Riddle me this one...

[jim r king]

So I have a 250 Gallon propane tank and I am hoping to make my second smoker build with.
The first one was about 150 gallons ,I finished that a couple years ago.
I found this one a month ago, valves open and setting empty in a field.
Filled it full to the rim of water and Dawn dish detergent. Let is set 5 days, Completely drained it, and completely filled it all the way up to the rim again with water until all of the soap bubbles had pushed out and only water was coming out.
I drained out about10 gallons and inserted the probe off my newly purchased lel propane/ natural gas/ methane sensor and it read 430 ppm...
I know that’s pretty low But I was a bit concerned because I did not expect any propane at all.
so I went ahead and drained off about 20 gallons. hook the hose back up and put it at the bottom of the tank and slowly filled the tank back up while taking a reading with the sensor.
So any air that was inside the tank is being pushed out the valve.
Took my last reading from the valve and the reading jumped from the original 430 to over 4500 ppm
Again this is after filling it completely full twice and letting it set and taking readings on the third fill.
any thoughts on why there might be this much propane still in the tank? Where is it coming from?
Picture attached shows the readings.. At one point it jumped up to over 4500 ppm

 

[chp]

So I have a 250 Gallon propane tank and I am hoping to make my second smoker build with.
The first one was about 150 gallons ,I finished that a couple years ago.
I found this one a month ago, valves open and setting empty in a field.
Filled it full to the rim of water and Dawn dish detergent. Let is set 5 days, Completely drained it, and completely filled it all the way up to the rim again with water until all of the soap bubbles had pushed out and only water was coming out.
I drained out about10 gallons and inserted the probe off my newly purchased lel propane/ natural gas/ methane sensor and it read 430 ppm...
I know that’s pretty low But I was a bit concerned because I did not expect any propane at all.
so I went ahead and drained off about 20 gallons. hook the hose back up and put it at the bottom of the tank and slowly filled the tank back up while taking a reading with the sensor.
So any air that was inside the tank is being pushed out the valve.
Took my last reading from the valve and the reading jumped from the original 430 to over 4500 ppm
Again this is after filling it completely full twice and letting it set and taking readings on the third fill.
any thoughts on why there might be this much propane still in the tank? Where is it coming from?
Picture attached shows the readings.. At one point it jumped up to over 4500 ppm

I would suggest taking a look at the technical notes for your detector and finding out what interferes. It may be the odorizer used that you are picking up. The mercaptan they add gives the gas its odor and can build up as an oil in old tanks. It takes a lot of soap to clear out that oil.

 

[indaswamp]

The soap could be throwing off the readings....

 

[jim r king]

Looks like both of your replies could be correct. Looking at the Lel monitor indicates that it measures a variety of gases including oxygen, ammonia , propane, methane, natural gas, formaldehyde hydrogen sulfide ,carbon monoxide, carbon dioxide ,hydrogen and includes the word “etc”.. so no telling what else it is measuring.
I looked up dawn dish soap and found out that it releases , formaldehyde, sodium chloride, sodium hydroxide, alcohol And a bunch of other stuff.
I put the sensor at the end of a bottle of dawn and tested to see if the sensor would go off... pic attached


I think what I’m going to do is duct tape a very large trash bag to the top , push water in the other port and collect a trash bag full of “ air” That came out of the tank. Take it far away from the propane tank , tape a 3’ long pc of visco detonation cord to it and see if the “ air” is flammable or not.
will see..

 

[OldSmoke]

Be sure to get a video!

 

[jim r king]

Be sure to get a video!

yep!! Plan on it .

 

[kmmamm]

Second what CHP said.
If the tank was setting level and fully filled with water such that it ran out of the opposite valve, the water would completely displace any residual petroleum gas.
Assuming the detector is picking up actual LPG there are a couple things that could explain why.....
First, if the tank was setting on an incline when filled. If that happened, is possible a small amount of gas could have trapped in an air pocket on the high side of the tank.
Second, it is also possible if enough water wasn’t added so that it was being forced out the top of those stand pipes, a small amount of gas may have remained inside the pipe and subsequently fell back into the tank when water was drawn out of the tank.
Think of it this way....under normal atmospheric pressure , LPG fumes are always heavier than ambient air and when not contained, will displace atmospheric air and (with the help of gravity), seek the lowest point of an enclosure. When water, (which is denser than air and LPG alike) is introduced into the vented enclosure, the less dense air and petroleum vapors will be displaced, essentially “riding” on top of the water and will be fully excavated only when the water has completely filled the container.
Here is a link to an industry article that explains in more detail:

Is Butane Heavier than Air - Is Butane Lighter than Air - Is LPG Lighter than Air

LPG gas is not lighter than air. LPG gas – propane – is heavier than air. In fact, LPG is over 50% heavier than air at sea level. So, LPG gas settles in low places…

www.elgas.com.au

https://www.elgas.com.au/blog/1973-is-lpg-heavier-than-air-is-lpg-lighter-than-air-propane

 

[bill1]

It sounded like you were purging from the bottom up? Propane is C3H8; air is N2. Molecular weight of 42 vs 28. So propane is denser than air. So is water and soap. To purge out heavy things, don't fight gravity. Fill from the top, purge out from the bottom. Make gravity your friend.

 

[kruizer]

Just light a match with it almost full of water a test it that way you might get a small flare if there is any gas left but that way you will know for sure,

 

[jim r king]

So tonight I dumped out 20 gallons of water, filled the tank slowly in one of the 2 ports, put the sensor in the other port as air was blowing out the hole.
sensor read around 4500ppm.
I duct taped a Empty 55 gallon bag to the second port, collected about 20 gallons of air and tied it off w duct tape , put in a pc of visco cord in the bottom where propane would collect, lit it..... nothing happened.
Tried to light the bag on fire.... again nothing happened.
I also filled a 2 L bottle 1/2 full with Dawn dish detergent and soap...
The sensor did not pick up any readings at all from the 2 L bottle while forcing the air out.
Pretty sure the sensor is reading the mercaptan In the tank...
Dumped the tank and plan to fill it up 4th time tomorrow and will start cutting on this weekend.
Thanks for everybody’s replies!
appreciate the feedback.