Pitot Probe Calcs and Radiused Inlet

[sabre1fv]

I'm interested in the basic equation behind the velocity calculations for Pitot style probes in the spreadsheets posted by both 86rocco and larrycavan. The calculations in the spreadsheets have been simplified so that I can't get to the basic equation. I'm considering using a manometer fluid with a higher specific gravity than water and need to see the complete equation so I can tease out what changes need to be made in the computation. Can anyone help or provide a link?

Also, radiused entries for testing heads and manifolds: how large should they be? And is there any benefit in having a straight section between the radius and the port entry? I suppose the radius may also be dependent on the test depression due to a higher flow rate... There is probably a suggested ratio of radius to port diameter/width.

Thanks in advance.

-Jim

[86rocco]

Here's what I used to create my spread sheet,

It came from , and here are a couple more references,
,

[sabre1fv]

Hi 86rocco,

Yes, this is for a water manometer only. I am unable to figure out how the 1096.7 figure is calculated. I realize we are dealing with Feet per Minute as a velocity, but we are inputting units in inches and pounds per cubic foot and producing feet per minute. I realize I'm just being anal about this, but as an engineer I want to understand the details... just so I understand the details.

BTW: I have learned a lot from your posts on many subjects, as well as those from many other contributors, for which I am very appreciative.

-Jim

[86rocco]

[1960flh]

Jim; The attached PDF might help, I think the Dwyer calculation incorporates the conversion from Inch's of water to Millibar (Millibar to Inches of Water 0.4014388) so if your monometer fluid is a different sg you will have to do the approprate pressure conversion, I THINK ??? .

Rick

[sabre1fv]

Hi Rick,

Appreciate the digging you did to find the info here. Unfortunately, the temperature and pressure measurements are used to calculate the air density (they are being used to calculate the fractional difference from a standard air temp and pressure which in this instance is 14.696 psi/760 mm Hg/1013 mbar and 68 deg.F/20 deg. C/293.15 K). Don't mean to be wise ass, but I looked this up to see if this was what I was truly looking for and thought that I might as well mention what I found in case someone else might be interested.

I've looked through several books trying to sort this out so I knew exactly where to find this info; probably never use it again...

I do agree with Ed (86rocco) that the spreadsheets provided elsewhere on the site do give fairly accurate numbers as I have seen the same information in Paul Van Valkenburg's book: Race Car Engineering and Mechanics.

Ah well, onward... and thanks.

-Jim

[larrycavan]

Go here Then scroll down to page 3. There's a chart there.

Now, in that chart, 1096.7 is velocity of speed of sound but at 42 degrees F. How that figures into the formula I have from Dwyer isn't clear to me. It's certainly not standard air as used in the formula...It might not apply at all..

Here's another link in which the value is uses as a coefficient. This may be what you're looking for..




PS....IMO, PDF documents should be banned from the Internet. Code it in HTML or don't bother putting it out there!
:angry:

Why are you heading down this road to a solution anyway? Dwyer manometers on SF benches use heavier fluid for the pitot manometer. You simply adjust your scale to fit the fluid using normal formula and you're good to go. You can do that with the spreadsheets available here....



Larry C

[1960flh]

Jim;

I would never think you or anyone on the site was an ass, unless it was a blatant flame, many of us are here to learn and some of us just to exercise the old grey matter. Its been 23 years since I sat in a Physics lecture. The equation Rocco has posted from the Dwyer site us for a Pitot Static Tube for measuring Velocity in FPM with a water monometer per Dwyer. So help me see where I am going wrong with your interpretation of the below equation which is supposed to take input from a Pitot Static Tube.

1013.25 is barometric pressure at sea level in Millibar
B is your current location barometric pressure in Millibar
T is your temperature at your location converted to Kelvin
293 is 68 degrees F
Pv is the Dynamic Pitot pressure or (Total Pitot Pressure) in Milibar
Ps is the Static Pitot Pleasure in Millibar

Since this device is taking a differential pressure reading I can see where the local barometric and temperature readings are being used for the correction of local air density but Pv & Ps are readings from your Pitot tube & monometer converted to Millibar and will give you a the result in 10ths of a Meter per second. If you were using a different density fluid in your monometer you will stil have to convert to either Inchs water or millibar depending on which calculation you choose.

Rick

[gofaster]

sabre1fv asked:
...Also, radiused entries for testing heads and manifolds: how large should they be? And is there any benefit in having a straight section between the radius and the port entry?

I use a radius of 1/2 of my port opening (round port). I used to use clay, but later switched to reusable aluminum radius fixtures. Due to mounting methods, my radius fixtures are about 2 3/4" long with a straight section of about 2". At 10" depression, (SF-110) there was no measurable difference in the results between the clay and the aluminum radius entries. When the new bench is finished, I'll re-evaluate at greater depressions. I have thought of making some flush mount fixtures for some time now, but haven't gotten around to it. If the results change at a greater depression, I'll have to do it.

[sabre1fv]

Larry C.

"Why are you heading down this road to a solution anyway?"
-I have a smaller bench (similar in size to a SF 110) and was hoping to use a fluid heavier than water so my manometer would be smaller. I think I will just go with a longer water manometer on the side and be extra careful.

"You simply adjust your scale to fit the fluid using normal formula and you're good to go. You can do that with the spreadsheets available here...."
-I guess I missed that spreadsheet! Or I misunderstood something; I thought the spreadsheets were for a water manometer. Could you please point me to the right one?

I have no idea how the speed of sound would play into this; certainly doesn't seem to fit into the equations I have seen (or understood).

Rick,

We are reading the manometer in inches (or feet); a Millibar is 1000th of a bar or 1000th of 14.7psi. So in the last part of the equation some conversion has been made from millimeters/centimeters/meters to millibar OR (easier for my simple mind to comprehend) inches to pounds per square inch; this is where I am having trouble. What conversion factor is being used and how does the fluid density fit in? I have been trying to follow the units (like inches to pounds per square inch) and that is where I am getting lost because they don't cancel out to feet/second (or inches/second)

I believe a factor here is that density is reported as pounds mass per cubic foot which is NOT the same as pounds per cubic foot. 1 pound mass = 1 pound(foot)-seconds^2/32.2 ft or more simply 1 pound-sec^2/ft = 32.2 pound mass. I didn't find this in my Physics book, but in my Statics and Dynamics book. UGH. I'm still confused, though using the above, the equation that Larry pointed out on page 46 of the PDF, the units do work out to feet/second (I think). I don't know if that helps at all, but I have spent several hours trying to make sense of it so I could explain it and I don't think I did a good job.
Sorry for that. Suggestions, comments?

gofaster,

Thank you for the tip. I'll give that a try!

Thanks to all for their efforts.

[SWR]

[1960flh]

[color=#000000]Jim;

If I understand correctly you have a manometer that say is 28

[sabre1fv]

Hi Rick,

The link you provided to Efunda was a big help, I now see how this all plays out; finally! The long hair math also shown there is a bit much for me though thankfully I don't need to worry about it. Thanks. I have decided against using something heavier than water; I had hoped there might be a simple solution (no pun intended), and it is water! The manometer will just be longer than I had originally hoped, but that is OK and for just the reason you mentioned.

Hi SWR,

The conversion from WC inches to psi conversion was something I missed; thanks for pointing that out. I actually stumbled on this conversion in a Dietzgen conversion table shortly after I saw your post. There are definitely a LOT of ways to get the velocity numbers as I have seen from the responses to my post.

Thanks again to all who took the time to look at this and respond.

-Jim