Selecting the Right Radius for Pitot Static Tube Measurement in Circular Tubes

[Su Solberg]

Hello,
I would like to know how to select the radius of a pitot static tube to test the air flow in side a circular tube??

Thanks in advence.

 

[FredGarvin]

What radius do you refer to? You can either mean...

1) The radius (diameter) of the tube used to construct the pitot tube.
2) The radius from centerline of the pipe in which to take the measurements.

#1 is the tougher of the two to answer. If you have no experience in instrumentation I would recommend looking to a company that manufactures them and getting their advice. There is a fair amount of research done to do this properly. This factor becomes pretty important depending on the type of flows being investigated.

#2 is up to you. Usually the centerline of the pipe is where the measurements are taken. However, pitot traverses across the whole cross section are done as well to get a picture of the velocity profile.

 

[russ_watters]

I would have thought that since there is no flow inside a pitostatic tube that the diameter of the tube would not matter much.

 

[FredGarvin]

It depends on what you are measuring. If your flow area is based on a .25" diameter pipe, you don't want a proble that will take up half of that flow area.

Pitot tubes' accuracy can be affected by their shape and size. For example, the shape of the front face determine's the tube's susceptibility to yaw induced errors. Other errors can be introduced especially when coming close to a solid surface like a wall.

 

[Cyrus]

It depends on what you are measuring. If your flow area is based on a .25" diameter pipe, you don't want a proble that will take up half of that flow area.

Pitot tubes' accuracy can be affected by their shape and size. For example, the shape of the front face determine's the tube's susceptibility to yaw induced errors. Other errors can be introduced especially when coming close to a solid surface like a wall.

I wonder how that works. If the pitot tube is undergoing yaw conditions, the freestream flow is not coming directly perpendicular to the tube. So, I'm trying to see why that would affect the reading but I'm not connecting the dots.

The air inside the tube is static. When its directly into the flow the static air blocks the freestream air from entering. As a result, the freestream air has to slow down, come to a stop and stagnate at the tip of the probe. Consequently, the static air inside the pitot tube has to be under compression to slow down the dynamic air trying to enter it. This causes a hike in the pressure sensor.

But when the flow is coming at an angle, it's still got to slow down and come to stop like before. The only difference is that its coming in at an angle and slowing down.

Edit:

I figured it out. Its only the cosine component of the flow that has to come to a complete stop when there is an angle. The sine component of the flow never changes. So you only read a fractional (cosine to be exact) component of the dynamic pressure.

So if you knew the dynamic pressure at zero yaw, and you meaured the a new pressure that was lower, you could deduce the yaw angle in flight. Of course this is unrealistic in practice, because how does the airplane know the pressure is reduced from the nominal zero yaw value as opposed to just flying slower? Both would cause the same effect.

 

[Su Solberg]

Thanks for your replies, Cyrus, FredGarvin and russ_watters.

but seems I still cannot fully get what I am looking for.

For example, I have a 10 cm diameter tube. how large should the pitot tube be.
5mm diameter? 2mm diameter?
btw, is there any standard(I prefer BS, ISO and AUS)of choosing the size of the pitot tube?

Thanks again for all your kind help ^^

 

[FredGarvin]

United Sensor Corp. has a nice little section that shows some of the more common error inducing conditions. It has a great plot that shows the accuracy dome against + and - yaw angle errors. It starts about 1/3 the way down. For anyone in testing or the like, I would highly recommend bookmarking this site.

http://www.unitedsensorcorp.com/pitot.html

For anyone that EVER has to put a piece of instrumentation in a fluid flow, I would say you have to have a copy of SAE AIR 1168-5

 

[Su Solberg]

United Sensor Corp. has a nice little section that shows some of the more common error inducing conditions. It has a great plot that shows the accuracy dome against + and - yaw angle errors. It starts about 1/3 the way down. For anyone in testing or the like, I would highly recommend bookmarking this site.

http://www.unitedsensorcorp.com/pitot.html

For anyone that EVER has to put a piece of instrumentation in a fluid flow, I would say you have to have a copy of SAE AIR 1168-5

Thanks for your information, I will take a look.
Actually, it's for capacity test of an air compressor.