A method of monitoring an analogue signal output from a sensor comprising: comparing the amplitude of the analogue signal to a first threshold pair using a first comparator, wherein the first threshold pair comprises a first high threshold and a first low threshold; setting a primary monitoring signal using the first comparator, to a first value when the amplitude of the analogue signal exceeds the first high threshold; setting the primary monitoring signal using the first comparator, to a second value when the amplitude of the analogue signal decreases below the first low threshold; communicating the primary monitoring signal on a first monitoring channel; comparing the amplitude of the analogue signal to a second threshold pair using a second comparator, wherein the second threshold pair comprises a second high threshold and a second low threshold and wherein the second high threshold is greater than the first high threshold and the second low threshold is less than the first low threshold; setting a secondary monitoring signal using the second comparator, to a first value when the amplitude of the analogue signal exceeds the second high threshold; setting the secondary monitoring signal using the second comparator, to a second value when the amplitude of the analogue signal decreases below the second low threshold; communicating the secondary monitoring signal on a second monitoring channel; switching the role of the second comparator such that the second comparator sets the value of the primary monitoring signal and communicates the primary monitoring signal on the second monitoring channel; switching the role of the first comparator such that the first comparator sets the value of the secondary monitoring signal and communicates the secondary monitoring signal on the first monitoring channel; comparing the primary and secondary monitoring signals by comparing times at which pulses occur in the primary monitoring signal with times at which pulses occur in the secondary monitoring signal; and determining from the comparison whether an error exists with the monitoring of the analogue signal. comparing the amplitude of the analogue signal to a first threshold pair using a first comparator, wherein the first threshold pair comprises a first high threshold and a first low threshold; setting a primary monitoring signal using the first comparator, to a first value when the amplitude of the analogue signal exceeds the first high threshold; setting the primary monitoring signal using the first comparator, to a second value when the amplitude of the analogue signal decreases below the first low threshold; communicating the primary monitoring signal on a first monitoring channel; comparing the amplitude of the analogue signal to a second threshold pair using a second comparator, wherein the second threshold pair comprises a second high threshold and a second low threshold and wherein the second high threshold is greater than the first high threshold and the second low threshold is less than the first low threshold; setting a secondary monitoring signal using the second comparator, to a first value when the amplitude of the analogue signal exceeds the second high threshold; setting the secondary monitoring signal using the second comparator, to a second value when the amplitude of the analogue signal decreases below the second low threshold; communicating the secondary monitoring signal on a second monitoring channel; switching the role of the second comparator such that the second comparator sets the value of the primary monitoring signal and communicates the primary monitoring signal on the second monitoring channel; switching the role of the first comparator such that the first comparator sets the value of the secondary monitoring signal and communicates the secondary monitoring signal on the first monitoring channel; comparing the primary and secondary monitoring signals by comparing times at which pulses occur in the primary monitoring signal with times at which pulses occur in the secondary monitoring signal; and determining from the comparison whether an error exists with the monitoring of the analogue signal.