Calibration of a Flowmeter

This instructible will provide step-by-step instruction in the calibration of bulk-flow measuring devices, such as Venturi meters, orifice-plate meters, and paddlewheel flowmeters.

Supplies

The devices to be calibrated operate within a system illustrated in Figure 2.

Step 1: Cailbrate the Validyne Differential Pressure Transducer

First, the transducers marked as VF in the setup diagram needs to be calibrated. This will be done with no flow and static pressures to prevent fulctuation in the measurements of the transducer. Fully open the bleed valve connected to the manometer. This will introduce a pressure difference across the manometer, which you must measure, along with the voltage reading from the transducer. Then, slightly close the bleed valve of the manometer to introduce a lower pressure difference, and record the measurements of the manometer and the transducer again. Repeat this process until you have 5 sets of measurements. If a plot of the measured pressure difference and the voltage provides a linear relationship, then you have created a calibration for the pressure transducer.

Step 2: Data Aquisition

Now we will collect some data with the hydraulic flowmeter and the paddlewheel flowmeter, and we will also take measurements for the weight-time flow rate in the system. Start by opening the discharge valve to the maximum within our calibration. Now, we must simultaneously measure the three values. Record the levels of the manometer and the voltages of the paddlewheel flowmeter and the pressure transducer. To measure the weight-time flow rate, close the basin drain valve and watch the balance beam. Once the beam reaches balance, start a stopwatch and add ano additional weight the beam Stop the stopwatch when the beam reaches balance a second time, and open the basin drain valve. Now, you must slightly close the discharge valve. You want to achieve 80% the flow rate we started with. You will need to use the manometer to determine what percentage of the original flow rate you are at. The pressure difference in the tube is related to the flow rate in an exponential function, so if you want to achieve 0.8 times the flow rate, you need the height difference in the manometer to be 0.64 times the original. At the 80% flow rate, repeat the procedure for the three measurements. Repeat this at every 20% interval down to 20% of the maximum flow rate.

Step 3: Analysis

By plotting flow rate as a function of the manometer deflection, you will create the calibration curve for the flowmeter under analysis. Notice that the relationship is not direct.

Step 4: Analysis

By plotting flow rate as a fucntion of manometer deflection under logarithm scales, you will create an alternate to the calibration curve. Notice that the data follows a straight line, which indicated that a power-law type relationship applies to the flow rate and the pressure difference of a system.

Step 5: Analysis

By plotting the discharge coeffieicnt as a function of the Reynolds number, the accuracy of your calibration can be determined. If the values for the discharge coefficient remain constant, then the flow measurement devices function accurately. If the values for the discharge coefficient are not entirely constant, then the flowmeter is inaccurate. Additionally, the theory used in the development of these procedures can be tested by comparing the theoretical ideal value of unity shown in Figure 6 to the values calculated from the experiment. If these values do not match, then there is some error in the theory. To correct this, additoinal consideration can be taken such as accounting for the friction between the fluid and the pipe walls or flow seperation. The values for flow rate can also be used instead of the Reynolds numbers as they are directly related and thus the shape of the graph will not be affected.

Step 6: Analysis

By plotting the voltage output of the paddle-wheel flowmeter as a function of the mass-time flow rate, we can determine the accuracy of the paddle-wheel flowmeter. From this graph, we can see that the paddle-wheel flowmeter seems the be generally accurate, but at both low and high flow rates the accuracy of the flowmeter is decreased.