Measuring CFM?

[cheesewonton]

cheesewonton,

Blackheart is correct...now onto other matters.

This will be my last response in this thread. You literally have no idea what you're talking about because you obviously have never been in a lab, written college grade reports and been overseen/graded by PhD professors. I have (think graduate level). I have also taught Math, Physics and various computer science (think programming/networking/hardware) courses in High School and College. I actually make great efforts to obtain the most accurate, repeatable laboratory grade measurements as a rule on my channel. That's why I started it back in May of 2017.

I have two vane type anemometers, a GM8901 and a HP-846A. The HP-846A has a built in calculation (CFM) function, the GM8901 does not, so you use the ratio I stated before. You will see these in my videos on my YouTube channel.

From cheesewonton: "If you are using a propeller anemometer your measurements are not accurate."

Completely false statement made by someone who isn't familiar with the equipment. Stop repeating it!

At this point I can guarantee that you don't know how to use your hotwire anemometer either. Have you calibrated it for your environment?
1) Elevation correction
2) Temperature/Humidity correction
And don't forget a standardized supply load voltage for your vacuums.

Through using a series of cone shaped adapters (easily made), ANY vacuum port can be fitted properly to ANY vane adapter. Basic Physics dude.

My test lab typical specs: loaded test voltage range (120-121 VAC), temp. 67-69 F, humidity 35%-45%, elevation 840 feet (close to USA populated avg. of 636 feet).

Here's an elevation versus pressure pdf
https://www.nwflowtech.com/media/0y0aizb3/nwft-barometric-pressure-vs-altitude-table-122120-v2.pdf

I've spent about 1000 hours in high-end labs (chemistry, physics, biology (yuck)) before I fully understood how to setup my own correctly.

Best of luck in your testing journey but you'd better learn how to use your equipment properly and refrain from posting false info which you have here.

Bill

Lol, I just retired from a career in one of the Navy's premier weapons labs. I made a career in R&D. I think I know my way around instruments and data.

 

[cheesewonton]

With the GM8901 to obtain an accurate airflow measurement you would multiply the speed in feet per minute by the area of the hose opening in square feet. Hoses have different diameters and thus different surface areas at the opening, so there is no one size fits all factor. If you are measuring a 1 1/4 inch diameter hose you would multiply the observed airspeed in ft/min times the surface area 0.0088 square feet. If you were measuring a 35mm hose on something like a Miele you would multiply the observed airspeed in ft/min by 0.0104. Ect. Each hose diameter will have a different square foot opening to use in the calculation.

Likewise with the HP-846A, directly from the operating manual, you have to manually enter the surface area of the hose opening of the vacuum being tested. If you are not entering the correct value the resulting CFM calculation will be incorrect. As before for accuracy there is no one size fits all factor to multiply airspeed by.

A general shortcoming with rotating vane anemometers are bearing friction and the amount of obstruction to airflow inherent in their design. Hot wire anemometers, like you find in automobile Mass Airflow Sensors do not create as much of an obstruction to the airflow so there are less losses and greater accuracy. HVAC pros will use capture hoods with differential pressure sensors or a thermal ( hot wire ) sensor.

 

[vaclab]

cheesewonton,

I simply had to respond to such a grossly ignorant comment. While I have GREAT respect for our armed forces, if what you claim is true, you couldn't pass an 8th grade Algebra or 9th grade Geometry test. Geez, what a disappointment. I posted these calculations many years ago on Vacuumland, but here they are again.

For the GM8901 anemometer:

Diameter of GM8901 Anemometer Detector = 2.1875 inches
Radius = 1.09375 inches = .0911458 feet
Detector Area = 3.1415926 x .0911458 x .0911458 = 0.0260990 (rounded)

Soooo, if I obtain a reading of say 5747 ft/min, that converts to about 150 CFM.
5747 (ft/min) x 0.026099 (area) = 149.9 CFM

I also noticed you refused to provide ANY of your lab specs after I stated mine (as expected). No pics, no vids, just false claims. Par for the course I suppose.

Lastly, you did mention a 9.5 on the Baird meter, which equates to 109 CFM. And you somehow can't manage to clean (dust) with 109 hose CFM? What world do you live in anyway? The typical hose CFM is around 80 CFM, so your Kirby has plenty of above the floor cleaning power.

Bill

 

[vaclab]

Here we go again. If you had bothered to read/understand my previous posts and/or view some of my extensive YouTube videos, you would clearly see the cone adapters I use. These type of adapters have been used in the industry for decades. They adapt (without interference) diameter "A" to diameter "B" very easily. Try and make one rather than whining so much. How on Earth do you think that airflow measurements were made say, 50 years ago?

Example #1: if a hose is 1" diameter and you want to measure its flow, you can with any measuring tool that is 1" or larger. Yup, you can create a 1" to 2" cone adapter and proceed to mate it to a 2" vane.

Example #2: if you have a large 12" tube/square (say an A/C vent), you can use the same 2" vane detector to measure part of the area (say 10%), then multiply by 10.

 

[cheesewonton]

You are multiplying airspeed in ft/min times 0.26099. That is the diameter of the fan. You are using the instrument incorrectly. You need to multiply airspeed by the diameter of the hose opening. The operators manuals for both anemometers ( I looked at them ) tell you to multiply airspeed by the surface area of the duct. In this case the duct is a vacuum hose. This explains your overly high airflow readings. That is also how my anemometer explains the airflow calc.

 

[vaclab]

Here we go again...and you typed 0.26099 instead of 0.0260990. Different by a factor of 10. So here's a picture of the least expensive way to measure vacuum airflow (i.e. the hose is smaller than the vane). Note: my wife isn't holding everything exactly straight.

The vane is a detector and the cone (properly sealed at both ends of course) will spin at a speed proportional to the flow at its given diameter. That same airflow enters the hose at a different diameter. The total flow of the air MUST be the same (minus about 0.25 CFM for the blade drag of the GM8901). Sooo, if 100 CFM enters the vane, 100 CFM MUST enter the hose.


One method of using a small vane to calculate airflow in a larger airduct is to first find the area of the duct (i.e. H x W). Find out the ratio of duct area divided by vane area and now you have a simple multiplier you can store in your head.

For example: if the area of the duct is 10 times the area of the vane, simply take your (calibrated CFM) measurement and multiply by 10. Say the vane reads 10 CFM, then the total airflow through the duct would be 100 CFM. Pretty easy huh?

Do you require another picture showing the vane being held up to a duct (area comparison)?

 

[cheesewonton]

Here we go again...and you typed 0.26099 instead of 0.0260990. Different by a factor of 10. So here's a picture of the least expensive way to measure vacuum airflow (i.e. the hose is smaller than the vane). Note: my wife isn't holding everything exactly straight.

The vane is a detector and the cone (properly sealed at both ends of course) will spin at a speed proportional to the flow at its given diameter. That same airflow enters the hose at a different diameter. The total flow of the air MUST be the same (minus about 0.25 CFM for the blade drag of the GM8901). Sooo, if 100 CFM enters the vane, 100 CFM MUST enter the hose.
View attachment 168426

One method of using a small vane to calculate airflow in a larger airduct is to first find the area of the duct (i.e. H x W). Find out the ratio of duct area divided by vane area and now you have a simple multiplier you can store in your head.

For example: if the area of the duct is 10 times the area of the vane, simply take your (calibrated CFM) measurement and multiply by 10. Say the vane reads 10 CFM, then the total airflow through the duct would be 100 CFM. Pretty easy huh?

Do you require another picture showing the vane being held up to a duct (area comparison)?

That is incorrect. The diameter that matters is the diameter of the hose opening. My anemometer is a simple probe. There is no shroud like the propeller ones. You don't use an adapter cone. Just hold the probe in front of the hose with the probe oriented for the direction of airflow ( a dot on the top of the probe gives you orientation ).

For use measuring airflow for an HVAC situation the technique is to take airspeed measurements in feet per minute at the four corners of a duct and at the center, then average those readings. If it is a round duct like my house has you would take readings at four of five points around the perimeter and at the center and calculate their average. Take the average airspeed you just calculated and multiply that by the area of the duct in square feet. For a smaller duct like a vacuum hose you do the same, but you have a smaller duct. But the salient area is the area of the duct, which is the hose opening.

When an HVAC tech takes an airflow measurement using a shrouded airflow meter they too use the surface area of the duct opening, not the smaller opening of the shroud, to calculate airflow. Look at some of the CFM figures shown at the beginning of this thread. The CFMs shown for the suction inlet of the vacuum are literally greater than the motors in them can produce based on the performance charts supplied by the manufacturers. These airflow values are literally impossible for those vacuums to achieve. The numbers I calculate are less than the maximum values for a given orifice size, which makes sense given airflow losses going from the motor to the hose end.

 

[cheesewonton]

Attached is the manual for my anemometer. Read page 4. It explains how to determine airflow.

 

[vaclab]

You're simply not grasping how measuring tools work here. You should know that your hot wire doesn't actually measure airflow directly just like a vane type. It measures airspeed and then you enter in the area calculation (and type, i.e. square, round). Hot wire anemometers also have to constantly correct for temperature and humidity since the wire has to report a temperature difference via voltage/resistance. Also, how the hot wire thermometer is used will affect the readings. For example, stuffing it straight into the flow versus a vertical position. The vane type is only used one way, which simplifies things somewhat.

The vane type only cares about the air flowing through itself, not the hose or airflow box hole, etc. That's a known calculation and is given above. Then, various cones are used translate the flow to the vane in the proper manner and thusly CFM is measured.

Let me say it another way. I have gathered and created data from many OEM's and not only compared them to my readings but also have been the actual SOURCE of verification for some of those OEM's as well. In other words, My measurements stand confirmed as accurate, even by third parties. I've even compared notes with the likes of Wirecutter (see my YouTube video).

To be used correctly, a hot wire thermometer must be calibrated with various known wind sources and I'll let you Google that for verification.

The vane type doesn't care about temperature or humidity and usually needs no extra calibration.

Bottom line, your false assumption that vane types are worse (not to be trusted) compared to hot wire types is quite disappointing.

Now onto duct measurements:

My simple example was exactly that. Something that assumes an area and approximately equal flow at all points in the space. I have no desire to type a dissertation length post. Obviously, with things like corrugations, angles, vents, splitters, turbulence is introduced.

Airflow CFM Measured with a Hot Wire Anemometer!


If you watch carefully, you see the tech perform a wide range of measurements that involve a HUGE amount of human error (speed of insertion/removal, tilting, etc. Then everything has to be averaged, yippee! While this method will certainly give a general measurement of airflow, I hope you see just how sloppy the entire procedure is. What would the error bar calculations be, plus or minus 10%? Larger? Not to mention performing measurements with several open holes, geez!

Anyway, good luck with your measurements but unless you make the attempt to fully verify them (as I have for years), your results won't gain any credibility.

Bill

 

[kenc184]

It's nice to see that at least one gentleman in this cfm debate is not casting insults at the other and "throwing his toys out of the pram".