Calibrating Flowmeters

First thing I would like to do is to congratulate you on your new position, my old position. Due to the fact I am the only one that can do this job, I have the pleasure of training you. Sadly, I am unable to do this in person so this Instructable will have to do. In this piece, I will teach you how to do my job, I will teach you how to calibrate various flowmeters. I will go through each process individually. Before doing so, I would like to show you the apparatus, as seen above in figure 1. The water comes in from the left and leaves the bottom, as the arrows indicate. Between those points, there is a flowmeter, in our case it is an orifice-plate flowmeter. Under this is a manometer, as well as a differential pressure transducer attached to this. Just past this part of the apparatus is the paddlewheel flowmeter, which has a digital reading of flowrate. This pipe is located on the ceiling of the lab, and drains into a weighing tank in the basement of the lab which I will mention later on. Figure 2 gives a better layout of the entire lab, so that you can get situated. The weight-time procedure must be done to have a standard to work with. The weight ratios for the weighing scales for the 3 computer stations are 200:1, 10,000:1, and 200:1 for F-1, F-3, and F-4, respectively. This type of measurement is the standard for calibration for each instrument. As a quick run-through of the weight-time procedure: the stopper hits the top, a weight is added, and the time it takes to hit the stopper again is plugged into the equation, used in previous sessions, to find the flow rate.

First, make sure the discharge valve is closed and make sure that the mercury in the manometer is at an equal level on both sides. For the manometer differential pressure transducer, you must calibrate the output voltage. This must be done when the fluid is static. It is vital that there is no flow in the test section, as it could raise some trouble in measurements later. When the fluid is static, the transducer should be at zero; if not, zero the transducer by the computer. Once this is done, while the discharge valve is closed, open the manometer bleed valve ("CAL VALVE") to create a reduced pressure in one of the manometer lines. While this is going on, you must take readings of the transducer output (V) and readings of the manometer (cm). The LabVIEW software should do this for you, so do not worry about doing all three actions at once. It will take 5 data points from 0 pressure differential all the way to the maximum possible differential with the bleed valve all the way open. This value, for reference, should not surpass 10 volts. The LabVIEW software will also do a least-squares analysis on the transducer data, and also store some data that can be useful later. Once the 5 points are taken, you can close the bleed valve.

The first thing that needs to happen for the paddlewheel flowmeters P1 and P4, is that it need to be set to 6.25 turns. This makes the potentiometer set to 625 ohms so that there is an output of about 10 volts. P3 needs to be set to 3 turns, so that the potentiometer is set to 300 ohms. Once this is done, you may open the discharge valve slowly until it is fully open. If the allowable manometer deflection is reached, then that is an indicator to stop opening the discharge valve. If this is not done correctly and with much caution, the mercury could leave the manometer. At the point the Signet paddlewheel voltage reaches a significant nonzero measurement, record the differential pressure voltage and the significant nonzero value spoken of previously. Also, when the maximum flow rate is reached, record the manometer measurement. With LabVIEW's help, you will be able to also record the time-averaged pressure transducer voltages. It would also be helpful to record the maximum manometer deflection. This process is to be done multiple times, decreasing the flow rate as you go. The total manometer deflections are about (0.9) max 2∆h , (0.8) max 2∆h , (0.7) max 2∆h , ⋅ ⋅ ⋅, ( . ) max 0 1 2∆h. These flow rate readings should be lessening by about 10% of the maximum flow rate. The LabVIEW software will give you the flow coefficient as soon as 10 data sets have been taken. It will be expressed in terms of the Reynolds number Re and the paddlewheel readings. The weight-time method is how the flow rate will be shown in the data. Above is the orifice-plate flowmeter that you will be working with.

In first of the figures above, a linear scale is used to show the data of the measured flow rate as a function of the manometer deflection. The best fit line, passing through the points, has its equation displayed above it. This curve is now the calibration curve for the flowmeter under analysis. You may use it to make sure your devices are calibrated correctly.

In the second of the figures above, the same information is presented, but this time, it is under a logarithmic scale. This information, when presented this way, seem to fall into a straighter line. You may also use this as a reference to check the calibration of your instruments. Because the data falls along a straight line, the power-law relation applies.

The third of the figures above shows the discharge coefficient as a function of the flow rate. For these calculations, the Reynolds number is found using the full diameter and the velocity found in the pipe.

2. The discharge coefficient stays, for the most part, constant over the Reynolds numbers tested. The values were all lower than the unity that was hoped for. This was expected, as it was told to us in the lab manual for this particular experiment. It could be assumed that the value of the discharge coefficient will be less than unity to help the theory, and make it more realistic.

4. The paddlewheel flowmeter was reliable between .3 and 20 ft/s. This range was given to us, and it is known that it is only good to use between these values. When the deviation from the linear path it was on, this is when it can be assumed that the data is not good after. The readings at lower flow rates seem more scattered than the measurements at high flow rates. This means that this type of flowmeter does better with high flow rates.

References

TAM 335 Lab Manual, "V. Open-channel Flow"