Building the TERC Rubber-Thread Ozonometer: Steps

Construction Steps

1. Draw a line from the center of one drilled hole to the center of the other drilled hole.

2. Mount the paper protractor and bearing assembly on
the board.

a. Cut out the protractor along the dotted lines.

b. Place the hole in the protractor on the baseboard over the 6.3 mm (1/4") hole which has been drilled in the board for the bearing shaft.

c. Insert the bearing shaft through the bearing cover (glued to the bearing), the spacer, and the protractor, in that order.

d. Screw the bearing shaft into the 6.3 mm (1/4") diameter pre-drilled hole in the baseboard. Tighten the screw far enough so that the bearing fits snugly against the metal spacer. The bearing should turn freely and not wobble.

e. Orient the 60deg. line on the protractor parallel to the top edge of the baseboard.

f. Tape the corners of the protractor to the board with transparent tape.

3. Place the conversion chart on the board.

a. Place the conversion chart in the lower right hand corner of the baseboard.

b. Tape the corners of the conversion chart to the board with transparent tape.

4. Insert the straw into the tube extending from the bearing cover.

The straw should fit into the bearing cover snugly. If the straw fits loosely, increase the outer diameter of the straw by wrapping a small amount of transparent tape around its end. Make sure the straw will not slip out of the bearing cover.

The straw should not touch the board as the bearing spins. If the straw touches the board, you will need to adjust the bearing cover: with two fingers, hold down the bearing cover firmly and use your thumbs to carefully bend the yellow plastic end of the holder upwards slightly. Be careful not to pry the bearing cover off the bearing. (If you do, you will need to re-glue it.)

5. Set the thumb screw by screwing it straight into the board so it goes through about 10 turns. It will be hard at first-you might need pliers. After tightening the thumbscrew, loosen it several turns and then rotate it back and forth until it can be easily turned with your hand. If there are splinters coming up from the baseboard where the thumbscrew enters, break them off so that later the rubber thread will not catch on them.

a. Before handling the rubber thread, clean your hands carefully. The small amounts of dirt and oil normally found on hands might weaken the rubber and add an unwanted variable to the experiment.

b. Cut a section of rubber thread approximately 300 mm (30 cm) long. The baseboard is approximately this long.

c. Lay the 300 mm long section of rubber thread out as straight as you can without stretching it. Keep it flat.

d. Carefully mark off a 230 mm (23 cm) length by making two marks with a pen or pencil exactly 230 mm apart. Do not use anything with a sharp point!

e. Cut a piece of string from the spool of sewing thread approximately 15 cm (6 inches) long. This string will be used to tie the loop of rubber thread. Lay the string down on the table and form the rubber thread into a loop on top of the string.

Line up the ends of the rubber thread so they overlap and the two marks you made at 230 mm meet. Fasten the ends of the loop together by tying the piece of string tightly around the rubber. As you tighten the knot, carefully line up the two marks at the knot. Try to keep the rubber thread untwisted as the loop is made.

f. After the knot is tight, cut the free ends of the rubber thread to about 5 mm, and trim the ends of the string.

Note: Every time an ozone measurement is taken a new loop of rubber thread is used. To make it easy to measure the rubber thread, you can draw a set of marks on the baseboard of the ozonometer indicating the 300 mm and 230 mm lengths. Also, if you plan to make a series of measurements, cut and tie all the rubber loops you will need at one time and store them in an air-tight plastic bag.

7. Place the loop of rubber thread around the bearing and the screw.

a. Place the loop of rubber thread around the bearing, making sure the rubber thread goes around the middle of the bearing and is parallel to the surface of the baseboard.

b. Next, guide the rubber thread down to and around the screw.

c. Make sure the knot in the rubber thread is not underneath the shield.

d. Carefully pull the rubber loop around the screw so that the pointer is roughly aligned with the 0deg. mark on the protractor. For coarse adjustments, use a paper clip to work the loop around the screw. For fine adjustments, turn the thumb screw in the baseboard about one turn in either direction.

8. Construct the ozone scrubber.

a. Cut the blotter paper along the solid lines. Do not cut on the dotted lines.

b. Next, fold the blotter paper lengthwise, along the dotted lines, into four equal sections. In order to make accurate folds, line a ruler up to each dotted line, and scribe along the line with a dull point (scribing is the process of marking a material so as to leave an indentation). Pressing hard with a hard fine-point pen works well.

9. Make a shield for the scrubber.

a. Cut along the solid black line on the transparent plastic ozone shield. Do not cut along the dotted lines!

b. Next, you will be folding the plastic along the dotted lines. However, before you fold, scribe along the dotted lines on the side of the plastic into which the fold is made. Pressing hard with a hard fine-point pen works well.

c. After scribing along the three dotted lines, fold the plastic sheet as shown below. Note: one of the folds is in a different direction from the other two.

10. Install the shield on the baseboard.

a. Line up the folded edge of the shield along the line you drew that runs from the bearing to the screw.

b. The left hand side of the rubber thread should run down the middle of the triangular part of the shield, without touching the sides. You may need to adjust the position of the shield slightly so that the thread doesn't touch the sides.

c. Adjust the shield carefully so that neither end touches the bearing or the adjusting screw. This is important for later adjustment and operation of the instrument. After getting the shield positioned use some transparent tape to hold it in place.

d. Install the plastic shield on the ozonometer baseboard using two push pins so that the shield is held down and the flap can be tucked under one side of the push pins. You may need a small hammer to get the push-pins into the baseboard.

11. Position the ozone scrubber in the shield.

a. Once you have finished installing the plastic shield and have the rubber thread in its position, make sure the flap of the plastic shield is out from underneath the two push pins.

b. Next, unfold the blotter paper, and slide one side of it under the length of rubber within the shield. Fold the blotter paper into a triangular structure. Fold the plastic shield around the blotter paper and slide the flap under the push pins.

12. Check the Ozonometer to make sure that the pointer and bearing are moving freely without binding.

a. To check the static friction of the assembly, first adjust the pointer to 30 degrees either by turning the thumb-screw or by slipping the rubber thread around the thumbscrew using a paperclip.

b. Now, pull the pointer gently all the way out to either edge of the protractor and let it go. Repeat this step several times.

Note: The pointer should return to within one or two degrees of the same spot each time. Watch the blotter paper as you move the pointer to each end of the protractor. The blotter paper should not move. If it does, that means the rubber thread is binding somewhere on the blotter paper. To fix this, look down the rubber thread as it passes through the blotter paper, and adjust the folded paper where the rubber is binding.

c. Adjust the pointer to zero degrees. For coarse adjustments, use a paper clip to work the loop around the screw. For fine adjustments, turn the thumb screw in the baseboard about one turn in either direction.

The ozonometer is now ready to take ozone measurements!