pitot tube velocity measurement
pitot tube velocity measurement
(OP)
Hello all.
I have an application where I need to measure the velocity of water that is stirring in a tank at the wall a certain depth below the surface. I have mounted a handmade pitot tube and pressure tap side by side against the wall underwater and have connected those to simple air line coming out of the water over the wall of the tank and than back down the outside of wall of the tank that connect to a manometer on the side of the tank. See attached sketch.
When stagnat, water fills the two pressure taps and up the air lines up to the water surface. When stirring, the water level in the tube elevates, pushing the air between the water in the air line and the oil in the manometer to give us a differential pressure reading. Using Bernoullis equation and the pressure being equal at both points I solve for V to get V = sqrt(((2(densityoil-densitywater)*g*h)/densitywater). h being the height of the manometer column. Using this to calculate the velocity of the flow produces a value I feel is innacurate. Do I need to take into account any of the other elevations somehow? Am I thinking about this correctly? Any thoughts would be appreciated in regards to this topic.
I have an application where I need to measure the velocity of water that is stirring in a tank at the wall a certain depth below the surface. I have mounted a handmade pitot tube and pressure tap side by side against the wall underwater and have connected those to simple air line coming out of the water over the wall of the tank and than back down the outside of wall of the tank that connect to a manometer on the side of the tank. See attached sketch.
When stagnat, water fills the two pressure taps and up the air lines up to the water surface. When stirring, the water level in the tube elevates, pushing the air between the water in the air line and the oil in the manometer to give us a differential pressure reading. Using Bernoullis equation and the pressure being equal at both points I solve for V to get V = sqrt(((2(densityoil-densitywater)*g*h)/densitywater). h being the height of the manometer column. Using this to calculate the velocity of the flow produces a value I feel is innacurate. Do I need to take into account any of the other elevations somehow? Am I thinking about this correctly? Any thoughts would be appreciated in regards to this topic.





RE: pitot tube velocity measurement
Independent events are seldomly independent.
RE: pitot tube velocity measurement
Why don't you just rent one of the propeller meters?
http://www.erdco.com/hp302.htm
RE: pitot tube velocity measurement
B.E.
RE: pitot tube velocity measurement
@BIMR: tried to find a propeller meter in the past but they were all too big. This one from ERDCO looks great but I called them and they no longer make such a thing....
RE: pitot tube velocity measurement
Hope this helps.
B.E.
RE: pitot tube velocity measurement
Bernoulli's equation does work here, but not in the way you trying to use it. The density of the oil is important in determining the pressure, but not the velocity directly (i.e., use the height change to get a pressure and then carry that number into Bernoulli).
If condition 1 is at rest (v=0) then:
P1/ρ(water)=V2^2/2+P2/ρ(water) and
V2=((P1-P2)/ρ(water))^0.5 (be really careful of the units here, that messes up this equation almost as often as misapplying it)
I would run my tube from the pitot tube to the closed side of the monometer and leave the other leg open (your static leg doesn't help here). Then
P1-P2= ρ(oil)*Δh*g
in one step:
V2=((ρ(oil)*Δh*g)/ρ(water))^0.5 (the densities are ratioed, not subtracted)
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: pitot tube velocity measurement
This might not be practical because if the velocity isn't within a certain range h might be so small that you can't measure it accurately enough or so large that you need an extension ladder to reach the top of the water column. If the latter is the case, you can get around this by making the tube connected to the dynamic port first bend down then rise up with a denser liquid in the bottom of the U-bend. The calculation becomes a little more complex in this case. In this scenario, like the simpler one I described above, leave both ends of your manometer tubes open to atmosphere.
RE: pitot tube velocity measurement
http://www.swoffer.com/pdf/3000bulet12.pdf
RE: pitot tube velocity measurement
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: pitot tube velocity measurement
RE: pitot tube velocity measurement
Good luck,
Latexman
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RE: pitot tube velocity measurement
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