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

r

%-. N

4 - ^ X ^

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

RI5A CIRCUIT DIAGRAM

FIG. 7

3©3