Last year my 4-year-old can of bear spray reached its expiry date, completely unused, and I wondered how sound the basis of that date was. Reddit showed plenty of others wondering or asserting on the matter, several outdoorsy sources reported on testing, and there was a journal article from researchers at Brigham Young University.
The good news starts with: the active ingredient, capsaicin, does not degrade.
It continues with: the other big concern is propellant leakage - and you can check that by weighing the can. Mine weighed 300g new (contents 225g), and still weighed 300g four years later. Bear Beware Solutions, based in Alberta, suggests discarding the can if the weight drops below 75% of original.
Unfortunately, none of the sites that tested old cans bothered to weigh them - so when there was sub-par function, we don't know for sure that they had leaked, and were underweight. (CBC news reported that two of four old cans produced thick, comparatively feeble streams, and mentioned that the nozzle fell off one of them, but they pushed it back on and tested anyway.)
Some sites did suggest a brief annual test fire to verify that the stream was up to par; I went with that. I sprayed a decent blast - about half a second, I reckon - and it was impressive: a vigorous yellow cloud, blasting out perhaps 6 metres. The cans hold about 7 seconds' worth of spray, and when I weighed mine it was down by 16g - 7% of the net weight, consistent with 1/2 second. So I'll be able to test 5 times before hitting Bear Beware's suggested limit - which will double the lifespan of the can.
This article had me scratching my head right away: the abstract said things that just didn't make sense - notably
We… documented that bear spray head pressure declines in a logarithmic, not linear, fashion; over half of a new (7‐sec spray time) canister's pressure was lost in the first 1 second of spray
They go on to say that pressure is nearly exhausted after 4 seconds; as a result, they recommend not test‐firing cans. Now, I'd seen plenty of video of people blasting entire cans out during tests - and the stream does not falter. What's more, these researchers mention filming 8 such trials... At the time I wasn't totally clear on the mechanism of aerosols, but had a reasonably strong impression that it was phase change that made it possible to cram a bucketful of shaving cream into a small can. This is true; the propellant is liquid inside the can (pressure circa 3.5 bar for bear spray's propellant) - and flashes to vapor as it exits the nozzle into the 1 bar environment. Droplets of propellant liquid become bubbles - and that's how you get foam. Numerically, the propellant in the little bear-spray can expands to 33 litres at 1 bar.
For that system, declining "head pressure" is not a thing. What exists is a curve that relates temperature to something called "saturation pressure". At a given temperature, the liquid will boil until the container reaches the saturation pressure, then stop: that's the stable state. These researchers were imagining the physics of a scuba tank, and using that to interpret an aerosol can. Now I was fascinated.
Turns out they had made a test rig that let the can (circa 200 ml) squirt into a piece of tube of around 24 ml, holding the valve open to let this system settle, then closing the valve, emptying the tube, and repeating - 128 times, it turned out, to empty the can.
It occurred to me that this would mean they were squirting into a space whose pressure was increasing from 1 bar to 3.5 during each cycle, meaning that the propellant would not all be flashing. The math on that was not something I felt like taking on, so I got Gemini to do it - and the answer was: "roughly 45% of the propellant evaporated to pressurize the rig while roughly 55% was expelled as liquid."
Proceeding... using impressive-to-them terminology such as "linear regression", they pronounce:
Our research shows that at 4 years roughly 7–8% of propellant will have escaped (Fig. 11). This loss corresponds to a 40% reduction in head pressure given pressure depletion curves (Fig. 10).
Reduction in (fictitious) head-pressure is a scuba-tank hallucination... they invented a story where the robust continuous sprays they had filmed were barely trickling out after 4 seconds - plotted as data, looking very real:
The sequential de‐pressurization of bear spray showed an initial steep loss of pressure followed by a much slower loss until all pressure was exhausted (Fig. 10). The relationship between trigger number and resulting head pressure was highly correlated (R2 = 0.94) and logarithmic. The logarithmic regression equation that best fit these data can be used to predict the loss of head pressure and contents as a function of triggering. For example, assume a person has fired a 1‐second burst of bear spray. Because a 225‐g can of Counter Assault™ will spray for approximately 7 seconds, a 1‐second burst (1/7 of the total spray time, or 1/7 of 128 triggerings = 18) can be used in the regression equation to show that 150 kilopascals remain after 1 second (44% of total pressure). The first second of spray released more of the contents than the remaining 6 seconds, and this suggests that test firing canisters quickly diminishes the ability of bear spray to protect the user.
The evidence of their own eyes was overwhelmed by naive math. If we carefully parse the rationale to locate the delusion, they assume that 18 of their leisured tube-fillings correspond to 1 second of actual spray. But the tube fillings are each removing (24/200+24) = 3/28, about 1/9, of the can contents. That will indeed give you a logarithmic curve - even though "head pressure" (so to speak) of a can in real-life use is roughly constant (there is some chilling, which does reduce saturation pressure) until the can is empty.
It is surprising, at least initially, how often accreditation turns out to be a Potemkin facade, until you get your eye in, and realize that these things propagate, and perpetuate, as the Potemkin pseudo-erudite become the accreditors of the next generation.
Last year my 4-year-old can of bear spray reached its expiry date, completely unused, and I wondered how sound the basis of that date was. Reddit showed plenty of others wondering or asserting on the matter, several outdoorsy sources reported on testing, and there was a journal article from researchers at Brigham Young University.
The good news starts with: the active ingredient, capsaicin, does not degrade.
It continues with: the other big concern is propellant leakage - and you can check that by weighing the can. Mine weighed 300g new (contents 225g), and still weighed 300g four years later. Bear Beware Solutions, based in Alberta, suggests discarding the can if the weight drops below 75% of original.
Unfortunately, none of the sites that tested old cans bothered to weigh them - so when there was sub-par function, we don't know for sure that they had leaked, and were underweight. (CBC news reported that two of four old cans produced thick, comparatively feeble streams, and mentioned that the nozzle fell off one of them, but they pushed it back on and tested anyway.)
Some sites did suggest a brief annual test fire to verify that the stream was up to par; I went with that. I sprayed a decent blast - about half a second, I reckon - and it was impressive: a vigorous yellow cloud, blasting out perhaps 6 metres. The cans hold about 7 seconds' worth of spray, and when I weighed mine it was down by 16g - 7% of the net weight, consistent with 1/2 second. So I'll be able to test 5 times before hitting Bear Beware's suggested limit - which will double the lifespan of the can.
The journal article: babes in blunderland
An Investigation of Factors Influencing Bear Spray Performance | BearWise
(Journal of Wildlife Management, 2020)
This article had me scratching my head right away: the abstract said things that just didn't make sense - notably
They go on to say that pressure is nearly exhausted after 4 seconds; as a result, they recommend not test‐firing cans. Now, I'd seen plenty of video of people blasting entire cans out during tests - and the stream does not falter. What's more, these researchers mention filming 8 such trials... At the time I wasn't totally clear on the mechanism of aerosols, but had a reasonably strong impression that it was phase change that made it possible to cram a bucketful of shaving cream into a small can. This is true; the propellant is liquid inside the can (pressure circa 3.5 bar for bear spray's propellant) - and flashes to vapor as it exits the nozzle into the 1 bar environment. Droplets of propellant liquid become bubbles - and that's how you get foam. Numerically, the propellant in the little bear-spray can expands to 33 litres at 1 bar.
For that system, declining "head pressure" is not a thing. What exists is a curve that relates temperature to something called "saturation pressure". At a given temperature, the liquid will boil until the container reaches the saturation pressure, then stop: that's the stable state. These researchers were imagining the physics of a scuba tank, and using that to interpret an aerosol can. Now I was fascinated.
Turns out they had made a test rig that let the can (circa 200 ml) squirt into a piece of tube of around 24 ml, holding the valve open to let this system settle, then closing the valve, emptying the tube, and repeating - 128 times, it turned out, to empty the can.
It occurred to me that this would mean they were squirting into a space whose pressure was increasing from 1 bar to 3.5 during each cycle, meaning that the propellant would not all be flashing. The math on that was not something I felt like taking on, so I got Gemini to do it - and the answer was: "roughly 45% of the propellant evaporated to pressurize the rig while roughly 55% was expelled as liquid."
Proceeding... using impressive-to-them terminology such as "linear regression", they pronounce:
Reduction in (fictitious) head-pressure is a scuba-tank hallucination... they invented a story where the robust continuous sprays they had filmed were barely trickling out after 4 seconds - plotted as data, looking very real:
The evidence of their own eyes was overwhelmed by naive math. If we carefully parse the rationale to locate the delusion, they assume that 18 of their leisured tube-fillings correspond to 1 second of actual spray. But the tube fillings are each removing (24/200+24) = 3/28, about 1/9, of the can contents. That will indeed give you a logarithmic curve - even though "head pressure" (so to speak) of a can in real-life use is roughly constant (there is some chilling, which does reduce saturation pressure) until the can is empty.
It is surprising, at least initially, how often accreditation turns out to be a Potemkin facade, until you get your eye in, and realize that these things propagate, and perpetuate, as the Potemkin pseudo-erudite become the accreditors of the next generation.