# Thread: VELOCITY PRESSURE TO CFM

1. ## VELOCITY PRESSURE TO CFM

I tested velocity pressure in a duct to find out the actual cfm for a fan coil unit, the readings of manometer(TESTO) is not stable, it does varies from .02 to .10 inches water column. Actually, it is my first test by using TESTO manometer. I depended some YouTube videos to measure the velocity pressure. My experience had solely with anemometer. Then I took the maximum value and then that’s square rout multiplied with value 4005 to get the AIR VELOCIY. Then air velocity multiplied with duct area. But final result is more than name plate value!! Somewhere mistake, any advice from you, thank you

2. Professional Member
Join Date
Feb 2016
Location
Louisburg Kansas
Posts
2,963
Post Likes
The problem you are having is not abnormal. Your calculations are correct. You will get better results with an inclined manometer because you can mentally average the readings. The problem is lack of continuity of mass which results from the low pressure of the fan. As your readings indicate the airflow is not stable.
I wish I had better words to explain this but don't.

3. Originally Posted by WAYNE3298
The problem you are having is not abnormal. Your calculations are correct. You will get better results with an inclined manometer because you can mentally average the readings. The problem is lack of continuity of mass which results from the low pressure of the fan. As your readings indicate the airflow is not stable.
I wish I had better words to explain this but don't.
thanks your reply,i meant the reading is not steady state, i checked in afresh air duct for testing purpose,it shows same variation. that reading is not come to steady state.

4. Professional Member
Join Date
Feb 2016
Location
Louisburg Kansas
Posts
2,963
Post Likes
The reading is not steady state because the air flow is not steady state. Both ducts you checked are low pressure and change with building pressure. The static pressure in the duct changes also but not as much as the velocity pressure. Analog gages are easier to mentally average and give better results on low pressure systems.

5. Originally Posted by WAYNE3298
The reading is not steady state because the air flow is not steady state. Both ducts you checked are low pressure and change with building pressure. The static pressure in the duct changes also but not as much as the velocity pressure. Analog gages are easier to mentally average and give better results on low pressure systems.
your wrote here" change with building pressure". what you mean. does velocity pressure vary as per the building pressure?

6. Professional Member
Join Date
Feb 2016
Location
Louisburg Kansas
Posts
2,963
Post Likes
Velocity pressure varies with the velocity of the air in the duct. Air velocity is determined by the pressure developed by the fan. The pressure developed by the fan is determined by the resistance of the duct due to friction. Fresh air is introduced into a building at low pressure because buildings are pressurized at around 0.05 inches of water. The fresh air duct you tested had to overcome the duct resistance and impact of airflow due to wind and opening and closing of doors as well as possible changes in exhaust fan airflow. Low pressure fans do not deliver air at a constant volume. Airflow surges and changes with slight variations in resistance. The variance in velocity pressure you are reading is telling you that but is not telling you the exact cause.
The airflow surges are seldom noticeable except when measuring velocity pressure and the most noticeable with digital meters. The wide variance in readings you got have to be averaged to calculate airflow within an acceptable range and that is not possible to do on digital meters.
If you use an inclined manometer and have a straight run of duct long enough to traverse the results will be much better. Record each reading and average them before calculating airflow.
Low pressure air movement and measurement is a difficult thing to explain and one of the most misunderstood areas in HVAC. ASHRAE has a lot of information on this subject but I'm retired and no longer have a source for the information.

7. Originally Posted by WAYNE3298
Velocity pressure varies with the velocity of the air in the duct. Air velocity is determined by the pressure developed by the fan. The pressure developed by the fan is determined by the resistance of the duct due to friction. Fresh air is introduced into a building at low pressure because buildings are pressurized at around 0.05 inches of water. The fresh air duct you tested had to overcome the duct resistance and impact of airflow due to wind and opening and closing of doors as well as possible changes in exhaust fan airflow. Low pressure fans do not deliver air at a constant volume. Airflow surges and changes with slight variations in resistance. The variance in velocity pressure you are reading is telling you that but is not telling you the exact cause.
The airflow surges are seldom noticeable except when measuring velocity pressure and the most noticeable with digital meters. The wide variance in readings you got have to be averaged to calculate airflow within an acceptable range and that is not possible to do on digital meters.
If you use an inclined manometer and have a straight run of duct long enough to traverse the results will be much better. Record each reading and average them before calculating airflow.
Low pressure air movement and measurement is a difficult thing to explain and one of the most misunderstood areas in HVAC. ASHRAE has a lot of information on this subject but I'm retired and no longer have a source for the information.
thank you sir, i took only the maximum value, i should have taken the avaerage value from the live readings

8. Professional Member
Join Date
Feb 2016
Location
Louisburg Kansas
Posts
2,963
Post Likes
You are very welcome Moideen. Low pressure air pulsates similar to compressed air produced by a reciprocal air compressor. That is how I think of it. When doing duct traverse I use the equal area method and take at least 16 readings which are then averaged.

9. Originally Posted by WAYNE3298
You are very welcome Moideen. Low pressure air pulsates similar to compressed air produced by a reciprocal air compressor. That is how I think of it. When doing duct traverse I use the equal area method and take at least 16 readings which are then averaged.
you brought good lights on this case, i was hasty when taking the readings, need patients, i had taken from 3 points only

10. Which testo manometer are you using?
If it’s the 510 or 510i the resolution is not appropriate unless you are reading over 700 feet per minute velocity.
A precision manometer may be required.

Sent from my iPhone using Tapatalk

11. Originally Posted by TruTech-Bill
Which testo manometer are you using?
If it’s the 510 or 510i the resolution is not appropriate unless you are reading over 700 feet per minute velocity.
A precision manometer may be required.

Sent from my iPhone using Tapatalk
i am using testo 510. do you mean, is the analogue meter precision meter?

12. ## VELOCITY PRESSURE TO CFM

The testo 510 is not accurate enough to read pitot tube pressures unless the air speed is over 700 feet per minute.

A precision manometer with an accuracy of +/-1 Pascal or better would work much better.

13. Originally Posted by TruTech-Bill
The testo 510 is not accurate enough to read pitot tube pressures unless the air speed is over 700 feet per minute.

A precision manometer with an accuracy of +/-1 Pascal or better would work much better.
Which brand is good in prcision manometer

Page 1 of 2 12 Last

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•