Flow Measurement
CHEG 2810 Data Acquisition
10/4/09
Ken McFarland & Pacscal Matengo
Abstract:
In this lab we constructed calibration curves for three different types of flow meters. A
rotameter, an orifice plate, and a magnetic flow meter. This was done by finding the flow rate
manually, and comparing it to the reading on each of the meters at that known flow rate. This
was done several times for each meter, to reduce errors, and to obtain multiple readings to
compare against each other.
We found the meter constant for the rotameter, and we were able to build a VI in
LabView that converted the output from the magnetic flow meter into the flow rate. We were not
able to finish the portion of the lab involving the orifice plate, however, due to mechanical
problems with the orifice plate meter that we were to use.
Introduction:
The purpose of this lab was to gain practice in creating calibration curves, and to
familiarize us with flow meters in general. It also provided us with a chance to create a LabView
VI on our own, which we had not done before. This was very helpful in making us more
comfortable using LabView on our own, since we had previously only used it to copy things that
we were watching Professor Hladky do step by step. The chance to use it on our own was very
enlightening, as we were not nearly as familiar with LabView as we had initially thought we
were after completing the first lab.
This lab also was an introduction to measuring fluid flow, which seems to be something
that we will continue to do, both in this lab and in the chemical engineering program in general.
It showed us that fluid flow meters do not show the flow rate directly, or at least the types that
we used in this lab do not. Instead, they gave some other value that was proportional to the flow
rate of the fluid, and we had to find the flow rate manually, so that we could find some constant
that equaled that known flow rate when multiplied by the value the meter gave us.
Results and Discussion:
The first meter we calibrated was the rotameter. We set up the system of pipes so that the
water passed through the rotameter, and only the rotameter, and then we recorded the flow rate,
which we found using a bucket and a stopwatch, and the height of the bob in the rotameter. We
found these two data points for seven different flow rates, and calculated the meter constant for
each of them individually. Then we discarded two of the first three values that were obviously
too high when compared to the others, most likely due to the fact that we had never done this
before. We then averaged the remaining five values, to get a final meter constant for our
rotameter of about 113. This was found with the height of the bob measured in meters, and the
flow fate of the water in kg/s.
We found the mass flow rate for all of our water samples rather than the volumetric flow
rate, as it was much more precise to measure the mass of the water collected using the provided
scale, and divide that by the time it took to collect it than it would have been to eyeball the
volume of water in the bucket and divide that by the time to find the volumetric flow rate.
The second meter that we were supposed to use for this experiment was the orifice plate.
However, when we examined the meter before starting the second portion of the lab, we noticed
that it was leaking mercury metal. We brought this to the attention of one of the T.A.’s, who
found Professor Hladky, and showed it to him. Professor Hladky instructed us to skip the second
portion of the lab, so we did, moving directly to the magnetic flow meter.
If we had done the second section of the lab, we would have been creating tow additional
calibration curves, for the two outputs of the orifice plate meter. This meter constricts the flow of
the liquid, and measures the difference in the pressures, or the pressure differential, between two
points, one in front of the obstruction, and one directly behind it. This pressure differential is in
some way proportional to the flow rate of the liquid through the pipe; however we were not able
to determine if it is directly proportional or not, due to the fact that we could not collect the data
for ourselves.
The third meter that we had to create a calibration curve for was the magnetic flow meter.
These meters work by inducing a charge in the liquid flowing through the meter, and then
measuring the liquid’s electric potential, as the liquid flows through the meter. The electric
potential is compared to a maximum electric potential that is a property of the meter, and does
not change. Therefore, the electric potential, or rather flow rate, is displayed as some percentage
of this maximum electric potential or maximum flow rate. This percentage can also interpreted
by a computer if they are sent to it. They will be sent to a computer as an electric signal with a
voltage that corresponds to the percentage on the meter.
The calibration curve for the meter was created in the same manner as it was for the
rotameter, and we found that the maximum flow rate that the magnetic flow meter could measure,
which would correspond to a reading on the meter of 100 percent, was 2.11kg/s. We also had to
create a VI in LabView to convert the voltage output by the meter into the flow rate. This
calibration curve was more complex than the others, because before we could create the
calibration curve, we had to build a VI so that we could we could see the readings from the meter
in terms of voltage. A picture of the VI we created, and the data it received in its raw form are
included in figure 1. After we had the raw data, we had to create a calibration curve in the same
manner as the first two were created, by comparing known flow rates to the meter readings at
those flow rates. This allowed us to multiply the voltage we received from the meter by a
constant to get the flow rate of the liquid. A picture of the VI with the constant and outputting the
mass flow rate in kg/s is attached in figure 2.
Procedures:
For the rotameter, we had to measure the height of the bob, and find the flow rate of the
water using the bucked and stopwatch for between five and ten different flow rates. We found
this data for seven flow rates, and ended up using the data from five of them to create our
calibration curve, for reasons explained earlier. We knew from the equations that were given in
our lab handout that the height of the bob varied linearly with the flow rate, or in other words,
they were directly proportional to each other. This meant that the height of the bob times some
constant would always equal the flow rate. We used our known flow rates and bob heights to
find the meter constant for our rotameter.
The magnetic flow meter gave the result as a percent of some theoretical (for our pipe
system, our pump topped out at about 65%) maximum flow rate. That maximum multiplied by
the percent the meter gave us was equal to the flow rate of the water. Or, in other words, some
known flow rate divided by its percent of the maximum flow rate is equal to that maximum flow
rate. We found that the magnetic flow meter we were using had a maximum flow rate of
approximately 2.11 kg/s. We also had to create an additional calibration curve for this meter,
relating the voltage it sent to LabView to the flow rate of the water. This was accomplished in
the same manner as the other calibration curves were created, and we found that multiplying the
given voltage by 0.32 would give us the flow rate of the water in kg/s.
Conclusions:
We found that the maximum flow for the magnetic flow meter was 2.11 kg/s, and the
voltage constant for it was 0.32. The meter constant for the rotameter was 113kg/(m*s).
After this lab we have had significant experience with creating calibration curves for
meters that are linearly related to whatever value they are measuring. If the orifice plate had been
working, we may have also gotten to create calibration curves for a meter that was not directly
proportional, however, we don’t know that for sure. We will, however, no doubt see many more
presentations on this lab by different groups, so hopefully we will be able to at least learn
secondhand about how to build calibration curves for orifice plate meters.
Recommendations:
If we had to do this lab again we would definitely take many more data samples than we
did for the rotameter. We would also be much more precise in finding the flow rate using the
bucket and stopwatch method, as that is where our data has the highest chance of error, and it
was the least precise part of entire lab for us. We would also do the orifice plate meter portion of
the experiment. As we will not get a chance to do redo that portion of the experiment, we will
instead have to make sure to pay very close attention when the other groups who do this lab
present their findings on the orifice plate meter.
Figure 1
Figure 2