Missoula Technology and Development Center: Shaping Solutions for the Forest Service
Simple Solution Stops Pepper Spray
Pepper spray stops bears. That's why Forest Service employees working in bear country carry it.
Pepper spray also stops humans. A 1-second burst of spray directly in someone's face could cause temporary blindness; induce choking, coughing, and nausea; and restrict breathing. If the spray accidentally discharged in a vehicle, the driver would have difficulty stopping safely.
The center was asked to develop a container that could be used to safely carry bear spray.
Mechanical engineer Dick Karsky not only needed to figure out a solution, he needed to figure out a realistic test to show that it actually worked.
Karsky sealed the bear spray canister in a PVC plastic container that would prevent any leakage. A pressure gauge showed that the container held 150 pounds of pressure per square inch for 4 hours without leaking. Karsky had calculated that 150 pounds of pressure could be generated if the canister were heated to 125 degrees, as could easily happen in a vehicle on a summer day.
A pressure gauge proved whether containers leaked.
For the test, Karsky rigged a handle on a threaded rod sealed into the cap. When he twisted the handle, the end of the rod fired the bear spray inside the container. The pressure rose to 60 pounds per square inch. The container didn't leak. The problem was solved.
But Karsky wasn't finished. He knew that the O-rings that sealed the container's caps could fail if they weren't installed properly or cracked with age. He also knew that it would be impossible to open the container if it were sealed at low altitude and taken to high altitude. He recommended that a tire valve be installed on the cap of all PVC containers so they could be tested to prove they held pressure and to relieve pressure when they were taken to different elevations.
Mark Matheny, founder of pepper spray manufacturer UDAP Industries in Bozeman, suggested storing pepper spray in a simple 2-liter Nalgene container. Meanwhile, Dave Dieziger from the Forest Service's Missoula regional office had tested terry cloth pouches. Dieziger found that the terry cloth trapped the pepper oil while allowing the propellant to escape. The pepper oil is not dangerous when it is a liquid.
Karsky combined the approaches. First he tested the Nalgene container. It held just 50 pounds per square inch of pressure before leaking, not enough to contain the aerosol if the container burst. He lined the container with 11.2-inch-thick open-cell, fine-pore urethane foam used for furniture padding. Karsky drilled a vent hole into the container's top and placed a 2-inch-thick circular plug of foam under the top.
The recommended storage container is a simple
Nalgene plastic bottle lined with open-cell furniture foam.
The foam traps the active ingredient while allowing
the propellant to escape.
For testing, he drilled another hole into the top of the container and used a rod to fire the pepper spray. The foam trapped the active ingredient. The problem was solved a second time.
This solution was inexpensive, simple enough for anyone to make, and relatively foolproof. This is the solution that Karsky recommended. Matheny now sells this container as an accessory.
Each year, Matheny hears about canisters of bear spray that have leaked or exploded after being left in a hot car. One canister left on a dashboard blew out the windshield.
Matheny has never heard of a problem from anyone who was using the container to store bear spray. Forest Service safety regulations now require that pepper spray be in one of the MTDC-approved containers whenever pepper spray is carried in a vehicle or in a light aircraft. The spray cannot be carried on commercial aircraft.
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