Understanding LP tanks and usage?

[MikeBob]

I know there are some experts here on LP tanks/volume ect, since reading in a few other threads here.
I have a LP heater in my garage shop, a Mr Heater Big Buddy, running on a 20lb tank.
http://www.mrheater.com/big-buddy-portable-heater.html
To my question, I have a gauge on the line at the tank. this one. Click on the pic for an enlargement and better detail to my question.
https://www.amazon.com/gp/product/B000AM...=UTF8&th=1
Why would I get more gas out of the tank on a cold day, then a not day if the tank is outside?
Thanks, MB

[TDKPE]

Why would I get more gas out of the tank on a cold day, then a not day if the tank is outside?

You don't.  The gauge reads the internal pressure while gas is being used, and that pressure depends on the ambient temperature, and the rate the gas is used.  The liquid vaporizes more readily at higher temperatures (since gas has to absorb external heat), so the gauges show different ranges for green, yellow, and red depending on the ambient.

But the amount of gas contained in the tank is the same regardless.  Only the rate it can vaporize changes, based on the level of the liquid in the tank (more liquid allows faster heat absorption), and the ambient temperature (ditto).

[therick34]

For what it's worth, those gauges are estimates, and not often good ones.

The best option if you really want to know what is in the tank is a scale and they do make one for 20 lb tanks. The tare weight is stamped on the collar of the tank and the OPD valve on the tank limits it to filling at 20 lbs, so there's not much room for error when using a scale.

Rick

[MikeBob]

Thanks both, ya I know about the weight thing, just dont have a scale to use.
So the gauge is just relative? On a hot day, I would get every drop out of the tank, a cold day some would be left in it, if I got that right.
I was just trying to get a handle on it. Where I fill them, they inspect, weigh and fill and only charge for what they put in, not a flat fee for a 20lb tank. Nice, every 6th fill is free with there "friendly fill" card.

[blackhat]

On a cold day, you could still empty the tank. You just may not be able to maintain the pressure your appliance requires.

[JimReed@Tallahassee]

These little propane tanks are handy but can be a pain when they run out. As said, weighing is the only accurate way to determine the remaining product. My BBQ tank seems to give up the ghost at very inconvenient times so I keep a spare handy. This two tank rotation system takes all of the anxiety out of the propane tank mystery.

[fredhargis]

These little propane tanks are handy but can be a pain when they run out. As said, weighing is the only accurate way to determine the remaining product. My BBQ tank seems to give up the ghost at very inconvenient times so I keep a spare handy. This two tank rotation system takes all of the anxiety out of the propane tank mystery.

That's the way I do it as well. I have one of those gauges and found it to be, well, useless. The last time I filled my tank I was talking to the guy at the pump about the gauge...he said it was amazing how many tanks are brought in to be filled that only take 1/2 gallon or so of LP....all due to those gauges. That would be a real screwing if you do the tank exchange thing.

[Roly]

These little propane tanks are handy but can be a pain when they run out. As said, weighing is the only accurate way to determine the remaining product. My BBQ tank seems to give up the ghost at very inconvenient times so I keep a spare handy. This two tank rotation system takes all of the anxiety out of the propane tank mystery.

 Mine too.  It only seems to run out when I am using it.
  Use the two tank system also.   Roly

[MikeBob]

I do have 2 and I dont do the bottle swap. I have a place a 1/2 mile away that weighs and fills them, and charges by weight of what they put in.