A sensor device for monitoring rotational machinery when attached thereto, the rotational machinery having a motor comprising a rotor and a stator, the sensor device comprising: - an outer enclosure with an outer surface; - a measuring circuit comprising a first sensor element and a second sensor element configured to measure a field during rotation of the rotor; - a data processor arrangement comprising one or more data processors and being configured to be in communication with the measuring circuit for receiving measured first field signals provided by means of the first sensor element and measured second field signals provided by means of the second sensor element; - one or more signalling units in communication with the data processor arrangement, wherein the data processor arrangement, the measuring circuit and the one or more signalling units are enclosed by the outer enclosure, wherein the data processor arrangement is configured for processing input based on the measured first and second field signals (432, 442) according to a predefined set of rules, and configured to provide an output based on the processing, the output indicating a machine state selected amongst a plurality of machine states, wherein said plurality of machine states includes information of the rotational direction of the rotor, wherein the one or more signalling units are configured for reflecting the selected machine state by means of one or more visual and/or audio signals based on the output provided by the data processor arrangement, wherein the first sensor element and the second sensor element are adapted to measure on the same field type and are arranged in mirror symmetry about a first axis and a second axis being perpendicular to the first axis, wherein the sensor device is configured with a mounting area arranged on the outer surface adapted for being attached onto a non-modified surface of the rotational machinery. - an outer enclosure with an outer surface; - a measuring circuit comprising a first sensor element and a second sensor element configured to measure a field during rotation of the rotor; - a data processor arrangement comprising one or more data processors and being configured to be in communication with the measuring circuit for receiving measured first field signals provided by means of the first sensor element and measured second field signals provided by means of the second sensor element; - one or more signalling units in communication with the data processor arrangement, wherein the data processor arrangement, the measuring circuit and the one or more signalling units are enclosed by the outer enclosure, wherein the data processor arrangement is configured for processing input based on the measured first and second field signals (432, 442) according to a predefined set of rules, and configured to provide an output based on the processing, the output indicating a machine state selected amongst a plurality of machine states, wherein said plurality of machine states includes information of the rotational direction of the rotor, wherein the one or more signalling units are configured for reflecting the selected machine state by means of one or more visual and/or audio signals based on the output provided by the data processor arrangement, wherein the first sensor element and the second sensor element are adapted to measure on the same field type and are arranged in mirror symmetry about a first axis and a second axis being perpendicular to the first axis, wherein the sensor device is configured with a mounting area arranged on the outer surface adapted for being attached onto a non-modified surface of the rotational machinery.