Particle sensor for detecting or analysing particles in a fluid, the sensor comprising: an emitter for emitting an electromagnetic wave, a modulator for modulating the emitted electromagnetic wave, a detection space for containing the fluid; a detector comprising a reference fluid chamber and a reference pressure or temperature sensor, wherein the reference fluid chamber contains a reference fluid, and wherein the reference pressure or temperature sensor is arranged to sense a pressure or temperature wave of the reference fluid in the reference fluid chamber, a further detector comprising a further reference fluid chamber and a further reference pressure or temperature sensor, wherein the further reference fluid chamber contains a further reference fluid, and wherein the further reference pressure or temperature sensor is arranged to sense a pressure or temperature wave of the further reference fluid in the further reference fluid chamber, and a controller for detecting or analysing the particles in the fluid based on the pressure or temperature wave sensed in the reference fluid chamber and based on the pressure or temperature wave sensed in the further reference fluid chamber, wherein the emitter and the detector are arranged such that the electromagnetic wave traverses the detection space and at least a part of the electromagnetic wave which traversed the detection space enters the reference fluid chamber, wherein the emitter and the further detector are arranged such that the electromagnetic wave traverses the detection space and at least a part of the electromagnetic wave which traversed the detection space enters the further reference fluid chamber characterized in that the reference fluid in the reference fluid chamber comprises a first isotope of a chemical particle, the further reference fluid in the reference fluid chamber comprises a second isotope of the same chemical particle different to the first isotope, the controller for detecting or analysing the particles in the fluid is configured to detect or analyse one or more isotopes of the chemical particle based on the pressure or temperature wave sensed in the reference fluid chamber and on the pressure or temperature wave sensed in the further reference fluid chamber. an emitter for emitting an electromagnetic wave, a modulator for modulating the emitted electromagnetic wave, a detection space for containing the fluid; a detector comprising a reference fluid chamber and a reference pressure or temperature sensor, wherein the reference fluid chamber contains a reference fluid, and wherein the reference pressure or temperature sensor is arranged to sense a pressure or temperature wave of the reference fluid in the reference fluid chamber, a further detector comprising a further reference fluid chamber and a further reference pressure or temperature sensor, wherein the further reference fluid chamber contains a further reference fluid, and wherein the further reference pressure or temperature sensor is arranged to sense a pressure or temperature wave of the further reference fluid in the further reference fluid chamber, and a controller for detecting or analysing the particles in the fluid based on the pressure or temperature wave sensed in the reference fluid chamber and based on the pressure or temperature wave sensed in the further reference fluid chamber, wherein the emitter and the detector are arranged such that the electromagnetic wave traverses the detection space and at least a part of the electromagnetic wave which traversed the detection space enters the reference fluid chamber, wherein the emitter and the further detector are arranged such that the electromagnetic wave traverses the detection space and at least a part of the electromagnetic wave which traversed the detection space enters the further reference fluid chamber characterized in that the reference fluid in the reference fluid chamber comprises a first isotope of a chemical particle, the further reference fluid in the reference fluid chamber comprises a second isotope of the same chemical particle different to the first isotope, the controller for detecting or analysing the particles in the fluid is configured to detect or analyse one or more isotopes of the chemical particle based on the pressure or temperature wave sensed in the reference fluid chamber and on the pressure or temperature wave sensed in the further reference fluid chamber.