I use Bear Spray quite regularly, and my staff and I have sprayed black bears many dozens of times during control work. In my experience, "60% of the time, it works every time". 
Effectiveness is based on the combination of the following conditions:
1. Strength - All bear spray's are not created equal. Look for the 1.0% Capsaicin, and a strong, rapid stream (I like Frontiersman).
2. Proximity - Less than 2m is best, more then 3m is useless. Some cans with a thicker concentrate produce a tighter cone and longer reach. Buy a couple of different brands and test them.
3. Wind - Medium to Strong winds render it virtually useless, regardeless of direction. It just disperses the vapor it too quickly.
5. Duration - The bear must "Breathe" it in for full effect. An excited bear breathing rapidly takes in far more than a calm bear just acting inquisitively. The calm bear may still be scared by the noise and action IF it is not too brave.
6. To maximize #5, buy the biggest can you can, and spray long bursts. A 325gram can will give you 7-10 seconds of continous spray, or two very long bursts (count to three mississippi, or just empty the damn bottle and then grab your partner's for any round two). Just like firearms, no point conserving ammo only to die with half a mag left.
Where you cannot carry a firearm, I would recommend a combination of Bear Bangers and Spray. The bangers are usually quite effective on non-conditioned bears. If the bear has been conditioned to bangers, they can still be effective if you can place them closely enough to "impact" the bear with the shock wave. I have yet to see even a bold bear stay put after a bear banger explodes within 15 feet of his head. Of course they are not completely accurate, and the range is slightly different for each brand, but if you practice, you can be pretty reliable with them. They say there is a danger if you detonate the banger behind the animal and cause it to run towards you, but I have yet to see that happen in the field. As long as the blast is within 20 feet of the animal, it can't determine the direction, and runs away from the "Known" threat.
Bangers in close proximity work on the principle that Bears hate to be "Touched", and similarly the animals can often be conditioned effectively and harmlessly with a sling shot or a pellet gun. Those are not practical or effective for a defensive scenario, but the banger with a spray backup will get the job done better than anything other than a firearm.
As for the cans losing pressure, we use our cans several times until they are empty, and we find that they will last at least six months after first discharge, and most are fine the next summer. I am a firm believer in testing your weapon before you need to rely on it, so I always test my can before I approach an animal. Of course that may not be effective in the field, but if you test your can in the spring, rinse the tip after, and replace it each spring you should be fine. Use the old can to practice or demonstrate the stream/effects for friends and family who may be going out with you. I buy inert cans when we plan to burn a lot of them during training, as they are a lot cheaper.
Effectiveness is based on the combination of the following conditions:
1. Strength - All bear spray's are not created equal. Look for the 1.0% Capsaicin, and a strong, rapid stream (I like Frontiersman).
2. Proximity - Less than 2m is best, more then 3m is useless. Some cans with a thicker concentrate produce a tighter cone and longer reach. Buy a couple of different brands and test them.
3. Wind - Medium to Strong winds render it virtually useless, regardeless of direction. It just disperses the vapor it too quickly.
5. Duration - The bear must "Breathe" it in for full effect. An excited bear breathing rapidly takes in far more than a calm bear just acting inquisitively. The calm bear may still be scared by the noise and action IF it is not too brave.
6. To maximize #5, buy the biggest can you can, and spray long bursts. A 325gram can will give you 7-10 seconds of continous spray, or two very long bursts (count to three mississippi, or just empty the damn bottle and then grab your partner's for any round two). Just like firearms, no point conserving ammo only to die with half a mag left.
Where you cannot carry a firearm, I would recommend a combination of Bear Bangers and Spray. The bangers are usually quite effective on non-conditioned bears. If the bear has been conditioned to bangers, they can still be effective if you can place them closely enough to "impact" the bear with the shock wave. I have yet to see even a bold bear stay put after a bear banger explodes within 15 feet of his head. Of course they are not completely accurate, and the range is slightly different for each brand, but if you practice, you can be pretty reliable with them. They say there is a danger if you detonate the banger behind the animal and cause it to run towards you, but I have yet to see that happen in the field. As long as the blast is within 20 feet of the animal, it can't determine the direction, and runs away from the "Known" threat.
Bangers in close proximity work on the principle that Bears hate to be "Touched", and similarly the animals can often be conditioned effectively and harmlessly with a sling shot or a pellet gun. Those are not practical or effective for a defensive scenario, but the banger with a spray backup will get the job done better than anything other than a firearm.
As for the cans losing pressure, we use our cans several times until they are empty, and we find that they will last at least six months after first discharge, and most are fine the next summer. I am a firm believer in testing your weapon before you need to rely on it, so I always test my can before I approach an animal. Of course that may not be effective in the field, but if you test your can in the spring, rinse the tip after, and replace it each spring you should be fine. Use the old can to practice or demonstrate the stream/effects for friends and family who may be going out with you. I buy inert cans when we plan to burn a lot of them during training, as they are a lot cheaper.
