Introduction: TAM 335 Partial Report Lab 5
When it comes to the calibration of flowmeters, the bulk of the process is understanding how to use measurement devices such as Venturi meters and orifice-plate meters to determine flow coefficients. The difference between the two meters is show in the figures above. We will be finding coefficients as a function of Reynolds number. The supplies necessary for the experiment include a differential pressure inducer, manometer(mercury), paddlewheel flowmeter, as well as the Venturi meters and orifice-plate meters mentioned above in a pipe system. The setup is shown below. As for specific clarifications for each device, the paddlewheel flowmeter is a Signet 3-8511-p0 "lo-flo" device with an operating velocity range of 0.3-20ft/sec. Also, it needs to be connected to a Signet 8511 digital readout transmitter that supplies a 4-20 milliamps current to a fixed resistance to produce the varying voltage reading. To acquire the data from this experiment, we will be using LabVIEW software. The requirements for this software is the correct calibration of the pressure transducers and it will result in readings on pressure difference across the flowmeters.
Step 1: Experiment Setup
The first step in the experiment is to make sure that the discharge valve is closed. After this has been accomplished, switch the focus onto the manometer and the central scale. Get down to eye level for the manometer and make sure that both sides are starting at the same height level. If one differs from the other, cautiously open and close the manometer valves until the height levels line up. The central scale must read zero before any weight is applied to it, if not then recalibrate it correctly.
Step 2: Calibration of Manometer Differential Pressure Transducers
After the setup is complete, now it is time to focus on the real lab. Calibrate the Validyne differential pressure transducer, which is done with no flow in the system. This means that in the diagram above, the VFn, for which n is an integer, is equal to 0. With the discharge valve closed, open the manometer valve to reduce the pressure in one of the sides, then record the transducer output voltage as well as the manometer height readings. These measurements are done through LabVIEW. Continue this 5 times and record the maximum voltage value reading when the valve is completely open. This max voltage should not exceed 10 volts.
Step 3: Data Acquisition
If everything has been followed in the past steps, this is where the main important data values are recorded. First check to see if the Gain Adjust control on the paddlewheel flowmeter is at 6.25 turns for P1 and P4, while set at 3 turns^2 for P3 (In the diagram featured in Step 2). Once this is checked, open the discharge valve slowly until one of two points have been reached: if the valve is completely open or if the allowable manometer deflection is reached, then stop. When observing the paddlewheel and the Validyne flowmeter readings, note down both measurements once the paddlewheel is a nonzero value. Starting with the maximum flow rate, record the following: manometer readings, paddlewheel flowmeter readings, weight-time measurement and the time-averaged pressure-transducer voltage (through LabVIEW). Repeat the procedures 10 times at 90% of max flow rate, 80%, 70%, etc. Once this is done, the flow coefficient, Cd should be displayed in LabVIEW as a function of Reynolds number.
Step 4: Results
Now that the data has been acquired and shown in the table above, the goal is to understand what the relationships are between different measurements. Looking at the flow rate Q as a function of the manometer height difference, deltaH for either flowmeter (Venturi or Orifice-plate), there is a relationship that arises. First using linear scales, the equation is a form of K(deltaH)^m where y = .0014x^(.5674), however if the scales are changed to logarithmic as shown in the second graph, the relationship becomes linear. It is an interesting takeaway from the lab where you can not only learn the function relationship between Q and deltaH but also how to manipulate the axis in order to produce a linear line. The next step is to observe how the discharge coefficient, Cd is a function of the Reynolds number. Looking at the third graph, even though it is not obvious, Cd stays at a relatively constant number of 0.55. As an extra note, to calculate Reynolds number, the equation is V1*D/v where V1 is the velocity of the flow, D is the diameter of the pipe, and v is the dynamic viscosity of the liquid. Focusing on the Discharge coefficient, you should notice that is a little above half of the ideal number of 1. In order to correct this value, there is the mathematical way and there is the geometric way. Algebraically, the diameter of the pipe should decrease as shown in the equation following the Discharge Coefficient vs Reynolds Number graph. Thinking of another way, the pipe system needs to be smoother around its turns (elbow) and it cannot be rigid or sharp. Finally, looking at the paddlewheel voltage, it should be a function of the flow rate. Take a note about the cutoff range for the paddlewheel which is .3 to 20 ft/sec. By observing the graph as well as taking into the account the range, it should be acknowledge that the last couple data points have a fluid velocity outside the specified range of the paddlewheel therefore the reading is inaccurate.
Step 5: Conclusion
Now that the lab has been concluded, the experimenter should be able to understand the core objective of this lab. Understanding the paddlewheel flowmeter as well as understanding the process behind calculating flow coefficients as a function of Reynolds number should be accomplished. Obviously in every lab there are errors that will arise but be sure to take these into account and think of ways to better the experiment to increase the overall accuracy of each measurement.