What is pressure at propane tanks?

[dresslered]

I'm wanted to add real pressure guages to my camper propane tanks (before the regulator) to determine remaining fuel. What is the pressure that I can expect from a full tank reading. TIA.

 

[Thomas]

About 110 psi at 70 degrees Farenheit, if the tank is 80% full or 50% full. You're looking at the vapor pressure of the liquid propane. The vapor pressure is dependant on temperature. There will always be propane in both liquid and gas states until the tank is empty. I don't think the pressure will change as long as the temperature is stable.

 

[B.G. Smith]

http://www.altenergy.com/propaned.htm Almost everything you ever wanted to know about propane. bg

 

[Thomas]

This page addresses your pressure question in fairly simple terms.

 

[dresslered]

Thanks for reminding me of my thermodynamics. I completely forgot about the liquid/gaseous states, but am curious now as to why some tanks come with cheap little indicators--must just tell you when it's empty. I've seen the little temperature strips (not too informative), but it looks like weight will be the best gauge. Thanks again,

 

[Loren]

David,



As long as there is ANY liquid propane in your tank, your pressure will depend on the temperature. You cannot use pressure as an indicator of the quantity in the tank.



This is also true for fire extinguishers that contain a liquid that converts to gas, including Halon.



The guages on some tanks use a float, usually on a twisted rod, to drive a needle on the gauge.



The temperature strips will work only while gas is being used. If the liquid is not boiling inside the tank, there won't be any temperature difference between the gaseous side and the liquid side. You can sometimes do the same thing the strips do by noticing condensation on the tank while you are using gas.



As you said, the best indicator is weight.



Loren

 

[jelag]

A standard tank..... 20 lb etc weighs about 17 to 18 lbs empty and 37 to 38 full... . so a simple pull scale,,(fish scale) would give you a good idea of how much is empty... . and if you know the BTU load of your BBQ you can figure about how much you have left... a standard gallon of propane has about 110K BTU's of energy...



Hope this helps... I'm sorry I don't have the information for the 7 gal tanks in memory... Can you tell I was caught out one night without gas... .

 

[DLBurns]

The temperature strips will work only while gas is being used. If the liquid is not boiling inside the tank, there won't be any temperature difference between the gaseous side and the liquid side. You can sometimes do the same thing the strips do by noticing condensation on the tank while you are using gas.



Loren[/QUOTE]



David,

I have used the plastic strip and consider them just about useless. I now light a burner on my stove or turn on the heater and then go out to the tanks and hit it with my Raytek Infrared thermometer. This gives me a pretty good indication of how much fuel I have left in the tank. Not perfect but easier than taking the tanks out to weigh them.

Dave

 

[dresslered]

Thanks again for all the good tips guys. Not sure what I was thinking, but perhaps when I saw some of the new tanks that came with pressure gauges, I thought that there was some meaningful data here. Basically, with the liquid/gas states, pressure is a constant (as long as temperature is a constant) until you completely run out. I see this all the time at work with my CO2 tanks (300-psi until I've depleted the liquid). I like the fish scale idea as well as the Raytek Infrared thermometer (been trying to justify one of those for a long time).

 

[plongson]

The easiest and cheepest way I have found to determine the liquid level in a propane tank is to take a pot of hot tap water and pour it over the tank and then run your hand up and down the tank to find the spot that seperates warm from cool. Above the liquid level, the steel absorbes the heat from the water and the spot below does not. VIOLA... that is how much is left.

 

[Thomas]

. . . Above the liquid level, the steel absorbes the heat from the water and the spot below does not. . .

Nitpick: I think the steel tank is heated both above AND below the liquid level. The temperature difference you notice is due to the greater thermal conductivity of the liquid propane vs. the gas above it. While the water is being poured on the tank, all other things being equal, the steel BELOW the liquid propane level will absorb more heat than the steel above the liquid level because of the liquid propanes better ability than the gas to conduct heat away from the steel.



I've used the hot-water method for forty+ years. Simple, cheap, and accurate.

 

[plongson]

Well, I'm not Mr. Wizard, I just know that it is warm above and cool down below... . And it doesn't require IR spot meters, fish scales, or costly mechanical internal floats and gauges...

 

[Thomas]

I think the hot water method is the best for most of us. I've watched this thread and wondered how long it would be before someone suggested it. As for being Mr. Wizard, your description is an excellent description of how things appear.