help! mysterious pressure gauge drop

We're testing a new copper line for leaks.
The line was purged of air, filled with water at 116 psi, and monitored with a pressure gauge.
After first night, pressure dropped to 102 psi.
After second night, pressure dropped to 92 psi.
This leads me to believe that there's a leak, however, there are no signs of humidity whatsoever.
Is it possible for a line that shows no humidity (i.e. a line that presumably has no leaks) to keep loosing pressure?
Is there a logical explanation for this?
Thanks in advance.
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renux wrote:

Has the temperature changed?
Might the joints (if any) be creeping?
Have you calculated the change in volume that would be needed to account for what you're seeing? Water's not terribly compressible.
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Yes, there has was a variation of about 20oF.

It's hard to say since most of the line is not exposed.

I haven't, but this morning we refilled the line and purpuosefully opened the faucet to experience a similar drop in preassure. The water lost was about a third of a mug.
We are actually testing 3 lines in this fashion: 1 cooper line for water and 2 copper lines for gas.
The good news is that the line for water has stabilized at 76 psi for the last 72 hrs. so I feel pretty comfortable with this.
For the gas lines I'll start another thread.
Thanks!
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You should cap off these lines and not have any faucets attached, then pressure test them with air.

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water cooled off and pressure dropped. heat water pipe and pressure will rise. or gauge problem.
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It's possible for the leak to evaporate without condensing on the pipe, especially in low humidity.

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assuming it was filled with cool water you have a leak somewhere. can you examine the entire line? is it exposed?
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Most of the line is not exposed. However, the line stabilized to 76 psi for the last 72 hrs., so it seems to be ok. Thanks!
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116 psi!!! Where did you get WATER at that presure? I always thought leaks were tested with air presure. Water is not compresible, and changes in temperature will change the presure, I think. --Phil
renux wrote:

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It's been a while so I may not be completely correct but here goes: Water is compressible and that is the crux of why pressure can change in a fixed volume system such as the copper lines. Water, like almost any fluid, contains air in solution (entrained air) and it is this air that gets compressed (or expands). Take the expression: Delta P=(Delta V)/V*BM where Delta P=change in pressure; Delta V =change in volume; V=Volume under pressure (copper lines); BM=Bulk Modules As the temperature of the fluid changes so does its density (lbs/cubic ft). Since there is no change in the weight of the fluid it's volume will change via compression/expansion of the entrained air. The temperature (density) does not change enough to account for a pressure change of approx 20% (116 to 92) so look for a leak somewhere. As a side note: the compressibility of water (or any fluid, like hydraulic oil) plays a significant role in high response control systems. Too much entrained air and the end result is poor response and in many cases an unstable system. MLD
Youngstown State University

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We used a home-made pump that uses a hydraulic component and a lever. We used water because it makes it very easy to spot a leak. Thanks!
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wrote:

Again, no experience, and I thought I would learn about this here. Some people here seem to have written as if using water is the normal procedure, but others hint that it's not, or say to use air.
I offer this for the OP's consideration. Water is only compressible to a slight degree, and the major thing they could be compressing is the slight amount of air that may be left in the pipes when they are is filled with water. Because that volume is small, tThat means that it would only take the loss of a little bit of air or a little bit of water (giving more room for the air), to multiply the available space significantly and make the pressure drop significantly.
Also, the air can get absorbed intio the water. This happens much faster at high pressures, and iiuc when the air is absorbed in the water, it takes up no space (amazingly) and thus the pressure would drop.
Look at the water prssure arrangements in 6-story buildings. (In nyc. There may be a different height where this applies in other places.) I think the same thing applies with houses that have wells and compression tanks.
Four or 5 stories and less, city water pressure is enough, Seven or 9 stories or more, they put a water tank on the roof.
At 6 stories they pump the water into a tank that is about 1/3 air after the air in the tank is compressed, and they use the air pressure to pump the water to the sixth floor (and the other floors, but for the sixth it's essential) After a while the air in the compression tank has been absorbed into the water, the pressure drops, and a separate air pump has to be used to add more air. Here the water is being replaced with new water every time someone uses any water, unlike the OP's sealed pipe, but there is still absorption to the extent the original water will take it.

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The normal way to pressure test heat systems and water pipes is to use air pressure. It's easier to re-solder if you have a leak.

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