How do I know if my expansion tank is water logged?


The story begins when I dicovered that the relief valve (of my home's heating system gas-based heater) discharges about 1/4 cup of water daily.
A technician from the gas company (called for a different problem) that saw this immediately pointed to the Amtrol expansion tank (Extrol model #30) as the culprit. He said he could tell just by knocking on the tank and listening whether it is a hollow sound or not.
After he went, I tried this myself but I actually noticed that the top part (which should be filled with water) does not sound hollow, while the bottom part (which should be filled with air only) sounds hollow. Isn't this the way it is supposed to be? If so, what made him say this is the expansion tank?
Also, while he said that the expansion tank should be replaced, I noticed a threaded air valve at the bottom of the tank, which looks like exactly the ones found in bicycle tubes. Is it possible that I can get away with simply re-filling some air in the tank, instead of going into the expense of replacing the tank?
It should be noted that I have no intention of replacing the tank myself, but rather let a professional heating plubmer do that. However, I feel that I need to gather as much information about the problem as possible, before going ahead with ordering a repair. One plumber I called said the expansion tank alone will cost me $250 (excluding labor!). I found that same exact tank for sale on the Internet $43.55 ...
Thanks! Sam
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snipped-for-privacy@yahoo.com wrote:

put an auto tire pressure guage on the air valve and check the pressure is still as specified in the owners manual.
If water comes out of the air valve you need a new tank , or a repair kit.
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Also to small a tank will cause the boiler to overpressure as the water heats. What pressure do you reach, could also be a bad pressure valve.
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m Ransley wrote:

does this model of tank have a bladder to seperate the air and the water?
Mark
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Mark wrote:

Yes, it does. Does this provide additional info that can help?
BTW, I followed marks542's suggestion: I put a bicycle air pump with pressure gage (an auto tire one won't work because the threaded valve on the tank fits a bicycle pump, not an auto tire one) and attempted to measure.
My first observation was that **no water is leaking** (that's good, right?)
However, the pressure gauge on my bicycle pump only starts to show calibrated reading from 20 psi and above. The Extrol 30 needs only 12psi. I heard some air discharging in the process and so I was afraid to depleat the air in the tank (which would exacerbate the problem - not good on a Sunday morning...).
BTW, the air that came out of that valve was stinky (like dead fish or something). Weird.
What do you suggest that I do next?
Thank you all for your help! Sam
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snipped-for-privacy@yahoo.com wrote:

if it has a bladder and significant amounts of water come out the air valve, that means the bladder has ruptured.... time for a new tank.... a very small amount of water is prob just condensation
and I always thought a bike tire and car tire used that same fitting so I don't undertand what you are saying about that.
Mark
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Mark wrote:

Mark, you are right. Thanks to your comment I double-checked... First, I found this:
http://www.sheldonbrown.com/gloss_sa-o.html#schrader
Which confirmed that I actually have a pump for the Schrader type, which is the standard automotive-style air valve.
That lead me to double-check why my first trial with the car tire gauge did not succeed. It turns out that I was trying to be so cautious, that I didn't press the gauge hard enough againt the valve.
So... I went ahead and used the gauge as recommended: The reading shows something like 11.8 psi.
This is close enough to the optimal 12 psi (as specified by Amtrol), so I must assume that the expansion tank's problem is a water-logging? The stinky smell coming out of that valve suggests that indeed it may be the result of water sitting there for a loooong time.
Comments? Suggestsion?
Thanks! Sam
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Myvz800 had written this in response to http://www.thestuccocompany.com/maintenance/Re-How-do-I-know-if-my-expansion-tank-is-water-logged-158625-.htm :
snipped-for-privacy@yahoo.com wrote:

-------------------------------------
I found your post and thought you might be interested in this. Factory pre-charged to 12 psig Pre-charge should be adjusted to equal minimum operating pressure at tank location
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