Smoke detector#Ionization

Ionization smoke detectors, ionization detectors or simply I- detectors are fire detectors, which operate with a radioactive substance, usually the isotope americium - 241. They were long time smoke detector of choice. Because of the radioactivity but they are gradually replaced by optical smoke detectors and heat detectors.

Principle of operation

A small volume of air is irradiated with an alpha - or beta- emitters and ionized thereby. The conductivity of the air is measured by two electrodes. To smoke is in the air so often meet ions on smoke particles and are deposited there on. The conductivity of the air decreases. Based on the conductivity decrease can be concluded that the smoke concentration. Reducing the current flow suggests the ionization alarm.

Frequently working with two measuring volume / chambers. The reference chamber is closed and can only slowly and thereby allow air to smoke. The measuring chamber is open ( one electrode is executed as a grid ) and smoke can penetrate. The power of the two chambers is compared ( voltage divider with two chambers ). Various disturbances such as Air pressure, humidity, etc. change only slowly, have the same effect on both chambers and are therefore not measured.

Pros and Cons

Ionization detectors are particularly sensitive to small smoke particles as they preferably in flaming fire, but also in diesel soot, occur. In contrast, optical fire detectors are better suited for the early detection of smoldering fires with relatively large and light smoke particles. The detection performance of both types of detectors must therefore be regarded rather as complementary to each other. A distinct advantage in terms of security against false alarms ( by steam, cooking fumes, cigarette smoke, etc. ) can be identified for any of these types of detectors. For use as a lifesaving smoke detectors, as they are already required in some German Länder, optical fire detectors are preferable to the ionization detectors, as the lethal fumes of a smoldering fire be detected before any flame.

Used alpha emitters

The first ionization smoke detectors used radium as an alpha emitter. Radium also emits a large amount of gamma radiation. There were only relatively large currents are measured. According strongly had to be the spotlight. In the past radium was applied by hand with brushes on a small area in the smoke detector; with appropriate radiation exposure of the workers. Later, the radioactive material was rolled as oxide matrix under a very thin but hermetically sealed precious metal cover. The discs or sheets were installed as so-called " Sealed radioactive substances" into the detector. Design approval, and thus the general applicability, in the present handling permit the installer's, so was not possible.

Later, xenon, krypton ( 85Kr ) and tritium was used in glass ampoules. However, the gases bargen especially in the manufacturing also risks.

The materials were later replaced by americium -241. It has a much lower proportion of gamma. But instead must be about five times the activity towards radium.

Amplifier

The ionization currents are very small. At the beginning of electron tubes were used. They were able to measure currents in the range of a few nano- amperes. Later, these field-effect transistors (MOSFETs ) have been repaid. With them it is possible to measure currents of less than 10 pA. This allowed the necessary radioactivity is reduced accordingly.

Isolation

When measuring small currents is the isolation of great importance. Water on the surface, dust and moisture lead to leakage currents. The insulation distances therefore had to be great and be made from suitable materials.

Problem

The regulations concerning radioactive sources have been tightened over time. Manufacturers offer to dispose of the radioactive emitter professionally. If a house torn down with a fire alarm system, probably no one remember that in every room is a radioactive source, which must be disposed of properly. The owner shall ensure that the ionization detectors are disposed of properly. Even the installer of the fire alarm system must be in maintenance document every exchange of a Ionisationsmelders and demonstrate disposal. For this reason, the use of ionisation is prohibited in the technical connection requirements for fire alarm systems in more and more counties.

Another delicate point is the corrosion of the springs. The radioactive materials can not be well encapsulated, otherwise the alpha radiation would be absorbed by the wrapping. In practice, the spotlight was often not even covered only with a very thin layer of gold or. Corroded the source with the time, could leak radioactive material. The radiation exposure of a mounted at some distance and intact detector is low. Only a small proportion of gamma radiation comes to the residents. However, if from radioactive material and is inhaled, for example, it can have serious consequences, because alpha-emitters in the body have a highly damaging effect.

Due to the radioactive radiation, ionization detectors are, however, only used in special cases, because the requirements are very strict. However, the hazard potential of a single detector is low under normal conditions of use and disposal.

Normally, the ionization detectors are completely safe because of their low activity. In case of fire, but the fire debris must be checked for missing fire detectors. If not all detectors are found, the entire fire debris must be disposed of as hazardous waste in accordance with the Radiation Protection Regulations ( at least in the EU ), which also results in significant additional costs for a use of the fire department. But the search for the detector is not always very easy. With Geiger counters you barely have a chance to see a film with a thickness of several centimeters. It is also a visual search. By means of the fire alarm installation plan can be estimated the approximate location of the fire alarm.

The most common are ionization in Anglo America, as they may be disposed of through the municipal waste there.