Measure the diameter (D) of the drinking straw using the ruler. The diameter is the thickness of the straw.
Calculate the cross-sectional area of the straw. The cross-sectional area (A) is given by the formula: A = (pi) x (diameter/2) x (diameter/2). For example, a straw with a diameter of 0.5 cm will have a cross-sectional area A = (pi) x (D/2) x (D/2) = (3.14) x (0.5 cm/2) x (0.5 cm/2) = 0.20 cm^2.
Calibrate the straw by calculating the separation distance (S) between 0.2 cm^3 division markings on the straw. The spacing between two adjacent markings S = (0.2 cm^3)/(A) = (0.2 cm^3)/{ (pi) x (D/2) x (D/2)}. For example, the 0.2 cm^3 graduation markings on a 0.5 cm diameter straw will be 1 cm apart: spacing = (0.2 cm^3)/{ (pi) x (D/2) x (D/2)} = (0.2 cm^3/{ (3.14) x (0.5 cm/2) x (0.5 cm/2)} = (0.2)/(0.2) = 1 cm.The choice of volume markings determines how accurately fluid volumes can be measured. Choosing larger volume divisions (for example, using 0.5 cm^3 markings) will make the measurement of volumes that are smaller than 0.5 cm^3 inaccurate.
Use the permanent marker and the ruler to measure and draw the graduation markings along the side of the straw. Make the first mark or tick a distance S from the bottom end of the straw. Label this mark as 0.2 cm^3. Mark the rest of the straw with ticks placed a distance S apart. Add 0.2 cm^3 to each preceding tick or marking to get the following graduation volume label. For example, the second label will be 0.4 cm^3 = first volume label + 0.2 cm^3 = 0.2 cm^3 + 0.2 cm^3. Once the markings are in place, the straw will serve as a simple, graduated 0.2 cm^3 pipet.
Lower the straw into the liquid and then place your index finger over the open end of the straw. Slowly lift the straw from the liquid without moving your index finger. If the straw contains too much liquid, gently move your index finger over the top end of the straw in a rubbing action. The level in the straw will fall slowly as air enters the top of the straw. Continue to move your index finger this way until the liquid in the straw has dropped to the desired level. Be careful not to lift your index finger completely off the top of the straw as this will cause all the liquid to drain from the straw.
Record the volume of fluid in the straw by noting the height of the column of liquid in the straw. Read the bottom of the meniscus (curved surface of the fluid column).
Release your index finger from the top of the straw to allow the liquid in the straw to flow out.