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Measuring Pressure Vessel Dimensions Procedure

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

  1. Stand the bottle on its base.
  2. 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.
  3. 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.

  1. Cut out a 2 cm wide strip of paper with at least one straight edge.
  2. Wrap the paper around the bottle so that it sits snugly up against it and so the straight edges meet.
  3. Make a marking on the paper where the end finishes.
  4. Remove the paper and measure the length between the marks.
  5. Record this as the circumference.
  6. 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

  1. Completely fill the pressure vessel with water.
  2. Place the pressure vessel on the Volume and dimension measuring stand.
  3. 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.
  4. Record the reading on the ruler.
  5. 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.
  6. 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.
  7. Depressurise the bottle.

Circumference Change

  1. Completely fill the pressure vessel with water.
  2. Place the pressure vessel on the Volume and dimension measuring stand.
  3. 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. 
  4. Wrap the strip around the bottle one full circumference.
  5. Hang the weight from the other end of the strip.
  6. Either record the reading on the strip where it starts the overlap, or draw a line on the strip.
  7. 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.
  8. 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.
  9. Depressurise the bottle.

Notes

  1. Appropriate protection should be worn when performing this experiment as it may involve getting close to the bottle under pressure.


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