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Tutorial on flow metering (Part 3)

31 Jul 2012  | Mohit Arora, Prashant Bhargava

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In Part 1 of this series, we discussed the fundamental concepts and principles incorporated by flow meters along with various flow measurement methods used in mechanical flow meters. Part 2 covers the pulse based counting method and the various sensors that are used in the industry, as well as the way they generate different pulse waveforms to be used in variety of flow meters. This part will take a deep dive on the methods to perform the measurement of these pulses.

Pulse sampling theory
Sensors based on Pulse Counting Theory (discussed in part 2 of the series) convert the motion of flow into mechanical movement which is converted to electrical waveforms that can be measured using electronic circuits. For flow measurement it is necessary to determine the current and previous states of sensor output (for flow direction) and accumulate every change in state (for determining amount of flow over a period of time). Hence the sampling process must be accurate and should not miss out any state change.

In order to be able to sample a given waveform, it is important to know about the various characteristics of the waveform being measured. Based on the details provided in part 2, we can conclude that the various waveforms generated by the sensors can be uniquified and the following properties can be deduced:

- Waveforms are encoded in one of the following schemes:

 • 2bit or 3bit gray or binary encoding
 • 2bit quadrature (90o separation) encoding
 • 2bit, 180o separation encoding
- Waveforms are periodic and the period of waveform varies with the rate of flow of fluid being measured

Figure 1 shows the various waveforms.

Figure 1: Waveform outputs from different sensors based on Pulse Counting Theory.

Each of the above waveforms also represents an angular position of the disk inside the flow meter. In addition to measuring the flow, the angular position must be detected in order to detect forward and reverse flows and perform measurements accordingly.

In order to sample the waveforms from sensors correctly, certain timing parameters must be established based on which a measuring circuit can measure the flow correctly irrespective of the rate of flow at any given point in time.

Sampling Period: By knowing the pipe diameter and maximum design pressure, the maximum flow rate can be determined. Once the maximum flow rate is known the maximum rotation speed of the disk can be determined. Knowing the rotation rate of the disk and the number of states on the perforated disk (e.g. consider optical sensor with 3bit sensor gray coded disk) the maximum number of changes per second can be determined. Provided the disk is sampled more frequently than the maximum number of state changes, every state change will be detected. This gives us the sampling period to be supported for accurate measurement of the flow. In other words, the sampling period is the minimum time available to sample one sector.

Sampling Window: This is the window within the sampling period when the sensor outputs can be sampled by MCU. This window is also the minimum time needed by MCU to sample its input pins connected to the sensor.

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