Static pressure tubes vs pitot tubes

[Chris J]

I've noticed when measuring static pressure in a duct most people use L shaped tubes with small holes in the sides, very similar to a pitot tube except it's missing the hole in the end for dynamic pressure.

What is the reason for using an L tube with holes in the sides for static pressure alone? Wouldn't just a connection into the side of the duct generally provide a fairly accurate measurement?

 

[Lnewqban]

That shape allows measurements deep inside the duct.

Please, see:
https://www.contractingbusiness.com/service/article/21212881/secrets-of-the-static-pressure-tip

https://www.linkedin.com/pulse/static-pressure-fans-ducts-best-length-vahid-tabatabaeifar

 

[russ_watters]

What is the reason for using an L tube with holes in the sides for static pressure alone? Wouldn't just a connection into the side of the duct generally provide a fairly accurate measurement?

It should, but it would require a second hole and probe. I've also seen L shaped static pressure probes though, and I think they want to be a little away from the side of the duct so there's no weird effects from the boundary layer or roughness.

https://www.contractingbusiness.com/service/article/21212881/secrets-of-the-static-pressure-tip