plastic tubing level doesn't match guage glass level?

jpf321

i rigged up a plastic tube from one of my boiler drains on the return side of the hartford loop .. for some reason, the level in the plastic tube is lower than the level in my gauge glass? any explanations would be appreciated. thx.



EDIT: fyi, my gauge glass is not clogged top or bottom.

Nick_C

When running

Is the level difference there at rest, when boiling or all the time?

jpf321

at rest .. should have mentioned

on a prefectly at rest, hasn't run in a few hours boiler .. gauge glass about 1in higher than plastic tube from boiler drain valve located on return side of hartford. strange huh? the only thing I can think is that there is 1inch of crud in the bottom of my boiler chamber. 

lutorm

Tube open, right?

I assume the plastic tube is open to the atmosphere?



Are you sure the boiler is unpressurized? No change if you open the relief valve?



That's very difficult to understand. No amount of crud anywhere can make the levels not equal, as long as it's not totally blocking the water. Only 3 things can make the levels unequal: non-atmospheric pressure in the boiler, non-atmospheric pressure in the tube, and capillary action in the tube (though that would make the water rise higher).

jpf321

yep ..

yep .. tube open to air.

opening pressure relief valve no change

0-20oz gauge is at 0.



you forgot 1 thing, but it's proportional to pressure .. different temp...the water in the boiler may be at a slightly warmer temp than that in the outside boiler pipes/tube. could it make that much of a difference? (see pics below)

Nick_C

Boyle's law

Capillary action would be negligible in a plastic tube of that size (and would work in the opposite direction as stated above).

Assuming the sight glass valves are fully open on the top and bottom, the only explanation is that the pressure from the "air" in the boiler and the return pipes, taken as a whole, is less than the atmospheric pressure in the basement.  Probably because the "air" inside the boiler and pipes has a different composition (saturated with water vapor) and temperature than the "air" in the basement.  Remember that Boyle's law (pV=nRT) only works for an "ideal" gas.  You are working with two different non-ideal gasses in your setup.

First thing I would check, though, is the top valve in the sight glass. To make sure you don't have a vacuum under cold conditions.  That's happened to me a few times due to crud.

jpf321

double checked

i dbl checked the valves all fully open.

jpf321

does everyone else's match?

just curious if anyone else that has ever hung a plastic tube off their return side piping has matching waterlines? thx.

Ex Maine Doug

It is an illusion

It might be from the weight of all that stuff at the bottom of your postings.  The pictures are being tilted so the water is really level but appears not to be.  A similar effect occurs in the right rear view mirror -  objects are not really as big as they appear or something like that.  ;-}

jpf321

it's not

it's not an illusion trust me .. if this wasn't what I was seeing, i wouldn't have bothered the wall .. and I wouldn't have asked prior to posting the pictures.

David Nadle

Contact angle

It's interesting that in your pictures the water in the gauge glass has a prominent meniscus but the water in the tubing does not. The difference in contact angle due to the different materials could explain why capillary action is acting more strongly in the glass tube. Maybe you could use a different tubing material, or add a length of glass tubing and place the water surface within.

 

Matthew Grallert

Temp?

I bet the reason the levels is different is the temperature difference between the two.  Have you tried this experiment with the boiler at ambient?

cattledog

density changes with temperature

I believe that you are seeing the effect of the water density change with temperature.



If you can determine the two temperatures, and the absolute heights of the two columns you should be able to use one of the online fluid calculators to see if the observed difference is consistent with the differing densities.

jpf321

temp...

i have not tried boiler ambient, and i probably won't until 1) my ambient is 170dF or 2) after heating season. but i did propose temp as the difference above. what i should do is try again when the boiler has been running a while and the return is temped up...but the problem there is that 1) the steam is then out in the system (lower gauge) and 2) the pressure will be higher on the returns (higher tube)