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Static pressure measurement

bob_50
bob_50 Member Posts: 306
economical path would probably be to hire a certified balancer . From my experience even professional balancers are lucky to be within 10% . bob

Comments

  • Mark_46
    Mark_46 Member Posts: 312


    Any suggestions on how to measure plenum duct static pressure at a resonable cost? I would like to measure in inches of water. Is there an inexpensive measuring device you know of on the market? Also, the same question for velocity...velometers?

    I suspect this hunt (and the solution) will take me to buying some used equipment on Ebay but any input would be appreciated. Thanks.
  • larry_15
    larry_15 Member Posts: 55
    Static Pressure

    The cheapest way is to use a manometer you can buy a well type for about $30.00 the will go both negative to a positive pressure I have used this way when I have high static presuure on the discharge of a fan 4 to 5 inches of static and I am testing the high static pressure switch.

    But for a small plenums in the residentail market I don't you will find that kind of staic pressure. The cheaper way is a magnehlic you can buy them in 1" 2" on up. the bigest you would need is a 5".

    I own a 1/4" inclined manometer and a 2" then I have an electronic that goes to 20" and a magahilic that goes to 5" plus a water tube (well type) that goes to 15" and i use them all. If you are an air balancer they have an inclined that goes up to 10". With my 1/4" I have static presusser probes that can reach about 30" into the duct which is nice if you are trying to figure out pressure drops over a coil I can use them with anyone of my manomeeters execpt the water tube. The more expensive ones were in the 2 to 4 hundard dollar range.

    I found an anometer at Gustave Larson for about 250.00 That I am every happy with I find more use for that than I thought I would. And it is small enough to fit in my service bag. Which is better that taking a hood everywhere yoou go.

    Larry
  • larry_15
    larry_15 Member Posts: 55
    Static Pressure

    The cheapest way is to use a manometer you can buy a well type for about $30.00 that will go both negative and to a positive pressure I have used this when I have high static presuure on the discharge of a fan 4 to 5 inches of static and I am testing the high static pressure switch.

    But for a small plenums in the residentail market I don't think you will find that kind of staic pressure. The cheaper way is a magnehlic you can buy them in 1" 2" on up. the biggest you would need is a 5".

    I own a 1/4" inclined manometer and a 2" then I have an electronic that goes to 20" and a magahilic that goes to 5" plus a water tube (well type) that goes to 15" and I use them all. If you are an air balancer they have an inclined that goes up to 10". With my 1/4" I have static presusser probes that can reach about 30" into the duct which is nice if you are trying to figure out pressure drops over a coil I can use them with anyone of my manometers execpt the water tube. The more expensive ones were in the 2 to 4 hundred dollar range.

    I found an anometer at Gustave Larson for about 250.00 That I am every happy with I find more use for that than I thought I would. And it is small enough to fit in my service bag. Which is better that taking a hood everywhere you go.

    Larry
  • Mark_46
    Mark_46 Member Posts: 312
    Larry

    Great info, thanks.

    I have a high velocity miniduct system which operate at about 1.2" of water. Of course CFM and pressure affect system output. So I want to measure each and get exact values for my systems. I'll then know what the system is 'producing'.

    I'm looking to do this fairly inexpensively because I dont see a need to measure these values all the time. Besides, they more or less wont change. I want to measure these valuse for set up purposes and then thats it. So I thought if I could somehow stay below $50 or so for each it would be worth it. But I see these tools are $150 (anemometers/velometers) and up except the magnehelic gauge which is about $60.

    You mention a $30 manometer. Could you tell me the brand or selling source?
  • will smith_4
    will smith_4 Member Posts: 259
    Static pressure

    I've always used Dwyer Magnehelics for measuring static pressure (I also use them for gas pressure). I've had a 0-5" and a 0-10" on my truck for the past 15 years or so-use 'em all the time. They measure positive and negative pressure, and never freeze!
  • Empire_2
    Empire_2 Member Posts: 2,343
    Dwyer \"U-Tube Manometer

    This is the perfect tool to measure static since it uses an oil base solution with a specific gravity of .826 which will truly stabilize your readings for an accurate result. Mag's are great to but are sensitive (very) they tend to jump around.

    Mike T.
  • Brad White_88
    Brad White_88 Member Posts: 11
    Static Pressure

    Mark, I concur with much of what has already been offered. Personally I like the Dwyer Magnahelics and the lowest scale (0.25 inches) is one of my favorites. I will feel a lot better when I find it; I think it walked...). This one in particular had a velocity scale in addition to SP and would measure up to 2000 FPM, fast enough for any residential system velocity. For SP I would use a 0 to 1.0 inch scale.

    The inexpensive Dwyer inclined manometers are fine for short term or even fixed in place use (such as a filter bank you are monitoring for loading).

    I came across one of the large acrylic block manometers given to me by an associate. Very accurate but takes a few minutes to set up.

    Day to day I use a Dwyer 477 electronic manometer plus some accessory Pitot tubes.

    Any of these I mentioned will measure velocity, total and static pressure hence will cover all that you need.
  • bob_50
    bob_50 Member Posts: 306
    Mark

    If this is a minimum use type deal which it sounds like. Go to the hardware store and buy some 1/4" ID clear plastic tubing and a roll of duct tape. Drill a 3/8" hole in the plenum where you want to measure the pressure. Tape the tubing on the side of the unit in the shape of a U . Make legs of the U about 12" long but leave one of them long enough to reach the hole you drilled. Put 6" of water in the U. Mark the level of the water on some tape. Stick the long end of the tube in the hole. Try to hold the end of the tube flush with the inside of the duct. Turn on the fan and measure the difference between the two column in inches. If you want a finer easier to read scale tape the U to the unit on an angle and expand the scale. If you want more accuracy you will neeed a pitot tube which will also allow you to read velocity pressure. You will have to do a pitot tube traverse and convert velocity pressure to velocity to find CFM. bob
  • Brad White_88
    Brad White_88 Member Posts: 11
    Not that simple, unfortunately....

    I used to hope for that kind of arrangment being workable....

    If your standard home furnace fan has a total SP of 0.75 inches, your eye could be off depending on angle of view and fluid viscosity by 30 percent or more.

    Mike T.'s post about 0.826 Sp. Gravity fluid is illustrative. The meniscus (the curved surface of the fluid) is completely different between water and "the red fluid". They hardly read the same at all in a meaningful way. At best you can say you "have pressure". Ok.

    With residential system velocities in the reange of 600 to 1000 FPM you are dealing with velocity pressures of between 0.0224 inches and 0.0623 inches, hardly the kind of resolution that my eyes can muster even on a good day. One has to get up to 4005 FPM velocity to achieve an inch of velocity pressure. (It rises and falls by the square of velocity, you see.)

    I would stick with calibrated instruments if I wanted any kind of repeatable accuracy at such low pressures and velocities.
  • bob_50
    bob_50 Member Posts: 306
    That's why

    I suggested laying it over on it's side. You can expand the scale to read in hundredths if you want. For a one time deal I can't see investing in professional instruments. IMHO. bob
  • Brad White_88
    Brad White_88 Member Posts: 11
    I got that, Bob

    The inclined method works OK for rough numbers but when one needs real numbers ("otherwise why use them?", he asked rhetorically...), I say invest a few bucks or better yet- rent or borrow. If Mark lived closer I would lend him my stuff.
  • Brad White_88
    Brad White_88 Member Posts: 11
    At over $80 an hour

    for a TAB journeyman in Local 17 with a four hour minimum, I will see you at the tool counter! :)
    And at those costs, ten percent just will not do...


    We specify 5% tolerance and if conditions are right with sufficient upstream and downstream distances to the measuring point, this can get to within 2%.

    My experience anyway!

    Best,

    Brad
  • Al D'Ambola_2
    Al D'Ambola_2 Member Posts: 7
    Manometers

    Mark,

    I would recommend a Dwyer Magnehelic gauge. The scale should go up to 1" of pressure, this will suffice for most residential and light commercial systems. Especially if you use one side of the manometer at a time. This will also indicate which side of the system is causing an issue, the high reading is your problem side.

    There is as of right now not a true accurate way to measure system delivered CFM on a HVSD system. You will have to rely on your total external static pressure and fan curve chart from the manufacture.

    Static pressure is such a critical measurement that should be conducted on every call, it is the same test a medical doctor performs they call it blood pressure. There are so many issues you can un-cover by diagnostic pressure testing!
  • Mark_46
    Mark_46 Member Posts: 312
    Mike

    Oooh, key point. I would be disappointed if I hooked up and got a jittery gauge. Thanks for the tip.
  • Mark_46
    Mark_46 Member Posts: 312
    Bob

    I've seen something like this through my online inquiries. My main question with these home made versions is what do you use as a scale? It seems thats the most important part. OK, so if the water level moved 1", does that equal 1" of wc pressure?


    You recommend 1/4" tubing but what if 1/2" was used? Doesnt the volume of water in the tube effect the reading? There would be more water mass in a larger tube and therefore harder to move from one side of the U to the other, no?
  • Mark_46
    Mark_46 Member Posts: 312
    Brad

    > The inclined method works OK for rough numbers

    > but when one needs real numbers ("otherwise why

    > use them?", he asked rhetorically...), I say

    > invest a few bucks or better yet- rent or borrow.

    > If Mark lived closer I would lend him my stuff.



  • Mark_46
    Mark_46 Member Posts: 312
    Brad

    Youre too kind. Thanks anyway.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Mark

    I use The "U" tube pretty much for Large Air rotation Units or Reznor MUA units. In most of our applications the MUA's are for Fire House applications where CO, Co2 and neg/pos press has to be controlled. Some of the MUA's are 100% / some use return and outside air.

    Mike T.
  • Mark_46
    Mark_46 Member Posts: 312
    Al

    you say there is not a true accurate way to measure HV systems, why?

    Yes...I do see in the spec sheets that motor amperage draw is another way of determinig a 'calculated' pressure. But I think I would prefer to take an actual pressure reading. It would seem to me that each installed system has different variables.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Tube diameter, if consistent

    does not affect the "height" differential reading. Pressure is equal across the full area whether 0.5" diameter, 0.25" diameter, or any other size.

    Fluid viscosity- yes, we know it is water, but the stuff is too thick :) to represent itself within a captive tube. So we use the 0.826 stuff....
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    HV System Measurement

    Any good manometer and pitot tube arrangement can measure any HV system. You just need good distances. Use of amperage and other corroborating data (fan RPM via a tachometer for example) is just another way to corroborate/triangulate what you are getting for velocity readings in the ductwork.

    Add to this: If you know your outlet total CFM and your duct traverse CFM, the outlet CFM is often less. The difference is leakage.
  • Empire_2
    Empire_2 Member Posts: 2,343
    Just a thought

    Not only is that just an approach method assuming that all duct work, both supply and return are as specified and designed, but what happens to any load meaning the motor in this case if you grossly under size or oversize for that matter, the duct? Amp draw DONT MEAN CRAP!. Q: if you were to put a piece of sheet metal over the top of a furnace completely to block it's air flow, do you think the amp draw would go up or down? In relation, is more work being done or less? Same with Hi Vol. Units... Even more so since the static is to remain at a set manufacturers refference. All I am trying to convey is that motors drive, but it is the design that ultimatly determines your total SP. period.....

    Mike T.

    Mike T.
  • bob_50
    bob_50 Member Posts: 306
    Mark

    I suggested 1/4" ID tube because I think it would be easier to get an accurate reading of static pressure. When you drill the hole in the plenum you want to drill it so the hole is perpendicular to the air flow. You do not want any of the velocity component of the air to impact the end of the tube. In my original post I mentioned to hold the end of the tube flush with the inside of the duct because sometimes the velocity over the end of the tube can reduce the static pressure. To avoid these potential problems they make pitot tubes, static probes and a little probe I like called a trail tail that is flexable and has fletching like an arrow so it's always perpendicular to the air flow. For what you are doing I think just a little care would be enough. I made a rough sketch to show how to tape the tubing to the unit. Just make a full scale drawing like my sketch, tape it to the unit and tape the tube over it. Read the vertical scale on the right. One inch vertical is 1"WC. You can see by mounting the tube on a 30 degree angle the scale is expanded 100% so it will be easier to read.
  • Jim Bergmann_2
    Jim Bergmann_2 Member Posts: 79
    Static Pressure

    The Testo 506-3 Auto ranging manometer has two scales, +/- 4" and +/-80", one button operation and it fits in you pocket. No leveling required, no gauge fluid to spill, fits in your pocket, and will take the place of three or four magnehlics, List it @175, but I have seen them cheaper at Johnstone, Sid Harvey, RE-Michaels. It uss two CR2032 Batteries available at Walmart that will last about 1-2 years. Perfect for pressure drops across coils filters, static pressures, setting gas pressures or checking pressure switches. Easy to read also. I have been using them for 4 years nnow with no failures in my lab with high school students as the primary users.

    Rotating Vane Anemometer

    For a highly accurate quick measurement, the rotating vane anemometer is the best way to measure airflow. Vane anemometers have several advantages over any other method. The primary advantages are speed, accuracy, and ease of use. Vane anemometers do not require air density compensation due to air temperature, humidity, or atmospheric pressure. The mini vane allows for a full duct traverse with an automatic calculation of the CFM in the duct if the dimensions are input into the instrument before the measurement is taken. It is imperative that the ducting attached to the appliance, and the base pan, if side returned is used, is sealed. Air leaks up-stream of where the measurements are made will significantly alter the actual reading obtained with this method. If done carefully the measurement error will be less than 3%. Changes in yaw and pitch of the probe head in the duct as much as 10% will result less than 1% error in the measurement making the mini-vane an ideal probe for field air measurement
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