How to measure air flow

[pstu]

Davidr said, of the trash bag and stopwatch method of measuring air flow:

>>Personally I view this method to be at the least very inaccurate.

There is "precision" and "repeatability" issues in addition to whether the measurement would agree with a flow hood. Could you speak to whether you consider this repeatable? I think it would be.

>>When placing a trash bag over any diffuser you immediately change the airflow
>>ditribution of the duct system due to an unidentifiable back pressure.

I submit that a garbage bag in the process of being filled, will have such little back pressure compared to other methods, that this is not a significant issue. The bag is absolutely limp to begin with, and should not begin to offer resistance until nearly filled. That's what I speculate anyway.

Let me add that operator error may be a big thing with any measurement method, including expensive flow hoods. I have had 2 separate flow hood measurements done on one of my systems, troubleshooting a reported problem. The results are so far different that I have to believe one or the other made a measurement error. One said 723 CFM (for a 3.5 ton system), the other said 1320. I firmly believe the skill level of my 2 sets of techs, was an issue. I trust that *you* would apply enough expertise with a flow hood, that your readings would be repeatable and reasonably accurate.

Best wishes -- P.Student

[davidr]

Originally posted by perpetual_student
Davidr said, of the trash bag and stopwatch method of measuring air flow:

>>Personally I view this method to be at the least very inaccurate.

There is "precision" and "repeatability" issues in addition to whether the measurement would agree with a flow hood. Could you speak to whether you consider this repeatable? I think it would be.

>>When placing a trash bag over any diffuser you immediately change the airflow
>>ditribution of the duct system due to an unidentifiable back pressure.

I submit that a garbage bag in the process of being filled, will have such little back pressure compared to other methods, that this is not a significant issue. The bag is absolutely limp to begin with, and should not begin to offer resistance until nearly filled. That's what I speculate anyway.

Let me add that operator error may be a big thing with any measurement method, including expensive flow hoods. I have had 2 separate flow hood measurements done on one of my systems, troubleshooting a reported problem. The results are so far different that I have to believe one or the other made a measurement error. One said 723 CFM (for a 3.5 ton system), the other said 1320. I firmly believe the skill level of my 2 sets of techs, was an issue. I trust that *you* would apply enough expertise with a flow hood, that your readings would be repeatable and reasonably accurate.

Best wishes -- P.Student

P.Student,you are correct in the fact that operator error is a huge factor in testing in the field.
When I first started performance testing I found my first readings not matching up at all.
The problem was with the hood settings not being in the proper direction of airflow,so I understand how this can happen.
The issue is with the lack of proper training & expierience in the field of many.
I found my mistakes due to the training I had recieved & not immediatly blaming the instruments as being inaccurate.
ASHRAE standard 111 states if a flow hood measurement comes into question a traverse must be performed of the branch feeding the diffuser. If I question my readings I traverse to be certain.
The trash bag method was conducted by the same scientist who claim that flow hoods are off by 30% or more.
If this was the case that means ANSI standards are unreliable & hood manufacturers are producing high dollar garbage.
You can see why I am skeptical of how well the research was carried out for the trash bag test,the results may be repeatable but how accurate are they?

[cem-bsee]

cut some strips of 25- 50# paper -- place over register in same location of each, measure offset of paper -- for comparison --

[dan sw fl]

Originally posted by b-astill
The air flow out of the each duct varies considerably.
... is there a device to measure the air flow at the registers. What airflow should we be looking for at each register and how much difference between registers is acceptable. All the registers are the same size 3.5" x10"

Brian,

Air Velocity Anemometer.

http://www.calright.com/_coreModules...ctMasterID=204

or similar from Dwyer or Davis Instruments

Air flow requirements per room depends on the room size, glass & orientation, function, ...

OLD 'rule of thumb' might be used if you wish to perform all the work yourself.
CFM= 0.8 * Room Square Footage
....... + More for windows
....... - Less for Interior rooms
Bath or Powder room .. < 50 CFM frequently.
Family ... 15 x 18' .. ~240 CFM



[Edited by dan sw fl on 06-18-2005 at 07:04 PM]