Transmitter Design Of Constant Temperature Differential Thermal Gas Mass Flowmeter

Thermal mass flowmeter can directly measure the mass flow of the measured gas,which has a large turndown ratio.Therefore the thermal mass flowmeter is widely used in chemical and power generation industries and other fields.In order to solve the problem of air flow measurement in the air conditioning and ventilation system of nuclear power plant,higher requirements are put forward for the design of thermal mass flowmeter due to its poor working conditions and large design flow range.Based on the principle of constant temperature difference,the hardware circuit of thermal mass flowmeter is designed,including a power supply circuit,a signal conditioning circuit,and a signal processing and output circuit.The constant temperature difference between sensors probe is maintained by the Wheatstone bridge and the negative feedback control circuit.Aiming at the influence of lead resistance on the measurement result,the principle of lead compensation circuit is obtained through theoretical analysis,and the lead compensation circuit is designed.Aiming at the problem of the small measuring range of the instrument,the conversion of control voltage to output current is achieved based on the LT1510 constant current drive chip,which replaces the triode in the traditional circuit and effectively improves the turndown ratio of the instrument.The raw data of heating current are analyzed in time domain and frequency domain.The results show that the heating current value approximately conforms to Gaussian distribution,and the frequency domain distribution of noise signal is wide.Therefore,the fourth-order Butterworth low-pass filter is used to filter the heating current data,which improves the repeatability of the measurement results.The nonlinear relationship between the heating current and the measured velocity is fitted by the piecewise 6th order polynomial according to the least squares criterion,in order to obtain the instrument characteristic curve under the minimum sum of squared errors,which improves the measurement accuracy of the instrument.A two-dimensional regression equation is used to construct a functional relationship between the fitted flow rate,gas temperature and actual flow rate,so as to eliminate measurement errors caused by gas temperature changes.Based on the modular design concept,the program of each sub module is designed,including a main monitoring program,a signal acquisition module,a flow calculation module,an interrupt module,a pulse output module,a host computer communication module,a LCD display module and an initialization module,etc.Each sub module cooperates with each other to realize the complete process from the signal sampling and the flow calculation to the result output.In order to verify the effectiveness of the designed circuit and algorithm,the gas flow calibration experiments and gas temperature compensation experiments were carried out.The experimental results show that the measurement results of the developed constant temperature differential thermal mass flowmeter meet the requirements of the 1.0-level accuracy when the gas temperature is constant,the maximum measurable flow rate of the instrument reaches 60 m~3/h,and the turndown ratio reaches 60:1,which effectively widens the measurement range of the thermal mass.When the temperature of the measured gas changes within the range of 20?60?,the temperature compensation algorithm effectively eliminates the influence of the gas temperature change,and the measurement result of the instrument meets the requirement of the 1.5-level accuracy.