Although this means that measurements can be taken more quickly and continuously compared to off-line techniques, much less physical information on the SoH of the PEFC is realised with each measurement. Therefore, computational models are integrated to assist in the diagnostic procedure, improving the accuracy of SoH estimations and providing meaning to the sensor data . On-line techniques can be classed as model-based (white-box), data-driven (blackbox), or a combination of the two (grey-box).