[en] The Harwell Carbon Meter (HCM) provides a continuous indication of the thermodynamic carbon activity in liquid sodium. This information is important with respect to the carburisation and decarburisation of steel components of the sodium circuits of sodium cooled Fast Reactors. A description of the HCM is given. It has been shown to respond rapidly to changes in carbon activity, to give an output which is in satisfactory agreement with spot calibrations using nickel 'tabs' and to operate reliably without failure for long periods. (author)