Quiz - Physics - Gauge Pressure

[Brett Hatch]

What does that definition have to do with (c)?

Never having heard of "gauge pressure," I did not understand the question so responded with "a. 2.55 ATM" . Was surprised to see so many responses of (c), so Googled "gauge pressure", found a similar definition. Now I understand both the question and the answer.

This notion of zeroing out a gauge to atmospheric pressure seems a little bonkers to me -- climbing a hill or going into an airplane is not the same thing as entering a vacuum! I suppose it is highly pragmatic in some industries that deal with measuring pressures all the time, hence the terminology. Is that right, and if so, which are the relevant industries?

 

[tursiops]

Never having heard of "gauge pressure," I did not understand the question so responded with "a. 2.55 ATM" . Was surprised to see so many responses of (c), so Googled "gauge pressure", found a similar definition. Now I understand both the question and the answer.

This notion of zeroing out a gauge to atmospheric pressure seems a little bonkers to me -- climbing a hill or going into an airplane is not the same thing as entering a vacuum! I suppose it is highly pragmatic in some industries that deal with measuring pressures all the time, hence the terminology. Is that right, and if so, which are the relevant industries?

You mean other than scuba? How about the pressure in your tires, for example?

 

[Ukmc]

Concept of gauge pressure is really important and goes right to the bottom of all pressure/volume calculations, sac calculations etc. The problem with a gauge pressure is that it doesn’t account for an air column, so you have to add the pressure of air column to your gauge (usually 1 atm but can vary) to get an absolute pressure, which You would use for all your calculations.

Never having heard of "gauge pressure," I did not understand the question so responded with "a. 2.55 ATM" . Was surprised to see so many responses of (c), so Googled "gauge pressure", found a similar definition. Now I understand both the question and the answer.

This notion of zeroing out a gauge to atmospheric pressure seems a little bonkers to me -- climbing a hill or going into an airplane is not the same thing as entering a vacuum! I suppose it is highly pragmatic in some industries that deal with measuring pressures all the time, hence the terminology. Is that right, and if so, which are the relevant industries?

 

[Brett Hatch]

Concept of gauge pressure is really important and goes right to the bottom of all pressure/volume calculations, sac calculations etc. The problem with a gauge pressure is that it doesn’t account for an air column, so you have to add the pressure of air column to your gauge (usually 1 atm but can vary) to get an absolute pressure, which You would use for all your calculations.

Why don't we have gauges that are calibrated to 0 instead of 1 atm? Seems like it would simplify things for the user.

 

[Ukmc]

Why don't we have gauges that are calibrated to 0 instead of 1 atm? Seems like it would simplify things for the user.

Sure. It would be more convenient for a gauge to read out 1 as the starting point at the surface, but I can see where it would present a problem for analog gauges, especially when you do altitude diving and have to go below 1 atm to calculate your absolute pressure. For digital gauges, I agree, I think the concept of gauge pressure is less important. For example my Teric reads out whatever surface pressure it is probably based on the altitude.
There maybe some other issues, I would love to see responses from the seasoned members of the board.

 

[Pedro Burrito]

Spoiler: Answer

c. 1.55 ATM/22.89 psi

Metric: 16 * 0.097 (the pressure change per unit of fresh water) = 1.552

Imperial: 53 * 0.432 = 22.896

 

[Couv]

Thanks @Pedro Burrito This was an interesting quiz question. Please keep them coming.

 

[HKGuns]

Ha! The streak continues. Thanks!

 

[Russjstewart]

No wonder people think I am an idiot. Just re read the question and noted the “fresh”.

Really glad I resisted the temptation to make a smart comment when I did the survey. Duhhhhhhhh!!!!!!!!!!!

 

[ToneNQ]

Sure. It would be more convenient for a gauge to read out 1 as the starting point at the surface,.......snip.......

Would it be more convenient? For all intents and purposes, ambient pressure is our zero (datum). It's key to what our TTS will be, surface off gassing rates, etc. Every pressure variation during the dive is relative to our zero. The absolute number ceases to be useful to the diver, even though its existence is essential to the calculations. I agree the concepts are not critical when the computer does it all for you and it does it very well, hence so many are not aware of it.

The few hours of flying that I did, one of the checks on ground was to zero the altimeter to adjust for the changes in atmospheric pressure. We would set the altimeter to the known altitude of the airstrip. Essentially calibrating gauge pressure to account for the variation in ambient pressure, against a known set point.

There aren't many things we measure that are absolute, even temperature is based on scales that are relative. Kelvin and Rainkine absolute, yet the great bulk of the world uses Celcius and a dozen or so use Fahrenheit.