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Rerun: How Smoke Detectors Work

Published Dec 6, 2023, 7:29 PM

Smoke detectors save lives, but how do they work? From photoelectric effects to nuclear radiation, we explore how smoke detectors let you know when there's a fire.

Welcome to Tech Stuff, a production of Iheartradios How Stuff Works. Hey there, and welcome to tech Stuff. I'm your host, Jonathan Strickland. I'm an executive producer with iHeart Podcasts and how the Tech are you. Well, I'm still vacating, which means I'm off in Florida being a crazy tourist type person. And in the meantime, I have an episode for you that we publish back on February tenth, twenty twenty. This one is titled How Smoke Detectors Work. I figured it was a good idea to do an episode about smoke detectors as we go toward the end of the year, because I've always tried to change out the batteries and my smoke detectors at the beginning of each year, because otherwise I just would forget to do it. But by making it like, hey, happy New Year, you gotta change out all the smoke detector batteries, I am less likely to forget. But how did those darned things work in the first place. Well, let's listen and find out. Now. Clearly, smoke detectors are incredibly important because we all know fires can be life threatening. They can spread quickly, they can cut off potential escape routes for the people who are caught in the way. So early warning systems that can alert us to danger before it becomes a mortal danger are fantastic inventions. And there are a couple of inventions I want to talk about before we actually get to smoke detectors. I'm sure that doesn't come as a surprise to any of you who have listened to Tech Stuff episodes before. I always like to set the ground well. The first one that I want to talk about is the portable electric fire alarm, as in a fire alarm that works on electricity, not a fire alarm that detects electric fires. Francis robins Upton, who is a physicist who was a partner and general manager of the Edison Lamp Works, as in Thomas Edison, developed this particular invention back in eighteen ninety. Now, according to the patent, the design would sound an alarm after detecting that the temperature had risen above some predetermined limit, so if it got too hot, this thing would go off. And the way this would work was really clever. You can actually learn exactly how it was supposed to work based on the patent that Upton got on this particular invention. So the way it works is that there are a pair of electrical contacts and if they are in contact with each other, if they touch each other, it completes a circuit. But normally there's a gap between them and electricity can't pass between the two. It can't cross that gap, so it's normally in the off position. However, one of those two electrical contacts is mounted on a fixed arm and the other is on an arm that's attached to a coil of biomet hallick material. Now I mentioned biometallic strips in the episode about lighters, and it's something you find in a lot of different technologies that depend upon changes in temperature as a variable, like thermostats have bimetallic strips in them. And essentially what it is is a strip of metal that consists of two different metals. And think of it as like a metal sandwich, a top side and a bottom side, and one metal is the top side, the other metal is the bottom side, and each metal has slightly different properties, and one of those properties is how quickly they expand when they're heated up. You know, we know that metal expands as it gets warmer. Well, using a bimetallic strip, you have one side that will expand faster than the other side, and when that happens, it causes the strip to curl, so it deforms. As it gets warmer, one side is expanding faster than the other and you get this curled metal as a result. So with Upton's proposed invention in the patent, this coil of bimetallic material would act as sort of an actuator. As it would get warm, it would expand, and this in turn would create a pushing force on the arm with the electrical contact on it that then can move toward the other electrical contact that's in a fixed position. And so once the coil had expanded enough, the two contacts would touch one another and it would complete a circuit. At that point, electrical current could actually flow from a battery that was connected to this device all the way through to activate an alarm bell. So once the temperature got hot enough the two contacts touch, you get a circuit. The alarm bell goes off. If it cools down, the contacts will slowly separate, and eventually they'll separate enough where that electrical circuit can't be complete. Anymore, the alarm bell would go off. And Upton's invention was, without doubt a very clever one, and it could help save property by sounding an alarm before a fire had raged completely out of control. But there are many dangers with fires, and heat is just one of them. Another very serious danger is smoke, which not only obscures your vision, but it also can suffocate you as well. So while the alarm as design would work, it wouldn't necessarily be enough to save lives, or it might not save enough lives because it would only go into action after the temperature had already increased enough to make this bimetallic strip expand to a sufficient degree, and by that time it might already be too late for the people in that building. Now, as far as I know, Edison's company didn't actually produce any of these fire alarms, but they did patent it. Over in Great Britain, there was a fellow named George Andrew Darby who patented his own fire alarm, also triggered by an increase in temperature. But Darby's invention did not depend upon bimetallic strips. It depended on butter or some other material that melts at higher temperatures. Say what, all right, So imagine you've got a seesaw like contraption, a lever in other words, and the heavy end of the lever is up in the air actually because you have a weight on the opposite end of that lever. So it's like a kid sitting on a seesaw that doesn't have another kid at the end of it. The kid at the end of the sea saw weighs it down, and the free end of the seesaw is up in the air. Well, in this case, the fire alarms seesaw. The arm that's up in the air has an electrical contact point on it, and if the arm of the seesaw were to come down, that contact would complete a circuit, and thus electricity would be able to run from a battery through an alarm, very much like Upton's invention. So weighing down the other end of the arm. The thing that's actually keeping the electrical contact up in the air is a block of butter or fat or wax or some other material that can melt at higher temperatures. So as the temperature rises, the block begins to melt away, and eventually it melts enough so that the weight isn't sufficient to keep the other end of the seesaw up in the air, and it'll sink down and the contact will complete the circuit and the alarm will go off. I was amused to find a butter based fire alarm. I wasn't surprised that it came out of England, but I was amused to find it. I can think of a few potential problems with such an arrangement. For example, it might start attracting pests that could eat the weight, so in those cases you wouldn't actually have a fire alarm, but you might have a rat alarm, or it would just turn rancid and smell offul Still a pretty clever approach, but not one that I think you would actually want to put in your buildings. But these solutions weren't practical for homes or anything like that. As I mentioned, they wouldn't really alert you to the presence of smoke, which on its own would be enough to be deadly. So this was really looked at as more of a solution for things like factories facilities, where you've got a lot of industrial operations going on, where the risk of fire is high and the risk of property loss is also high smoke detectors also trace their history back to chemists, physicists, and inventors, and in fact, you could say that smoke detectors were made possible not just through exploratory science, but also happy accidents. And will begin with a super smart Swiss physicist in the early twentieth century and try saying that three times fast. Super smart Swiss physicist. His name was Heinrich Grinecker or Grindacre if you prefer study a lot of different stuff, including radioactivity. Wilhelm Runtgen had discovered the existence of X rays in eighteen ninety five, and Grindacre devoted a good deal of his work toward getting a better understanding of X rays and other forms of radiation, and to do that he had to overcome some practical obstacles. For example, he needed a device to help measure the intensity of X rays, and such a device didn't really exist yet, so he got to work inventing one. X rays are a type of ionizing radiation that means that when they encounter molecules or atoms, they can ionize them. And an ion is a molecule or an atom that has a net electrical charge, so it's either a positive particle or a negative particle. And a positive ion is one that has more protons than electrons, so you have a net positive charge. A negative ion would be the opposite, has more electrons than protons and has a net negative charge. Now, one thing Grindacre did was to create ions by building a grid of wires through which he could stream high voltage current. And you would pass air molecules essentially through this grid, and the current would strip electrons off of those particles, creating positive ions. So the grid was in this chamber through which gas could move, and as I said, the current would ionize the gas. But the other really neat thing he did was he invented a voltage multiplier circuit because he needed to generate this really high voltage around two hundred and twenty volts actually, and that could take incoming alternating current electricity. That's the type of electricity that power plants typically send out because it's easier to send out alternating current over long distances than direct current. Well, Grindacre's invention would bring in alternating current and then convert it to direct current and run the direct current through a circuit with stuff like capacitors and diodes, and the whole process is a bit complicated to explain, particularly without the use of visual aids, And also it goes beyond today's topic. But in the future I will have to talk about voltage multiplying circuits more specifically, because they are important in all sorts of technologies, including super cool bleeding edge science stuff like particle accelerators. But for the purposes of this episode, the important thing to remember is that it made it possible for Grindacher to create an ionization chamber. Now, Grinacre's concern was radioactivity, and so we're going to leave off his part of the story at this point because that was his big contribution, was creating a feasible way to make an ionization chamber. We need to focus more on the folks who actually made the first smoke detectors. Maybe I'll do a full episode about Grindacre and other early physicists in the future, since their work would lead to a deeper understanding of atomic physics and by extension, quantum physics. But for now, we're going to get back to smoke alarms. So that brings us to our next Swiss smarty pants person, Walter Jaeger. Now back in nineteen thirty, Yeager wasn't setting out to build a smoke detector. That wasn't his goal. Instead, he had developed a hypothesis. He thought that perhaps using a device with an ionization chamber like the one Grindacre had made, he could build a poison gas detector. So how did he think he could do this? Well, this gets into how smoke detectors actually work, so we're going to dive into it, all right. So, as I said before, you've got your ionized particles. That's a basic component of a large number of smoke detectors. There's actually a different type of smoke detector that doesn't use an ionization chamber at all, but i'll cover that later in this episode. So, these ionized particles are positively charged. They've had electrons stripped off of them, so they have more protons than electrons. They're positively charged. A battery can to two metal plates creates a positively charged surface on one side and a negatively charged surface on the other side. So the electrons that are stripped away from the molecules will then move to the positively charged plate, and the positive ions are going to move to the negatively charged plate. Because opposite charges attract. This movement of electrons is electricity. That's what electricity is. So it's not a lot of electrical current, but it's consistent. Jaeger hypothesized that poison gas would interfere with that current of electricity, and that if you detected a drop in current, that would set off the detector, and Jaeger would say, oh, there's poison gas here. So he started testing it, except it didn't work. The poison gas did not set off the detector, and as the story goes Yaeger was getting frustrated and stressed out and he decided to smoke a cigarette and think about the problem. And so he lights up the cigarette, he starts puffing away and the next thing he knows is detector's going off. Now, the poison gas had not interacted with the ions, but the smoke did, so what's going on. Well, there are particles in smoke that can bond to ions, neutralizing them, so negatively charged particles that can bond with the positive ones. And when that happens, you get a drop in current between those two electric plates I was talking about, and that's what sets off the detector. Another scientist named Ernst Maley improved upon this design by using a cold cathode tube. I've talked about cathode tubes in the past. Here's a quick rundown. It's essentially a device that emits electrons. Looks a lot like a light bulb. You've got a filament that's encased inside a vacuum tube, and the ideas that you pass an electric current through the cathode tube's filament. The cathode tubes filament heats up due to electrical resistance, and as it heats up, it starts to emit a stream of electrons due to thermionic emission. Essentially, that electrical resistance means that the flow gets impeded. You convert some of that energy over into heat. The heat itself strips electrons away from the tungsten filament inside, and then you get your stream. Cold cathode tubes work on a different principle, though they aren't necessarily actually cold. So you've got a cathode that's the electrode that would emit electrons, and on the opposite end of the tube you have an anode that's the side that accepts electrons. So it's the positively charged part of this particular device, and these are both sealed in a tube, and that tube also has a gas inside of it, and applying a sufficient voltage between the cathode and the anode a difference in electrical charge. If it's sufficient enough, it'll cause a discharge between the two and the gas will act as a carrier for that electrical current. So a neon light is an implementation of the cold cathode tube technology. Melee's objective was to boost the signal from the detection circuit so that the signal could be strong enough to trigger an alarm, So not just that it would detect a drop in current or a change in the electrical current across these two plates, but that it would also have a strong enough signal so it could power a loud speaker or activate a physical bell. And a separate circuit in the smoke detector activates upon this change in electrical current, sending a signal to the cold cathode tube, which acts like an amplifier. It takes that incoming signal amplifies it enough for it to do some of the useful work like powering a loud speaker and thus alerting you that there is in fact smoke in the area. Now, Jaeger, being an enterprising sort, partnered with Melee to launch a business offering smoke detectors in the late nineteen forties. However, these detectors weren't terribly practical because they required that high voltage to operate to create that ionization chamber, and homes were not wired for that kind of high voltage, so that meant that you couldn't really get one for your house. They were mostly used, again in big industrial settings where you could wire things up for that kind of power. The solution to this problem was already set in motion, though at the time it was top secret. I'll get to that after this quick break. Now, while the high voltage smoke detectors weren't seen as practical from an implementation standpoint for your average homeowner, it was undeniable that they were useful, and this was particularly highlighted in the United States by a tragedy that took place on December first, nineteen f eight, at the Roman Catholic school in Chicago called Our Lady of the Angels School. Now, the origins of the fire have never been verified. No one knows exactly what started it, but it appeared to begin in the stairwell for the school, and the school had limited fire prevention measures in place. The school itself was an older building in Chicago and had been grandfathered into Chicago's safety standards because it had previously met earlier regulations. So when the city updated its fire safety regulations, one of the policies there was that older buildings that had passed the previous ones were considered safe. Students and staff were unaware of the danger of the fire until it became a critical threat, and more than ninety children died in that tragedy. It was a truly horrifying event and it illustrated the need to develop better fire detection and prevention methods. Research in Canada and the United States began looking into various catastrophic fires that had happened over the years, and according to doctor Jim Milk of the University of Maryland, these studies found evidence suggesting that had smoke detectors been available at the time, there would have been forty percent fewer deaths in those catastrophic fires. There were more than three hundred of them that they were looking at, and rise of heat detectors, which I'll talk about in a second, would have decreased the number of fatalities less than ten percent. And these were hypotheses, mind you, there was no way of knowing that that in fact would actually have been the way it played out. We only know what actually happened, not what might have happened. But the scholars were pointing out how the cause of death wasn't always due to direct exposure to fire itself, which rise of heat detectors would help you avoid, but exposure to smoke, and smoke detectors might activate well in advance of a rise of heat detector, giving people precious time to evacuate a building. So that rise of heat detector, that's like the kind I was talking about at the top of this episode. It's the type that can monitor increases in temperature and at a certain temperature they'll activate, they'll sound an alarm, but by then it might be too late. So the high voltage requirement for smoke detectors that ionized air that had an ionization chamber was still a problem at this point. It made them impractical and expensive, particularly for homeowners. It was again more common for really big buildings like manufacturing plants and factories and that kind of thing, and the rise of heat detectors like the ones I first described in this episode would also be used in those facilities. So the solution to this problem of requiring high voltage would have its roots in a top secret program that had a very different aim. See back in nineteen forty four, there was a guy named Glenn Seborg who had been asked to join the highly classified Manhattan Project. Now, this was, of course, the United States's effort to find a way to weaponize atomic energy, to split the atom to release an enormous and destructive force that could be used as a weapon. Seborg led a team that researched radioactive materials as they tried to determine which of them would be the most useful in a weapon like an atomic bomb. They discovered and created numerous radioactive elements in the process. Elements that are above ninety two on the elemental table were all synthetic. They were all created in labs and their very unstable atoms. Now, among the ones that they worked on was one called amerasrium. So ameraserium is a synthetic element that was produced in the lab in a cyclotron experiment in Berkeley, California. The specific variant that we're interested in for this podcast is an isotope of amerasyrium. It's ameraserrium two forty one. And you know I mentioned what ions are, but what is an isotope case it's been a while since you've had basic science because I always have to look these up. I'm not trying to shame anybody. I get my stuff mixed up, so I got to look it up. Well. You know, an ion is an atom or molecule that has a net electric charge, right. That means it either has too many or too few electro electrons for it to balance out with the protons. That's an ion. Isotopes are different. You have the right number of protons and electrons, but you have different numbers of neutrons between two different two or more different variants of the same element. So ameraserrium two forty one and a marasyrium two forty two are nearly identical. They have the same number of protons and the same number of electrons, but ameraserrium two forty two has one neutron more than a maraserrium two forty one. It changes the atomic mass of that particular atom, but otherwise as the same protons and electrons as the other isotopes of that element. So the maracerrium two forty one is radioactive, which means it decays and gives off radiation. It's ionic radiation, so that means that the energy that's given off, the radiation that's given off is energetic enough to strip electrons off of atoms or molecules and ionizing them. A small amount of a maraserrium two forty one will ionize atoms like oxygen and nitrogen, just through the natural process of radioactive decay. So what do I mean by a small amount. I'm talking about one five thousandth of a gram, so a super tiny amount of radioactive material. Now, smoke detector manufacturers did not immediately jump on a maraserium as a replacement for a high voltage circuit. That would take some time and a lot of study before determining that a maraserrium was pretty safe to use under specific parameters. The type of radiation gives off is primarily alpha radiation. There's alpha, beta, and gamma radiation. I'll talk more about that in a subsequent episode, but alpha radiation is the emission of alpha particles, and an alpha particle is essentially the same thing as a helium nucleus. The nucleus of a helium atom includes two protons and two neutrons. That's an alpha particle two protons and two neutrons. An alpha particle has good ionization power, but it also doesn't have a lot of penetrative power. It can't go through matter very easily. It's a massive particle in the grand scheme of things and moves more slowly than other types of radiation, so an alpha particle is too weak to pass through a thin sheet of paper. It can only go through a few centimeters of air before it loses energy and can't move anymore. So while ameras serium two forty one is radioactive, it's considered relatively safe in small amounts and if kept in isolation. You wouldn't want to come into direct contact with the stuff, and you definitely wouldn't want to inhale or ingest any mrror serium or get it in an open wound because it is carcinogenic. But it would have to get past barriers. It's not even strong enough to get through the skin, but it is strong enough if you were to ingest it or breathe in some dust, it could potentially cause cancer. It could certainly increase your risk of developing cancer, so there is a danger to it. So your typical smoke detector actually has some radioactive material in it to create the ions that flow between two charged plates. The ions behave just as the ones did with Jaeger's high voltage device. It's the same sort of stuff. It's charged particles, So smoke particles will still interact with them, just as they would with Jaeger's invention with the high voltage grid, and they will still bind and cause a drop in current, and that's what triggers the circuit that powers the actual alarm. It would be nearly two decades between the invention of a maraserium to forty one and its application as an ion generator in a smoke detector. The United States Atomic Energy Commission would grant a license in nineteen sixty three authorizing the use of a maraserium to forty one in smoke detectors. And the thought was that the amount of radioactive material would be so tiny that it would not really stand to be a hazard, and it was just a very low risk, but there was a very real risk of fire and smoke, and so when you weigh it against each other and said, the risk of fire is high, and the risk of something happening because of this very tiny amount of radioactive material is low. It makes way more sense to err on the side of detecting fires. So two years later, in nineteen sixty five, a guy named Dwayne D. Persall introduced a battery powered smoke detector. This eliminated the need for that high voltage circuit, and Percell came by this accidentally. A lot of inventions really ended up being created as a consequence of some other unrelated effort. So in nineteen sixty three, that same year where the Atomic Energy Commission granted the license, Percell had been working on some tough problems with his employees. Not problems with his employees, but with his employees. They were working on tough problems. Purcell was in a rough spot, so he had taken out a second mortgage on his house to create a company called Persol Company and a spinoff company called Satratol or Sata Troll rather Sata Trill Corporation in Denver, Colorado. I find that name incredibly difficult to say properly. In fact, I'm sure I'm saying it incorrectly. But anyway, The company's main business was selling heating and air distribution equipment for commercial buildings, so sort of like an HVAC company for big, big, big buildings. But the product is engineered were working on was what he called a static neutralizer. The idea was he was going to create an ion generator, and the idea was that these ions would neutralize static electricity build up on equipment, and that's something that could be a real issue for industrial operations and clean rooms and stuff. So you need to have a way to neutralize static build up or else you can have a discharge that could ruin tons of work. However, his team had encountered a problem. They saw that their ion generator was getting clogged up pretty quickly. The ions were attracting particles like dust and stuff, and as it was attracting dust, it was starting to make the entire device as a whole less effective. An engineer named Lyman Blackwell was running tests to see what could be done to keep the ion generation going. While testing the system, they noticed that the ion meter they were using to monitor performance would occasionally fluctuate, and one of the technicians running the equipment was a smoker. He was chained smoking during the whole testing process. Eventually they figured out that the meter was detecting fluctuations in the ion flow whenever smoke was getting pulled in through the fan on the generator and inserted into the ion stream. So essentially they were making the same discovery that Jaeger had made decades earlier. But the big difference was that Percell's ion generator didn't require that high voltage to run because it was depending on that small amount of amerserium two forty one. So he had the bright idea to take this unexpected result and turn it into an actual product. Two years later, he had the first battery operated, ionization based smoke detector. He called it the Smoke Guard seven hundred, but it wasn't quite ready for the home market yet because government regulations had not yet caught up to the technology. We're going to get into that as well, because, as it turns out, it's not enough just to make tech that works. You have to make tech that works within the boundaries of laws and regulations. Meanwhile, a pair of inventors named Donald Steel and Robert Emmerk came up with an alternative method for detecting smoke with a device and this approach wouldn't used ionized particles at all, so there's no need to create any sort of ionization chamber. Instead, Steel and Emark created a smoke detector that relied upon light and photo detectors. So photo detectors. Those are that's light sensors, right, so sensor that detects light. They are two basic categories of light sensors. The first type are sometimes called photovoltaics. These are devices that emit electrons when they're exposed to light, so solar panels are a type of photovoltaic cells. They generate electricity when exposed to light. The other type of light sensor would be the photo resistor or photoconductor. These sensors have electrical properties that change if they are actually exposed to light. So, for example, a photo resistor has a relatively high electrical resistance when it's in the dark, so that material resists the flow of electricity going through it. However, as the material is exposed to light, the electrical resistance decreases and electricity can pass through it more readily. So if you place one of these devices in a circuit and you have a voltage detector also attached to that circuit. The detector will pick up changes in voltage as light hits the sensor. Now, you could try and build a smoke detector that works by having a light shining on a photosensor of some sort that's at least partially open to the air outside of the detector, and if something like smoke were to enter that pathway the direct detector's vents, and it somehow gets in between the light and the sensor, then you would have smoke obscuring or blocking some of that light, and if it were enough, then the alarm would go off. But that actually wouldn't be a very sensitive smoke detector. It wouldn't work unless the smoke was thick enough to really cause an issue. It's actually pretty tricky for a sensor to detect dips in light intensity, and by the time it would the smoke might be thick enough to already be a major threat to people's safety. But there's a clever workaround to this. So instead of an alarm that goes off when a light no longer is shining on, it, design a system where the alarm goes off if the sensor detects light. Even a small amount of light can create enough of a signal for it to send a message to start the alarm. So in this version, you've got a light that's shining down a pathway. So imagine a chamber. You got a light at one end, and it's open so that air can come into the chamber at a nine degree angle, like a perpendicular angle to this light, and tucked away just a bit, you have a little alcove where there's a sensor. So under normal conditions, the light is going through the chamber, but the sensor is tucked in at a right angle, so it's not picking up any light. The sensor stays in the dark. However, if smoke enters the chamber that the light is passing through, some of that light hits the smoke and starts to scatter. And this is the same sort of effect you would see if you were driving a car on a really foggy day. It's why you're not supposed to use the high beams on your head lights when you're in the fog. The light will hit the fog and scatter, and it's more likely to make it harder for you to see rather than easier. Well, on the smoke detector, some of that scattered light hits the sensor, which then activates a signal to the alarm, and that approach allows for far more sensitivity in a smoke detector. They can go off much faster than one that would require the smoke to block the light. So a dark sensor picking up on light is just more reliable than a lit up since they're trying to detect a dip in brightness. However, it's also not fool proof because vapor or dust could cause false alarms. Now, during all this innovation, changes were starting to happen on regulatory levels around the world, and when we come back, I'll talk more about how that played out in the United States. But first let's take another quick break. All right, So now we've got the basic technology of smoke detectors understood, let's talk about regulations. See, if you're going to produce and market something that's meant to protect lives, it's often the case that a government agency or two will take notice and they want to make sure that the thing you're making does what you claim it does. If lives literally hang in the balance, it's important. So this is the same basic underlying philosophy we see in agencies that monitor stuff like food processing, pharmaceutical development and production. The era I'm talking about in the late nineteen sixties in the United States was a particularly tumultuous time. You had the civil rights movement, you had America's involvement in Vietnam, and other really politically charged events playing out in the US. And these were sparks pun intended that ignited civil unrest across the entire country, and in turn that was testing the limits of police forces and fire prevention and fire extinguishing services. So this prompted the US Congress to re examine policies around safety to better protect citizens. One thing to come out of this was the Fire Research and Safety Act, which was signed into law by then President Lyndon Johnson. The President was citing some figures that suggested as many as twelve thousand Americans had died in fires in nineteen sixty. Later estimates adjusted that number down significantly to like eight thousand, but that's still way too many people. The Act called for the creation of a twenty person panel to study the challenges and make recommendations for new safety standards that could be carried out at the federal level and require any building in the US to follow certain processes to make sure that they were safe for people, and it took a few years for all of this to actually coalesce into a report. I mean, it was a big task, and we also happen to know that things in the US government don't go super fast. But in nineteen seventy three, the panel released a report. The report was titled America Burning, which is pretty sobering all on its own. And America was leading the way in industrial nations when it came to per capita deaths and property loss due to fire. So the argument they were making was that America is this incredibly advanced country, Why the heck are we losing so much to fire? We should be doing better than that, And eighty percent of the deaths were occurring in people's homes. The report also contained pictures of Percell's smoke Guard detector. He had come to overcome challenges to make his detectors practical and safe. Now. His original design did not have a battery. Instead, it was to be hardwired into the electrical system of a building. It didn't require two hundred and twenty volts like the earlier smoke detectors off Switzerland did, but it still was an expensive proposition, and it made it an unlikely candidate for home adoption because every unit would set someone back about one thousand dollars at the time, which would be a lot more than that today. So his team was able to create a version that would operate on battery power, which was already an engineering triumph. But they need to figure out how to make this a reliable one, or to convince people that it was reliable, because batteries, as we all know, eventually exhaust themselves. The chemical reactions inside a battery are what produces electrons, and over time, you get to a point where there's been enough of the chemical reaction going on that you don't have the active ingredients necessary to sustain that supply of electricity anymore, and not the proper voltage anyway. And we're talking about something as critical as a smoke detector, that's a real problem. So his team solved this issue by creating circuits that would send a chirping alarm to the smoke detector if it detected a drop in voltage across the primary circuit of the smoke detector. So the chirp wouldn't require very much energy of itself, and it would be repeated until the voltage across the circuit returned to the proper level, in other words, until the battery was replaced. In addition, Percell included a small card in the box for the smoke Guard seven hundred, and customers were meant to take the card and then fill out little forms on the card with their own information, including their address and the date that they installed their smoke detector, and Personal's company would actually mail out an annual reminder to its customers saying, hey, it's time for you to replace the battery and your smoke detector. In order to keep it operational, Personal worked closely with safety officials and organizations both to improve his smoke detectors and make them more useful, and also to help shape policy so that these detectors would be recognized as effective and a good option to help curb the problem of fatalities due to fire disasters, and the work paid off both for Personal and for people in general. He was able to convince officials that a battery powered smoke detector was effective if it had the ability to alert occupants of a dying battery. The government actually would mandate that the chirping alarm sound should last at least seven consecutive days in an effort to alert homeowners to change battery. And this was specifically in case someone might be out of town when the battery starts to give out. They wanted it to last long enough so that you would have time to get back and find out, oh, gosh, I need to switch out the batteries on my smoke detector. The government also had to balance out the cost of installing fire prevention systems and homes, including in newly constructed homes. Initially, plans called for both smoke detectors and rise of heat detectors. However, after numerous studies, the government concluded that rise of heat detectors weren't really practical if you were looking at trying to save lives. They just weren't good enough to do that, and they were really expensive, and that smoke detectors were much better for the purpose of preventing fatalities. And so that meant that they got rid of the rise of heat detector requirement, and that helped bring the cost down of implementing fire protection systems and homes, and that in turn increased the likelihood that people would actually follow the rules and adopt smoke detectors. The regulations paved the way for Personal to manufacture, market, and sell his smoke detectors to the American public, who could be reassured that the devices would actually provide a valuable and potentially life saving service. He scaled up his company to meet demand before eventually selling it off in nineteen seventy seven. And from everything I've read about him, it sounds like he was motivated not only by an entrepreneurial spirit, although he certainly had that, but also a genuine desire to make his community and the world a better place if he could. And I think that's pretty cool. Since the introduction of the optical and the ionization chamber based smoke detectors, we've seen some innovations, but the basic principles all remain the same. There are smoke detectors that incorporate both types of methodologies, meaning there are smoke detectors that have independent systems to detect the presence of smoke, and we've seen some incorporate other types of tech, such as network connectivity in the form of products like the Nest Protect smoke detector, and those smoke detectors add a little more functionality to the basic kind. They work on basically the same principle, but they have some more features. For example, they can send information across a local area network wirelessly and then that network can send an alert to you on an app on a smartphone, and that could be valuable if, for example, you're away from your home when an alarm goes off. To give you a notification, you can perhaps either call home, or if no one's home, you might even call a fire department to go and check on your home to make sure that everything is all right. And most homes have multiple smoke detectors. In fact, you're supposed to have one outside of every bedroom, for example, as well as maybe one in the kitchen. My own home has six of the darn things, and it can be an issue of figuring out which detector is going off, and that could be of vital importance. So with connected detectors, then you get a notification saying toctor number three is going off, and you know that number three happens to be outside the guest room, so you would be able to very quickly figure out what's going on, as opposed to trying to determine which of your numerous alarms is going off. It also means that if a battery is running out and a smoke detector is chirping, or maybe a battery was just a bad battery and starts to chirp, you can more quickly track down which detector is making the chirping noise. This really applies to folks like me because I live in a townhouse that has a few floors, and the center of the townhouse is essentially like a chimney. There are stairs that go from the bottom floor, and the stairwells open all the way to the top of the townhouse, so it's like an echo chamber inside my house, which means when something like a smoke detector starts to chirp, I can't easily identify whether it's on the floor I'm on the floor above me or the floor below me. And I have six smoke detectors. If it's time to replace the batteries, that's one thing, But if it's just that a battery is going bad early, then I have to figure out which of those detectors is making the problem. And exacerbating this issue for me is the fact that I have a cute little doggie named tibult and the chirping smoke detector noise causes him intense distress, like he starts to shake with fear. So I get really upset when one of my smoke detectors starts to chirp prematurely. There's no smoke or anything. It's just giving me a chirp alarm. But that's a me problem. One other thing that a lot of smoke detectors can do these days is they can also perform as carbon monoxide detectors. So carbon monoxide is an odorless and colorless gas, so human beings can't easily detect it, and it's also toxic. It's a byproduct from burning carbon based fuels like gasoline, heating oil, or natural gas, and in confined spaces it can be really dangerous stuff, so like a garage, for example. And while we humans can't really detect carbon monoxide with our own senses, there are a lot of other ways to see if the stuff is around. So carbon monoxide detectors can work using one of a few different methods. Upon detection, they basically do the same thing as a smoke detector. They send a signal to sound an alarm, but the way they detect the carbon monoxide can be a little different. So there are three basic approaches to this, and one you might have what are called biomimetic sensors. These sensors mimic us the name some sort of biological function, such as hemoglobin, which interacts with carbon monoxide. So these sensors have a gel inside of them, and that gel can absorb carbon monoxide. As the gel does absorb carbon monoxide, the gel changes color. Separate sensor that's monitoring the color of the gel, and if the gel changes, then the sensor picks up on that change and sends a signal to the alarm. These sensors can actually be reset. The gel will return to its original color once it gets rid of that carbon monoxide, but it has to be set in an environment that's free of carbon monoxide for several hours in order to reset. The next type is the metal oxide semiconductor sensor. This has components that have a certain level of electrical resistance, very much like the optical smoke detectors I talked about earlier. So these components react with carbon monoxide in a way that lowers the material's electrical resistance and so meters are monitoring a voltage across a circuit, and if it detects this change in voltage, then it will send a signal to the alarm. And the third type of sensor that you could find in a carbon monoxide detector as an electrochemical sensor. These sensors also detect changes an electrical current in the presence of carbon monoxide, but they have electrodes that are inside a chemical solution, so they're actually engulfed in a chemical solution. Around these electrodes and the chemicals in the solution react very very quickly in the presence of carbon monoxide, and that changes the electrical qualities of the solution, which means that you are able to detect a change in the circuit very very quickly. In fact, this stuff is used in professional settings. It's a very sensitive kind of alarm. Today, there are a lots of smoke detectors that double as carbon monoxide detectors with separate components monitoring the environment. I hope you enjoyed that episode from February tenth, twenty twenty, how smoke detectors work. As a reminder, I will be back next week with all new episodes. I have no idea what they'll be about because currently I'm on space mountain or something. I hope you're all well, and I'll talk to you again really soon. Tech Stuff is a production of iHeartRadio's House Stuff Works. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.

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