Development Of Nuclear Thermal Mass Flowmeter Based On Constant Power Principle

Thermal mass flowmeter is a flowmeter that can directly measures the mass flow of the measured gas by using the principle of heat transfer.It has the advantages of large range ratio,high measurement accuracy,small pressure loss and high reliability,which makes it is widely used in petroleum,chemical,metallurgy,and nuclear power and other fields.In order to realize the air flow measurement of the safety assurance system in the main control room of the nuclear power plant,the sensor structure and the transmitter circuit of the 1E thermal mass flowmeter are designed according to the 1E safety requirements and measurement system requirements of the electrical equipment of the nuclear power plant.The sensor includes:a Pt1000 platinum resistance temperature sensor(speed measuring element)and a heating element forms a speed measuring probe,which is used to measure the temperature of the speed measuring element;another Pt1000 platinum resistance temperature sensor(temperature measuring element)and a mass equalizer forms a temperature measuring probe,which is used to measure the temperature of the fluid.This kind of sensor design can not only realize the constant power heating of the speed measuring element,but also directly measure the voltage difference signal proportional to the temperature difference signal of the two probes without using the microprocessor and the analog-to-digital converter.The transmitter circuit is developed using pure analog devices,including constant current source circuit,signal conditioning circuit,non-linear correction circuit,4-20m A current output circuit and power supply circuit.The constant current source circuit includes a driving current source and a heating current source,which are used to drive the speed measuring element and the temperature measuring element to generate voltage signals and to heat the heating element with constant power,respectively,so that a temperature difference is formed between the speed measuring element and the temperature measuring element.In the selection of the size of the constant current source,the 1m A current source is selected as the driving current source,which can not only avoid the resistance value change caused by heating the platinum resistance,but also ensure that the output voltage of the platinum resistance has a higher signal-to-noise ratio;Choosing a 75m A current source as the heating current source can not only avoid the high temperature of the speed probe and accelerate the oxidation of the probe,but also ensure that the sensor characteristic curve has a higher resolution.The signal conditioning circuit includes a differential amplifier circuit and a low-pass filter circuit,which are respectively responsible for differentially amplifying the voltage across the speed measuring element and the temperature measuring element and filtering out high-frequency interference in the amplified voltage signal.In view of the serious non-linear problem of thermal mass flowmeter sensor characteristics,according to the principle of heat transfer and King’s law,combined with its own sensor structure and transmitter circuit,the transfer function between the flow rate and the sensor output signal is derived,and A non-linear correction circuit is designed through the real-time analog computing chip AD538 to realize the transfer function.Aiming at the problem that the effect of non-linear correction is easily affected by factors such as sensor manufacturing process,platinum resistance heat dissipation capacity and ambient temperature,it is proposed to divide the flow interval and set different power values in each interval to realize the correction of the sensor signal in the entire flow interval.The nonlinear correction circuit includes an analog reciprocal,a subtraction circuit,a voltage segmentation circuit,an analog power operation circuit,and an isolated output circuit.This part of the circuit is responsible for performing a series of analog operations on the voltage signal output by the signal conditioning circuit to obtain the flow rate.The 4-20m A current output circuit converts the output voltage signal corrected by the non-linear correction circuit into a current signal that has a linear relationship with the flow rate and outputs it;the power supply circuit is responsible for supplying power to each module circuit.In order to verify the performance of the instrument,a real gas flow calibration experiment was carried out in Chong-Qing Chuan-Yi Automation.The experimental results show that the developed 1E thermal mass flowmeter can reach the accuracy of(1%RD+0.5FS)in the flow range of 0-100m~3/h,which meets the design requirements of the 1E technical indicators.