Propane Bottles and Air Pressure Gauges

Propane is a liquid when it is in the tank. The pressure of the gas above the liquid depends on temperature. A pressure gauge won't tell you how full the tank is.

It's measured by weight, when it matters.

Get a second tank.

Keep your wife happy.

Some good and accurate infos for which I give you a GA.

By the way, often (always in my experience!) a pressure bottle has a Tare weight, this is the empty weight of that particular bottle. A Bottles weight can vary slightly from bottle to bottle due to manufacturing tolerances.

If you get a mechanical weighing machine (old fashioned type which doesn't switch itself off!) they often have an adjusting ring to set "0" accurately. I have noticed that these often have an extremely wide adjustment range, you may find it possible to "Null" out the Tare weight so that only the gas weight will be shown.

Another slightly better way, if that adjustment is not enough is to take the scales apart and turn the scale to achieve the same effect, to a new position....but do set it to a known position that is close to the tare weight of the bottles.

For example:-

all bottles have a Tare weight between 18.5 and 20 lbs. Then set the scales to 18lbs and dial in the difference for each and every bottle each time it is changed.....its easy...

For anyone wanting to read more about Tare there is a short Wikipedia dissertation at:-

Tare weight

Bathroom scales are made to weigh 200 lb or more. They will work for a propane tank, but are not very precise.

Go to an RV store. They have spring scales made to tell you how much propane is left in the tank.

The stick-on strip in another comment is also good, but only works when gas is being drawn off.

.... also here is another way of indicating level for about $10, self-adhesive version of water on the bottle using temperature difference for liquid/gas level.

http://www.gasgauge.co.nz/shopping_cart.php

The original poster needs to realise that this particular gauge, because it depends upon differential temperature for its indication, will only work while propane is being drawn from the bottle, or for a short while afterwards until the temperature of the gas and the temperature of the liquid become equal. It won't indicate if the bottle has been in storage for a long time, as there will be no difference in the temperatures of the liquid and the gas. The temperature difference arises while the bottle is being used because there is an energy change on evaopration, which is expressed as the Latent Heat of Evaporation. It is a feature of all sub-critical liquid/vapour systems.

If a tall propane bottle is being drawn on for a long time, a frost can sometimes develop on its outside lower parts where the inside is in contact with the liquid, which is another possible indicator and indicates the principle at work with the above self-adhesive gauge.

Weighing the bottle and comparing the weight with that of an identical empty bottle is probably the most reliable indicator.

In my experience the tapes are poor unless discharge rate is high generating gas/liquid temperature difference

You could suspend the bottle from a cheap spring weigh scale & mark it at the tare weight or a little above . Marked a little above the tare weight would ensure you change out before empty This would need a flexible hose to allow weight only change to be detected

The pressure of a bottle of propane is independent of its volume provided there is liquid propane remaining in the bottle, because the propane in the bottle exists in a liquid/vapour equilibrium. Drawing off some vapour will lower the pressure, and the temperature a little bit, and some liquid will evaporate to maintain the equilibrium. When outside influences have recovered the bottle temperature, the pressure will be the same as before the vapour was drawn off, though the liquid level will be lower.

Please be aware that not all pressure gauges are suitable for all purposes. In particular, if there is any prospect of the substitute gauge venting, it is unsuitable for use on fuel gases: a leak of propane vapour will eventually find a source of ignition with potentially explosve results!

All air pressure gauges have a right-handed thread. Please be aware that some gas bottles use a Left-hand thread for fueL, and a right-handed thread simply will not fit the thread on their bottles.

One reasonably accurate way to know if the bottle is coming to an end is to give it a tap with a spanner. The sound of an empty bottle is quite different from a full one. If you tap it at different levels you get a different ring. With a bit of experience, you get to know if its coming to an end. But best is to have a spare bottle next to the one in use. The propane should not be stored in the kitchen. We used to keep it in the utility room next door with lots of ventilation. There was enough room for a second bottle.

For camping, there is a good system that switches automatically from an empty bottle to a fully one.

It indicates clearly which is the empty one.

It allows replacement of the empty one while the full one is being used.

It switches back to the replaced bottle when the present full bottle is empty.

It just needed checking from time to time and any empty bottle replacing......

I did not notice where you live in the world or I might have been able to give you slightly more accurate infos......

Hi Andy,

I found this http://151.1.209.147/Reca/sh_doc.asp?ID=20863 which is the sort of thing you are referring to. I found one for $38.

Chas

Instead use a simple weighing m/c . Find the difference in Weight from Tare Wt painted on Bottle.

Store the bottle on a bathroom scale

Yes, bathroom scale could be the best for this purpose as well.

Discussing a simple physics fact here is worthwhile.

Immaterial of level, the pressure at gas space inside the bottle will stabilize to a particular pressure. This pressure varies with liquid to liquid and prevailing amibiant temprature. Once the liquid in the bottle gets empty and the bottle is only with gas, the pressure would start reducing there later.

This is true for any liqufied gas.

Thanks to everyone who answered my question.