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Overview
This procedure is used to measure the dimensions of a
pressure vessel. Because pressure vessels can stretch while
pressurised the dimensions change. The change in dimensions is
important for design purposes. |
Caution
Due to pressures involved, this experiment should be carried
out behind a safety barrier.
Equipment
Procedure A - Unpressurised
This procedure measures the height and circumference of the
pressure vessel in the unpressurised state. This information is
useful for example when matching bottles to see if they will fit
into one another.
Height
- Stand the bottle on its base.
- Balance a set square on the top of the bottle thread
so that an accurate measurement to the base can be made
using a ruler.
- Record the measurement.
Circumference and Diameter
Most bottles have a number of protrusions around the
circumference of the bottle. Knowing the dimensions of these
helps in calculating how bottles will fit together.
It can be difficult to measure the diameter of the bottle
directly because they are soft and often slightly distorted. It
is easier to measure the circumference and then convert that
into a diameter using the formula d = circumference / pi.
Often bottles will have a cylindrical section that is
slightly narrower than the top and bottom sections of the
bottle. See diagram 1.
The middle measurement is useful for splicing bottles
together, and the top and bottom measurements are useful for
determining the rocket's cross sectional area when calculating
drag, or fitting the rocket between launcher guide rails.
- Cut out a 2 cm wide strip of paper with at least one
straight edge.
- Wrap the paper around the bottle so that it sits snugly
up against it and so the straight edges meet.
- Make a marking on the paper where the end finishes.
- Remove the paper and measure the length between the
marks.
- Record this as the circumference.
- Repeat the above steps for the top, middle and bottom
sections of the bottle.
Procedure B - Pressurised
This procedure measures the change in height and change in
circumference of the pressure vessel when pressurised. This
information is useful for understanding how much the rocket will
stretch under pressure and how it may effect any fittings fitted
to the rocket or the launcher.
Height Change
- Completely fill the pressure vessel with water.
- Place the pressure vessel on the
Volume and dimension measuring stand.
- Attach the needle to the top of the bottle. (see
diagram) and make sure it lines up with the ruler on the
side of the stand.
- Record the reading on the ruler.
- Connect the air supply and slowly increase the
pressure to the desired level.
NOTE: You should have performed the
burst test on
the bottle prior to this test in order to not exceed the
burst pressure.
- When the pressure reaches the desired value take the
reading on the ruler. The difference between the two
measurements is the height increase at that particular
pressure.
- Depressurise the bottle.
Circumference Change
- Completely fill the pressure vessel with water.
- Place the pressure vessel on the
Volume and dimension measuring stand.
- Attach one end of the measurement strip to the
desired section of the bottle with a piece of tape. The
middle section of the bottle is the most often useful.
- Wrap the strip around the bottle one full
circumference.
- Hang the weight from the other end of the strip.
- Either record the reading on the strip where it
starts the overlap, or draw a line on the strip.
- Connect the air supply and slowly increase the
pressure to the desired level.
NOTE: You should have performed the
burst test on
the bottle prior to this test in order to not exceed the
burst pressure.
- When the pressure reaches the desired value take the
reading on the strip or make a second line. The difference between the two
measurements is the circumference increase at that particular
pressure.
- Depressurise the bottle.
Notes
- Appropriate protection should be worn when performing
this experiment as it may involve getting close to the
bottle under pressure.
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