In case anyone is interested I checked some details on my 7.3 gallon LP tanks.

Empty they weigh 25.3 pounds. Full they weigh 55.3 pounds

Propane is approx 4 pounds per gallon. When needed now you can get an exact measure of remaining LP with a fish scale. Just weigh the tank subtract 25.3 for the tank and divide the remainder by 4 to know how much you have left.

What I find iratating is taking a half empty bottle in for refilling and they charge you for a full tank. I try to make sure mine are empty. I always carry three bottles plus a 100 lb bottle for winter.

I've never had a dealer charge for a full tank when they just partially refill it. All of them around here have either a gallon meter or a scale.

I just put a gauge on my RV tank. It only cost $12 at Home Depot and that's a lot easier than taking the tanks off to check them.

During the last oil boom in the Permian basin, all the propane dealers were charging by the tank, not buy gallons. It was $25 - $35 per 7 gallon tank. You paid the young girl inside then got in a line (usually 4 or 5 people deep), show your receipt to the guy operating the pump and filled your tanks. They must of made a fortune because west Tx looked like a used RV parking lot. Ran into the same problem in eastern Ohio. Solved that by renting a 200 lb tank they would deliver too in RV parks.

That sounds like a bigger screw job than the high priced Blue Rhino exchange tanks.

How accurate is that gauge, I've not seen one that showed much better then it had fuel or was empty! Because the volume of the tank is liquid, and the gauges measure pressure not fuel level. They usually go from having some level of fuel because the pressure is still there to empty in minutes when the fuel is gone and the pressure drops.

Knowing the weight is the only way of externally knowing the level without an internal float gauge. However I think that a pressure gauge has merit for determining if you have any pressure at all. The other issue will be seeing far more pressure above 80 deg then you will below 40 deg. This will make the reading quite complicated to make sense of. You could be very low in the tank volume, with an outside temp of 90 deg still showing high pressure on the gage. It's not apples and apples trying to determine the amount of liquid by measuring the amount of pressure.

MY camp trailer has,..... and I think many newer systems have a two tank system with an automatic pressure sensor that switches tanks when the pressure is low in the first one. I wonder of you can put the gauge on both tanks or will it read the pressure after the auto switch? I'll have to look into it. Thanks for the idea.

I just put on a few weeks ago and haven't used it much at all so I don't know yet how accurate it is. I agree that weight is far more accurate but my bottles are a pain to take off and weigh. Mainly I just want to know when the 1st one is about empty so I can switch without having to bleed the gas through the lines if I run out.

Hey look here, this is the ticket, however I think it will be a struggle to fit under the tank and within the steel bottom tank framework.

http://store.tvlintl.com/index.php/gaswatchtm/smart-gaswatchtm-scale-model-gw214.html

It may also have a problem with the clamp down bracket which possibly pushes the tank down in addition to the weight of the tank.

The FAQ's say that it works only with a 20lb tank. My 30's are too heavy. It won't work with a normal tank tie-down anyway. It's just a fancy scale.

Dump some hot water on the side of the tank, you will be able to feel the level of lp. Works even better when you have been drawing gas from the bottle for a while.

I am thinking about turning off the second tank on my RV (right now it will automatic switch if the first tank goes empty) If I do this then I will know when the first tank is empty. Of course the draw back is waking up freezing in the middle of the night or running out of gas a in the middle of cooking dinner.................

That's why I added a bbq gauge for $12. It might not be totally accurate but at least I'll know when it's getting down so I can switch.

I think the gauge has value to diagnose what is in the tank without taking it off.

My system has an auto switch over between tanks. I guess I need two.

My camper is 40 years old. There's no switchover valve between the tanks. I just turn one off and the other on. I did add a valve so I can close the main line when I take a hose off a tank so I don't have to bleed the gas all the way through the lines each time.

Just got back from a 3 day antelope hunt. As I said before, I have 2 tanks without a switchover valve between them. A couple weeks ago I put a pressure gauge on tank #1.
I was going to put a 2d valve on the tank #2 but I learned that it's not needed. If I shut off the #1 where the gauge is and open #2, the pressure from #2 will bleed over to the gauge and it will show the pressure from #2. The gauge got down in the red on #1 and when I switched tanks, it came up to the middle of the green.
So, I don't need to get fancy. A $12 gauge will tell me all I need to know. It doesn't have to be perfect.