Which type of gas should I be using?
What is LPG and how does it work?
Liquefied Petroleum Gas (or LPG) is commonly used in caravanning for cooking, heating, and where no other power source is available, running the fridge. Petroleum gas is created as part of the process of refining crude oil - it is collected and compressed into a liquid before being bottled. The LPG will remain in its liquid form for as long as it is held under sufficient pressure. If that pressure is lowered (i.e. some gas is drawn off) an amount of the liquid will boil into gas until the pressure is restored. In some applications, the LPG is drawn from the bottle and used in its liquid state, however for most (including caravan gas appliances) it is allowed to boil and then used in its gaseous state.
What are the safety considerations?
As with any flammable gas, there is a serious risk
of explosion in the event of an LPG leak. As the gas itself is colourless
and odourless, a small amount of 'stenching agent' is added so that
the leak is likely to be detected by smell.
Gas bottles should always be stored upright, so that the valve is not submerged in the liquid. The reason being that a liquid LPG leak is considerably more serious than a gas leak, as the liquid converts into very large volumes of gas.
While it is not actually illegal to tow a caravan with gas appliances lit, it is illegal to enter certain areas (such as tunnels or petrol station forecourts) with any kind of naked flame. In any case, towing with gas appliances lit can only increase the risk of a serious fire or explosion in the event of an accident. The caravan fridge should be capable of running from 12 volts when attached to the car - if your car does not have this facility, it would be better to have it fitted than to run the fridge on gas while towing. The gas installation should be serviced annually by a qualified engineer, and any compartments containing gas bottles, pipes or appliances must have their bottom vents kept clear. LPG is heavier than air, and a leak will result in a dangerous build-up of gas if there is not adequate floor-level ventilation.
What types of gas are available.
For the caravanner, two types of LPG are generally available - Propane and Butane. Propane canisters are invariably red, and use a universal type of connector. Butane canisters vary in colour from supplier to supplier, but are most commonly blue. The connector for Butane also varies between manufacturers, with some (for example, Calor) using different types of connector on different sizes of bottle. In each case, the gas pressure inside the bottle is much too high for the pipes and appliances inside a caravan, and so a regulator must be fitted to the bottle. Regulators are available both as 'high pressure' and 'low pressure' models - for a caravan application, it is vital that you select a 'low pressure' model (28 mbar for Butane and 37 mbar for Propane.)
Which gas should I use, Propane or Butane?
The physical properties of the two gases are very similar, and when regulated to the correct pressure, they will perform almost identically. However there are some important differences.
Of the two gases, Butane has the most advantages.
It is less toxic and so can legally be used and stored indoors.
Litre for litre, it contains around 12% more energy than Propane and so you can squeeze more running time into the same sized bottle. (Butane is heavier than Propane though, so weight for weight it's a pretty close call.)
Butane also burns cleaner than Propane (although this isn't normally a serious issue in caravanning.)
Finally, while it's not strictly a property of the gas, Butane canisters generally use clip-on type connections. These are far more convenient than the Propane screw type connections, especially if you swap bottles around regularly (as you might if you also use your caravan bottle to run a barbecue.)
Conversely, Propane has only one advantage over Butane - but it's a big one!
In order to be usable, the liquid in the bottle must
be able to boil into a gas.
In the case of Butane, this will happen at any temperature above -2C, whereas with Propane, this figure is much lower, at -42C.
In the real world, it's not so clear cut. Whenever some of the liquid boils into gas, the remaining liquid cools. It is therefore possible for the temperature of the liquid to drop to several degrees below ambient. This can easily prevent a Butane canister from producing a useful gas supply, even when the outside temperature is several degrees above 0C. A compromise can be reached by mixing Propane with Butane, but as far as I'm aware, none of the UK 'big bottle' suppliers actually do this. The small gas cartridges that are produced for camping stoves and gas lamps are often Propane/Butane mixes. So choosing the right gas pretty much boils down to whether you need to use it in freezing (or near freezing) conditions. If this is likely, then Propane is a must. If not, then Butane has the edge.
How do I tell how much gas is in the bottle.
Due to the need for strength, gas bottles can easily weigh as much as the gas that they contain. This makes it very difficult to judge how much gas is left by handling simply by feeling the weight of the container. There are, however, several ways that you can find out.
Weighing the bottle.
Most gas bottles have an empty weight stamped somewhere on the bottle. In the case of Calor bottles, it is normally on a metal disc around the valve. While Calor bottles are sold by the kilogram, the empty weight is still in LB's and oz. So, first you need to convert the imperial measurement into metric. Once you've done that, simply weigh the bottle on your bathroom scales and subtract the empty bottle weight. You should now have a good idea of how much gas is left.
Sloshing the bottle around and listening to the amount of remaining liquid.
A little less scientific, but it can give a good indication. Once
you have an ear for it, you can at least get a good idea of when you're
about to run out.
If the ambient temperature is cool enough, and you have been drawing gas from the bottle continuously for some time, condensation may form on the outside of the bottle. The height at which the condensation stops gives a fairly accurate indication of the amount of liquid remaining.
The Truma Sonatic system.
As far as I'm aware, this is the only device that actually gives you a true reading of the contents of the bottle. A transceiver unit beneath the bottle sends and receives ultrasonic pulses to sense the physical level of the liquid. Unfortunately, technology such as this comes with a pretty hefty price tag. You have to decide whether it's worth around £100 just to know how much gas is left in your bottle.
Also worth a mention:
The GasLow gauge.
While this device doesn't tell you how much gas you
have left, it can tell you when you're getting close to running out.
The GasLow is available as a gauge that screws behind the regulator, or as a gauge and regulator combined. When fitted, it monitors the pressure of the gas inside the bottle, giving an idea of how well the bottle is keeping up with demand. The problem is that even a very low bottle will reach full pressure while no gas is being drawn, and once you start drawing the gas, it takes considerable time for that pressure to drop. For that reason, the GasLow's reading is only meaningful once you've been drawing gas from the bottle continuously for several minutes. This works well with barbecues and such, as you can keep an eye on the gauge while you're using the appliance.
With the gauge stuck in the gas locker though, you're likely to run out of gas without spotting the warning. The GasLow doubles as a basic leak detector, allowing you to prove that your system is capable of retaining pressure, even with the gas supply switched off. You may consider it worth buying just for this.