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Design and Application of Pulse Flow Sensor Calibrator

The calibration of the traditional pulse flow sensor instrument coefficient is generally completed by a volumetric flow standard device, a counter, and a timer. Its instrument coefficient is defined as the number of pulses emitted by the sensor when a unit volume of fluid passes through the flow sensor. The unit is usually 1L (number of pulses per liter) or 1m3 (number of pulses per cubic meter). According to the provisions of literature [1], in order to ensure the effectiveness of the flowmeter instrument coefficient, it should generally be ensured that the absolute value of the relative error of the number of pulses output by the flowmeter in a calibration is not greater than 13 of the repeatability of the flowmeter being tested. Since the counting error of a general counter is ±1 pulse, within the calibration time interval, the counter should collect a sufficient number of pulses N to achieve the required calibration accuracy.

For some large-diameter flow sensors, the instrument coefficient is generally small (for example, a 200 turbine flowmeter has an instrument coefficient of only 1.5/L; a vortex flowmeter with a large diameter has an even lower instrument coefficient of only about 0.2/L. ). For such a flowmeter, in order to collect enough pulses, it will take a long time to calibrate, and it will require larger calibration equipment (larger standard container). Due to various limitations, the counting pulses always fail to meet the requirements. Dual-time counting technology is currently a relatively common pulse interpolation technology in the world. It can use smaller calibration equipment in a shorter calibration time and still ensure sufficient technical accuracy when the total number of counting pulses is small. Pulse interpolation technology was earlier used in micro-volume tube flow standard devices.

The “Pulse Flow Sensor Calibrator” we developed is a dual-time method calibrator composed of a traditional counter and dual-time method measurement and control technology. Tests have shown that the calibrator is convenient and reliable to use, can shorten calibration time, use smaller standard containers to calibrate larger diameter flow sensors, and has higher technical accuracy than conventional calibration methods.

1. Principle of dual-time counter

Pulse interpolation technology is an effective way for the piston calibration device to increase the output signal resolution of the flow meter, thereby reducing the size of the calibration device. Usually, in order to obtain sufficient pulse numbers for calibrating a flow meter, two approaches can be taken. One is to improve the output signal resolution of the flow meter so that as many pulses as possible can be obtained within the limited calibration time; the other is to increase the calibration The measuring effective volume of the device. Generally, the number of pulses output by a unit volume of fluid passing through a flowmeter is limited (such as the turbine flowmeter and vortex flowmeter mentioned above), and the effective measurement volume of the calibration device cannot be made very large. Pulse interpolation technology solves this problem very well. It has several methods such as dual-time method, four-time method and phase-locked loop method. Using a “small volume” (effective volume of the device) to collect 500 pulses for the piston calibration device can achieve the same accuracy as a large-volume calibration device collecting 10,000 pulses.

In the formula: N is the number of pulses of the flow sensor signal recorded by the counter; N1 is the number of pulses interpolated by the dual-time method or the four-time method; Δt1 is the time interval from the detection start signal to the detection stop signal; Δt1 is the time interval from the detection start signal to the detection stop signal. The entire pulse cycle time interval between the rising edge of the first pulse after the start signal and the rising edge of the first pulse after detecting the stop signal.

When the stability of the flow standard device complies with the standard regulations, the flow pulse signal period can be considered stable, so the pulse interpolation number obtained by formula (1) should be valid.

In addition to the two-time method, the four-time method can also be used to determine the pulse interpolation number.

This article uses the dual-time method as an example to design a pulse flow sensor calibration instrument.

2. Hardware design of the calibration instrument

This calibrator does not use a microprocessor and has good working reliability. The control signal can be a very narrow pulse signal or a level signal using the single-pole double-throw switch K1. When using level signal control, switch K2 can be used to select high level control or low level control.

When the control signal is a pulse signal, switch K1 selects pulse control. Assume that the Q terminal output of the initial state flip-flop TR1 is low level L (it does not matter if it outputs high level H), and the terminal outputs high level H and is fed back to D end. Switch K2 selects high level control or low level control.

When the control signal is a pulse signal, switch K1 selects pulse control. Assume that the Q terminal output of the initial state flip-flop TR1 is low level L (it does not matter if it outputs high level H), and the terminal outputs high level H and is fed back to D end. Switch K2 selects high-level control (if the Q-terminal output of the initial state flip-flop TR1 is high-level H, K2 can select low-level control), the input terminals of NAND gates B and C and the D terminal of flip-flop TR2 Both are low level, so gates B and C are closed, the Q terminal output of flip-flop TR2 must also be low level under the action of the flow pulse signal, and gate E is closed. The counter and timers T1 and T2 are both in stopped state. Use the reset button to return the counter and timer to the initial zero state and display all zeros.

When the “start counting” control signal pulse (the first control pulse) arrives, since the D terminal of TR1 is high level H, the control pulse triggers TR1 to make its Q terminal output high level H, and immediately turns on the The NOT gates B and C cause the counter and timer T1 to start counting and timing. At this time, the NAND gate E has not yet opened, but the D terminal of the flip-flop TR2 is already at high level. The first rising edge of the flow signal after the front edge of the control signal triggers TR2, causing its Q terminal to output a high level and open the NAND. Door E, timer T2 also starts timing.

When the “stop counting” control signal pulse (the second control pulse) arrives, TR1 is triggered again and the Q terminal outputs a low level L, thus immediately closing the NAND gates B and C, causing the counter and timer T1 to stop counting. and timing. However, the NAND gate E is not closed immediately. It is not until the first rising edge of the flow signal after the leading edge of the “stop counting” control signal pulse that TR2 is triggered and low level L is output. The NAND gate E is closed and the timer T2 Stop the clock. By substituting the data obtained from timers T1 and T2 into equation (1), a more accurate pulse interpolation number can be obtained.

When the control signal is a level signal, switch K1 selects level control, which is equivalent to crossing the flip-flop TR1 and directly controlling the NAND gates B and C and the D terminal of the flip-flop TR2. The selection switch K2 points to high-level control or low-level control respectively for high-level action or low-level action. The rest of the action is exactly the same as pulse control.

3. Indicators and results

3.1 Calibrator indicators

In addition to being a controllable counter and timer as mentioned above, the calibrator also has the function of measuring signal frequency and period. It is not used to calibrate flow meters, but can be used alone as an instrument to measure signal frequency or period.

The specific indicators are as follows:

① Timer, 6-digit LED display, resolution 1ms;

② Timer (including frequency and period), 8-digit LED display, the highest resolution is frequency 1Hz, period 0.1μs, counting ±1 pulse;

③ Measuring range: frequency is 10Hz ~ 100MHz, period is 0.5μs ~ 10s, counting capacity is 99 999 999, timer is 1ms ~ 999.999s;

④ Manually switch display between T1 and T2.

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