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Thread: Airflow capture hoof

  1. #1
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    Airflow capture hoof

    I am looking for suggestions on a hood for a non-TAB mechanic. We have been running across a lot more situations where I need have airflow checked.
    I would prefer to call someone that knows this field, not myself, but the boss wants to use us to see if flow is approximately what it should be.
    If it needs proper paperwork, a professional would be called in.

    It would not be for commissioning, it would be for old work, VAV's, damaged duct work,troubleshooting etc. What would you guys suggest for something simple and reliable ?


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    I have an Acubalance by TSI in like new condition for sale. I goes to 2 ft x 2 ft. I am retiring. Digital, measures supply and returns. $900.00
    Doug

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    Having a flow hood is a good idea. You need to learn how to use it because for some applications they are useless. For your use the old analog hood would be best. Digital hoods have more problems with low pressure systems than the analog.
    Analog hoods are both simple and reliable. You should also consider having pitot traverse capability. Use an inclined manometer for that. Again simple and reliable.
    No man can be both ignorant and free.
    Thomas Jefferson

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    Quote Originally Posted by WAYNE3298 View Post
    Having a flow hood is a good idea. You need to learn how to use it because for some applications they are useless. For your use the old analog hood would be best. Digital hoods have more problems with low pressure systems than the analog.
    Analog hoods are both simple and reliable. You should also consider having pitot traverse capability. Use an inclined manometer for that. Again simple and reliable.
    Do you recommend any books, literature or any on-line reading to get properly trained on this equipment ? I was always interested in this field because some of the problems I come across is not the equipment, it is the water or air flow issue. Understanding this better helps to put everything into perspective.




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    Quote Originally Posted by chander View Post
    Do you recommend any books, literature or any on-line reading to get properly trained on this equipment ? I was always interested in this field because some of the problems I come across is not the equipment, it is the water or air flow issue. Understanding this better helps to put everything into perspective.




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    I had an advantage when I got into TAB because I did design for 25 years, mostly water and steam. Because of that I have books up the rear. If you join NEBB they have a lot of info that is useful but get a copy of SMACNA including their duct leakage test manual if you don't already have them. For piping I also like the ITT Grinnell pipe design manual because it has a lot of info related to TAB.
    When you decide what you want let me know because I have enough TAB instruments to support a company. I closed down and retired a while back. My stuff is older but does the job.
    No man can be both ignorant and free.
    Thomas Jefferson

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  8. #6
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    Quote Originally Posted by WAYNE3298 View Post
    Having a flow hood is a good idea. You need to learn how to use it because for some applications they are useless. For your use the old analog hood would be best. Digital hoods have more problems with low pressure systems than the analog.
    Analog hoods are both simple and reliable. You should also consider having pitot traverse capability. Use an inclined manometer for that. Again simple and reliable.

    I wasn't going to offer anything because........ The proper tools are one thing, but along with them comes the responsibility to learn how to use them properly. And as soon as you have the tools on your truck, whether you are, or not, you are the company "expert".

    I've seen a couple guys in your situation, who did their best, and took on the responsibility. They thought they knew what they were doing, but did not, and it wasn't fair to them.

    New, flow hoods are not cheap, not even the mini's. Personally, for what you will want to do with it, I would put the word out that I was looking for a serviceable used Analog Shortridge, or Alnor flow hood.

    Occasionally they surface here and there in some very unusual places. One time I found one in an old Army surplus store. The only part missing was the spider. They had no idea what it was, or what it was worth.

    Check the pawn shops, Craigslist, Facebook, all the usual places, as well as mention it in all of the websites where us knuckleheads tend to lurk. Eventually you'll find one. I'd probably have it calibrated at least once before you start using it, just to make sure it's relatively accurate.

    And, per Wayne's advice, I'd probably get something to do a simple traverse with. I would however not recommend an old incline manometer, or a set of Dwyer analog manometers. (sorry Wayne) Way too much hassle.

    I'd recommend either a 0-10 inch electronic magnehelic (Fluke higher end....Extech lower end), with the tubing and a couple different pitot tubes, or a hot wire anemometer (expensive, and somewhat delicate, but a very good time saving tool).

    And definitely,,,,,get yourself some training on how to perform traverses and how to set up and take readings with a flow hood..

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    Quote Originally Posted by Artrose View Post
    I wasn't going to offer anything because........ The proper tools are one thing, but along with them comes the responsibility to learn how to use them properly. And as soon as you have the tools on your truck, whether you are, or not, you are the company "expert".

    I've seen a couple guys in your situation, who did their best, and took on the responsibility. They thought they knew what they were doing, but did not, and it wasn't fair to them.

    New, flow hoods are not cheap, not even the mini's. Personally, for what you will want to do with it, I would put the word out that I was looking for a serviceable used Analog Shortridge, or Alnor flow hood.

    Occasionally they surface here and there in some very unusual places. One time I found one in an old Army surplus store. The only part missing was the spider. They had no idea what it was, or what it was worth.

    Check the pawn shops, Craigslist, Facebook, all the usual places, as well as mention it in all of the websites where us knuckleheads tend to lurk. Eventually you'll find one. I'd probably have it calibrated at least once before you start using it, just to make sure it's relatively accurate.

    And, per Wayne's advice, I'd probably get something to do a simple traverse with. I would however not recommend an old incline manometer, or a set of Dwyer analog manometers. (sorry Wayne) Way too much hassle.

    I'd recommend either a 0-10 inch electronic magnehelic (Fluke higher end....Extech lower end), with the tubing and a couple different pitot tubes, or a hot wire anemometer (expensive, and somewhat delicate, but a very good time saving tool).

    And definitely,,,,,get yourself some training on how to perform traverses and how to set up and take readings with a flow hood..
    Thanks for your words of wisdom, I would prefer to just call someone that's knows what they are doing. That sometimes doesn't work well when troubleshooting a service call and we have to re-schedule with the customer, balancer and ourselves to come back to the site. It could delay a call for weeks. I just feel like I am becoming a Jack of all trades and a master of none.

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    I respect your opinion Artrose. I suggested the incline because there is no calibration expense and because they are not as easily fooled as any other gage. When doing a traverse the biggest potential for error is "reversed flow". That is in quotation marks because the tricky ones are best seen to identify. I agree with most of what you said but I hated my hot wire. Nothing wrong with it I just didn't like it.
    One caution with the inclined manometer is in extreme weather let it reach room temperature before using it.
    I double and triple your advice concerning learn what you are doing.

    On one job the engineer insisted that one of the duct systems was static regain design and didn't need balanced. I tried to tell him it wasn't properly designed for regain but if that was his intent I couldn't put money in the job to balance it or read all the diffusers. He agreed but it didn't work. The contractor called and insisted I read the diffusers with a flow hood. I told him I didn't have money in for that but they couldn't be read with the hood anyway. The mechanical contractor read the diffusers and gave them to a consulting engineer (not the system designer). The consultant knew the flow hood wouldn't work and rejected the readings. They then ask me to obtain the airflows and I did by pitot traverse ahead of each diffuser. The consultant redesigned from those readings and the system was modified accordingly. Had he redesigned using the bogus readings there would have been another disaster.
    No man can be both ignorant and free.
    Thomas Jefferson

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    Quote Originally Posted by chander View Post
    Thanks for your words of wisdom, I would prefer to just call someone that's knows what they are doing. That sometimes doesn't work well when troubleshooting a service call and we have to re-schedule with the customer, balancer and ourselves to come back to the site. It could delay a call for weeks. I just feel like I am becoming a Jack of all trades and a master of none.

    Sent from my ASUS_P00J using Tapatalk

    Jack of all Trades......

    That's actually something you should be very proud of.

    All that extra knowledge and experience makes you very valuable.

    Don't short change yourself. You're likely already a master of more than you might realize.

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  13. #10
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    Quote Originally Posted by WAYNE3298 View Post
    I respect your opinion Artrose. I suggested the incline because there is no calibration expense and because they are not as easily fooled as any other gage. When doing a traverse the biggest potential for error is "reversed flow". That is in quotation marks because the tricky ones are best seen to identify. I agree with most of what you said but I hated my hot wire. Nothing wrong with it I just didn't like it.
    One caution with the inclined manometer is in extreme weather let it reach room temperature before using it.
    I double and triple your advice concerning learn what you are doing.

    On one job the engineer insisted that one of the duct systems was static regain design and didn't need balanced. I tried to tell him it wasn't properly designed for regain but if that was his intent I couldn't put money in the job to balance it or read all the diffusers. He agreed but it didn't work. The contractor called and insisted I read the diffusers with a flow hood. I told him I didn't have money in for that but they couldn't be read with the hood anyway. The mechanical contractor read the diffusers and gave them to a consulting engineer (not the system designer). The consultant knew the flow hood wouldn't work and rejected the readings. They then ask me to obtain the airflows and I did by pitot traverse ahead of each diffuser. The consultant redesigned from those readings and the system was modified accordingly. Had he redesigned using the bogus readings there would have been another disaster.
    How/why, were the flowhood readings not acceptable Wayne?
    I think one can see different flows at times, dependent on how air exits the diffusers ,but is that what you are refering to?

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    The diffuser design airflows exceeded the hood capacity. They tried to read half the diffuser at a time. That was bad enough but they had to cut a cardboard and install it on the hood to fit the hood to the diffuser. The system was also low pressure and they read the diffusers with a digital hood. The airflow readouts were unstable. Before they took readings I traversed the duct and their total wasn't close to the traverse.
    No man can be both ignorant and free.
    Thomas Jefferson

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  16. #12
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    Quote Originally Posted by WAYNE3298 View Post
    The diffuser design airflows exceeded the hood capacity. They tried to read half the diffuser at a time. That was bad enough but they had to cut a cardboard and install it on the hood to fit the hood to the diffuser. The system was also low pressure and they read the diffusers with a digital hood. The airflow readouts were unstable. Before they took readings I traversed the duct and their total wasn't close to the traverse.

    There are situations where a flow hood just doesn't work so well, despite the different size canopy's, and your ability to customize the canopy with cardboard and tape.

    I had been working with a hood for a few years, and thought I had it down pretty good. Was even starting to develop a little carpal tunnel in my right hand, and was already wearing old people work shoes because my feet were pretty screwed up from all the ladder time.

    We were balancing a small shotgun style auditorium unit with the air handler centered on one end, with the air handler discharge being sent up through a splitter, then out and down each side the length of the structure. There were 1x4 sidewall diffusers about 14 feet off the floor tapped directly off the duct, equally spaced the length of the room.

    We took multiple flow hood readings, and no total was the same, and none of the readings came close to design.

    There was some serious head scratching that day. Never encountered this before. Turns out the hood was creating too much backpressure, and the air simply stayed in the very large supply duct, and moved over to another discharge where there was less resistance to flow.

    We traversed the unit, and walked away. Another lesson learned.

    Don't forget to zero your analog meter before you take sidewall readings.

    Learning. It's not confined to the classroom.

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  18. #13
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    Your post Artrose demonstrates my point about low pressure systems especially when read with digital flow hoods. High airflows in a low pressure system will make you pull out your hair. You can't even establish a flow factor to correct the readings.
    To be totally up front I have never seen that problem addressed by TAB certifiers.
    No man can be both ignorant and free.
    Thomas Jefferson

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    Quote Originally Posted by Artrose View Post
    There are situations where a flow hood just doesn't work so well, despite the different size canopy's, and your ability to customize the canopy with cardboard and tape.

    I had been working with a hood for a few years, and thought I had it down pretty good. Was even starting to develop a little carpal tunnel in my right hand, and was already wearing old people work shoes because my feet were pretty screwed up from all the ladder time.

    We were balancing a small shotgun style auditorium unit with the air handler centered on one end, with the air handler discharge being sent up through a splitter, then out and down each side the length of the structure. There were 1x4 sidewall diffusers about 14 feet off the floor tapped directly off the duct, equally spaced the length of the room.

    We took multiple flow hood readings, and no total was the same, and none of the readings came close to design.

    There was some serious head scratching that day. Never encountered this before. Turns out the hood was creating too much backpressure, and the air simply stayed in the very large supply duct, and moved over to another discharge where there was less resistance to flow.

    We traversed the unit, and walked away. Another lesson learned.

    Don't forget to zero your analog meter before you take sidewall readings.

    Learning. It's not confined to the classroom.
    Would a powered flow hood have helped in this situation. I'm hearing that for supply measurements a powered flow hood is more accurate than the passive ones. I have thought about getting one.

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    szw21 By powered I assume you mean digital. If so in this situation they are worse. One of my competitors read a railroad signal house airflow with a digital hood and the controls overheated when tested due to the lack of air. The digital hood readings indicated there was enough air. I was ask to drive the 70 or 80 miles to read the airflows when there was a TAB company about 5 to 8 miles from them. That made me curious so when I got there I read the airflow with the analog and digital hood. The digital indicated enough airflow but the analog indicated the flow was short. The engineer designing the system said the airflow indicated by the analog was close to his calcs. He redesigned using the analog flows and all was well.
    No man can be both ignorant and free.
    Thomas Jefferson

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    Quote Originally Posted by WAYNE3298 View Post
    szw21 By powered I assume you mean digital. If so in this situation they are worse. One of my competitors read a railroad signal house airflow with a digital hood and the controls overheated when tested due to the lack of air. The digital hood readings indicated there was enough air. I was ask to drive the 70 or 80 miles to read the airflows when there was a TAB company about 5 to 8 miles from them. That made me curious so when I got there I read the airflow with the analog and digital hood. The digital indicated enough airflow but the analog indicated the flow was short. The engineer designing the system said the airflow indicated by the analog was close to his calcs. He redesigned using the analog flows and all was well.
    Wayne - no I don't mean digital vs analog. There are digital passive and powered flowhoods. A powered flowhood has a fan to compensate for the backpressure of having a flowhood and supposedly has far better accuracy than the passive. The only thing is that I believe the powered flowhood is only good for measuring lower airflows so they are good for supply measurements. For return measurements you would need the larger flowhood like the Alnor EBT731.

    This is based on a talk I attend a while ago by the CEO of one of companies that makes airflow measurement stuff including flowhoods.I cannot recall which company it was offhand - I think it was The Energy Conservatory.

    Here is an example. https://retrotec.com/flow-finder-mk2.html

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    I have never seen a powered flow hood and didn't know there was such a thing. It will be interesting to see if anyone here has used one.
    No man can be both ignorant and free.
    Thomas Jefferson

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    The talk I attend presented results of tests of various flowhoods for supply measurement. They took us through the test setup and then the results. For lower supply airflows the larger flowhood were off by as much as 30%. The smaller passive flowhood were off by about 15% and the powered flowhoods about 5%. One thing they did not do in the test was to test the large flowhood with a smaller hood designed for smaller registers. If they did I suspect the accuracy would be closer to 15%.

    Analog was not mentioned. I think the digital instruments of today are probably way more accurate than when they first came out.

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  25. #19
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    Forgot to add that the digital passive large flowhood - EBT731 - does have a backpressure compensated measurement mode to account for the insertion loss of the flowhood. I have used it on return air measurements but on larger flows it did not make much difference. I will see if I can get a hold of a powered flowhood for experiments and also measure some supplies in BP compensated mode with the 731.

  26. #20
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    Quote Originally Posted by szw21 View Post
    The talk I attend presented results of tests of various flowhoods for supply measurement. They took us through the test setup and then the results. For lower supply airflows the larger flowhood were off by as much as 30%. The smaller passive flowhood were off by about 15% and the powered flowhoods about 5%. One thing they did not do in the test was to test the large flowhood with a smaller hood designed for smaller registers. If they did I suspect the accuracy would be closer to 15%.

    Analog was not mentioned. I think the digital instruments of today are probably way more accurate than when they first came out.
    How often are you taking supply air measurements that are over 500cfm?
    Most high volume hoods are highly accurate with returns, they just suffer accuracy on supply air.
    "The only true wisdom is in knowing you know nothing" Socrates

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