B.A.R.C-625
O
GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION
RADIOISOTOPE SMOKE ALARM (RISA)
byK. Krishoamurthy, S. M. Rao, G. S. Ramakrishna
R. S. Deshpande and Allan RaskinaIsotope Division
BHABHA ATOMIC RESEARCH CENTRE
BOMBAY, INDIA
* 1972
B.A.R.C.-625
I
u
GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION
RADIOISOTOPE SMOKE ALARM (RISA)
by
K. Kri@hnamu?ihy, So Mo Rao, G, S. RamakrishnaR. So Deshpande and Allan Rasklna.
Isotope Division
BHABHA ATOMIC RESEARCH CENTREBOMBAY, INDIA
1972
ABSTBACT
This report describes th© saliei.t features of the
dsvelopmeat of a radioieotope emok© alarm for use in ent'ly
firs warning systems. The unit consists of a suitatsla teitiiso
source-ion chamber aBsero'bly eith the assooiated electronicfi.
The unit possesses high sensitivity for smoke detection. Its
reliability and radiologies! safety have also been studied.
RADIOISOTOPE SMOKE ALARM (RISA)
by
K. Krishnamurthy, S. M. Rao, G. S. RamakrishnaR, S. Deshpande and Allan Raakina
1. INTRODUCTION
1.1. The radioisotope smoke alarm is a very sensitive system
available for the detection of smoke and gases resulting from
fire and hence can be used to give an alarm even before the actual
fire occurs. Due to its high sensitivity it is ideal for clean
rooms5 electronic equipment rooms, computer rooms etc. Further,
since the sensitivity of the unit is adjustable it can also be
advantageously used in industrial sites* However, the effect of
industrial gases on the response of the system must be ascertained
before installation.
The fire in any ordinary combustible material usually appears
in the following sequence of
Combustion gasss —Smoke -—-—Flame •—--Heat
For protection of personnel and property, detection of fir©
oust take place before appreciable heat or flame develops. Many
fires develop through the first two stages over a considerable
length of time which ie known as the incipient time. Detection of
fire must take place during this incipient time if the damage is
to be kept to a minimum.
A deteotion system which depends, for its operation upon
the rate of rise of air temperature or a fixed temperature will
i 2 I
give a signal only after the fire i8 well established, while a
system wnich detects combustion gases and smoke will provide
warning at the incipient stage itself thus minimising the
damage to a great extent. This point can be explained clearly
with the help of fig- 1° This figure indicates the relative
stages at which varit us types of thermosensitive detectors and
smoke detector respond.
She ionization type smoke detentor is unique, in that
it responds to invisible combustion particles and smoke irrespective
of the presence of flame or rise in ambient temperature»
2. PHYSICAL PRINCIPLE
The sensing element of the ionization smoke d'etre tor
head is an ionization chamber* In this chamber, the air is mt*de
conductive using the alpha or beta radiations from a radioactive
source. These radiations have the property of Ionizing the air,
thus producing -we and -ve ions. If an electric field ia applied
across this ionized air, it results in a current flow. This curreat
strength depends on the ionization produced by the source, on the
applied electric field and on the magnitude of recombination of
ions produced.
At low voltages applied to the electrodes, only a part of
the lone produced are collected, fflie remaining IOHB of eitb.es
polarity collide and recombine thus getting neutralised. As we
increase the voltage applied to the electrodes, rate of ion
collection increases and recombination process rate decreases.
It is only when the applied potential reaches a certain limit
that all the ions fonued are collected by the electrodes* This
is known as the saturation point. Beyond this level of voltage,
the current remains constant regardless of the increase in the
applied voltage.
8 3 «
Apart from these factors the current in an ion chamber
dependa on the composition of the gas medium between the
electrodes. The rate of drift of the ions is closely related
to their <ize and mass. Heavier the ions slower will br> their
drift tr,»ards the electrodes. All combustion particles nre
about thousand times larger than air molecules- Some of them
are visible as smoke but most of them are still too small to
be seen. When combustion particles enter the ion chamber the
ions tend to attach themselves to these particles resulting in
the reduction of ion mobility, in the applied electric field.
This leads to a higher rate of re-combination and in a sharp fo11
in the current. In Hadioisotope Smoke Alarm (lllBA) this drop
in current ia used to actuate an alarm (Fig. 2.).
There are atlet-dt two ways of achieving this. One way ie
to use an electrometer D.C. amplifier for actuating an electrical
relay, if the current in the ion chamber falls. The second method
is to use a specially designed cold cathode trigger tube to rospond
to thiB varying current. The latter type uee alpha Bburces like
Americium-241. In the present report we have discussed the first
method of detection vfith the indigenously available tritiisi
sources.
3- DESCRIPTION OF THE UNIT
Figo 2 shows the biocic diagram of the unit.
3.1. Xoniaation Chamber
The bottom perforated portion of the sensing unit shown
in fig.3 contains the ioniswition chamber which is also schematically
shown in fig.6. The chamber itself consists of two circular plates
parallely placed and separated by an optimum distance experimentally
arrived at. A Buitable tritium disc source is mounted on one of the
plates. She chamber is mounted inside a perforated housing to
I 4 3
provide protection to the source as well as to allow free
entry of air and smoke. Collection voltage is selected for
optimum performance. A voltage of about 80V is applied to
one of the plates and the other plate (collector) is
connected to the electrometer which is housed above the ion
chamber unit.
5.2. Amplifier
The current flowing in the ionization chamber is very
low of the order of 10 Amp, and direct measurement of the
current is difficult due to the unavoidable reflected
impedance from the input circuit of the amplifier. The ampli-
fier is designed to overcome this difficulty by using an
electrometer preamplifier stag*8., followed by two stage D.C.
amplification. Due to the low grid leak current of the
electrometer tube and using a high -ve feedback, high stability
is achieved. High insulating material (teflon) is usied to
insulate the collecting plate and the control grid of the
electrometer tube from the rest of the circuit. Voltage at the
output point of the amplifier is adjusted to about -1J volte.
When current in the ionization chamber falls° the transistor
(fig. 7) at the output stage conducts and output voltage
changes in the +ve direction. Shis voltage change is used to
actuate the relay. Change in the voltage at the output point
depends on the amount of smoke that enters the ionization
chamber. lo enable adjustment of the sensitivity of the
detector, a level detection circuit is used.
3«3» Level Detection Circuit (Fig. 7)
Output voltage of the amplifier is directly given to the
base of T3« The common emitter voltage of T3 and. T4 is
I 5 8
adjustable for sensitivity selection. This voltage is such that
23 remains non-conductingo Hence initially TJ is in "OFF"
state and T4 is in "ON" state. This keeps the relay E1 in the
noxzsally energised condition^ When the output voltage of the
amplifier changes to -12? or less due to the entry of smoke
in the ion chamber, T5 conducts and changes to "ON" state and
1*4 changes to "OFF" state = Relay R1 in the collector circuit
of 14 g@ts deenergised. Normally closed contact of R1 is used
to actuate another re-lay R29 which operates the alarm. When 32
is actuated, power to this is held through the contact of R2
ite<bl£ ead hence it remains in the actuated state irrespective
of the changes in the rest of th© circuit and the alarm continues
till resetting is done by interrupting the power to the relay B2.
4» S P E C I A L F E A T U R E S
The unit gives an alarm signal, whenever
a) produots of combustion or smoke are present in theenvironment.
b) any component fails
o) power to the main unit i s not available
Th© detector gives & isual indication (flickering sad
light) alongwith an audio indication of a bel l ringing.
Th© unit i s also provided raith automatic ssiteh over to
feattojgr power operation in case of mains power failure»
5« RELIABILITY EVALUATION
Th® unit has been subjected to nuattesded an$ ce&tisueus
for a period of 3 months under industrial environment.
i 6 i
She observations area
i) thera has not been any false alarm due to componentsfailure during this period*
ii) no significant variation in tha standing current iathe chanter, indicating tha stability of the tsltiuasource and electronics«
iii) response was always good during testa nith smoke..
She unit ie now undergoing customer evaluation tests at
il/se Mazagaon Bocks Limited«
6. RADIOLOGICAL, S A F E T Y
The radioactive source used is a tritium target with
tritium absorbed on a fine layer of titanium backed by a copper
disc.
Experiments have revealed that there is little or no
escape of tritium gas during normal operation,. It is also
estimated that even if any degassing takas piece5 the concentration
levels in the environment will not rise significantly as the souse©
is always in free atmosphere.
Physical approach to the source by unteaiaad persons
is avoided by enclosing the system in a perforated shell with a
radioactivity warning displayed on it. Ho direct exposure to the
source is possible.
The Directorate of Sadiation Protection fcas approved
the system and found it safe for installation.
7. CONCLUSION
The uait at the present stags of development appears
promising as a sensitive eaoke detector for an early fig© sasniag
system. It is also considered safe from the radiological health
point of vies*
8 7 8
For fisal industrial us@s the unit is being designed
to incorporate sis to twelve detector heads connecting to
e single electronic control console unit.
8o ACKNOWLEDGMENTS
Shanks are due to Br« ?«>K. lys , Head, iBotops Division,Shs-i ?©o^.8eskar9 Chief Hre Officer and other officers ofS»A»£»Co fire teiga&Q fog thoir keen interest in tha<iev©lopaeat of the unito
THERMAL DETECTORS
FIXED TEMP.
RATE OF RISE IN TEMP.
RISA
1 2 3 4 5 6 7 8
TIME OF RESPONSE in orbiiary units(AFTER IGNITION)
FIG. 1
RADIO ISOTOPE SMOKE ALARM (RISA)
BLOCK DIAGRAM
ION1ZATIONCHAMBER
AMPLIFIER SENSITIVITYSELECTOR
ALARMCIRCUIT
HG. 2
•JMr »'«•?! REE 396
RADIO ISOTOPE SMOKE ALARM
ALARM
SENSITIVITY
MAINS OFF
ADJ.
BESET
jSOTOPED I V I S I O N BHABHA ATOMIC RESEARCH CENTRE / TBOMBAY / BOMBAY / INDIA
FRONT PANELFIG. -4
I=3~ -MALE FIXTUREFOR CABLE A
MESH
MAIMS POSE POt QETgRCABLE • Pi
r.HETER O7F FORPS BEU.
REAR PANEL
FIG. 5dhar 7 11 71
REF398
RADIO ISOTOPE SMOKE ALARM
DETECTOR HEAD
un.
SEI
\A \ \/A
162mm.-SOURCEELECTRODESELECTROMETEROUTPUT
1000
HOUSING
FIG. 6REP 399