This article is based on ORNL-5040 (Corrected), “The KFM, A Homemade Yet Accurate and Dependable Fallout Meter,” by Cresson H. Kearny, Paul R. Barnes, Conrad V. Chester, and Margaret W. Cortner, published by Oak Ridge National Laboratory in January 1978. The full document, including scale-accurate patterns, is in the public domain and freely reproducible.
There’s a piece of survival gear so effective that Oak Ridge National Laboratory spent years testing it, proved it in the hands of untrained families, and then gave it away for free. It measures nuclear fallout radiation about as accurately as the $600 commercial instruments of its day. No batteries. No electronics. A shelf life measured in decades.
You can build one from an empty Pringles can, some aluminum foil, a strip of drywall, and a roll of Scotch tape.
That’s the Kearny Fallout Meter, or KFM. Cresson Kearny and his team developed it at ORNL in the 1970s to solve a real problem: In a nuclear emergency, almost nobody would have a working radiation meter. Civil defense gear sat in storage in the cities. Radio broadcasts would report averages from miles away. The people who most needed to know the radiation level around their own shelter would be flying blind. The KFM fixes that. You can build one in an afternoon. I did, and I’ll pass along the places I stubbed my toe, so you don’t have to.
How It Works
The KFM is an electroscope, one of the oldest radiation-detection instruments there is. The concept is simple. Inside a sealed can, two tiny leaves of layered aluminum foil hang side by side from insulating threads. You charge those leaves with static electricity, the same charge you build rubbing a balloon on your hair. Because both leaves carry the same charge, they repel each other and spread apart. The bigger the charge, the wider the spread.
When gamma radiation passes through the air in the can, it knocks electrons loose from air molecules. Those charged particles bleed the charge off the leaves, and the leaves drift back together. The faster they close, the higher the radiation level.
You read the meter by looking straight down through a clear cover at a millimeter scale. Measure the gap between the leaves before and after a timed exposure, subtract, and look up the result on a chart. That gives you the dose rate in roentgens per hour. No power, no calibration against a radiation source. Built to spec, the geometry and weight of the leaves set the calibration permanently.
Skip the Soup Can
The original instructions call for a standard 8-ounce steel can, about 29⁄16 inches inside diameter. Good luck. Cans have gotten bigger since 1978, and the scale patterns in the old report no longer line up with anything on a grocery shelf.
Here’s my fix: a single-serving Pringles can. The diameter lands close to the original spec, and the part that sold me is that the clear plastic lid is exactly the rigid, see-through cover the design needs. No stretching thin film over the top (Saran wrap is far too flimsy), no hunting for a scrap of Plexiglas or sacrificing a piece of Tupperware. The can’s foil-lined interior does the job of the metal wall, so you don’t need to line it with anything. One container solves two of the hardest sourcing problems in the build.
What You Need
Most of this is already in your house.
For the meter:
• One single-serving Pringles can, emptied and wiped clean — lid included
• About 2 square feet of standard aluminum foil. Regular, not heavy-duty
• 6 inches of doorbell wire or any thin insulated wire
• About 3 feet of the thinnest, clean thread you can work with. Fine twisted nylon or unwaxed nylon dental floss is ideal. (More on thread below — I learned this one the hard way.)
• A piece of drywall (gypsum wallboard), about half a square foot, to bake into your drying agent
• Tape: cloth duct or masking tape for assembly, plus a roll of Scotch Magic Transparent Tape for charging. A little Band-Aid tape or other thin, flexible tape for small joints.
• A wooden pencil, a toothpick, and two strong rubber bands
• Glue, if you have it. One-hour epoxy is best; model airplane cement works.
• Nitrile or latex gloves
For charging: a roll of Scotch tape, wide packing tape, or a piece of hard plastic (ruler, draftsman’s triangle, Plexiglas, etc.) plus dry typing paper.
Tools: 12-inch ruler, scissors, a large sewing needle, a small sharp knife, a hammer, pliers, and a small nail you can sharpen to a point. You can also use a small drill bit instead of making holes with the hammer and nail.
Step 1: Make the Drying Agent
Start here. It takes the longest, and it’s the single most important part of the build. Don’t take my word for it. When I first assembled my meter, I tried charging the leaves before adding any drying agent. Nothing happened. Not a flicker. Then, I baked a batch of gypsum, dropped it in, gave it 15 minutes to pull the moisture out of the can, and tried again. The leaves snapped apart on the first pass. That’s how much this matters.
Here’s why: The foil leaves have to stay bone dry. Humidity conducts electricity and bleeds the charge off the leaves before radiation ever gets a shot at them. The fix is anhydrite, a desiccant you make by baking the gypsum out of drywall.
Cut a piece of drywall about 12 by 6 inches. Peel or soak off the paper facing. Break the white gypsum core into pieces no bigger than half an inch. Spread them one layer deep on a pan or a folded foil tray, and bake at your oven’s highest setting, 400 degrees F or higher for one hour. (No oven? Heat them in a pan over a flame until the pan glows dull red.)
The moment they come off the heat, the pieces are anhydrite and they drink moisture fast. Seal them in a Mason jar while still warm — every minute of air exposure costs you drying power. Never substitute calcium chloride; it’s corrosive and will eat the foil.
Step 2: Prep the Can
Wipe the can clean and dry it. You’re not lining or wrapping anything; the foil interior is your chamber wall. You need four small holes for the stop-threads: insulating threads strung across the inside of the can that keep the charged leaves from swinging into the wall and dumping their charge when the meter gets bumped. Place them symmetrically around the can, an inch or so below the top rim. Two holes opposite each other for one thread, two more at 90 degrees for the second. The old report includes exact-scale patterns, and they’ll get you close, but on a Pringles can I gave up on the wrap-around template and just eyeballed the placement. It’s forgiving.
Punch each hole with a sharpened nail or use a drill, bracing a stick or wooden handle inside the can so you don’t crush it. Make the holes barely big enough to pass a needle, then work the needle around to fold the sharp edges back. Run one clean thread through all four holes to form two crossing stop-threads. Tie tiny wooden toggles (3⁄8-inch slivers of wood) to the ends on the outside to lock the tension, and tape over the holes to seal out air.
One rule runs through this whole build: Never touch the parts of any thread that live inside the can. Skin oil wrecks the thread’s insulating value and drains your charge.
Step 3: Make the Aluminum Foil Leaves
This is the fussy part, and it’s where most first attempts fail. Read it all before you touch the foil.
You need two identical leaves, each eight plies of standard foil. Cut a piece about 4 by 8 inches. Fold in half to 4 by 4 (two-ply), again to 2 by 4 (four-ply), and again to 2 by 2 (eight-ply), lining up the edges exactly on that last fold so one corner comes out perfectly square. Flatten the stack with your fingers until it looks like a single sheet.
Lay your leaf pattern on the foil with one corner on that square corner, trace it with a sharp pencil, and cut it out. Along the thread line at the top, fold the edge up to vertical, then back down into a flat hem. That hem is where the suspending thread rides.
Now the thread, and here’s where I’ll save you some grief. I used the innards of 550 paracord for mine and regretted it. Those little strands cling to each other and fight you the whole way. Use the thinnest thread you can manage. Fine twisted nylon or unwaxed dental floss. Thinner is always better here.
Cut an 8.5-inch piece, lay it along the thread line, and secure it with tiny pieces of Band-Aid tape or a dab of epoxy on a toothpick. The thread should exit both upper corners. Do not touch the inch of thread on either side of the leaf. Repeat for leaf two.
Step 4: Install the Leaves
Hang each leaf inside the can by draping its thread across the rim to the opposite side. The hem side faces out toward the wall; the smooth sides of the two leaves face each other.
Uncharged, the leaves should hang side by side and touching, upper and lower edges level. Look straight down: You want to see a single thin line, no gap, no curl. If they don’t look right, remake them. This is the part that decides whether the instrument works.
Tape the threads to the outside of the rim only — never inside. Then, make the “seat:” cut a 1-inch length of pencil and tape it to the side of the can, just below where the lid grips, with its top about ¾-inch below the can’s top. That’s your ruler rest for taking repeatable readings.
Step 5: Cover Scale and Charging Wire
This is where the Pringles can pays off. The lid is your cover, so there is no film to stretch, and no skirt to trim. Cut the paper millimeter scale from the pattern and tape it to the top of the lid, centered over the leaves and perpendicular to them, with the zero mark directly above the point where the leaves touch. This scale transferred to the lid with no trouble. (The dose-rate table won’t wrap a Pringles can cleanly the way it did the original. Just print it and tape it to the side or keep it next to the meter.)
Make the charging wire from your 6 inches of insulated wire: Strip both ends and put a small right-angle bend at the bottom. Tie two 2.5-inch adjustment threads to a point about 2 inches down from the top. One bare, one with a small sticky flag of tape. Punch a hole in the lid ½ inch from the rim, directly above the centerline between the leaves, and push the wire through until its bare lower end hangs about 1⁄16-inch above the leaves.
Finally, drop your anhydrite in. A single layer on the bottom, no pieces stacked, none tall enough to touch the leaves. Snap on the lid. When you’re not using the meter, store it in an airtight jar with extra drying agent; humidity creeps through over time.
Charging the Meter
Two methods. Tape is faster.
Tape method: Set the charging wire so its lower end sits about 1⁄16-inch above the leaves, held by the sticky adjustment thread. Pull a roll of Scotch Magic tape from its dispenser and hold it an inch or two from the meter. Quickly unroll 10 to 12 inches. Holding the unwound tape taut and perpendicular to the wire, slowly sweep both hands to the side, past the wire, over about 2 seconds. The static charge jumps a spark gap to the wire and runs down to the leaves. You may see or hear a faint spark, and the leaves spread. Aim for at least 15mm.
After charging, lift the wire up via the adjustment threads so it parks near the underside of the lid. That gap keeps the charge from leaking back out. Don’t touch the wire.
Plastic-and-paper method: Fold about 20 sheets of dry paper into a 4-inch square. Pinch the outside firmly and rub a piece of Plexiglas or a plastic ruler briskly on the inside fold, keeping your fingers off the edges. Pass the charged end slowly past the wire, ¼ to ½ inch away. Repeat until the leaves spread. This method dies above about 85 percent humidity. The tape method holds to about 90 percent. Above that, you need a dry-bucket.
The Dry-Bucket
A dry-bucket is a large plastic-covered bucket with a cup of anhydrite in the bottom and two bag-lined hand holes you reach through without breaking the dry seal. It lets you charge the meter when outside humidity would otherwise stop you. It takes about an hour to build. The original ORNL report walks through it in full.
Reading and Using It
Set the meter on a flat surface. Stand a 12-inch ruler on the seat, touch your eyebrow to the top of it, and look straight down. Read where the lower edge of each leaf falls on the scale and add the two numbers — that’s your reading. Never read a leaf touching a stop-thread and never use a reading below 5mm.
To measure a dose rate: Charge the meter and record the reading. Expose it for one of the timed intervals in the table, then read it again. Subtract the after-reading from the before-reading, find that difference in the table, and read across to your exposure time. That’s your dose rate in roentgens per hour.
Example: Before exposure, 17mm. After a 1-minute exposure, 5mm. Difference of 12mm over one-minute equals 9.2 R/hr.
Outdoors, hold the meter about 3 feet off the ground. If the difference comes out under 2mm, recharge and use a longer interval. If the after-reading drops below 5mm, recharge and use a shorter one.
What the Numbers Mean
A few reference points from the ORNL guidance for a healthy adult with no prior significant exposure:
• 6 R per day can be tolerated for up to two months without losing the ability to work.
• 100 R in a week or less is unlikely to cause serious sickness.
• 350 R in a few days is likely fatal when medical care isn’t available.
• 600 R in a week or less is nearly certain to be fatal within weeks.
The encouraging part the researchers stressed: Fallout fades fast. A spot reading 2,000 R/hr an hour after a nearby nuclear disaster drops to roughly 200 R/hr seven hours later and under 20 R/hr within two days. A shelter with 2 to 3 feet of earth overhead can cut your exposure enough that most people in hard-hit areas could ride out the first two weeks and then step out into tolerable levels. The KFM is the tool that tells you when that window opens, and how long you can stand in it.
A Few Critical Reminders
Your drying agent is doing its job if the charged meter, sitting idle, loses no more than 1mm over 3 hours. More than that, swap the anhydrite.
Keep every thread clean and never touch the sections inside the can. Contaminated thread drains the charge and gives you garbage readings. Thinner thread beats thicker every time. Again, skip the paracord guts.
Build one now, in calm conditions, and practice charging and reading it before you ever need it. Then, build a second. In ORNL’s field tests, the families who succeeded were the ones who practiced.
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