Flowrate Measurement: Experimental Uncertainties

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The uncertainty of the flowmeter measurements is a constant


2% of the flowmeter full
-
scale (e.g. 0.02

1.12 gpm).
The uncertainty of the volume collection calculation must be derived and calculated for each data po
int

THERMODYNAMICS

Winter 2005





ME 241



Guessous/
Oakley






Laboratory Assignment #1


Flowrate Measurement: Experimental Uncertainties



Rotameters












The purpose

of this laboratory exercise
is

to introduce you to fluid flowrate measurements, cal
ibration,
and associated experimental uncertainties. You will be comparing volumetric flow rate measurements,
taken directly from two different rotameters, to those d
etermined indirectly using a volume collection
method that involves a graduated cylinder
and a timer
.


Experiment

Specifications
:

1.

With the experimental apparatus provided, set up the experiment as shown in the accompanying
sketch.

2.

Use the volume collection method t
o indirectly measure flowrate.
Conduct steady
-
flow
experiments, monitori
ng both flowmeters and simultaneously measuring collection
-
time and
collection
-
volume, at 10
-

15% intervals over the full range of the smaller flowmeter. Use two
methods:

a)

Hold collection
-
volume,


, constant at 9
00 ml, and record the variable collectio
n
-
time, t.

b) Hold collection
-
time, t, constant at 8 sec, and record the variable collection
-
volume,

.


NOTE:
To insure repeatability, repeat each test two times (for a total of
3

trials each) and use
average values when calculating your flowrate.

Tabulate

your data by collection method.

4
.

Plot two separate graphs, one for each flowmeter, plotting
the
volumetric flowrate

measured
directly on the flowmeter
,
, ml/sec., on the abscissa, and
the
flowrate
s

measured indirectly with
the volu
me collection methods,
, ml/sec., on the ord
inate
. Use the same scales for ordinate and
abscissa and plot the results from
both

methods on each graph.

4.

Derive equations for the uncertainty and relative uncertainty of the indirectly

measured volume
flowrates. T
abulate the uncertainty values
and show the uncertainty for each data point on the
above graphs. Use error bars for one volume collection method and uncertainty bands for the other

method.


5.

W
hich
volume collection technique
is the most accurate, the constant collection
-
volume or the
constant collecti
on
-
time?

Explain why.

Timer


Graduated

Cylinder


0.81

gpm

1.12 gpm

H
2
O

Fluid Measurement and
Supply Unit

The uncertainty of the flowmeter measurements is a constant


2% of the flowmeter full
-
scale (e.g. 0.02

1.12 gpm).
The uncertainty of the volume collection calculation must be derived and calculated for each data po
int

6.

Draw a 45

straight line through the d
ata on each of the two graphs.
Is the volume collection
technique sufficiently accurate as a means of calibrating th
ese flowmeters? How can the
experimental uncertainties
associated
with the method be improved?


Laboratory Report:

Over the course of the first three laboratory assignments, you will learn to write a complete engineering
laboratory report. In this first
lab you will focus on a few of the most important elements. For this
report, you are asked to include the following:

a)

Title page with Abstract, (1 page)

b)

Theory including the Uncertainty Analysis, (½


¾ page)

c)

Experimental Results and a brief Discussion, (½
page for discussion and page count as required
for graphs and table)

d)

Appendices (copy of handout, original signed and dated data sheet, sample calculations)


Do not exceed the page lengths shown above.


Each of the above elements is described in more detai
l in the
Guide to Laboratory Report Writing.

Worthy of repetition are the following points:

-

The abstract is a
clear

and
concise

summary of the information already stated in the report

-

The theory should be presented with enough detail so that it is clear t
o a reader who is
unfamiliar with this experiment.

-

Experimental results, presented in tabular or graphical form, should be able to stand alone within
the context of the report. That is, no one should have to refer to the body of the report to
identify a l
ine, data point, or other element in a graph or table.

-

Finish your report early so that: a) all group members have a chance to review the complete
report, b) the abstract can be written, and c) any errors or omissions can be found and
corrected. Remember
that
no late lab reports are accepted
.