function. Coriolis meters can handle a wide range of flow rates and have the unique ability to output mass flow - this gives the highest accuracy of flow measurement currently available for mass flow measurement. Since they measure flowing density, coriolis meters can also infer gas flow rate at flowing conditions. American Gas Association Report No. 11 provides guidelines for obtaining good results when measuring natural gas with a coriolis meter. Heating value The volume of gas flow provided by a gas meter is just that, a reading of volume. Gas volume does not take into account the quality of the gas or the amount of heat available when burned. Utility customers are billed according to the heat available in the gas. The quality of the gas is measured and adjusted for in each billing cycle. This is known by several names as the calorific value, heating value, or therm value. The calorific value of natural gas can be obtained using a process gas chromatograph, which measures the amount of each constituent of the gas, namely: methane ethane hydrogen carbon monoxide water vapour Additionally, to convert from volume to thermal energy, the pressure and temperature of the gas must be taken into consideration. Pressure is generally not a problem; the meter is simply installed immediately downstream of a pressure regulator and is calibrated to read accurately at that pressure. Pressure compensation then occurs in the utility's billing system. Varying temperature cannot be handled as easily, but some meters are designed with built-in temperature compensation to keep them reasonably accurate over their designed temperature range. Others are corrected for temperature electronically. Indicating devices Any type of gas meter can be obtained with a wide variety of indicators. The most common are indicators that use multiple clock hands (pointer style) or digital readouts similar to an odometer, but remote readouts of various types are also becoming popular — see Automatic meter reading and Smart meter. Accuracy Gas meters are required to register the volume of gas consumed within an acceptable degree of accuracy. Any significant error in the registered volume can represent a loss to the gas supplier, or the consumer being overbilled. The accuracy is generally laid down in statute for the location in which the meter is installed. Statutory provisions should also specify a procedure to be followed should the accuracy be disputed. In the UK, the permitted error for