Quote:
Originally Posted by winkosmosis
That's was my whole point. The inside of the tube that points into airflow is higher pressure than the surrounding air, not lower.
Pressure - Wikipedia, the free encyclopedia
Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation (note: Bernoulli's equation only applies for incompressible, inviscid flow).
The pressure of a moving fluid can be measured using a Pitot tube, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressures. |
Hi Wink,
Well, reading what it says from Wikipedia versus actually working with pitot tubes and calibrating them for air flow are two different things.
Yes, you are correct about the stagnation pressure, but if you take a close look *inside* the pitot tube you will be presented with a series of small drilled holes that measure the *dynamic* pressure, which is the velocity component of the air flow.
And like I've mentioned already, proper usage and calibration of pitot tubes requires a differential sensor designed to measure both pressure taps.
If I must tell you, we have such devices at work, and yes, I have calibrated them with precision sub-sonic nozzles, so everything is very straight-forward on the theory of the device and actual usage (1).
By the way, the pitot tube is only good at measuring air velocity, and is not a good *mass flow* device. It does not measure air molecules and their relative mass, only the velocity of the particles.
Pitot tubes are of a family of devices that output a signal velocity-squared to the actual air velocity. For example, if you flow the air at 100 feet per second, you will measure a differential pressure of let's say 10 inches water across the two pressure taps. If you increase the air stream to 200 feet per second, the differential pressure does not double, but instead goes up by the square of the air velocity, or to 40 inches water difference. Because of this, pitot tubes have only a 10:1 usable range for differential pressure transducers measuring a range of 100:1.
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(1) One of our precision sub-sonic nozzles is an ASTM designed and custom built nozzle. And guess what, it has two pressure taps on it. One for upstream inlet pressure and what else, but a series of .040 inch diameter holes spaced 90° apart and located at the throat of the nozzle by design. These four holes are communicated to a annular rubber hose that then connects to a 3 foot tall classical water manometer.
When using the nozzle, all calibrated measurements are accomplished by taking the *differential* pressure between the inlet pressure and the throat vacuum. At full tilt, the nozzle generates almost 30 inches of vacuum at the throat! Pretty impressive to watch and demonstrate.
Remember that any time the air is moving quickly, the air pressure around the object is reduced due to the velocity component. If one is fixed on only the stagnation pressure, then you deserve a trip to a good calibration lab to witness first hand the pressure reducing effects of high air velocity.
The Bernoulli Effect is at work here and can be demonstrated with a straw and a piece of paper. Hold the paper on one edge and let it hang towards the floor under it's own weight. Now bring the straw close to the paper. About 1/8 of an inch should do it. Now blow smoothly through the straw and notice which way the paper moves. Does it move away or closer?
If you said closer then blow a little harder. If done properly the paper will be pulled toward the paper almost every time. The change in pressure due to the high velocity air causes the paper to move toward the straw.
Airplane wings create lift using the same phenomena.
There is *stagnation pressure* (read high) at the front of the wing and *vacuum pressure* on the top (read low).
Hope this helps.
Jim.