There are two types of smoke alarm. One of 'em ain't so good.
Before we get started I want to let you know that I will not be setting any of the alarms off in this video. When you last went shopping for smoke detectors, did you notice that there were two different detection technologies available to put your dollars towards? Let me tell ya, even though I shop for smoke alarms every other Wednesday it hadn’t occurred to me. That’s a lie, this is just a silly setup. In fact I only rarely go shopping for smoke detectors, but the last time I did was to replace one of mine because SOMEbody had installed the wrong type! The wrong type? There’s a wrong type of smoke detector? Well… suboptimal might be a better word. See, the detector outside my bedroom is in a hallway that leads to the kitchen. And it’s right next to a return vent for the building’s HVAC system.
That’s not ideal from the start but it’s hardwired and interconnected to the other alarms here so moving it isn’t an option. Anyway, being almost in the kitchen and having kitchen air pulled near to it, I was getting nuisance alarms every time I made toast. And I am not exaggerating. Every. Single. Time.
And often when I dared to simply use the oven! Didn’t even matter if I had put something in it yet. What was going on? Well, that smoke alarm (which is now right here) used the good old fashioned ionization chamber for its smoke detection abilities. It’s slightly radioactive! Now that’s a tried and true method for detecting smoke, in fact in some cases it works a little too well, uh but these days it’s revealing its weaknesses and according to some these should be considered obsolete. I replaced it with a photoelectric smoke detector and not only have the nuisance alarms completely stopped but I will probably have more advanced warning of a fire should one happen. At least, depending on the fire. Ah! A nuanced discussion! Those go great with the internet.
Let me start out by saying that regardless of what smoke alarms you might have in your home the most important thing is that you have them and that you know they are functional. If you can’t remember the last time you checked your smoke alarms, do it right now. Seriously, pause the video, make sure you have them, and if you’re able to test them without setting off a central monitoring system, do it. Most every fire-related tragedy we hear about these days stems from a home without working smoke alarms, meaning those tragedies were almost certainly preventable, and all too easily. You should have a smoke alarm in every bedroom, in locations immediately outside of sleeping areas such as hallways, and in addition to that there should be at least one on every level of your home.
Consult your local fire authority for more specific recommendations and requirements in your area. Let’s start with a bit of history. Smoke detectors are a fairly recent invention and the household smoke alarm didn’t hit meaningful mass production until the 1970’s. We needed to clear two hurdles before the smoke alarm would be commonplace: First, a reliable and relatively cheap smoke sensor needed to be devised. And secondly, we’d need to figure out how to make some cheap and mass-producible electronics which could monitor that sensor and sound an alarm if smoke was detected.
As it happens, the principle on which these first smoke sensors operate was discovered by accident in the 1930’s by Swiss physicist Walter Jaeger. He was trying to invent something which could detect poison gas, which didn’t work. But his device did react to the smoke particles coming from his lit frustration cigarette. Or so goes the story. The sensor used in the first commercial smoke alarms — and plenty that are still produced today — operates on the same principle as Jaeger’s experimental device.
It is made of a pair of electrodes that span an air gap, and sitting below a hole in one of the electrodes is a teensy little bit of americium-241 which emits alpha particles by way of being, ya know, radioactive. As the americium decays and flings out those particles, they collide with the nitrogen and oxygen atoms that make up the bulk of air and knock loose some of their electrons. This results in some charged gas molecules between the two plates. And with the help of a power source to maintain a voltage potential between the plates, often a 9-volt battery, those now charged molecules become attracted to the electrodes, and the end result once they move towards them is a lil’ bit of current flow. And I do mean a lil’ bit, about 100 picoamps. However, when something’s burning and releasing smoke particles into the air, once those smoke particles get between the two electrodes, they start absorbing or blocking the alpha particles coming from the americium.
That prevents the ionization of the nitrogen and oxygen molecules, which stops the flow of current between the plates. A relatively simple electronic circuit can monitor for current flow and sound an alarm if it stops, and with the development of the MOSFET and its cheapening in the 1970’s, suddenly we had everything we needed to produce an inexpensive, automatic device for warning of the presence of smoke. Now, if you’re anything like me, you might think that resorting to radioactive isotopes seems... a bit much. It’s not dangerous or anything, alpha particles are easily stopped - the radiation can’t even get through the plastic shell of the alarm.
But, it still feels like a rather… exotic technique. After all, we can see smoke particles with our eyes. And our eyes work because of light. And since we can see smoke using our light-sensing eyes, there’s probably a way to use a light sensor to detect smoke.
Indeed there is. All the way back in 1972, Donald F. Steele and Robert B. Enemark devised an optical smoke detector, which worked using essentially the same principle as modern-day photoelectric smoke detectors.
Their idea was to put a light source and two light sensing photocells in a detection chamber which would allow air to freely move through it, but which contained a series of light traps to prevent ambient light from getting in. FYI, photocells weren’t by any means a new thing at this point - this wasn’t groundbreaking tech, just a clever idea. Anyway, the sensors were arranged so that only one of them could “see” the light source. The second sensor had no line of sight to the light source, with a series of light-blocking vanes in the way. When smoke particles entered the chamber, though, light from the light source would reflect off those particles in every which way and become scattered. Think of it like a laser beam in a smoky room.
The second sensor could now “see” the light coming from the light source, thus smoke was detected, and an alarm is sounded. These early photoelectric alarms used two sensors as a means of testing the functionality of the light source. If no light was detected in the first sensor, this would indicate that the light source had failed. Which was pretty likely as the early designs used incandescent lamps. And no, they were not battery-powered, which is likely a reason the ionization sensor was preferred for so long. These days, though, a much simpler and way less power-hungry arrangement of infrared LED and single photodetector is used.
The photodetector is positioned in the detection chamber so that it can’t see the LED, and every few seconds the detector will put out a little blip of light. It may also blip a visible LED on the exterior to give indication that it’s powered on and functional. If it can't see the blip in the photodetector, then the air must be clean. If, however, the chamber has smoke in it, the smoke particles will scatter the light and some will be picked up by the photodetector. That usually won’t trigger an alarm right away, though.
Modern designs are often programmed upon first detection to begin an additional and more frequent series of blips to guard against false alarms. If, for instance, a piece of dust happened to float into the chamber, well you wouldn’t want the alarm to go off just for that. So it will perform a routine where it checks a few more times in rapid succession for scattered light.
It may also be looking for an increasing signal amplitude and thus thicker smoke over time before committing to an alarm condition, though that’s speculation on my part. In any case, if it keeps seeing light hit the photocell after a pre-programmed test period, it will sound the alarm. So, we have two commonly-produced and easily obtainable smoke detection technologies at hand: one using commodity LEDs, photodetectors, a bit of electronics and some plastic, and the other using some plastic, a bit of electronics, a couple of metal plates, and an exotic synthetic radioisotope of americium. Why are we still making this second kin- [exasperation noises] That’s a good question, but before I move on to the pros and cons here, I want to share that the principle of optical smoke detection is as flexible as it is simple. While spot-sensing smoke alarms work as I’ve described, another option is to shine a beam of light across a large distance and measure its received intensity. You can modulate that beam in some way to allow a sensor to pick up a signal through ambient light, and a sudden dropout of that modulated signal or simple intensity loss can indicate smoke.
These are often used in buildings with large open spaces like atria, and in such buildings you might have seen a funny-looking device at one end of a ceiling pointing across the building to another one on the other end, or perhaps a retroreflector. There’s a pretty good chance that was, in fact, a smoke sensor incorporated into the building’s fire alarm system. Alright, with two very different detection methods, it’s probably no surprise that the two technologies respond differently to different kinds of smoke. This may in part explain why ionization alarms are still on the market - remember how this one was sensing smoke whenever I made toast? Toastmaking is really just slightly burning bread, and even though I don’t like my toast all that dark, little whisps of smoke are produced as the surface of the bread gets singed. These smoke particles are really, really small though.
So small that they can barely be seen. However, those particles are very good at absorbing the alpha particles emitted from the americium, and so not a lot of that kind of smoke is needed to set off one of these alarms. These very small, often invisible smoke particles are commonly emitted from things that are actively on fire. And this makes the ionization alarm technology particularly sensitive to the flaming stage of a fire. In fact, even apparently clean-burning flames can set them off - that may have been why just using the oven was prone to causing an alarm in my case. Car exhaust can also trigger these alarms, which is why smoke alarms are rarely recommended in garages.
This has been a particularly puzzling thing to me as, ya know, cars catch on fire sometimes and I would think a smoke alarm in the place where you keep your car is a decent idea. Anyway, while ionization alarms are really good — arguably too good — at detecting fine smoke particles, they are absolutely crap at detecting large smoke particles. If a room is slowly filling up with visible smoke, an ionization alarm may very well do nothing. You might think, "oh, so what? You said it’s good at detecting smoke from a flaming fire, and so long as it’s gonna wake me when a fire’s actually happening what does it matter?" Well, most fires don’t just suddenly happen. They start slowly. And in the initial smoldering stages of a fire, the smoke particles tend to be the large, visible kind.
If all you have in your home are ionization alarms, they may not react at all to that situation. And that’s bad! Studies comparing ionization and photoelectric smoke sensing technologies have routinely shown that in the smoldering stage of a fire, photoelectric alarms respond much more quickly than their ionization counterparts. Some experiments have shown photoelectric alarms responding to the early stages of fire more than an hour before an ionization alarm does. Early warning of such fires is obviously valuable, and may give occupants time to find the source of the smoke and prevent a destructive flaming fire altogether.
And let’s not forget that smoke itself is very dangerous, with many fire-related fatalities happening due to smoke inhalation. In an Australian 60 minutes program which aired back in 2014, a photoelectric alarm was activated about seven minutes into a simulated fire, meanwhile three ionization alarms didn’t respond at all to the very smoky test environment. Which is astounding! A smoke alarm which doesn’t react to a smoldering fire and a smoke-filled room isn’t a very useful smoke alarm. Some jurisdictions have reacted to this new knowledge by mandating photoelectric alarm technology and discouraging the use of ionization detectors.
However, not all of them have. Ionization alarms still have that one narrow advantage; they do react more quickly to a flaming fire, and a photoelectric alarm needs large-ish particles which can be seen, so fires which develop suddenly may not trigger them right away. Because of this situational difference, here in the US neither the The National Fire Prevention Association nor the United States Fire Administration take a firm position on the two technologies. They recognize the pros and cons but say that, since all fires are different, making a recommendation either way doesn’t make sense. And I sorta get that. If an ionization alarm is better in some fires, well then maybe it’s good to have them.
But there are several problems with fixating on that situational speed benefit. The first is that fires which are suddenly flaming without having any smoldering action aren’t exactly common. It’s not like they never happen, one possible case might be if a cat knocks over a lit candle onto a tablecloth, but especially when people are sleeping, I don't think it's that common for a flaming fire to come out of nowhere. I spent some time looking for statistics and came up empty. If you know of a good source of information regarding this please share it below.
The second and perhaps most important issue here is that while ionization alarms are generally faster at detecting a flaming fire than their photoelectric counterparts… it’s a matter of seconds, and not minutes. When a house fire’s going on it's not gonna be a nice clean burning fire for long at all, and smoke thick enough to trigger a photoelectric alarm will accumulate pretty quickly. Surely there have been cases in which the seconds mattered for survivability, but of course the counterargument is that a photoelectric alarm may have given you an hour’s advanced warning in a different situation. Now I can hear you asking; why not both? Smoke alarms are pretty cheap, so why not have both detection technologies? Well, the smoke alarm manufacturers are way ahead of you offering dual-sensing alarms. There’s a problem with these, though.
They aren’t exactly clear on how those sensors get used. The packaging of this one implies that it would sound the alarm if either sensor detects smoke, but the included user guide doesn’t clarify whether this is in fact the case. It simply says the alarm sounds when combustion products are detected. Some alarms like this may not go off unless both sensors agree that there is smoke - and in that case, you’re not actually getting any benefit at all as it will always be the slowest response between the two technologies. And the reason why they might be programmed this way has to do with the final major issue with ionization alarms; false alarms. A smoke alarm which goes off when it shouldn’t is a nuisance that most of us have dealt with at some point.
And ionization alarms, thanks to their hyper-sensitivity towards certain kinds of invisible smoke particles, are particularly prone to false triggers - especially when placed near kitchens. False alarms aren’t just annoying, though, they are actually dangerous. That's because one of the most common ways people address a smoke detector which keeps detecting erroneous smoke is to take it down or otherwise disable it. And a disabled smoke alarm is exactly as effective as an imaginary one. Photoelectric smoke sensing is much less prone to false alarms.
They’re not impervious to the problem - as a matter of fact I had one myself which developed a newfound sensitivity a few years after I first put it up and started going off whenever I used the dryer. However in that case it was an electric dryer which vented directly into the hallway right where the alarm was so it wasn’t an ideal situation from the start. I think moisture probably killed it somehow. Anyway, as a more relevant example, since replacing this alarm near the kitchen with a photoelectric unit, I haven’t had a single nuisance alarm. And that was well over a year ago at this point. So where does this leave us? The answer seems kinda murky.
Ionization smoke alarms work very well in specific circumstances, but hardly work at all in others. They’re also more prone to false alarms which may make occupants more likely to disable them. Photoelectric alarms outperform them in smoldering fires and are less prone to false alarms, but they aren’t quite as good at alerting you to fires-in-progress.
So what should you do? Personally, I don’t think ionization smoke alarms present enough of a performance difference in active fires to justify their continued use at all. Since 2008, the International Association of Fire Fighters has recommended against the use of ionization alarms, and as I’ve already said, some authorities have taken action and mandated the use of photoelectric alarms. In fact, an ever-increasing number of authorities across the globe, including some US states. But ionization alarms are still routinely sold in other places, including my home of Illinois. And unless you know what the differences between the two technologies are, you probably haven’t paid any attention to this. Making matters worse is that many alarms that are out there in the wild don’t make it clear what technology they use.
If you take a look at the ones on your ceilings or walls, there may be no indication at all! These brand new alarms, which are photoelectric, don’t say that anywhere except the box! This brand-name ionization alarm does say it’s ionization on the back, so that’s good I guess. The combo alarm also says it’s both. To tell what you have, take it off the wall and see if there’s a label somewhere. Hopefully there is but if there isn’t, well if you can see the insides of the alarm the ionization chamber is a pretty recognizable thing; usually it's some sort of cylinder and will have a radiation label on it, but it’s not always possible to get at the insides. [voiceover] Two quick things; I was apparently wrong about there being a radiation label on the sensor body itself, and, speaking of the sensor body, the ionization chamber didn’t quite look like what I expected it to since apparently the outer shell forms one of the electrodes these days. Such are the perils of writing a script and shooting the talky bits before actually taking the things apart.
However, at least here in the US, if it’s an ionization alarm there’s probably gonna text regarding the fact that there’s americium-241 in there, with permission from the Nuclear Regulatory Commission. I don’t know what labels you might find in other countries, but if you see a warning about radioactive material, that’s an ionization alarm. Now, there is still the fact that ionization alarms may still be faster at alerting you to a sudden, flaming fire. It just doesn’t look to me and many other like this potential benefit outweighs the various downsides of the technology.
However if you think it’s valuable to have an ionization alarm just in case, my recommendation would be to purchase a separate alarm and make sure to place it far away from your kitchen to prevent nuisance alarms. I personally wouldn’t trust one of these combo alarms because it's just too unclear how the sensors actually get used in the logic - and two separate alarms are usually cheaper than one of these, anyway. For what it’s worth, I didn’t replace one of the ionization alarms here because it’s far enough from the kitchen to not be affected by cooking and having photoelectric alarms elsewhere makes me feel plenty safe.
But when it’s no longer functional, I will probably go with a photoelectric alarm to replace it. I don’t know if I would go so far as to call the ionization chamber an obsolete form of smoke detection, but it’s definitely flawed. The more people that are aware of its weaknesses, the better.
Before I go, you’ve probably seen recommendations to replace smoke alarms every 10 years. In fact, these days smoke alarms with single-use lithium cells which are designed to last 10 years are pretty common - and in some places, those are the only battery-operated kind available. Now, the recommendation has nothing to do with the americium.
The half life of americium-241 is over 400 years, so the sensor could easily outlive you. Instead the recommendation is to guard against aging electronics causing unit failure. I’m sorta on the fence about this, because 10 years is a pretty dismal life expectancy. But on the other hand, these are cheap and cheaply made so maybe it’s fair. What I wish were far more common was actually testing your smoke alarms with a canned smoke product. Commercial fire alarm systems get their smoke sensors actually tested this way.
You might have seen someone going around with a little spray can of magic smoke and puffing it into the sensors while radioing someone else at the control panel who's looking for a response. It seems to me like cans of that stuff ought to be available for the home and that we should be recommending using it. I mean, I’m sure you can find the stuff online somewhere but it has always seemed odd to me that we don’t recommend that strategy for the home.
Seems to me like actually proving operation of the sensor bit is more valuable than hitting a button and making sure it starts beeping. But what do I know, I just make YouTube videos. ♫ alarmingly smooth jazz ♫ Let me tell you, eve - well, that’s uh… [clears throat] I wrote it as “let me tell you” but I wanted to say it as “let me tell ya” and it’s those little details that’ll trip ya up. Well, that smoke alarm [clunk] which is now… yeah. Consult your local file ath….
File authority! Consult your local fileafloridaeyhamorffhhaffifceuuuigigrechoirmeninyourarea That knocks loose some of their electrons, resulting in some gas charged mole … frark! When those smark… smark porticles? That may have been why just using the oven was prone to causing this alarm. To alarm. Shoot. You didn't actually pause the video and check your alarms, did you? Well, here I am again tell you to do it.
The video's over now, you don't have any more excuses. DO IT.
2022-07-09 18:32
