# Propane Tank Pressure Gauge Meaning



## saltypat (Oct 8, 2006)

Last summer we bought an old boat that had a new stove and propane tank hookup, but no documentation. I did get stove info off the net

The small propane tank has a pressure gauge that goes from under 100 something to over 300 something (only looked at it briefy). Brifely, what is the gauge measuring and what would you think the pressure would read if it is getting near empty?

(We took the tank for our backyard grill tank in onetime to be refilled, and the serviceman said he desired it to be empty before he refills it.)

One option is that as a backup I carry some food which needs no cooking......but I am hoping you sailnet gurus will put me on the path of enlightenment.

Thank you for your time, SaltyPat


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## negrini (Apr 2, 2008)

There is no reliable way to measure propane quantity, except by weigh. LPG remain liquid under pressure, so only the dry part of your tank is full of gas at almost constant pressure; as you liberate the gas, the liquid surface replace it transforming from liquid to gas;what is reduced is the liquid part, so your pressure gauge will read the same for a long time, and when it drops, indicate your tank is out of LPG. Too late. The gauge could help you detect leaks, at most.


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## Rockter (Sep 11, 2006)

Yes... you have to weigh it. The guage means next to nothing. It tells you there is some LPG in the tank, but not how much.


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## sailortjk1 (Dec 20, 2005)

negrini said:


> The gauge could help you detect leaks, at most.


That's about all it does.
A drop in pressure over a period of time means you better get the soapy water out.


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## JimsCAL (May 23, 2007)

As long as there is liquid in the tank, the tank pressure will vary with the ambient temperature, not the amount of LPG in the tank. The liquid "boils" at a pressure that increases as the tank temperature increases.


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## CharlieCobra (May 23, 2006)

Propane boils at -40 F, which is why they don't use it in real cold climates. That's how it gets it's pressure. You will have pressure right up until the gas is gone but it will fall slightly as the tank gets really low. Rises in temp will cause changes in pressure as well.


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## JohnRPollard (Mar 26, 2007)

Salty Pat,

Suggest you review the thread below in the link. There is a lot of discussion in there about the purpose of the pressure gauge. It is a CRITICAL component of your propane system, but it does not measure the amount of LPG in the tank. It's sole purpose is for leak detection. As others have pointed out, remaining LPG is measured by weighing the tank:

LINK


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## btrayfors (Aug 25, 2006)

SaltyPat,

I agree that there's no reliable way to know how much propane is left in the tank, except to weigh it. And, I believe a pressure guage is a good thing to help detect leaks.

But I believe that what most of us fear is running out of propane during a cruise, when there's not usually a filling station nearby. That actually happened to me a couple of years ago....ran out on the first day of a 5-day trip!

Now, I have a solution: I purchased a brass adapter which adapts the regular propane hose fitting (the one that screws onto your tank) to accept small propane bottles, such as are used in camp stoves, bar-b-que grills, etc. Boat US and West Marine have these adapters.

Then, I purchased two small propane bottles -- the squatty kind (Coleman, I believe) -- and built a mount for them next to the main propane tank. These little bottles are cheap, and will tide you over until you can find a place to refill the large tank.

Gotta have that coffee in the morning 

Bill

PS...find another place to refill your tank. Most places I've found will definitely fill a tank that's not empty yet. They simply place it on the scales, and fill it to the proper weight.

B.


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## eherlihy (Jan 2, 2007)

Below is an excerpt from an email that I sent the owner of a boat that I chartered. I think that it explains the meaning of the presure guage fairly well: 

"When I picked up the boat, I noticed that one of the two propane tanks was low. I made this determination based on the weight of the tank. You probably know that at 70 deg F, at sea level, propane liquefies at 110 PSI. This means that at 70F the pressure gauge should never read higher than 110, but the weight of the tank will indicate how much liquid propane is contained in the tank. If the gauge reads less than 110, the contents of the tank have vaporized, and you will soon run out. In this case, the tank in question read 100.

While on this topic, the tank connectors require a 1.25" wrench. The adjustable in the tool kit only opens to 1". I brought a few of my own tools which included a large adjustable."

-Ed


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## sailingdog (Mar 19, 2006)

Believe that you should have said: "This means that at 70 deg F the pressure gauge should never read LOWER than 110...." since if there is any amount of liquid propane, the pressure should be 110 PSI.


eherlihy said:


> Below is an excerpt from an email that I sent the owner of a boat that I chartered. I think that it explains the meaning of the presure guage fairly well:
> 
> "When I picked up the boat, I noticed that one of the two propane tanks was low. I made this determination based on the weight of the tank. You probably know that at 70 deg F, at sea level, propane liquefies at 110 PSI. This means that at 70F the pressure gauge should never read *higher* than 110, but the weight of the tank will indicate how much liquid propane is contained in the tank. If the gauge reads less than 110, the contents of the tank have vaporized, and you will soon run out. In this case, the tank in question read 100.
> 
> ...


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## eherlihy (Jan 2, 2007)

sailingdog said:


> Believe that you should have said: "This means that at 70 deg F the pressure gauge should never read LOWER than 110...." since if there is any amount of liquid propane, the pressure should be 110 PSI.


Assuming that he intended to send me out with a fully equipped boat, then yes, the guage should not have been _lower _than 110PSI. That's an assumption that I didn't make. (As I remember, the dock hand looked at it and said; "100 pounds, that should hold you for a week." )

The guage can not read higher that 110 PSI if the tank only contains liquid propane. The guage could read lower, however, if there was no _liquid _fuel remaining in the tank, and the vapor was being used up.


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## Rockter (Sep 11, 2006)

The guage will indicate the vapour pressure of the propane at the then tank temperature. It does not vary much, unless you cool it out of sight. Someone said that it boils at -41 degF, so the guage would cease reading at that moment. If you took the top off the tank at that moment, then in theory the propane would just puddle there and would not come out.

At sane temperatures, the moment that the pressure begins dropping sharply, you will have very little indeed in the tank. Indeed, if the pressure dropped to (say) 100 psi, you would only have about 100/14.5 = 6.89 times the volume of the tank, and you have to leave one volume behind, all measured in standard volume units (typ cubic feet).
Would approx 5.69 times the tank volume in standard cubic feet last a week?
I doubt it.


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## Rockter (Sep 11, 2006)

The guage will indicate the vapour pressure of the propane at the then tank temperature. It does not vary much, unless you cool it out of sight. Someone said that it boils at -41 degF, so the guage would cease reading at that moment.

At sane temperatures, the moment that the pressure begins dropping sharply, you will have very little indeed in the tank. Indeed, if the pressure dropped to (say) 100 psi, you would only have about 100/14.5 = 6.89 times the volume of the tank, and you have to leave one volume behind, measured in standard volume units (typ cubic feet).
Would approx 5.69 times the tank volume in standard cubic feet last a week, with the pressure dropping all the time?
I doubt it.


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## TSOJOURNER (Dec 16, 1999)

The only reliable way to avoid running out of propane is a back up supply. That 100 psi tank would probably be gone before the water bolils for your coffee, so either take the tank off the boat and weigh it before your trip, or provide a back up! The suggestion was made to use a couple of those little green coleman bottles, but if you have room a second 10 pound bottle is less expensive in the long run because it can be refilled. My Endeavour is blessed with two propane lockers, one in each side cockpit coaming. One of them has the stove feed and solenoid, the port one is only a storage site. Trust me, there is a full gas tank strapped inside! For a liveaboard it would seem practical to purchase (or build) a second propane locker and vent it, because with daily gas usage you will run out and use those disposable bottles often!


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## CharlieCobra (May 23, 2006)

I don't know. Mine's been reading about 75 PSI lately and we cooked five meals and had coffee for four mornings with that level. It's now down to around 60 PSI. I did notice the flame is lower though so I imagine it's running out.


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## lbdavis (Apr 23, 2007)

Did anyone see the write up PS did on this little guy?








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You can see how much propane you have left. They are DOT approved and made by two different manufacturers. One is slightly larger than a typical 20lb-er and the other slightly smaller.

PS had nothing bad to say about them.


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## sailingdog (Mar 19, 2006)

The composite propane tanks are pretty cool...available in 10, 20 and 30 lb. sizes. The 10/20 lb. tanks are about $100, but not legal for use in many countries, including Canada IIRC.


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## saltypat (Oct 8, 2006)

*Kudos for all!*

Sailors.

Well, a big THANK YOU for all the propane tank info. Previously I always used alcohol.

It was all very interesting and enlightening, and complete.

I really appreciate the knowledge sharing on this site.

Also nice is the feeling of a sailing community with a positive social impact - for example, I have seen very few mean messages posted among participants. You can usually log off sailnet feeling a little better about the world.

Thanks again, SaltyPat


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## chucklesR (Sep 17, 2007)

I have the fiberglass tanks and I'm here to tell you being able to look and see the liquid level is the best way and only reliable way to tell how much is left. I've got a guage and it is absolutely meaningless.


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