What You Should Know About Type Approval Ionization Smoke Detectors
Translated from German by Abha Vardhan (2019)
An ionization smoke detector is a tool that helps to detect smoke. A normal ionization smoke detector usually comprises of two plates that are located close together with a radioactive material, which is a very little amount of radioactive substance, called Americium.
The plates are attached with a battery and it applies voltage to the plates, which charge both negative and positive plates. The radioactive material continuously emits alpha particles that release the electrons from nearby atoms in the air and consequently ionizes the oxygen and nitrogen atoms in the detector chamber.
The positive ions move towards the negative plate while the negative ones move towards the positive plate, and this movement of ions creates an incessant electric current. One can easily measure this small ionization current by the electronic circuit fixed to the plate.
Let’s learn about the working of the ionization smoke detector in the presence of smoke. When the molecules of smoke move into the ionization compartment, the smoke particles get fixed with the ions and deactivate them. This drastically decreases the total amount of ionized particles in the compartment.
This decrease in ions reduces the current in the chamber, which is detected by the electronic circuit. The reduction in current around the plates activates an alarm and also lights up the red LED lights when the alarm is triggered.
The Features of Ionisation Smoke Detectors
Ionization smoke detectors are effective in reacting quickly to blazing fires that have small ignition particles. This is possible because of their response to even the smallest smoke particles. However, they usually create false alarms in comparison to other detectors such as photoelectric detectors, which work best for smoldering fires.
Ionization smoke detectors are more affordable than most of the other detectors including photoelectric ones. They are also more secure and safe to use. If the battery gets weaker in an ionization smoke detector, the current drops and it causes an alarm in order to warn the owner to replace the battery before the device becomes unusable.
Type Approval Ionization Smoke Detectors (ISD)
Ionization smoke detectors (ISD) are smoke detectors that make use of radioactive substances predominantly americium-241, with activities up to 40-kilobecquerel. According to the German Radiation Protection Act, a permit is needed to handle devices incorporating radioactive substances.
However, one can use such instruments without authorization if they are type-approved. It is in accordance with the Federal Office for Radiation Protection (BFS), a proficient specialist that issues type approval.
Principle of Functionality
The radioactive substance is typically electroplated on a carrier layer, which is made of foil and firmly bound. This radioactive foil is attached firmly inside the ISD casing with the help of holders. The air present inside the ISD casing is ionized with the emission of the radiation released by the foil.
An ionization current is generated by electric voltage, which is applied within the ISD. The strength of the ionization current alters if fire aerosols enter the ISD through the smoke inlets.
This change is recorded electronically and is used to trigger the alarm. Different models of ionization smoke detectors (ISD) vary essentially in the number and design of the measurement and reference chambers, as well as in the electronic system used.
Type Approval or Licensed Ionisation Smoke Detectors
Ionization Smoke Detectors contain radioactive substances and therefore they need a license to operate. Nevertheless, these instruments can also be operated without a license if they are type-approved. The proficient agency that issues type approval equipment is the Federal Office for Radiation Protection (BFS).
The Necessities for Issuing Type Approval Equipment
In accordance with the German Radiation Protection Ordinance, it is important that Ionization Smoke Detectors (ISD) must meet several requirements to be type-approved.
- It is crucial that the rate of the ambient dose should not surpass 1 microsievert per hour at 0.1 meters from any touchable surface of the equipment.
- The radioactive material must be tightly enclosed in a sealed flask in a safe manner that nobody can touch.
- Even in the incidence of fire, the Ionisation Smoke Detector should be sufficiently fitted so that it does not release enclosed radioactive substances.
- The productive life of the Ionisation Smoke Detector is generally limited by the manufacturer to 10 to 15 years. Since there is no regulatory control on type approved ISDs, it is vital that it is manufactured in a manner that doesn’t require any extra leakage analysis for the radioactive material during its lifetime. However, ISDs will need the acceptance analysis by the constructor.
The authority that issues type-approved ISDs may describe deviating guidelines in their approval certificate.
Limitations for Type Approval ISDs
In accordance with “Annex 4 Table 1 Column 2 StrlSchV”, the issuing authority only grants a type approval ISD when the activity of the radioactive substance inserted into the device does not surpass ten times the exemption limits. It is worth noting that ten times the exemption limit for Americium-241 is 100-kilobecquerels.
If the need arises to open or dismantle the casing of the ISD containing the radioactive material, it must only be done with special tools to avoid removing the radioactive foil by mistake.
As per authority agencies, the possessor of the type approval ISD must return it to the authorization holder after use. Alternatively, the ISD device should be given back to the body designated by the proficient authority or to the State Collecting Facility.
The type approval ISD only permits the license-free usage and storage of ISDs. However, people or agencies will need a license, even when the ISD is type-approved or if they want to fix, remove or maintain it.
The Safety Regarding Radioactive Material of ISDs
It is not possible to emit radioactive material under standard operating conditions. Due to the pretty low dose rate in the surrounding area of the Ionization Smoke Detectors and the normal operating space from it, a person is only open to a dose of a few tens of microsieverts every year.
On average, the normal radiation exposure in Germany is approximately 2 millisieverts per year. Thus, the health threat associated with the intended use of ISDs is insignificant under standard working conditions.
The testing is conducted for only a single sample of ISDs. This is why all factory-made copies of the ISD must be similar to the initial sample in accordance with their radiation protection characteristics. It is because of this crucial reason that the company should operate a quality system that is controlled by an expert, and this expert must be appointed by the authority that issues type approval ISDs to foresee the quality of each device.
Duties for Issuing a Type Approval ISD
Anyone who needs a type approval ISD must send an application to the BFS.
The BFS includes the Federal Institute for Materials Research and Testing (BAM) for issuing type approval ISDs. Before granting the equipment, BAM conducts several specialized examinations to assess the use of materials, leak tightness, the design of the area where radioactive material is placed, and an overall quality check.
The analysis and tests include the mechanical strength (shock, vibration and drop test), temperature resistance, and the leak tightness of the equipment for incidences of fire.
The Applications of Ionization Smoke Detectors
In Germany, the use of ISDs is restricted to a few specialized jobs. They are typically applied in hazardous surroundings, on ships and other similar locations. This is because nowadays optical smoke detectors or combined detectors are widely used as alternate smoke detection systems in numerous application areas.
It is worth noting that in countries like the United States of America and the United Kingdom, Ionization Smoke Detectors are still being applied in private homes.
Source: bfs.de