Pitot Tubes for velocity measurement - Has anyone made their own?

[86rocco]

I didn't use a flow bench to test it, that's why I said it was not a controlled, rigorous test. I used a shop vac with a 8" long nozzle which tapers from 3" diameter down to 1" at the inlet, as I moved the probe about inside the nozzle, the velocity pressure changed in a very predicatible and repeatable manner which makes me quite confident that I'll get good results from it once I do get my flowbench up and running.

[Rick360]

When testing at 28" I've seen well over 40"dp (~425fps) on my J-type pitot velocity probe. That is too fast, but is not uncommon with stock port location heads where the short turn is too sharp.

The average velocity (calculated by CFM*2.4/CSA ) can be around 260-280fps(or more on raised runner race head), the local velocity that is measured with a pitot probe should be kept under 350fps if possible. Sometimes its very hard to get it less than 400fps over the short turn on heads with stock port location. The air flows much faster on the floor of the short turn and much slower at the roof. Typical numbers on sbc head might be 220fps at roof and 400fps at floor.

Make your manometer at least 50" if you flow at 28" to be safe.

Rick

[Unkl Ian]

THAT is what I've been looking for.Thanks.

[sheppard00]

me too thanks

[talon]

Good morning all

I am new to this forum. I have a Super flow 110 that I bought used. the previous owner put an auxillary verticle monometer on it. this is acutally a brass well with 5/16 or 3/8 aquarium tubbing attatched( this is filled with red fluid). there are two additional tubes for hooking up pitot tubes. I bought both types - one for exaust ( 90* bend ) and one for intake ( j bend). I have been working with these for the past week or so to help resolve some turbulance issues I have in a SBF E7 exhaust port.

What I would like to know - is - can I add a inch scale and place it beside this verticle piece of 3/8 tubing with the 0 on the scale at the fluid level while in the static position?

I would like to start measuring actual velocities.

Thanks
Jim

[86rocco]

It's not quite as simple as that because as the fluid rises in the tubing it drops in the well and the measurement need to be from the surface of the fluid in the well to surface of the fluid in the tubing. Fortunately, this happens in an easy to predict and calculate fashion based on the relative areas of the tube and well. If for example, your manometer tubing is 5/16" I.D. and your well is 1 1/2" I.D. a rise of 36" is the tubing would result in a drop in the well of 1.5625" for a total of 37.37.5625". Now on this manometer if you were to use a normal yard stick, with the zero at the static level, you would need to multiply your reading by 1.0434 =(37.5625/36). I got those numbers by plugging 5/16, 1 1/2 and 36.0 into the manometer worksheet in and simply made the scale vertical height the same as the scale length.

If you wanted to, there nothing to prevent you from creating your own scale which takes this into account and reads directly in fps. The inclined manometer spreadsheet I already refered you to is a good starting point, that scale working in a similar manner to a fps scale. For example, using the same numbers as before, a 37.5625" WC reading from a pitot tube corresponds to 405 fps, multiply 405 by the percentages on the scale and you'll have a fps scale.

[Mouse]

Flow Performance will be introducing a new type of velocity probe very soon.

The new type of probe is a huge improvement on the typical velocity probe being used today.

[hollywood63]

If I'm reading this correct according to your math. I have been testing at 28 in with a tube of .250 a well of 3 in. By playing with the spreadsheet I have in fact been testing at 31.58 in :p
1.1276=(28/24.83) and here I thought I wasn't dropping that much in my well. :p

[86rocco]

Smaller diameter tubing might be in order.What I've been using for my manometers that works really well is 1/8" i.d. rigid clear acrylic tubing that I got from an aquarium supply store. It comes in 36" lengths and with careful application of heat is also bendable. Here's a picture of the bend I put in the bottom of my inclined manometer.

[Rick360]

Talon,
If you are using the red .826 fluid, don't forget to include that in your calculations as it will increase the fluid height by that amount. reading x (86rocco's multiplier) x .826

Rick

[86rocco]

[hollywood63]

Rocco, I forgot to add I have a fluid density of 1.120 does that still add up
Sorry for the miss... funny thing is I woke up from a dead sleep last night and said to myself you for got to add the density. Is this a fun habit or what
Art

[86rocco]

Yes, that makes the numbers work, with the denser fluid, 28" on your scale does indeed correspond to 31.58" WC.

[talon]

Good morning gents

I'll have to ask the previous owner and see what type of fluid it is. I also have two different size tubes hooked in-line. One size aprox 3/8( haven't had time to measure yet) for the verticle monometer and then it necks down to the small size supplied with the pitot tube. That doesn't make a difference does it? the fluid head is confined in the larger tube.

My manometer is a verticle one. Your spread sheet asks for a length( as if calculating a inclined). I can't put 0 there - do I just put a small value?

Thanks
Jim

[86rocco]

In the spreadsheet, for a vertical manometer, just enter the same value for both the scale length and the scale vertical height

A large diameter tube just means there be greater movement in the well of the manometer. And also because the fluid in the manometer takes time to move and a larger tube means a larger volume to move, it will probably reduce the responsiveness of the pitot tube, which may or may be desirable depending your particular situation.