Amateur Scientist: Measure the Height of the Ozone Layer - Make:
1. CleanCleanliness and a dust-free room are essential when assembling an ultra-high-gain twilight photometer. While holding the circuit board by its edges with clean hands, thoroughly wipe both sides with a. . .
1. CleanCleanliness and a dust-free room are essential when assembling an ultra-high-gain twilight photometer. While holding the circuit board by its edges with clean hands, thoroughly wipe both sides with a clean paper towel soaked in isopropyl alcohol. Allow the alcohol to stay on the board for a few minutes. Then hold the board securely by its edges and blow away the alcohol with a blast from a canned air duster. 2. Assemble CircuitYour best friends here are a hands-free method of holding the circuit board, along with disposable vinyl gloves and a head-mounted magnifier with an LED illuminator. Begin assembly by placing the dry board atop a clean saucer with the copper pads facing up. Keep it in place with a bit of masking tape. 2a. CapacitorNext, solder C1 to the board. Be sure to hold the tiny capacitor with tweezers and do not touch it or the surface of the board with your bare fingers. Apply a thin layer of solder to one of the two pads for C1. Then carefully place the two ends of C1 over its two pads and secure it in place with tweezers or masking tape. Carefully touch the soldering iron to the junction of the chip and the solder-covered pad. The capacitor should remain in place. Touch a solder wire to the junction of the second pad and the circuit board. The solder should flow around this end of C1 and secure it to the board. For more tips on surface-mount soldering, see the last step of this article. 2b. Amplifier chipThe gain of the ADA4530 is so high that the chip is extremely sensitive to anything touching pin 8, the input pin to which the signal from the LED used as a photodiode will be connected. Therefore, pin 8 must be very carefully bent, from straight down to straight away from the chip, before its remaining seven pins are soldered to the board. Bending pin 8 requires considerable care to avoid dropping the expensive chip, which might bounce off the paper towel and disappear on the floor. My approach is to hold the ADA4530 with the narrow jaws of clean needlenose pliers and use clean SMD tweezers to carefully bend pin 8 outward so it is perpendicular to the chip. (Be sure to wipe the pliers and tweezers with isopropyl alcohol before using them. ) I have used this method for a dozen or more ADA4530 SMD chips, and none were dropped. If you have any spare SMD ICs, practice this procedure before doing so with the expensive ADA4530. Set the ADA4530 on a clean paper towel. On the circuit board, carefully place a thin layer of solder over the pads for pins 1 and 5 of the ADA4530. Remove excess solder with solder wick. While wearing disposable vinyl gloves, secure the chip to the board with a bit of masking tape across pins 3, 4, 5, and 6. Then very carefully touch your soldering iron to where pin 1 of the IC touches its solder-coated pad. If the ADA4530 is properly aligned, carefully remove the tape and solder pin 5 to the board. Complete the installation of the ADA4530 by touching the remaining pins with the iron and solder wire. Use your magnifier to be sure all seven pins are properly soldered. 2c. Mini connectorNext, insert the four pins of a 4-conductor mini connector socket into their holes on the top side of the board and then solder them in place on the back side. 2d. LEDCut a square of vinyl tape or Velcro-style hook-and-loop tape and apply it to the back side of the board where the LED will be installed. This insulates the LED from the board. Be sure to leave the holes through the board open. A 620nm red LED is soldered to the back side of the board. I have used both wire-lead and surface-mounted LEDs with extended, flat terminals, the latter of which I prefer. The positive pin, marked with a “+” sign, is soldered between the two center pins of the mini connector on the back side of the board. While still wearing vinyl gloves, add solder between the two center pins of the connector. Then press the LED’s anode terminal against the two connector pins and heat with your soldering iron until the terminal is melted into the solder between them. Carefully remove the insulation from both ends of a 1" length of thin hookup wire and solder one end to the LED’s cathode terminal. (Bend the terminal slightly upward if it is touching the board. ) Insert the other end of the wire through the pass-through hole in the board. It will be connected later. This shows the back side of the board with the LED soldered in place. Lower side of circuit board showing the LED. Note that the LED anode is soldered to the junction of two center tabs from the socket on the front side of the board. 2e. Feedback resistor and capacitorIt is essential to wear vinyl gloves for the next step to avoid touching RF, a 5GΩ or 10GΩ resistor, and CFwith bare fingers. First, carefully solder the leads of CF, a 10pF or 20pF capacitor, across the leads of RF. The capacitor should be just below the resistor but not touching it. Clip off CF’s excess leads. Soak the combined RF+ CFfor several minutes in a clean water bottle cap filled with isopropyl alcohol to remove all impurities. Agitate RFand CFwithin the alcohol before removing them. Then hold RFand CFwith clean tweezers and blow the alcohol away with a duster. Very carefully insert one lead from RFthrough the hole between the pads for C1 and CFand solder the lead in place. Bend the second lead from RF+ CFacross the board and solder it to the thin wire from the LED that you inserted from the back side. The board is completed by carefully soldering a short length of insulated wrapping wire between this junction and the unconnected pin 8 of the ADA4530. Top side of circuit board, before assembly. Top side of circuit board, after assembly. The C2 capacitor (100nF) is used only when the circuit is powered by two 9V batteries. These photos show the board’s top, bare and assembled. The large orange component is RF, a 5GΩ feedback resistor (use 10GΩ for more sensitivity). The round object below it is the feedback capacitor CFsoldered to the leads of RF. These components and the board itself should be touched only when you are wearing disposable vinyl gloves. The circuit diagram shows how the various connections are made between the circuit board, 4-pin plug and socket, the power switch, the 9V battery, and the Onset 16-bit data logger. After the external connections have been made, dip the entire assembled board into a small container filled with isopropyl alcohol and gently shake the board for 30 seconds or so. This will remove dust and other contaminants that will greatly reduce the circuit’s sensitivity. Remove the board and spray away the alcohol with a burst from a duster, and install the board into the end of the lens tube. 3. Enclosure and LensFor best results, the photometer’s LED should look at the twilight glow through a magnifying lens with a narrow field of view. While you can use an inexpensive plastic Fresnel lens, I used a high-quality glass lens and lens tubes from Thorlabs identical to those in the NASA photometers (see Materials list). These provide stability, but they can be replaced by an inexpensive Fresnel lens about the size of a credit card, cemented 1 focal length away from the LED. Thorlabs sells a wide variety of threaded tubes for their lenses. These tubes come with internal threaded rings. The rings that came inside the tubes I ordered fit perfectly around the 7/8" circuit board designed by Don Wilcher. Inserting the assembled board into a threaded ring permits the focus to be adjusted, and also allows the circuit to be removed for troubleshooting or for changing the LED. Select a ring that fits tightly around the circuit board. While wearing disposable vinyl gloves, carefully cement the board to the inside of the ring with a very thin line of cement. I used Gorilla Clear Grip. Make sure no cement touches the threads, for that will prevent the ringed board from being inserted into a lens tube. I assembled my photometer inside a 2¾"×2¾"×5½" tin enclosure from Hobby Lobby. I drilled a 1" hole through the center of the enclosure’s bottom using a step drill. This is best done by inserting a 4" scrap of 3×3 lumber into the enclosure. Hold the enclosure straight up with the wood roughly centered inside and resting on a flat surface. Begin carefully drilling through the center of the enclosure’s bottom until the hole is 1" across. Wear leather gloves to avoid scratching your fingers. After the hole is carefully smoothed, insert the threaded end of a Thorlabs lens tube into the outside of the hole and screw it into a second lens tube inside the enclosure. Then push a 4" section of hollow black plastic pipe insulating foam over the internal lens tube. This provides a firmly centered mount for the circular circuit board installed inside a threaded ring. Next, drill two 1/8" holes near the base of the enclosure, one for the power switch and one for the data logger cable. The wires from the 4-conductor connector are then connected as shown. Coil the input cable for the data logger and insert it between the foam cylinder and the enclosure wall as shown . The 9V battery can be mounted to the wall with 1" squares of self-stick velcro tape. Circuit board installed in lens tube and connected to on-off switch, 9V battery, and data logger cable. While wearing vinyl gloves, carefully screw the assembled circuit board into the end of the 3"-long tube that mates with the end of the interior 3" tube. It is extremely important to avoid routing any of the external wires over the circuit board. Besides contaminating the board, they might press RFand CFonto the ADA4530. Therefore, all external wires must be placed around, and never over, the exposed circuit board. Finally, install the Thorlabs lens over the end of the external tube. First, blow away dust from both sides of the lens. Use a Thorlabs lens cap or a small plastic bag to protect the lens when the photometer is not in use. This image shows the assembled twilight photometer before an evening twilight session. Completed twilight photometer. Using Your Ozone PhotometerThe completed photometer should be placed on a flat surface well away from artificial lights. I place a plastic level across the lenses of my photometers and use plastic furniture levels to make sure they are perfectly level before taking twilight measurements. Various websites provide the exact time for sunset and sunrise at your location. I set up my instruments and switch them on around 10 minutes before sunset and an hour before sunrise. At my location (29. 6 degrees North), an observation time of 1 hour will provide data from the ground to around 150km (93 miles). While a clear sky is ideal, data can often be collected through thin clouds. Processing the DataMyprevious twilight photometry article inMake:provides full details on how to interpret the instrument’s output during twilight. Back then my major interest was finding bulges in the data plot caused by aerosols in the upper atmosphere from African dust, forest fire smoke, and the aerosols around 20km known as the Junge layer. (For those aerosols, near-infrared LEDs work best. )The peak height of the ozone layer varies over time. At my latitude of 29. 6 degrees, the peak can range from 20–30km. Here are three typical plots during cloud-free twilights with an average peak of 27km. Fluctuations in the traces are mainly caused by aerosols, which is why I am experimenting with simultaneous measurements at 885nm, where ozone absorption is very low and aerosol effects can be problematic. Three ozone layer peaks measured by the twilight photometer from Mims’ Texas field. The average peak of 11 of my twilight ozone measurements made while preparing this article was 25. 13km — almost equal to the NASA average over my site of 25km. More ozone science:epa. gov/ozone-layer-protectionHow to Install Surface-Mount ComponentsTwenty years ago, Heathkit hired me to develop a surface-mount training product complete with two manuals and parts. While some of the components I used back then are now smaller, the assembly components I used back then have not changed. They include a head-mounted magnifying lens or lenses (preferably with a built-in lamp), SMD tweezers, needlenose pliers, a sharply pointed soldering iron, and a dose of patience. Some SMD tool suppliers include a practice board and a strip of SMD parts with their products. This allows you to practice preparing an SMD board, mounting the parts, and soldering them in place. Begin a practice session by placing a clean sheet of textured paper towel on your work table, and tape its corners to your work surface. Place your soldering iron and tools on either side of the paper towel. Next, carefully clean both sides of the SMD circuit board while holding it by its edges between your thumb and forefinger. Do not touch the circuit side of the board after it is clean and shiny. Use a small square of masking tape to hold the board with its conductive traces facing up to the clean side of a 3×5 card placed on the paper towel. Next, place the SMDs on the paper towel. (Don’t even think about sneezing!) SMD resistors and capacitors are easily soldered across two pads on the board. Touch the point of your iron to one of the two pads for a second or two, and then touch a thin length of solder to where the iron touches the pad. Your goal is to place a thin layer of solder on the pad. If the solder layer bulges upward, remove excessive solder from the pad with desoldering wick. Next, place the SMD component over the two pads and hold it in place with tweezers or a piece of masking tape. Then touch the point of your soldering iron to where the component rests on the second pad and melt a bit of solder between the unsoldered pad and the SMD part. Soldering SMD ICs is a bit trickier but can be achieved with practice. My approach is to apply a thin layer of solder to the pads for two opposite corners of the IC. I then hold the chip in place with a bit of masking tape and solder those two pins. When the pins are soldered exactly in place, I then very carefully solder the remaining terminals. Avoid excess solder. If you accidentally bridge two adjacent contacts and pads with solder, remove the bridge with desoldering wick. This article appeared inMake:Volume 98.