Calibrating Flowmeters

This ‘Instructable’ will teach the process of calibrating bulk-flow measuring devices. Following a specific process, calibrating these flowmeters is not a very difficult task, but it does require precision and certain observations. If not read correctly, there may be spillages or damage to pipes.

To begin, confirm that the two levels of mercury are even. If they are not, slowly open and close the two manometer drain valves until the levels are even. This is a very important step to begin our measurements for the rest of the experiment.

This key explains every symbol that may be required throughout this process.

First, the output voltage from the Validyne differential pressure transducer must be calibrated. Zero the transducer output on the VFn interface box located next to the computer. Open the manometer bleed valve to reduce the pressure in one of the lines. Take readings of the transducer output (V) and manometer levels (cm) and record the results on LabVIEW. Track five data points, from zero pressure differential to maximum possible with the valve fully open. LabVIEW will nicely store the slope and intercept for these points. This information will be used later.

Confirm the Gain Adjust control of the paddlewheel flowmeter is set to 6.25 turns for P1 and P4 and 3 turns for P3. Then use the Zero Adjust control to zero the paddlewheel flowmeter output. Slowly open the discharge valve until its either fully open or the allowable manometer deflection is reached. At the instant the Signet paddlewheel voltage takes on a significant nonzero reading, record both the Validyne differential pressure and the Signet paddlewheel voltages. Once the maximum flow rate is attained, record the manometer and paddlewheel flowmeter readings. Take a weight-time measurement and use LabVIEW again to record the time-averaged pressure-transducer voltages. Note the maximum manometer deflection ∆h(max)).

Repeat this process under the conditions that the total manometer deflections are approximately (0.9)^2*∆h(max), (0.8)^2*∆h(max),..., and (0.1)^2*∆h(max).

After the ten data sets have been acquired, use LabVIEW to set up a function of the flow rate expressed in terms of the Reynolds number Re. This represents the flow coefficient Cd.

From the differential manometer measurement ∆h, the pressure difference can be determined.

For the Venturi flowmeter, the diameter d2 is the throat diameter d, so Q can be also be expressed like this.

LR1) The left graph represents a linear scale for the flow rate vs. the measurement deflection for the Venturi meter. The line through the data points represents the calibration curve for the flowmeter.

LR2) The right graph represents a logarithmic scale for the flow rate as a function of the manometer deflection. The line through the points represents an alternate calibration curve for the flowmeter. When analyzing, its clear that this data falls in a straight line, implying a power-law relationship of Q = K(∆h)^m.

LR5) The left graph represents the discharge coefficient Cd as a function of the Reynolds number Re on a linear-log scale. As more energy is maintained in the system, Re increases.

The equation is how the Reynolds number was calculated. D is the full pipe diameter, V1 is the velocity in the pipe, and v is the viscosity calculated in LabVIEW.

LR6) This graph represents the discharge rate Q vs. the paddlewheel voltage output. In accordance with the linearity of the graph, there is no rising or falling cutoff flow rates.

1) In this procedure, the discharge coefficient Cd has a direct relationship with the Reynolds number Re until our data ended at 0.61. This is because Re is proportional to velocity and velocity is proportional to Q. The experimental value of Cd may be close to the ideal value of unity, but since inviscid flow was assumed, it cannot be perfect. To obtain more realistic values of Cd, values should be received from higher flow rates to avoid the friction developed at low rates.

2) The paddlewheel flowmeter is reliable because it removes any sources of human error. It is more accurate at higher flow rates because it can be affected by friction if the flow is small.