propane gauge

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pedataa136

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Aug 2, 2009
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Hi folks.Does anyone know if and where I can get a gauge to check propane capacity in my tanks?My old class c had a read out monitor but my fancy tt does not.I'm thinking something that I can just screw on and off.Thanks,Steve.
 
The ones you screw on and off won't do any good.  They just measure tank pressure.

Straight gasses like oxygen, nitrogen, etc. are stored in gas form under pressure, so measuring the tank pressure tells you how much gas is in the tank.

However, propane is stored in liquid form with a vapor pocket on top.  When you pull some vapor, some of the liquid boils into vapor to take it's place.   Thus the tank pressure stays constant until all of the liquid is gone.   Since liquid propane is 200 times as dense as vapor, the tank pressure (and the meter reading) won't drop until the tank is completely empty.

You need a gauge with an attached float that resides inside the tank to get an accurate reading.  This will involve having an accredited propane supplier replace the existing valve with one that includes the float and gauge.

There are other ways to determine how much propane remains in the tank.   The easiest is to pour about a cupful of hot water down the side of the tank.   The liquid acts as a heat sink so the tank above the liquid will be warmed by the water, while the metal below the liquid line will remain cool.    Run your hand down the side and you'll feel the difference in temperaure.  You can also buy a liquid crystal strip that attaches to the tank and works the same way - when you pour hot water on it the portion above the liquid warms up and changes color.

You can also weigh the tank.   Every tank has a tare weight stamped on it - that's the weight of the empty tank.   Subtract this from the total weight of the tank and you'll get the weight of the propane inside.   Divide this by 4 and you'll get the number of gallons remaining.
 
Your old Class C had a permanent mount tank with a remote electric gauge attached to its internal float gauge. The portable tanks on your trailer do not have any way to attach a read-out to the internal float valve, so no accurate gauge. A screw-on pressure type gauge will give you a ball park figure, though. 

Most trailers have dual tanks, so running one out is a non-issue. If you don't already have it, you can add an automatic cross-over regulator, that automatically switches from the empty tank to the full one.
 
As Gary mentioned, if you have two tanks there is usually a cross over valve that will switch automatically. However, if you do not check the indicator occasionally to see if one tank is empty, you could run both out of propane. My suggestion is to keep one tank turned off and wait until the first one is empty. Then switch over and remove the empty tank to refill. Be sure to switch over first before removing the empty tank. If you don't the switch will not take place and the full tank will leak out of the open hose.
 
Thanks guys.Learn something new on this site everyday.Literally.Have cross over gauge,indicator light,all that.Just thought having on demand checking capability would make life a little easier.After a couple of trips tend to lose track.Getting a little forgetfull.
 
The screw-on pressure gauges are enough to give you a clue as to remaining volume. I wouldn't trust them much below 3/8 of a tank, but that's enough to tell you to be looking out for a place to refill. You don't have to run a tank dry to refill it and you can run off the second tank while you do so.
 
What I learned from propane while working for a gas company (I never got certified, just memory), matches exactly what Lou explained.  So if the pressure is the same until all the liquid has boiled/evaporated to vapours, how can a screw-in gauge be accurate, it will show full until there is no more liquid, then will drop real quick.

While it is a different gas, I have noticed the same thing from my CO2 tank.  It's at 850 PSI until all the liquid had boiled/evaporated to vapour. Then it goes from 850PSI to 0PSI rather quick.  Sucks to run out of CO2 in the middle of a keg in the middle of a nice evening with friends.

 
The internal tank pressure does not seem to stay the same from 0-100% full, though I cannot explain why that would be except to say that few gasses are "perfect gasses" in the Gas Law (physics) sense. In my experience with this type of gauge, it reads "full" for a long time and then starts to fall and drops fairly rapidly after that.  That much is consistent with theory - the pressure stays pretty much the same until the tank gets fairly low -  but then it starts to drop off some.  That's why I said it gives a clue rather than a reliable reading.
 
RV Roamer said:
The internal tank pressure does not seem to stay the same from 0-100% full, though I cannot explain why that would be except to say that few gasses are "perfect gasses" in the Gas Law (physics) sense. In my experience with this type of gauge, it reads "full" for a long time and then starts to fall and drops fairly rapidly after that.   That much is consistent with theory - the pressure stays pretty much the same until the tank gets fairly low -  but then it starts to drop off some.  That's why I said it gives a clue rather than a reliable reading.

Gary,

If propane usage is static, i.e. no propane is being used, the pressure should remain constant regardless of the amount of propane in the tank. However, if you start using it, a tank which is very low will experience significant cooling as the the propane vaporizes. This decrease in temperature decreases the evaporation rate and hence the effective pressure in the tank. If the tank is full, evaporation does not reduce the temperature of the liquid to a significant degree and the tank pressure remains relatively constant for the "ambient" temperature. This is why in very cold temperatures, one should keep the tank relatively full. In a nearly empty tank the liquid contents would cool further due to evaporation and may not even produce enough pressure to provide the desired propane vapors (pressure).

Using this cooling effect I measue the amount of liquid remaining in my propane tank on my grill. I use an infrared remote-reading thermometer to determine the level of the liquid in the tank. This only works after having the grill on for a period of time. It takes much longer to get a consistently measurable temperature difference when the tank is full vs when it is getting relatively low. Producing a certain amount of vapor produces the same amount of cooling, however the smaller the heat sink (low liquid level) the greater the decrease in temperature.

 
There are a few different types of propane gauges

one is the "in-line" guage, it normally has a red/green dial, if it's green there is liquid in the tank,, RED = No liquid in the tank or.. No liquid in the SELECTED tank if you have an auto-change over, valve in front of your regulator

The next type is a tank mounted gauge.. This is usually a float, and how accurate it is.. I can't tell you.. floats in theory are dead on but in practice... Not always

Next is the magnetic liquid crystal thing.. (or adheasive) you slap it on, pour boiling water over it (Heat the water with propane from the tank, this improves performance), and the color change IS the level of liquid in the tank.. No question about it.. NOTE: my dad used to "Sound" the tank (Thunk it and note the level at which the sound changes. on the big 100lb bottles at home.. About the same accuracy.. Touch can be used too (The liquid spot is much cooler than the gas above it)

Next we have a fairly new/old entry in the field.. Scales

When I re-filled those old 100lb bottles at the sore.. They used to scale them.  (Very old beam balance)

Today, there is a company that makes a stress-guage, scale.. This "Gauge" weighs the tank, if you know the tank weight (programed in when installed) and there is nothign that is affecting it (Like ahold down bracket) it can tell you exactly how many pounds of propane you have left.

But the tank mounted "Screw in" gauges I've seen were all float type.  Kind of rare on bottles, but at least one company makes bottles with 'em.
 
Stick around a while, and you'll see more of it (them).
 
John,
The type of gauge we are discussing is for portable (DOT) propane tanks and looks like this:

Pressure-type  inline Propane Gauge

It works on the assumption that the vapor pressure in the tank decreases as the volume declines.  It was alleged here that the vapor pressure is exactly the same whether the tank is 10% full or 80% full (and the Gas Law suggests that should be so), but practical observation indicates otherwise. It's not real accurate, but the gauge does decline as gas is used from the tank. Apparently propane is not able to maintain enough vapor pressure to keep the same psi at 20% as it can at 80%. Richard has a possible explanation in his post.

Science also told us that bees can't fly, but they seem to do an excellent imitation of it. When our brains reach a conclusion at odds with the observed facts, it merely means that we simply do not understand all the factors involved.
 
Digital fish scale 50 lb $17.96 Walmart.  Who knows you might end up around Ocala  someday and need it for a fish.
 
Thanks Gary, As I said a "Red Green" dial (Small slice of yellow as the last of the liquid evaporates and the pressure in the tank starts to drop)

So it says "you have gas" but no way can it tell how much

(in short. I agree with you, the pressure does not drop till the tank is dry)

Seems we both read the same science books :)
 
John From Detroit said:
As I said a "Red Green" dial...

For a second, I thought you were talking about Red Green himself.  He'd make some sort of contraption with duct tape and spare parts to check how much propane is left, if it does not blow up first.
 

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