settle a dispute

[Ray from Indiana]

Oh, there is a difference, all right.
You are pulling a vacuum on different quantities of air (inside the tubing).

Wrong...re-writing the laws of physics?
The vacuum created will allow the column of liquid to rise to the amount of difference between atmopheric pressure and the absolute pressure at the vacuum tap.

Longer sensing line would only take a little longer to reach equilibrium...but in practice a foot or two longer sensing line won't make a difference in the static head.

[robert]

Oh, there is a difference, all right.
You are pulling a vacuum on different quantities of air (inside the tubing).

Wrong...re-writing the laws of physics?
The vacuum created will allow the column of liquid to rise to the amount of difference between atmopheric pressure and the absolute pressure at the vacuum tap.

Longer sensing line would only take a little longer to reach equilibrium...but in practice a foot or two longer sensing line won't make a difference in the static head.

We have a winner!!!

[r90s]

You are assuming that the air inside the tubing is an incompressible medium.

The air inside the tubing acts like a spring.

Think about it.

[Deleted User 287]

The verb in the title of this thread was SO wrong!

[Garnet]

The verb in the title of this thread was SO wrong!

This is Physics class, English is around the corner.

[robert]

You are assuming that the air inside the tubing is an incompressible medium.

The air inside the tubing acts like a spring.

Think about it.

Put a spring on a scale, zero the scale then put a 50# weight on the spring. It will register 50# when the spring is compressed.

[r90s]

Let's do some high-school physics.

From this website:

http://hyperphysics.phy-astr.gsu.edu/hb ... degas.html

We can get the info on the Ideal gas law:

PV = nRT

Where...

P = pressure
V = volume
T = temperature
R = universal gas constant
n = the number of moles of gas molecules

Since the temperature and universal gas constant are the same for both tubes in our case, we can ignore them to get this equation:

PV = n

... and we are focusing on pressure, so to isolate that number:

P = n/V

or Pressure equals the number of moles of gas molecules divided by the volume in which they are constrained.

To find the change in pressure for each tube, we calculate the change in number of moles of gas molecules divided by the volume of each tube.

Lets put in some arbitrary numbers. Lets say that the tubes have volumes of 100 and 150. Let's say that our intakes are balanced, and use the same number moles of gas molecules for each side: 1.

Calculate the change in pressure on each side: 1/100 and 1/150.
P1 = .010
P2 = .006

...showing that we will read different pressures on each side.


You can play with this by entering different numbers on this page:

http://hyperphysics.phy-astr.gsu.edu/hb ... sc.html#c1

cheers,
Jon-Lars
Beverly, WA

[r90s]

You are assuming that the air inside the tubing is an incompressible medium.

The air inside the tubing acts like a spring.

Think about it.

Put a spring on a scale, zero the scale then put a 50# weight on the spring. It will register 50# when the spring is compressed.

True.
Would you agree that the spring is now shorter in compression?

And if you put a different spring on there with a higher compression rate, and put the same weight on it, the scale will also read 50#, but......

.....the length of this spring will be different in compression than the first spring.

Cheers,
Jon-Lars
Beverly, WA

[r90s]

The bottom line on all this is that the different volumes of tubing will result in a very slight difference in your readings, but...

....a bigger problem will show up from the effect of the flexing of the tubing walls. When you pull a vacuum on plastic or rubber tubing, it collapses a little bit.

I would keep the lengths the same to eliminate this variable as much as possible, and keep that tubing fresh.

JL

[Rob Frankham]

My take is this:

1. Since we're talking about pressure (or perhaps more accurately, depression) here, the MEAN reading will not be affected by the different lengths of tube.
2. Any tendency of the tubes to reduce in diameter due to the effect of the depression will also not affect the MEAN reading even if the collapsibility of the tubes is different from side to side
3. Both of these factors will affect the transition times of the meter reading (i.e. the time the meter takes to respond to any change in pressure).
4. Since a number of test using vacuum gauges involve changes in vacuum with changes in throttle settings, the transition times are significant for those tests even if they aren't for static readings
5. It follows that using tubes of equal lengths and made out of similar materials is certainly best practice, if not essential

Rob