One of the questions that we get asked again and again is: where do you measure the flow in a Parshall flume?
We've seen flow measured:
- In the throat
- In the converging section (random location)
- In the converging section at a distance 2/3 the length of the converging section measured back from the throat along the longitudinal axis of the flume
- In the converging section at a distance 2/3 the length of the converging wall measured back from the throat
- Upstream of the inlet of the flume
But which is right? Are they all right? Are none of them right?
Remember that:
Under free-flow conditions, short-throated flumes (such as the Parshall) control flow in a region of the flume that produces curvilinear flow. As such, there is only one point of measurement at which the level-to-flow relationship holds. This point is within the zone where subcritical flow is being accelerated towards critical flow.
So, there is only ONE correct location to measure the level / flow in a Parshall flume! That means four of the five locations listed above aren't right!
How a Parshall Flume Operates
The purpose of the Parshall flume is to accelerate slower, subcritical flow to faster, critical / supercritical flow in a controlled manner so that a distinct level-to-flow relationship exists. Looking at the hourglass shape of the flume, it is easy to visualize that the flow is fastest in the throat. Logically that means that the flow entering the flume is slower, subcritical flow.
The downstream diverging section after the throat can be eliminated; as it is here that the accelerated flow is slowed before it rejoins the downstream channel.
Point of Measurement Location
So, we have located the section of the Parshall where measurements must occur, but where in that section is the single, true point of measurement? For that we turn to Dr. Parshall and his development of the flume. Dr. Parshall determined that flow should be measured at a point that 2/3 the length of the converging wall measured back from the throat. It is important to note that this distance is NOT simply 2/3 of the distance back from the throat, but 2/3 of the length of the sidewall.
The graphic below shows the difference between measuring back from the throat and measuring along the sidewall. For a 12-inch Parshall flume (shown on the graphic) there is a difference between the two measurements of 45/64”.
While this may not seem significant, remember that the water level in a Parshall flume drops as flow moves from the inlet of the flume towards the throat. Taking a reading upstream of the point of measurement will mean that the indicated flow rate will be higher than the actual flow rate.
The correct location for the point of measurement, Ha, determined, the question then becomes: must it be measured at the sidewalls or should it be down the centerline of the flume.
The answer is: if the flow is properly conditioned coming into the flume, either location is fine (and off to the side makes it easier to access the flow meter). If the flow is not as well conditioned, then the centerline of the flume may give more representative readings, but that statement depends upon turbulence and velocity profile perpendicualr to the direction of flow. Keep in mind that centerline readings are practical for manual readings on small flume and for ultraonic sensors. Bubbler and pressure transducer flow meters typically require the sensing element be located along the flume wall for maintenance and calibration access. Also, stilling wells, by the nature, have their intakes at the wall of the flume.