Manometer calculations

[prkaye]

Is the difference in water levels in the manometer (tank test) linearly related to the difference in pressure between inside and outside the tank? I think it must be, but my engineering training was a long time ago (and wasn't Mechanical eng).
If so, we have the following formula:
P = AD+B
Where P is the pressure difference, D is the water height difference and A and B are constants. From two of Vans test-kit instructions, we have the two data points (P=2.4 PSI, D=66") and (P=1 PSI, D=27"). Solving for A and B, we get the formula
P=0.036D+0.031
where P is in PSI and D is in inches. Does this seem right to the engineers out there?

The reason I want this formula is I'm doing an extended-time pressure test, and want to be able to compensate for changes in atmospheric pressure. If I keep a barometer in the shop, I should be able to compute updates of what the difference in pressure between inside and outside the tank should be (assuming the tank is airtight and making the simplifying assumption that the tank skin is inflexible), and then use the above formula to compute how the water levels in the manometer should be changing. Of course, I should probably factor in the temperature, which would affect the pressure in the tank to some degree.

Overnight I had the water separation D decrease by about 2.5", and I'm not sure if it's reasonable to expect this is just due to the atmospheric pressure change. The atmos press increased by about 0.2kPa(=0.029PSI) overnight (according to weather info), which by my formula should only mean a -1.7 inch change in D. Except I haven't accounted for temperature, and I'm not sure how to.

[IowaRV9Dreamer]

Here is a calculator for pressure click here

The density of water varies a tiny bit with temperature, but not enough to matter. Here is a link for that click here

You're right, the biggest variable is ambient pressure, but remember that the manometer reads differential pressure (difference between tank & ambient) - so you can subtract out the ambient pressure and it's variations.

Hope this helps - I can't wait to get to leak testing.

[terrykohler]

Phil:
I applaud your efforts, but in the end you're just trying to determine if the tank has integrity. You've already found that the tank will survive overpressurization and have resolved leaks around the filler caps. Now, keep it simple and use the balloon test method. If you overpressurize again, the baloon will protect the tank. If after 3-5 days the balloon is still inflated, you're good to go (slight variations in size will account for pressure/temp. differences).
"In the course of every project, there comes a time to shoot the engineers and launch the project." (not sure who gets credit for this factoid).
Terry (not mechanical either)
RV9A
N323TP

[rv8bldr]

Originally Posted by terrykohler

Phil:
I applaud your efforts, but in the end you're just trying to determine if the tank has integrity. You've already found that the tank will survive overpressurization and have resolved leaks around the filler caps. Now, keep it simple and use the balloon test method. If you overpressurize again, the baloon will protect the tank. If after 3-5 days the balloon is still inflated, you're good to go (slight variations in size will account for pressure/temp. differences).
"In the course of every project, there comes a time to shoot the engineers and launch the project." (not sure who gets credit for this factoid).
Terry (not mechanical either)
RV9A
N323TP

I agree with Terry, Phil. I think you're using sledge hammer to put in a thumb tack. Put the ballon on the tank. Put enough air in the tank that the balloon begins to fill (i.e. it is obvious that there is pressure in the tank). Let it sit for a few days, the balloon will appear to deflate and re-inflate with changes in atmospheric pressure. If the balloon never appears to re-inflate, you probably have a leak. Just give it a week or so to be sure.

This is how I did my tanks.

Cheers

[Mel]

One other comment about the balloon. All balloons leak somewhat on their own. Try to take another balloon from the same package, inflate it to the same size for comparison. If both balloons go down the same amount, it's the balloon and not the tank.

[prkaye]

I realise I'm putting in more effort than is required, but it's fun. And it's an educational exercise (this whole RV project is one big educational exercise). Not only learning the theory and skills of airplane construction, but this is teaching me more about testing methodology.
Although my undergrad education was in Engineering, I'm actually a mathematician by trade, so most of my professional knowledge and experience is rather theoretical... much of the applied science of building an aeroplane is still rather new to me.

By the way, to ensure zero leaks through the cap to get a really well-controlled pressure experiment, I bought a 2-inch "test plug" from Home Depot (intended for plugging 2" PVC I think). It's a rubber plug sandwiched by two metal plates. A wingnut over a bolt joining the two plates tightens the plug, expanding the rubber. Basically a crude version of the fuel caps. I got this idea from Kevin Horton. With this plug I was able to get a complete and total seal. Previously, even with tonnes of aluminum tape and packing tape, and grease around Vans fuel cap, I was not able to get a 100% seal. Always a tiny soap bubble stream leaking out of the tape somewhere.

As for tightening the fuel cap itself... that nut is pretty darn tight... Holding the cap in my hand, I was not able to adjust teh nut with a wrench in the other hand... I didn't want to put too much muscle into it for fear of breaking the cap.

[rv6ejguy]

Just watch temperature variations. This will probably affect your results more than baro changes- air expands a lot.

[az_gila]

Originally Posted by rv6ejguy

Just watch temperature variations. This will probably affect your results more than baro changes- air expands a lot.

Yes.... the previous posting about water density changing little with temperature is correct, but the tank is filled with air, and that does change volume with temperature....

Properties here...

http://www.engineeringtoolbox.com/ai...ies-d_156.html

But a little extra volume in the 37 (?) gallon tank will make a big change in you 1/4 inch (?) tube....

Enough numbers here to keep you out of trouble for a while...

gil in Tucson

[Mel]

Originally Posted by prkaye

I realise I'm putting in more effort than is required, but it's fun. And it's an educational exercise (this whole RV project is one big educational exercise). Not only learning the theory and skills of airplane construction, but this is teaching me more about testing methodology.
Although my undergrad education was in Engineering, I'm actually a mathematician by trade, so most of my professional knowledge and experience is rather theoretical... much of the applied science of building an aeroplane is still rather new to me.

By the way, to ensure zero leaks through the cap to get a really well-controlled pressure experiment, I bought a 2-inch "test plug" from Home Depot (intended for plugging 2" PVC I think). It's a rubber plug sandwiched by two metal plates. A wingnut over a bolt joining the two plates tightens the plug, expanding the rubber. Basically a crude version of the fuel caps. I got this idea from Kevin Horton. With this plug I was able to get a complete and total seal. Previously, even with tonnes of aluminum tape and packing tape, and grease around Vans fuel cap, I was not able to get a 100% seal. Always a tiny soap bubble stream leaking out of the tape somewhere.

As for tightening the fuel cap itself... that nut is pretty darn tight... Holding the cap in my hand, I was not able to adjust teh nut with a wrench in the other hand... I didn't want to put too much muscle into it for fear of breaking the cap.

The nut does not adjust the cap. It is a "jam nut". To adjust, loosen the nut, turn the threaded base to tighten or loosen, then re-tighten the nut.

[MTBehnke]

To add to the temp issue, I believe the compressor heats the air up a little as it runs to pressurize the tank, and may be affected as you pressure the fuel tank as well. Aside from obvious air leaks, I'd ignore any water level changes for the first hour or so to allow the air inside to normalize to outside air temperature.