How does an LPG tank gauge work?

[karamss]

Hi
I have read that the propane gas tank gauges that rely on pressure are useless because as you use the tank part of the liquid propane turns to gas raising the pressure again inside the tank.

What id like to know is then how do these gauges work?

And also I would like more detail on how propane gas work inside propane tanks.
Like when pressure is low in tank how long does it take for liquid propane to turn to gas? Is there a time window?

How much is one cubic ml of liquid propane in gas form?

Any help or references are greatly appreciated.

 

[DaveC426913]

I have read that the propane gas tank gauges that rely on pressure are useless because as you use the tank part of the liquid propane turns to gas raising the pressure again inside the tank.

At a given temperature the vapour pressure of propane will be approximately constant.
When you use up gas, some of the liquid propane will vaporize until the vapour pressure re-stabilizes.

What is problematic with a gauge is that vaporizing propane is endothermic, meaning it steals heat. This cools the tank, resulting in a different vapour pressure.

But if you let the tank warm up to a standard temp again the gauge will be accurate.

 

[karamss]

At a given temperature the vapour pressure of propane will be approximately constant.
When you use up gas, some of the liquid propane will vaporize until the vapour pressure re-stabilizes.

What is problematic with a gauge is that vaporizing propane is endothermic, meaning it steals heat. This cools the tank, resulting in a different vapour pressure.

But if you let the tank warm up to a standard temp again the gauge will be accurate.

Thanks for the reply dave! Two points
How long does it take for the particles to cool down?
And if the pressure stays constant (after stabilizing) how does the gauge figure out the amount?

 

[hutchphd]

What is problematic with a gauge is that vaporizing propane is endothermic, meaning it steals heat. This cools the tank, resulting in a different vapour pressure.

It seems to me that what is problematic is that vapor pressure is independent of volume or liquid surface area. It does depend upon T but why does that help? The only thing it will register reliably is an empty tank.
I quess at a high usage rate the T (and hence the vapor P) will fall at a slightly lower rate for a full tank, for a while...
Do they really make these?

 

[Averagesupernova]

A level gauge in a propane tank does just what is says. It measures the level. Does so with a float. I have one apart right now. I will try to remember to take some pix.

 

[hutchphd]

A level gauge in a propane tank does just what is says. It measures the level. Does so with a float. I have one apart right now. I will try to remember to take some pix.

This is what the OP is talking about I think

https://www.amazon.com/dp/B07ZMH7VR2/?tag=pfamazon01-20I quess I am not surprised. There are air pumps for pressurizing soda bottles, too

We should call all such inutile products "placebo devices"

 

[Averagesupernova]

My guess is all that pressure gauge is good for is saying you have plenty of gas until you have almost none.

 

[Tom.G]

Several years ago my wife and I were trailer-camping with a relative. We were in the mountains with nice daytime weather but the nights were definitely cold. The bedroom was a pop-up with canvas walls, meaning the inside temperature approached the outdoor temperature... it felt like the canvas was transparent to infrared.

There was a central propane heater that was fired up at night. Trouble was it cycled On and Off all night, never staying on long enough to get the place comfortable.

After a few nights and some investigation, we concluded the outdoor temperature was low enough that, when the propane was used, the tank temperature, and pressure, dropped. It dropped enough that the safety shut-off in the heater would shut down; when the tanks warmed up the cycle would repeat.

The owner was one of those people that didn't particularly believe in understandable causes and was also comfortable at lower temperatures.

We finally moved to a motel for the rest of the vacation.

Subsequent online research found the obvious (and well-known) solution: Put a fireproof blanket over the propane tanks and include a 60 Watt under the blanket.

So yes, low temperature most assuredly reduces propane tank pressure down to the point of uselessness.

Next time we'll know better.

Cheers,
Tom

 

[karamss]

A level gauge in a propane tank does just what is says. It measures the level. Does so with a float. I have one apart right now. I will try to remember to take some pix.

Thanks for the reply avgsup. I think what you are talking about are tanks with built in floater.
The ones i am talking about use pressure. You install them at the nozzel.

 

[Averagesupernova]

Thanks for the reply avgsup. I think what you are talking about are tanks with built in floater.
The ones i am talking about use pressure. You install them at the nozzel.

See my post #7.

 

[karamss]

This is what the OP is talking about I think

https://www.amazon.com/dp/B07ZMH7VR2/?tag=pfamazon01-20I quess I am not surprised. There are air pumps for pressurizing soda bottles, too

We should call all such inutile products "placebo devices"

Yes that is it. They are everywhere and claim to gauge your gas level :/

See my post #7.

Yes my guess is as good as yours too. By the way is your tank size is 20lb? Because i think not all tank sizes or type have the bulit in float gauge.

 

[Averagesupernova]

Tanks I refer to are spec'd in gallons. Larger stuff. Large truck chassis, tractors, etc.

 

[russ_watters]

And also I would like more detail on how propane gas work inside propane tanks.
Like when pressure is low in tank how long does it take for liquid propane to turn to gas? Is there a time window?

Here's a graph of saturation pressure vs temperature (3 graphs in different units):
https://www.engineeringtoolbox.com/propane-vapor-pressure-d_1020.html

If we're doing IP units, you can see at 30C, the pressure is around 10bara and at -20C it is around -2bara. Now, while that's a huge difference, the pressure is regulated down to a small fraction a bar. So it should still be ok unless the flow rate needs to be pretty high. But there is an equilibrium where the boiling rate has to equal the flow rate, so the temperature of the liquid will drop until the heat flow is enough to boil it fast enough. For unusually cold weather (or just Alaska or Canada in winter), it can get cold enough that the pressure drops close to atmospheric and you'll lose flow. But it would need to be about -45C to lose pressure completely.

This brochure has a ton of info including vaporization rates for different temperatures:
https://www.bergquistinc.com/sites/default/files/academy_guides/Handbook.pdf

...though that is for larger tanks. These are in IP units, so to pick one: at 30F/-2C ambint, it is 135,000 BTU/hr or 40 kW. But that's for a 150gallon tank. If we scale down to a standard grill/camper tank, it should be about 1/10th that, if I did the surface area to volume calc right. 13,500 BTU or 4 kW would be starting to become problematic for a grill or camper heat in winter. I don't know what temperature the propane drops to at that; that depends on the heat conduction of the tank itself.

How much is one cubic ml of liquid propane in gas form?

Google tells me the "expansion ratio" is about 270:1 and probably room temperature conditions.