Apparatus for measuring the efficiency of combustion appliances

This disclosure relates to apparatus of the kind suitable for taking spot measurements of the heat loss or stack loss and/or efficiency ( eta ) in flue gases (stack loss) and comprises respective sensors (5, 3) for producing output signals which vary with the temperature and the concentration of a constituent gas e.g. O2 of the flue gases and microprocessor-based computation means (10) arranged to derive measurement values of (and numerically equal to) the measured temperature and constituent gas concentration, from the two sensors and to apply these measurement values in the computation of a predetermined formula relating the stack loss or efficiency to the measured quantities. In accordance with the invention, the apparatus is arranged to automatically calibrate each sensor signal from a test measurement prior to deriving the measurement values. This may be achieved in the case of a sensor having a non-linear response, by performing a calculation of a formula defining the non-linear response of the sensor using a coefficient derived from the test measurement. The sensor output signal is thus automatically calibrated and "linearized" from a single test measurement. The predetermined stack loss or efficiency formula may be modified for different types of fuel, and temperature and O2 (or CO2) values used in the calculation as well as the result of the calculation may be presented on a visual display (24).