With the continuous improvement of the production technology of methanol and formaldehyde in our country and the scientific development, the research of the flowmeter has been paid more and more attention by the automatic control industry. Especially the instrument with precise temperature compensation function has attracted the attention of various instrument manufacturers. The research and development of precision temperature compensated flowmeters includes many aspects such as mechanical manufacturing, microcomputers, detection circuit hardening, software, performance calibration, etc. We have conducted research only on microcomputers, detection circuit hardening, software, and performance calibration techniques to design precision. Temperature compensation flowmeter.

1. The design idea of ​​precision temperature compensation flowmeter

1.1 The basic problems that must be solved to achieve the temperature compensation of liquid fluids

To achieve the temperature compensation of liquid fluids, on the one hand, the data transmitted from the medium temperature sensor to the micro-controller must be accurate and reliable, and on the other hand, the data processing speed of the micro-controller should be faster. When the calculation of the flow data is completed, the temperature data must be promptly processed. Process and precisely compensate for flow data.

1.1.1 Obtaining the medium temperature

In order to ensure that the medium temperature is accurately and reliably transmitted to the micro-chip, we use the platinum thermal resistance Pt100 which is more mature at home and abroad as the medium temperature sensor, and the manufacturers use the Japanese “CRZ” series products. The linearity, accuracy, and reliability of the temperature are guaranteed.

1.1.2 Improve Data Processing Speed

Because the flow sensor sends a signal with a higher frequency, the data processing speed of the micro-controller is required to be faster. In order to enable the flow signal and temperature data to be processed in time, 2 micro-chip microcomputers (dual CPUs) are selected and the crystal frequency is increased at the same time.

1.2 Mechatronics design is implemented to ensure that the circuit has low power consumption and the normal working time of the instrument is 3 to 8 years (no failure time of core components).

1.2.1 Selection of Flow Sensors

At present, the domestic and foreign instrument market, the turbine liquid flow meter still dominates the mainstream, with the continuous improvement of mechanical manufacturing technology, turbine liquid flow meter mechanical life is longer and longer, and the measurement accuracy is high, the structure is simple and easy to maintain. Therefore, a more mature liquid turbine was selected as a flow sensor.

1.2.2 Low Power Design

The circuit components are all surface-mount (SMD), and the microchip adopts Microchip products of the United States. The entire circuit operating state is controlled by the software written in the microchip microcomputer. When there is no flow signal, the software will automatically identify and enter the sleep low power consumption. status. The large-capacity lithium battery is used to ensure long-term stable operation of the instrument. When the battery voltage is lower than the factory set value, the instrument display will have a “battery!” flashing at a frequency of 2 Hz, and the meter can also work normally for more than one month.

2. Hardware design of precision temperature compensation flowmeter

2.1 Mechatronics design.

From the body, electronic meter, flow sensor, temperature sensor, as shown in Figure 1. The body is made of stainless steel to meet the needs of various fluid media. The head is aluminum alloy, which makes the circuit anti-interference ability. Explosion-proof rating is ExdIIBT4,ExiIIBT4. The temperature sensor is a stainless steel jacket.

2.2 Precision temperature compensation flow meter circuit design and working principle.

Circuit consists of flow preamplifier, temperature preamplifier, sensor excitation power supply, micro-controller 1, micro-controller 2, touch keyboard, flow output 1 (unit is 4-20mA) circuit, flow output 2 (in litres), liquid crystal Display and other components. Liquid crystal display shown in Figure 2. The microchip 1 sends the temperature-compensated flow data to the micro-controller 2 and the data is processed by the micro-controller 2 and sent all the way to the D/A and converted to 4-20MA for output. The other way is converted to digital quantity to rise (KG ) Output.

3. Software design of precision temperature compensation flowmeter

Modularization is divided into initialization, main program, interrupt subroutine, keyboard subroutine, data acquisition subroutine, data processing subroutine, display subroutine, and data output subroutine. The main program flow chart is shown in Figure 3. Users can modify various parameters in the software, such as medium temperature coefficient, flow coefficient, fluid density, etc., as required.

4. Metrological performance calibration

4.1 Frequency Response Test.

When using the Hong Kong "Longwei" brand signal generator and the United States "TeKtronix" oscilloscope to detect the frequency response performance of the precision temperature compensation flowmeter within the range of 5 ~ 3000Hz (M segment), the output voltage amplitude ≧ 3.1Vp-p can fully meet the requirements Flow sensor requirements. The frequency response curve is shown in Figure 4.

4.2 Metrological performance verification.

Using a 15mm diameter flow sensor, with a flow coefficient of 392.4I/L, measurements were continuously made 10 times at different instantaneous values ​​in a 100 litre vessel, resulting in an accuracy level of ±0.2%, a linearity of ±0.18%, and repeatability of ±0.03%.

4.3 Temperature verification.

Use Shanghai Electric Instrument Factory 0.02 "standard DC resistance box", input different temperature range (-40 °C ~ 90 °C), LCD display accuracy of ± 0.3%.

5. Conclusion

The precision temperature compensation flowmeter adopts two micro-chip microcomputers (double CPU), which makes the flow and temperature data processing speed faster, ensures the measurement accuracy of the temperature compensation flowmeter, and selects a more mature liquid turbine flow sensor to realize mechatronics. The installation is extremely convenient and easy to overhaul. Because of the intelligent design, all the complicated things are completed by the micro-chip microcomputer (double CPU), so the operation is simple and the anti-interference ability is strong. The circuit and software are designed for low power consumption, so they have high reliability and long service life. The RS-232 communication interface can communicate with the host computer. The pulse output function can be combined with the intelligent constant value controller to form a precision filling system. The 4-20mA output function can be connected to various flow calculation devices to achieve remote transmission instructions.

From the perspective of the technical performance and operating conditions of the precise temperature compensation flowmeter, the precise temperature compensation flowmeter can achieve a good temperature compensation for various fluids with high accuracy and good linearity. Effectively reduce the economic losses caused by measurement errors of liquid chemical products due to temperature. It is one of the most ideal flowmeters for measuring liquids. The product can be widely used in metallurgical, chemical, petroleum, urban construction, environmental protection, food and other fields, reliable measurement of water, tap water, sewage, diesel, gasoline and low viscosity crude oil and other liquid volume flow. Especially in methanol, formaldehyde flow metering, with domestic and foreign technology leading level.

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