23 July 2004

Where there's smoke, there's (not always) fire

An inside look at Smoke Detectors

Mike Dziekan, Connecticut Analytical Corporation

Part 3. Ionization smoke detectors

The ionization type smoke detector utilizes ionic current as a detection mechanism. The source of ions is air in a sensing chamber that has been ionized, generally by a radioactive source. If smoke particles enter the sensing chamber, the ionic current is reduced. This reduction is sensed by a circuit that triggers an alarm.

Radioactive Sources

One of the most common radioactive sources used in ionization smoke detectors is an isotope of the manmade element Americium known as Americium 241 (Am 241). The atomic number of Americium is 95.

Americium 241 has a half-life of 432.7 years. As it decays, it releases primarily alpha radiation (helium nuclei) along with some gamma radiation. as it gradually becomes neptunium-237, which is also radioactive. Americium 241 eventually decays into its final form, bismuth 209, which is not radioactive.

Americium is produced when plutonium atoms absorb neutrons in nuclear reactors. It's also produced as a result of nuclear weapons tests. Americium was discovered (made) in 1944 by Glenn T. Seaborg , along with his colleagues at the University of Chicago. Its discovery provided Seaborg with the unique distinction of holding the shortest patent claim ever written. The claim consists of only two words: "Element 95.

The amount of radiation emitted by Am 241 is very small. For more information, see this link: EPA Radiation Info . The Government requirements pertaining to Americium can also be viewed there. For information about many common sources of radiation, explore the EPA radiation sources webpage.

Operation of ionization smoke detectors

Ionization smoke detectors operate on a different principle than the photoelectric type. Photoelectric smoke detectors are more sensitive to visible smoke, while the ionization type is more sensitive to smoke composed of very small particles that are often invisible. A photoelectric smoke detector will generally have a greater sensitivity (faster response time) to slow, smoldering fires, while the ionization type will respond more quickly to fast burning, flaming fires.

A technical paper that explores the response time differences between ionization and photoelectric smoke detectors is Response Time Comparisons of Ionization and Photoelectric Detectors . System Sensor has a System Smoke Detector PDF document that discusses differences in the sensitivity of each type of smoke detector. This document states,

The characteristics of an ionization detector make it more suitable for detection of fast flaming fires that are characterized by combustion particles in the 0.01 to 0.4 micron size range. [One micron is a millionth of a meter (10 -6 meter).] Photoelectric smoke detectors are better suited to detect slow smoldering fires that are characterized by particulates in the 0.4 to 10.0 micron size range. Each type of detector can detect both types of fires, but their respective response times will vary, depending on the type of fire ."

When the ionization and photoelectric detector chambers become contaminated over time, the result is a more sensitive detector. Although this may sound like a good thing, it isn't, and will most likely lead to numerous false alarms. To understand why, consider this graph of chamber value over time:

As smoke or additional contaminants collect inside the ionization chamber, the headroom between the normal chamber background value and the alarm threshold value is decreased. The result is that, as time goes by, a very small amount of smoke or contaminates can cause an alarm or an alert (pre-alarm) condition.

Intelligent fire panels have special algorithms that can compensate for this slow increase in detector background level. An indication is also displayed on the fire panel (if so equipped) indicating when a specific detector has a high chamber value, and should be checked, cleaned, and/or replaced immediately.

Ionizing sources for smoke detectors

To understand more clearly exactly how an ionization type smoke detector works, we must first understand what is meant by ionizing radiation and how it creates ions. A very well written hands-on article about building your own ionization chamber is at the Techlib website, Fun with ion chambers .

Air molecules are neutral (i.e. they don't possess any net positive or negative charge). To make ions, we have to alter the equal ratio of electrons to protons. This is easily done by removing an outer electron.

There are several ways to ionize air. One method is to generate a high voltage discharge, but this adds expense, complexity and radio-frequency noise to the detector.

A second method is to use a radiation .source. One could use intense ultraviolet light (electromagnetic radiation), but that, too, would add expense and complexity. One could also use X-rays (also electromagnetic radiation). I won't even begin to discuss the complexities that one would encounter here!

Heat can also cause ionization, but imagine the difficulties in designing a small, plastic smoke detector that uses heat as an ionizing source while functioning continuously on just a few milliamps (10-3 Amps) or current

A much simpler ionizing source is a radioactive material that emits alpha particles. Alpha radiation has a very limited free path in air (i.e.. it won't travel more than a few centimeters in air). It has very little penetrating power and, thus, does not pose a health risk external to the human body. And it is very efficient at ionizing air molecules. As long as one does not ingest or inhale any alpha emitting material, then there are no serious health risks. A sheet of paper can effectively stop alpha particles.

A beta emitter can also be used as an ionizing source. However, the ionizing ability is not as great as that of alpha radiation. The alpha particle is a positively charged (+2) helium nucleus, while the beta particle is a fast moving electron or positron. The mass difference between the alpha and beta particles is enormous. Beta particles can pose a significant health threat if ingested or inhaled.

When a neutral molecule or atom is ionized, an outer electron is removed. The removal of the electron causes the production of an ion pair, the ejected electron (negative) and the remaining molecule or atom, now with a positive net charge.

The ability to ionize can be characterized by Linear Energy Transfer (LET) and Quality Factor (QF). LET is measured by ionization density, or the ability to create ion pairs per cm in a specific medium or tissue. If a particle or photon has a high LET, then its ability to cause biological impact is also higher, and thus its QF is also higher.

Some examples of typical values of QF are:

X-ray, Gamma ray, and Beta particles: QF = 1

Alpha Particles: QF=20

As you can readily see, the "safe" alpha particles have the highest ion pair creation ability. This means that they will generate lots and lots of ions. Now that we have a method for creating ions, the next problem to overcome is recombination.

Recombination can occur over time when ejected electrons recombine with ionized atoms and molecules. To prevent (or limit) this, a voltage is applied between two plates inside the ionization chamber, as shown in the nearby image.

When a potential difference is applied between the two plates of the ionization chamber, positive ions will migrate toward the negative plate, and negative ions will migrate towards the positive plate. The potential difference will keep the ion current flowing inside the ionization chamber. As long as the chamber is clear (i.e. no smoke or particulate matter) then the current will be at its maximum level. If smoke or particulate matter are introduced into the chamber, they will cause a reduction in the ionic current flow.

Ionizing chamber design

It should be obvious that including more gas molecules inside the chamber (pressurized gas) will provide more ions, and more ionic current will be available. The drawback is that the chamber would have to be sealed, and no smoke could enter. In the real world, as the ambient pressure rises and falls, there will be a slight change in ionic current, due to the increase or decrease of available air molecules.

The normal (no smoke present) ionic current is typically in the picoamp range (10 -12 amperes). As more smoke particles or contaminants enter the ionization chamber, the ionization current will be reduced. When the ionization current falls below a predetermined threshold, an alarm will sound.

I stated earlier that any aspirating detector or duct detector will most likely use a photoelectric detector. In the ionization type smoke detector, the cloud of ions could literally be blown out of the ionization chamber if the ambient airflow is too great. This is why a photoelectric detector is used in these high airflow situations. Newer ionization detector chambers have been developed that are less sensitive to rapid airflow, but generally a photoelectric type will be utilized.

If careful ionization chamber design and external air sensing housings are used, an ionization detector can be used to sample air traveling through a typical building air handling system without the risk of blowing the ionized air out of the ionization chamber

Because the ionization chamber ionizes air molecules, it is affected by atmospheric pressure changes and humidity. These influences can emulate the presence of combustion products. To make a better ionization detector, a better ionization chamber is required. The improved ionization chamber is known as the dual chamber design, and it has all but replaced the single chamber design.

Dual ionizing chamber design

In the dual-chamber design, one chamber is opened to ambient air (sensing chamber), while the other (reference chamber) is partially closed off so that it is affected only by atmospheric conditions, such as humidity and barometric pressure. The reference chamber allows air molecules and water vapor to enter, but blocks smoke particles. The holes in the sensing chamber are too small to allow smoke particles to enter.

The reference chamber will produce an ionization current that is unaffected by smoke, but affected by changes in humidity and barometric pressure. The two chamber currents are compared, and a large difference between the two triggers an alarm condition. If large, sudden atmospheric changes occur, both chambers will have a similar response and the difference between the two will be negligible. If smoke or products of combustion enter the sensing chamber, a large difference will be noted between the reference and sensing chamber, indicating an alarm condition.

The nearby illustration shows how smoke particles are prevented from entering the reference chamber, while easily entering the sensing chamber. The difference between the two chambers determines the alarm condition.

When smoke is present, the reference chamber maintains maximum ionization current, while the sensing chamber experiences a reduction in ionization current. If the atmospheric pressure or humidity changes abruptly, then both ionization chambers will fluctuate in a like manner. The difference between the two will be negligible. A single ionization chamber smoke detector might experience a false alarm condition, while the dual chamber design will not.

For best results in detecting smoke, both photoelectric and ionization smoke detectors can be combined into a single system. With the combination of both technologies in one detector housing, a fast responding detector can be realized. The result is a single detector that responds to slow, smoldering fires as well as fast flaming fires. With a "combo detector" that combines photoelectric and ionization sensing technology, much faster and more reliable detection capabilities are available.

Newer combination detectors have the combined abilities of photoelectric, ionization and heat detectors in a single detector. When the detectors are connected to an intelligent fire panel, additional capabilities can be given to the individual detectors.

Part 4

In Part 4 I will conclude this series on smoke detectors with a look at intelligent smoke and fire alarms and false alarms. The false alarm discussion will apply to both residential and industrial alarms