Help Understanding Pressures

[ramius-ii]

Hi All!
In the wonderful world of pressure sensors there are basically 3 types. Absolute, Gage, and Differential. All are easy to understand as to what they do and what they measure. The question is when you are using a differential sensor from what I understand you have two different pressures being applied to a single membrane thus one pressure is subtracted from the other. Thus your sensor is providing only one pressure measurement number? If this is true then how can you convert this one pressure number into speed? Inquiring minds wish to know. Thanks, Ed

[roedj]

Re: Help Understanding Pressures

I believe what you're basically looking for is a pitot tube. See the wikipedia article here:

http://en.wikipedia.org/wiki/Pitot_tube

Good luck as this isn't going to be easy especially at the sizes of models we're talking about.

Dan

[stuartl]

Re: Help Understanding Pressures

Hi All!
In the wonderful world of pressure sensors there are basically 3 types. Absolute, Gage, and Differential. All are easy to understand as to what they do and what they measure. The question is when you are using a differential sensor from what I understand you have two different pressures being applied to a single membrane thus one pressure is subtracted from the other. Thus your sensor is providing only one pressure measurement number? If this is true then how can you convert this one pressure number into speed? Inquiring minds wish to know. Thanks, Ed

Absolute: Reads the pressure relative to absolute zero pressure (i.e. perfect vacuum).

Gage: Reads the pressure relative to a fixed internal reference (usually 1 bar, check the datasheet).

Differential: Reads the pressure relative to a second pressure reading (as you describe above).

All three give exactly one pressure reading.

[ramius-ii]

Re: Help Understanding Pressures

Thank you!
Yes I understand this part and the basic question is if you are using a differential sensor you only have one pressure reading so how do you convert this to something like speed or does it take two sensors? As for size the pitot tubes for boats that are in use are small. Best, Ed

[stuartl]

Re: Help Understanding Pressures

I haven't thought about using a differential sensor to measure speed before but conceptually it sounds quite simple.

If you were to feed each side of the sensor from a separate pitot tube, each pointing the opposite direction, then the pressure would increase on the upstream side and decrease on the downstream side as the vehicle accelerated. The differential sensor would, I think, give a reading proportional to speed...

You could probably achieve a similar thing at very low profile by angling tubes inside the hull with a small mount exposed. You would have to expose the ends of the tubes enough to avoid the effects of eddy currents on the surface of the hull but I don't see that being a major issue.

You could even use the sail as the housing for your pitot tube, i.e. drill a hole in the front and back and pipe each one to your differential sensor.

[ramius-ii]

Re: Help Understanding Pressures

Thnaks Stewart!
One possible idea is to take incoming pressure from the nose (bow) and compare the reading to a static source such as the depth sensor located in a flooded area where the water does not move. Best, Ed

[tom dougherty]

Re: Help Understanding Pressures

It's called a pit log:

http://en.wikipedia.org/wiki/Pitometer_log

[Dominique]

One more type

It's called a pit log:
http://en.wikipedia.org/wiki/Pitometer_log

That article left out one more type. It's an MHD effect sensor. Basically it is a rod with an electrical contact on each side and a magnetic coil driven with AC inside with the mag field verticle(i think). It induces a voltage across the contacts proportional to the speed. It may however only be useful in salt water due to its need for the water to be conductive.

[tommydeen]

Hmmmm interesting