Do you buy motor fuel in Canada - this includes gasoline, diesel, propane, ethanol, avgas & jet fuel? If so, you are affected by the industry practice of Automatic Temperature Compensation (ATC or TC). If you live in the USA, you are on the verge of major changes in how fuel is sold. Either way, you need to understand ATC. We would be interested in hearing about fuel sales in the rest of the world. Are you temperature compensated?
There is much discussion and speculation as to exactly what ATC is or is not. At the wholesale level, all fuel sales are temperature compensated. This compensation takes into account a very well understood physical principle. That principle is that an object will expand as it is heated and contracts as it cools. Although there are some notable exceptions (if you are interested, consider why ice floats on water), most objects will follow this simple rule - fuel is no exception. For this reason, wholesale shipments of fuel are temperature compensated to a base or reference temperature of 15°C (or 60°F). What this means is that on a hot day (above 15° C), the fuel has expanded and you will receive less dense fuel. Less dense fuel contains less energy by volume and you will have to consume more to obtain the same results. Conversley, on a cold day (below 15° C), the fuel contracts and becomes more dense. In these conditions, you will receive fuel which contains more energy per litre (gallon, etc.).
"So what?", you ask. Well, according to one study by the Kansas City Star, U.S. consumers spent about $2.3 billion more for gasoline and diesel in 2006 than they would have had to if the pumps had been temperature compensated. In Canada, all major retail stations, with the exception of one notable hold out, have converted to temperature compensated fuel dispensers. When you buy fuel in Canada, you will receive a litre of fuel that has been temperature compensated. What this means is that the dispenser will adjust the customer volume display to reflect the volume compensated to 15° C. The dispensers contain a thermal probe immersed in the fuel supply line close to the metering chanmber. This probe constantly monitors the temperature of the fuel and adjusts the customer volume display accordingly. The customer sees Net or compensated volume. There is also a Gross or uncompensated volume display available to the service technicians and government inspectors. It is important to understand that the meter is still measuring actual volume and only the customer display is being affected by the ATC unit.
There is much debate as to whether or not temperature compensation is a benefit to the consumer. Many organizations claim that the average temperature is not consistent with the base temperature used (15° C / 60° F). These same groups claim they would only support temperature compensation if the base temperature is adjusted to a value more appropriate for their locale. In fact, this is not a good argument and would result in an incomprehensible mess. The consumer would be faced with a compensated litre being different in each location. There would be no consistency. The temperature of 15° C is internationally accepted as the base temperature for compensation. It makes little difference if the average temperature in your locale is 15° C or not. Currently, your litre of fuel is denser in the winter and therefore, all other variables being consistent, it will contain more energy. Conversely, the fuel is less dense in the summer and, again, all other variables being consistent, will contain less energy in the summer. What compensation does is to establish the litre at the base temperature. The rate of expansion (or contraction) of the fuels based upon temperature is well understood. Compensated dispensers merely adjust the customer (Net) volume display to account for this expansion (or contraction). In fact, the base temperature, once established, is irrelevant. The concern of course is that when compensation is started, the price per unit should be adjusted to reflect the change in actual average temperature for the location. Of course, this is unlikely to happen.
Temperature compensation of a fuel dispenser ensures that extremes of temperature of the fuel do not result in significant changes in the energy content of that fuel. Only temperature is accounted for and it must be understood that different fuel grades and fuel production at different refineries and at different times of the year will also affect the energy content of the fuel purchased. Winter fuel will have a lower vapour pressure than summer fuel to ensure that it will work reliably in your vehicle.
There are many studies that draw opposing conclusions as to whether temperature compensation is good or bad for the consumer. Most of these studies contain one fundamental flaw. They look at the average ambient temperature for the location they are studying. They then assume, mistakenly, that this ambient temperature is representative of the fuel temperature. These values are used to make the calculations to support their studies. In fact, average fuel temperature at a given location is dependant upon a variety of factors. Locations close to a refinery are likely to receive warm fuel year round. Locations that have fuel trucked in over a substantial distance will have fuel that is closer to ambient temperature delivered to them. In both cases, if the fuel is delivered to underground storage, it will slowly assume the temperature of the underground storage. Most underground storage tanks vary less than approximately 15° C between summer and winter. In the summer they will be cooler than ambient while in the winter they will be warmer than ambient. Those locations that store their fuel above ground will have fuel temperatures much closer to ambient temperature. In addition, fuel travels through supply lines to the dispenser. These lines may be quite long and may be buried beneath the black asphalt of the station. In these cases, fuel in the lines and the dispenser may be at temperatures significantly different than in the storage tanks. If the dispenser was used recently, then the fuel in the lines will be close to the temperature of the fuel in the tank. However, if the dispenser has not been used for sometime, the temperature of the fuel in the lines will be closer to ambient temperature than storage temperature. In the summer, the fuel in the dispenser and the delivery lines may be very warm, even in Northern climates. This fuel has been 'baking' in the sun and has expanded significantly. The temperature of this fuel may be substantially higher than ambient temperature and in extreme cases may vapourize in the lines. Temperature compensation of fuel and fuel oils makes purchasing fuel more consistent (and therefore fair) for everyone.
It should be noted that fuel and fuel oils (gasoline, aviation gas, diesel, kerosene and stove oils) are only moderately affected by temperature. Fuels such as liquified petroleum gas (LPG or propane) are affected much more signficantly. These fuels are almost always sold compensated. If you have natural gas service in your home, this will also most likely be metered with temperature compensation. Natural gas deliveries will also be adjusted for energy content on a regular basis as there is much difference in the quality of the gas from various sources.