any help to you.

Well thanks for the input at any rate ;0)...

Also, One more question when a 6" pipe shrinks down to a 4" on a bend...how would I calculate the effect of this? Or would it even matter assuming I started using this device correctly =0)

CstmIndctn said:
Yea wow I am retarded...as far as measuring at the filter housing goes. I am tryin to determine the difference in my intake vs the stock intake. I figure the only was is to hold the air supply a certain distance away (approx 9 inches away gives a 50mph wind) and then measure the flow rate threw the air filter and housing, etc to see how much the actual engine is getting. I know the calculations to determine the Volumetric Efficiency and how much CFM the engine needs at a specific RPM (Volumetric FlowRate)...how else would I determine which intake flows better other than this method?
Silly question, but are you using a handheld anemometer or is it an automotive part that is preinstalled in your intake assembly?

Handheld

## 1. What is an anemometer and how does it measure airflow?

An anemometer is a device used to measure the speed and direction of airflow. It typically consists of three or four cups that rotate in the wind, and the rotations are converted into a numerical value to determine the airflow speed. Some anemometers also include a wind vane to measure the direction of the airflow.

## 2. What are some common applications for airflow calculations and anemometers?

Airflow calculations and anemometers are commonly used in various industries such as HVAC, meteorology, environmental monitoring, and air pollution control. They are also used for building ventilation and air conditioning systems, wind energy assessments, and research purposes.

## 3. How accurate are anemometers in measuring airflow?

The accuracy of an anemometer depends on various factors such as the type of anemometer, wind speed range, and calibration. Generally, cup anemometers have an accuracy of ±2% for wind speeds between 5-25 meters per second, while hot-wire anemometers have an accuracy of ±5% for wind speeds between 0.5-30 meters per second.

## 4. Can anemometers be used to measure airflow in different units?

Yes, anemometers can measure airflow in various units such as meters per second, miles per hour, kilometers per hour, feet per minute, and knots. Most anemometers have a unit conversion feature that allows the user to switch between units.

## 5. What factors can affect the accuracy of airflow calculations and anemometers?

The accuracy of airflow calculations and anemometers can be affected by factors such as air density, temperature, humidity, and wind direction. It is important to calibrate and regularly maintain anemometers to ensure accurate measurements.

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