1 Introduction Turbine flow meters have been used in industry for over fifty years. They measure flow by detecting the turbine's rotational speed using an internal magnetic sensor and are a widely used flow measurement instrument [3]. With the development of fiber optic sensor technology, this paper combines a reflective fiber optic sensor with the traditional turbine flow measurement principle to fabricate a turbine flow meter with dual fiber optic sensors. Compared with the traditional internal magnetic turbine flow meter, the design enables the flow meter to measure both forward and reverse flow. In terms of detection principle, the fiber optic sensor overcomes the influence of the magnetic attraction of the internal magnetic sensor. Experimental results show that the range ratio of the turbine flow meter is effectively expanded. 2 Structure of Dual Fiber Optic Sensor Turbine Flow Meter A pair of reflective fiber optic sensors and a photoelectric conversion circuit are designed to detect the rotation of the turbine blades. The turbine blades are modified to make their end faces suitable for reflecting light (Figure 1). Figure 1. Schematic diagram of fiber optic turbine flow meter. The reflective fiber optic sensor uses multimode glass fiber with a single core diameter of 200 μm and a numerical aperture of 0.3. It consists of two fibers, including a light emitting fiber and a light receiving fiber. The detection end is fixed in an aluminum alloy sheath and can replace the internal magnetic sensor on the turbine flow meter [1][5]. In order to improve the signal-to-noise ratio of the reflective fiber optic sensor and ensure the resolution of the received reflected signal, the light source emitting circuit in the photoelectric converter is designed to output 10-12 kHz modulated light. The reflected signal of the modulated light is received by the receiving fiber through the turbine blades via the emitting fiber. After filtering, it is converted into a flow pulse signal with a signal response time of less than 0.2 ms and a detection distance of 1 mm. 3. Principle of bidirectional flow measurement of dual fiber optic sensor turbine flow meter. Traditional internal magnetic sensors are limited by their structure and can only detect the rotational speed of the blades in turbine flow meters. Since the reflective fiber optic sensor is small in size, two reflective fiber optic sensors are mounted side by side on the turbine flow meter. In this way, the two sensors can detect the reflected signals from different positions of the same turbine blade, and the signals of the two sensors do not interfere with each other (Figures 2 and 3). The flow signal output by the fiber optic sensor is: f10 = f10 f20 = f10 + Φ Q = (f10 or f20) / K Where f10 is the output signal of fiber optic sensor No. 1, Φ is the phase difference between f20 and f10, f20 is the output signal of fiber optic sensor No. 2, and K is the flow coefficient. The f10 and f20 signals output by the sensor are processed by the signal phase discrimination circuit [1] to output the forward flow measurement signal f1 and the reverse flow measurement signal f2 of the flow meter. At the same time, the flow pulse signal f and the flow direction status signal D can also be output. For forward flow: D = 1, 0 < Φ < 90°, for reverse flow: D = 0, Φ > 90°. 4 Test of reflective fiber optic sensor and flow meter 4.1 Response of reflective fiber optic sensor Test results of reflective fiber optic sensor and internal magnetic sensor using DC motor and standard tachometer show that: the signal characteristics of reflective fiber optic sensor: 1Hz ~ 6kHz; the signal characteristics of internal magnetic sensor: 30Hz ~ 1.2kHz, with obvious leakage pulses in the low speed range (Figure 4). 4.2 Bidirectional Flow Measurement of Dual Fiber Optic Sensor Turbine Flow Meter When the flow meter is installed in the forward direction, the photoelectric conversion circuit outputs: Q10＝f10/K; Q20＝0; Q0＝f10/K; D＝1. When the flow meter is installed in the reverse direction, the photoelectric conversion circuit outputs: Q10＝0; Q20＝f20/K; Q0＝f20/K; D＝0. 4.3 Flow Measurement Range Ratio of Dual Fiber Optic Sensor Turbine Flow Meter [2][4] Using a reflective fiber optic sensor and an internal magnetic sensor combined with the same 50mm diameter turbine flow meter, the flow signal output characteristics tested with compressed air show that: The output signal range of the reflective fiber optic sensor is 2～6.4Hz, and the converted flow range can reach 0.3～120m3/h. Without considering the resistance torque generated by the turbine viscosity, the range ratio reaches 1∶400. The effective signal range of the internal magnetic sensor is 41Hz to 1.1kHz, which translates to a flow range of 1.1 to 60 m³/h. Without considering the resistance torque generated by turbine viscosity, the range ratio is 1:60. 5. Conclusion The dual-fiber optic sensor turbine flow meter enables bidirectional flow measurement. The two flow signals are differentiated by a discrimination circuit to detect bidirectional flow. For unidirectional flow measurement, the measurement error caused by the reciprocating vibration of the turbine due to water hammer can be eliminated through the algorithm of the flow computer, as shown in the following formula: Q = (∑f1(D=1) - ∑f2(D=0))/K. Experiments show that the dual-fiber optic sensor turbine flow meter can accept turbine signals with frequencies from 1Hz to 6kHz, with a range ratio of 1:400 and a full-range error of ±1 pulse; while the internal magnetic sensor turbine flow meter has a range ratio of 1:60. This is mainly because the fiber optic sensor does not have the error caused by the magnetic resistance generated between the internal magnetic sensor and the turbine blades at low flow rates, and it also overcomes the problem of signal saturation in the high flow range of the internal magnetic sensor. The modulation parameters of fiber optic sensors can be varied according to overall design requirements, making them suitable for different applications and facilitating turbine design. Because fiber optic sensors are inherently explosion-proof and have no direct electrical signal contact with the flow meter, they are suitable for flow measurement of transparent media such as coal gas and light oils.