A method of adaptively sampling data to determine the start of injection in a solenoid actuated valve of a fluid injector includes, in an operating cycle or portion thereof of the valve, sampling the signal of current through a solenoid of the valve at sampling points having a pre-defined interval therebetween. At each sampling point, determining the value of the first derivative of current and detecting the sampling point at which the first derivative achieves a maximum as the start of injection. Values of the first derivative of the sampling points immediately preceding and immediately following the start of injection are determined. In a subsequent operating cycle, synchronisation of sampling is altered to shift sampling times depending on the values of the first derivative of the sampling points immediately preceding and immediately following the start of injection.