A question about the use of pressure cookers in the Boston bombings

[Adsos Letter]

Is it because they have locking lids?

Does the split-second of extreme pressure caused by the explosive detonating also cause the lid to lock down tighter for that millisecond, or does the locking system respond that fast?

[Robb]

1. It's more commonly seen in high elevations.

Punjabi jihadi used them because they were ubiquitous, but also increase the explosive yield. You have the relatively high pressure inside speeding up the reaction, so to speak. More bang for your buck; but at sea level, I think you'd be adding a level of complexity that wouldn't be justified by the end result.

[Adsos Letter]

5. Thanks Robb

I wondered if that might be the case.

[Xithras]

10. Perhaps, but once someone is trained in the technique....

Pressure cookers are common in Pakistan, Kashmir, and Afghanistan because so much of the population lives at such a high elevation. The technique was perfected there because it was a readily available supply, and is widely taught in the Taliban training camps. If someone were trained in one of those camps, they may be hesitant to tweak a bomb design even if the efficacy of one of the components is in question. While the pressure cooker might not ADD anything at sea level, an inexperienced bomb builder might not be able to calculate the effects of REMOVING the pressure cooker and could decide to include it anyway, just to be safe.

Besides, pressure cookers also have one added bonus. Because they're sealed airtight, a pressure cooker is also an extremely effective way to keep odors IN. If the outside of the cooker were thorougly cleaned after being sealed, its use could dramatically reduce the odds of the bomb being detected by explosives sniffing dogs.

[Robb]

11. Eh. Dogs find pressure cooker bombs in India often enough.

I'd like to see another source on this angle, too; TeT denied involvement pretty quickly, in contrast to Times Square.

[Xithras]

13. Yeah, that's the only thing that gives me any pause about it.

They're usually more than willing to claim credit. The denial is a bit odd.

And yes, dogs can still find pressure cooker bombs. But if its well cleaned, the pressure cooker reduces the dogs range by quite a bit, often requiring them to directly sniff the bag containing the bomb. An unprotected bomb can sometimes be detected several meters away. It's not foolproof, but it reduces the chances of being caught.

At the moment, I'm leaning towards a Taliban trained lone wolf. The TeT isn't exactly a highly centralized organization, and it's entirely possible that we may be looking at something that was carried out by a bomber trained in their camps, but not operating under their orders. I suppose that we won't know that bit until the bomber is caught.

[Robb]

14. Again, I'd wait for confirmation on the pressure cooker bit.

It's not the simplest way to make a bomb -- much less two that go off at the same time.

[tech3149]

2. the locking lid provides a complete seal that magnifies the force of the explosion

That's why they have a pressure release valve for normal use.

[backscatter712]

8. You just reminded me of a Mythbusters episode where they were blowing up hot water heaters.

They were replicating the conditions of a poorly/incompetently maintained water heater - they sealed off the pressure release safety valve, disabled the safety devices, filled the heater with water, and turned it on, until it blew up. Fairly impressive explosion, which launched the heater tank hundreds of feet into the air.

[jberryhill]

3. Get some gunpowder, light a match

The gunpowder will burn quickly, but will not produce an explosion.

In order to do that, you need some sort of pressure vessel or confinement. That's the basic idea behind pipe bombs. You confine gunpowder to a sealed piece of pipe, and the chemical reaction of the rapidly oxidizing gunpowder will release enough gas to build up pressure until the pipe fails, and explodes.

A pressure cooker is a little nicer than a pipe bomb since sometimes one can accidentally set off a pipe bomb while making it, either due to mechanical pressure from pinching off the pipe, or from friction due to getting gunpowder into the threads of fittings.

[snooper2]

20. that's how I learned the difference between fast/slow acting black gunpowder

I used a shaving cream can though

not fun picking gunpowder out of your face

[backscatter712]

4. I'd say it creates a moment of vastly increased pressure...

just before the pressure cooker ruptures - all the better to throw the shrapnel more violently.

IOW, it creates a nastier explosion.

[Adsos Letter]

6. Thanks for the answers, everyone.

The physics behind this make sense, even though nothing else about this tragedy does.

[rdharma]

7. What was said above plus.......

..... creates a lot of shrapnel and seals in scent and chemical signature (making them harder to detect).

[MADem]

9. These are common IEDs in SW Asia--Pakistan, Afghanistan, Nepal, India, etc.

They are commonly used in cooking too, to cut down on fuel costs and to tenderize less than optimal chunks of meat.

[davepc]

12. It's because they're cheap pressure vessels

Store up a lot of pressure and let it all out at once....

Ever open a shaked can of soda? Same idea.

[HockeyMom]

15. They have far more safety devices

than they did from 40 years ago when I first started cooking with them. Yet, you still have to be careful. They still can blow up.

[1-Old-Man]

16. Well, if it was a bomb built into a pressure cooker it may tell us a thing or two

First off there is nearly no advantage to be had by using pressure cookers to build the bombs in question if the explosive was one of the modern plastics. You'd need some kind of containment if you were just filling the can with gun powder, a very primitive way to make a bomb I'd think. But if you were a well versed bomb-maker wouldn't you have got yourself access to modern materials. What's that stuff the Army uses, C-4? The use of modern materials would seem to me to suggest an organization that was much more serious did this terrible thing but if the bombs didn't use modern technology, in a country were it can be had in the snap of one's fingers, I'd have to guess it was the product of an individual or very small group.

Oh, another thing to make my point, how do you hide a pressure cooker in a crowd? The thing would look so out of place it would have to provoke inquiry. Plastics, on the other hand, could be molded into any shape at all and easily hidden.

So to me pressure canners either don't make any sense or point to something less than an international conspiracy behind the bombings.

[femmocrat]

17. Another question.... How did they rig up two to go off almost simultaneously?

Could one person do that?

Also, could it have had a timing device or be set off by a cell phone, as some reports have suggested?

[FarCenter]

19. A couple of cheap digital travel alarm clocks should be accurate to 10 seconds over a day.

[SoCalDem]

23. Someone yesterday told of many devices that could be used .

Garage door openers..cell phones..car alarm key fobs, any of them would cause no concern if detected once the bombs were placed..

I know that backpacks are ubiquitous these days, but perhaps future public gatherings should be "no-backpack" zones. People carrying pressure cookers (or other suspicious-looking things) would be easier to notice.

I can still remember the first time I ever saw a backpack..I was about 10 & it was when we went to the mountains & saw some hikers.. That was the ONLY time I saw one until my own kids were told they had to have one for school.. I remember being confused & wondered why on earth school kids needed backpacks...

In "my day", women carried smallish purses/pocketbooks/handbags and professional men carried a briefcase, but people walked around all day long without carrying a ton of "stuff" with them..

Maybe a new Mom had a diaper bag, but that was about it.

These days, people everywhere routinely carry around provisions & clothing changes with them ...at a time when there seems to be "stuff" available to them every 10 feet

[politicat]

18. I've been looking at mine today...

I made bean soup with it yesterday so I was washing it when I heard NPR say it was a pressure cooker.

They're small and easily available. I think mine came from Target for under $50. That makes them somewhat ubiquitous.

They've got a decent amount of capacity -- mine's a 6 quart. It fits easily in a daypack -- I've taken it to work full of soups and chowders. It also fits in a small mailing box, which makes it easy to disguise.

They don't spill easily. See above, about carrying liquids in a backpack to work.

They're aluminum, so they don't spark, limiting the chances for premature detonation from static electricity.

Out of curiosity -- because this is how I cope -- I put my phone inside it, sealed it, and called it. The signal went through, so no Faraday cage effect to disrupt a trigger signal.

They're basically a big, premanufactured, non-conductive casing for a pipe bomb.

I've heard independent reports of the scent of cordite (gunpowder) from the scene and the ER docs. So likely not someone with access to more advanced explosives. It's a hardware store version of a portable mine.

[k2qb3]

21. Most low-order explosives smell like cordite.

The propellant could be a number of things.

HE don't require a pressure vessel, a pressure cooker isn't a particularly good pressure vessel but that's what it is.

All it really tells us is it was a low-order explosive, which was obvious from the video.

[FarCenter]

22. Does the bottom of the pressure cooker help direct the explosion?

Can the gunpoweder be layered underneath a layer of shrapnel so that when the lid blows off the shrapnel sprays out in a cone like a Claymore mine?