Testo 417 large vane, not reading correctly?

[mgenius33]

I've been using my CEM-619 for a back up flow meter in hard to reach vents where the flow hood can't. The consistency is pretty reasonable, and close to what I get for the flow hood.

So, I decided that a higher end vane may serve me better with more accurate readings. Well, I guessed wrong.
The 417 is all over the place. I was taking a measurement with the floor supply grille off for an accurate sqin area of 48. The flow hood measured most consistently between 145 & 150 cfm with the 14"x14" skirt.
With the 417 I took many timed averages, and point averages. They are reading from 160 to 200 cfm. Not happy, I'm considering sending it back.
Air flow from return grille is also way off from 750 to 950 cfm. The flow hood reads a consistent 800-810 cfm.
The air is 67°F ~38%RH, without heat or AC operating

[mgenius33]

Update:

I don't think the Testo 417 is measuring wrong, I think that you can't measure a square duct with a round vane.

I tested again with a 12"x4" x 6" round boot sealed to the opening. The mean measurement with the round opening is very consistent with ~146 cfm.
The round opening is consistently ~170cfm. That's almost 15% error.

I believe that the mini-hood / funnel needs to be used with this tool, or the readings will be useless. We know the greatest velocity is in the center of the duct. Well, if you drag the vane across the center from left to right, you're going to get a high reading.

So, a traverse must be made from side to side and top to bottom with the vane going at least half way out of the opening of the duct.

[mgenius33]

Update:

I don't think the Testo 417 is measuring wrong, I think that you can't measure a square duct with a round vane.

I tested again with a 12"x4" x 6" round boot sealed to the opening. The mean measurement with the 6" round opening is very consistent with ~146 cfm.
The12"x4" squareopening is consistently ~170cfm. That's almost 15% error.

I believe that the mini-hood / funnel needs to be used with this tool, or the readings will be useless. We know the greatest velocity is in the center of the duct. Well, if you drag the vane across the center from left to right, you're going to get a high reading.

So, a traverse must be made from side to side and top to bottom with the vane going at least half way out of the opening of the duct.

Correction

[mgenius33]

Another measurement with another grille, this time a Lima 800 2"x14" floor vent.

Measured timed average 478 fpm across the grille face. Lima shows an ak of .115 for this grille which would = ~55cfm

With the grille still in place I connected an up-side-down boot and 6" round pipe and read a timed average with the vane at ~465fpm = ~91cfm

Performed Pitot 12 point traverse in 6" pipe averaged .0125"wc Vp which = ~447fpm = ~88cfm

So, either I'm getting the ak waaay off or I'm not using a proper method to traverse a supply air grille. Because it's a 4" vane 1/2 of the vane is never in the direct path of the supply air being thrown from the 2" wide grille. So, what do you do in this case? I'm going to make a 2"x14" to 4"x14" sheet metal transition tomorrow, and see if I get a higher grille face velocity reading.

[mgenius33]

I'm finding that the Testo 417 "FPM" reading is very repeatable on small diffusers. Unfortunately, using a mfg. provided, ak or simply measuring the free area does not produce an accurate "CFM" result. I actually found the Testo 405i hot wire anemometer to be more accurate using free grille area measurements.

Because diffuser mfgs. derive the ak by using different standards, and depending on the diffuser you're testing, the vane anemometer can read very differently. This is because the free area, is not the ak. The ak is a representation of the flow characteristics of a diffuser and the free area.

I don't find the vane to be a very accurate way to measure all different types of diffuser face velocities unless you develop your own k factor for each individual type of grille your testing. Using a proper flow hood to measure total volume, and then using that volume divided by the vanes measured fpm will provide a very accurate and repeatable result with the Testo 417.

[WAYNE3298]

The problems you are running into is why I quit trying to use the rotating vane. I have cheap ones and high dollar ones and hate both. They are only reliable if the airflow pattern from every diffuser is identical and that is rare if ever the case.
You get better results by removing the sticks on the flow hood and bending the skirt to get on the diffuser. That isn't perfect but is repeatable and closer to the real flow than you might think.
On round diffusers you have to be aware that the airflow can swirl on occasion. If you suspect swirl make a cross out of cardboard (vortex breaker) and put it inside the skirt. That will give you repeatable and accurate readings.
Flow hoods can give bad readings if skirt washing is present.
I ignored AK's and compared traverse and flow hood readings. I only did that if my hood readings were questionable because with the hood you don't need the AK.