Pressure gauge reading 3

[flamearrow]

Hi any Savvy,

Would anyone explain to me what the pressure gauge in the fluid pipe reads (either analog or digital), I mean static or total pressure? I found from some web sources that it reads only static and we need to add the velocity pressure.
Thanks

 

[LittleInch]

Static only.

Correct.

In reality it's only important for low pressure high velocity locations.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[flamearrow]

Thanks, LittleInch, but I am still baffled coz despite the reading mechanism of the gauges in the fluid flow, the effect will be the total pressure (if the velocity pressure is not negligible as you mentioned) which is going to be converted into the reading, isn't it?

 

[LittleInch]

Please use proper English on this site, you're an Engineer, not talking to someone down the club "coz" it puts people off.

Errr no. The pressure guage measures the static pressure. If the pressure is low, but the velocity high the velocity head can be significant ( > 10%) of the static pressure.

Pump vendors though do add the velocity head to the static head on their charts so low head high velocity pipe can seem as though the pump isn't producing the correct head.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BrianPetersen]

It can read the total pressure if the probe is a Pitot tube which is correctly oriented and positioned.

 

[zdas04]

Therein lies the rub. The take-off for the pressure instrument is at the pipe wall. The no-flow zone and boundary layer are also at the pipe wall. Consequently, the instrument does not sense any of the dynamic pressure. If you extended a stinger into the flow and connected that to a pressure instrument the perturbations caused by the bluff body in the flow would mask (either cancel or multiply) the effect of dynamic pressure. Even the pitot tube idea (which can work) fails more often than it succeeds since they are very difficult to properly position, properly orient, and (in my mind most important) calibrate.

Whenever I have a problem with a velocity greater than 0.1 Mach (if I'm using the Bernoulli Equation) or 0.3 Mach (if I'm doing something where very small pressure changes matter) or 0.6 Mach (always) I add a calculated dynamic pressure to the static pressure, it really is the only way.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[dgallup]

The OP has given no indication about the geometry of the pressure tap. If it's flush with the wall then it is only static pressure but if it's a correctly positioned Pitot tube it will read total pressure. Flush taps are much more common because they are cheap & easy and with low velocity the dynamic pressure is negligible. I've seen it done both ways so with out additional information it is impossible to give the OP a definitive answer.

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[flamearrow]

I Thank all you guys for the reply, and LittleInch sorry I will correct the language, thanks for correction.

 

[TheTick]

Lighten up a skosh, LI. Gee whiz.