Fig shows Schematic arrangement of a combined pitot tubes
Combined pitot tube (Prandtl Tube, Pitot-static tube), in which both the two hollow tubes installed together in one casing as a single device. The Pitot-static tube provides both stagnation and static pressure measurement in single package. The inner tube (pitot tube) is pointed to the upstream and is parallel to the flow while the outer tube (static tube) had opening perpendicular to the axis of flow. The central tube kept separated from outer tube; several small holes are drilled to the outer tube. The inner tube has the impact opening for measuring stagnation pressure while the outlet tube has one or more holes on the side for measuring the static pressure.
Pitot-static tube (Prandtl tubes) has some disadvantages that are same as the pitot tube. (Read: Pros and cons of pitot tubes). Similar to pitot tube it cannot use in dirty condition because of the clogging of dirt in the opening of tubes.
Double tip pitot tube
In this case, two holes are formed in pitot tube which is diametrically opposite to each other. The tube is inserted into the stream with one orifice facing upstream and the other downstream. The connections from the two orifices are separately taken to the manometer tubes through two separate holes formed inside the pitot tube. In this case, no static pressure connection is required.
- Pitot-static tube also called Prandtl tubes