NBSIR 74-591

Some Problems Noted in the use of

Taguchi Semiconductor Gas Sensors as

Residential Fire/Smoke Detectors

Richard W. Bukowski and Richard G. Bright

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

December 1974

U S DEPARTMENT OF COMMERCE

NATiOIMAL BUREAU OF STANDARDS

NBSIR 74-591

SOME PROBLEMS NOTED IN THE USE OF

TAGUCHI SEMICONDUCTOR GAS SENSORS AS

RESIDENTIAL FIRE/SMOKE DETECTORS

Richard W. Bukowski and Richard G. Bright

Center for Fire Research

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

December 1974

U.S. DEPARTMENT OF COMMERCE. Frederick B. Dent. Secretary

NATIOIMAL BUREAU OF STANDARDS. Richard W. Roberts. Director

SOME PROBLEMS NOTED IN THE USE OF TAGUCHI SEMICONDUCTOR

GAS SENSORS AS RESIDENTIAL FIRE/SMOKE DETECTORS

Richard W. Bukowski^

and

Richard G. Bright

This paper reports on the performance of some Taguchi semiconductor gas sensors for use as residential fire/smoke detectors. The sensors were found to have difficulty detecting fires

involving complete combustion and also had a greater than normal

propensity to false alarm in other than fire conditions. Both

of these problems raise serious questions as to the suitability

of the sensors tested as residential fire/smoke detectors.

Key words: False alarm; fire detector; fire test; residential

smoke detector; semiconductor gas sensor; sensor contamination.

1. INTRODUCTION

The National Bureau of Standards is engaged in developing performance

specifications for optimum effectiveness of residential smoke detectors.

As part of this program studies have been conducted of the fire and smoke

response characteristics for most of the residential smoke detectors on the

U.S. market.

These examinations have included both full-scale and laboratory smallscale fire tests. In addition, analyses of false alarm tendencies have

been investigated for all types of detectors. An extensive review of the

available literature, both published and unpublished, has been conducted

as well for all types of detectors.

Most of the smoke detectors examined have been of the conventional

photoelectric or ionization-chamber type, but several of the detectors

examined have utilized the Taguchi semiconductor gas-sensing device. These

sensors were first developed and are being manufactured in Japan where they

are sold as combustible gas detectors. In the U.S., however, these sensors

are being employed in detectors which are being marketed as fire/smoke

detectors. These sensors are commonly referred to as Taguchi gas sensors,

named after their inventor.

This report covers the performance of detectors using the Taguchi gas sensor as the detecting element. The data is taken from that collected

in a broad scale investigation of commercial detectors.

At the time this paper was written the author was a research associate

at the National Bureau of Standards. Mr. Bukowski has returned to

Underwriter's Laboratory.

1
2. CONSTRUCTION OF DETECTOR SENSING ELEMENT

The gas sensing element consists of two helically-wound coil electrodes formed of a metal which is nonoxidizable at high temperatures, and

a block of high temperature organic electrical insulating material having

concave grooves on two side faces. The radius of the grooves conforms with

the peripheral surfaces of the electrodes. A high melting point, inorganic

bonding agent is used to fix the electrodes to the concave grooves of the

block. The structure is then embedded in a metal oxide semiconductor to

form the complete structure. The element is supported by the four coil

leads on posts embedded in a mounting base so as to allow free air flow on

all surfaces of the semiconductor coating. A fine mesh wire cage is affixed

over the assembly to protect it from physical contact and to prevent flame

propagation should a combustible gas/air mixture ignite in contact with the

heating coil (see fig. 1) .

3. THEORY OF OPERATION

The semiconductor crystal used in the device is an N-type in which

electricity is conducted principally by the flow of free electrons. Since

the number of free carriers is a function of the surface temperature, the

embedded heater coils are used to maintain the semiconductor at a temperature of approximately 250''C (482°F) so as to increase the number of free

carriers. The elevated temperature also serves to prevent the condensation

of water vapor on the semiconductor surface (which would tend to reduce the

surface conductivity) and to burn off any residue.

As was stated above, the structure is N-type semiconductor primarily

a metallic oxide coating (generally tin dioxide). The resulting coating

has a highly porous surface on which oxygen molecules are normally trapped

(chemisorbed) . When the sensor is exposed to an atmosphere containing an oxidizable (reducing) gas, the molecules react with the trapped oxygen

causing the release of electrons into the conduction band. This reduces

the surface resistance of the element, thus triggering an alarm.

4. RESPONSE TO FIRES

Based both on the principle of operation and tests conducted by several

individuals and laboratories [1,2]^, the device detects fires only by

sensing the oxidizable gases released by burning materials. This would

indicate that these devices will only sense fires burning with incomplete

combustion. If combustion of the material is complete, the oxidizable

gases liberated from the burning materials are consumed by the flames and

therefore are not available to the sensor. This can be easily demonstrated

by the burning of gasoline. The sensor will detect the gasoline vapors

Numbers in brackets indicate the literature references at the end of

this paper.

2
3
before ignition, but not afterwards. If the flow of oxygen to the gasoline fire is limited the sensor will again be able to detect the fire. In

fact, Figaro Engineering, Inc. , the major producer of the semiconductor

gas sensors, states in their product literature [3], "Since the TGS

(Taguchi Gas Sensor) is literally a gas sensor, it is of no use for

detecting smoke which does not contain any volatile or gaseous by-product."

"As for a fire sensor, the current one (sensor) is not good since its

sensitivity to CO is poor."

When tested in a small-scale smoke sensitivity test chamber, such

as used for sensitivity determinations with photoelectric and ionization

chamber smoke detectors (see fig. 2 for a typical small-scale test chamber),

the Taguchi gas sensor has equivalent response to the other types.

Table 1 shows the comparative sensitivity response of a Taguchi gas

sensor, a photoelectric detector, and an ionization chamber detector in

the NBS small-scale smoke sensitivity test chamber. Note the similarity

of alarm points of the three detectors at various air velocities from 15

fpm (4.6 metres /min) to 150 fpm (45.7 metres/min). This similar response

is due mainly to the fact that combustibles used in these small-scale

tests are generally smoldering cellulosics (punk sticks or cotton lamp

wicks) and produce insufficient thermal energy to cause complete combustion

of the oxidizable gases emitted.

Table 1. Small-Scale Sensitivity Tests

Alarm Point

^o-ft."-*- (OD-m"

Smoke Velocity Taguchi Gas Photoelectric Ion Chamber

FPM (m-min""^) Detector Detector ^ Detector

15

(4.5)

2.23

(0.03)

2.0

(0.029)

3.17

(0.046)

30

(9.2)

1.63

(0.02)

1.40

(0.020)

1.85

(0.026)

50

(15)

1.25

(0.018)

1.34

(0.019)

1.23

(0.018)

100

(30) • 0.79

(0.011)

0.98

(0.014)

0.85

(0.013)

150

(45.7)

0. 75

(0.010)

0.92

(0.013)

0.57

(0.008)

%-ft_^ = Light obscuration over five-foot length

OD-m = Optical density per metre

4
5
The lack of response of the semiconductor gas sensor as a fire/smoke

detector is shown in large-scale testing with flaming combustibles, where

an adequate supply of oxygen is available for complete combustion. This

lack of response was observed in a series of full-scale fire tests conducted

by NBS on several different types of conventional smoke detectors including

a Taguchi detector [4], Twenty-six full-scale test fires were conducted

in a large room of about 55,000 cubic feet (1,557 cubic metres). Ten

detectors including photoelectric and ionization chamber detectors, and

one Taguchi detector, were located on the ceiling of the test room approximately 21 feet from the fire center. The 26 test fires included shredded

paper, wood cribs, gasoline, polystyrene, polyurethane, and cotton. All

test fires were flaming fires except for the cotton fire which was a smoldering fire. Table 2 describes the test fire series.

Most of the conventional smoke detectors responded to nearly all of

the test fires. The Taguchi detector detected and alarmed for only one of

the shredded paper fires. The Taguchi detector was equipped with an analog

metered output in addition to its alarm horn. There was a small analog

output for the other shredded paper fires and the smoldering cotton

fire, but the detector did not approach its alarm point. The Taguchi

detector was tested in the small-scale sensitivity test chamber before

and after the full-scale tests. Its alarm point ranged between one-and

two-percent-per-foot in terms of light obscuration (an optical density

of 0.014 to 0.028 per metre), approximately the same sensitivity range

of the conventional smoke detectors used in the full-scale test fires.

See table 3 for a summary of detector alarm respo.nse times.

5. LONG TERM STABILITY

The device, as being used today, has only been available for approximately three years. Since there is no known way to conduct an accelerated

aging test on these devices, it is only known that they can exist in a pollutant-free atmosphere for approximately three years and still remain

operational. There is, however, some variation in the sensitivity of

the device to certain gases over a period of time.

On the short term, the basic manufacturer states that the sensitivity

of the device varies from day-to-day as a function of the ambient temperarture and humidity conditions. It has been reported that the sensitivity

to other gases slowly decreases with time. For example, the sensitivity

to hydrogen goes up while the sensitivity to carbon monoxide goes down.

The rate of increase or decrease also varies from material to material.

It is for these reasons that the long term stability of the device would

be in question and would be a function of the ambient conditions in which

the device is installed. This would make it difficult to predict the long

term stability of the device in a given installation.

c6
Table 2

Test Nos. Types of Test Fires

1,2,3,10 Eight ounces (250e) of shredded paper in hardware

cloth basket. Match ignited in bottom center.

i Class A wood brand ignited by 100 cc of ethyl alcohol.

6 100 cc of motor gasoline, match ignition.

7 200 cc of motor gasoline, match ignition.

8,9,27 Two ounces (57g) of polystyrene packing material,

ignited by 50 cc of ethyl alcohol.

11,12 Two Class A wood brands ignited by 100 cc of ethyl

alcohol.

13,14,21 Two Class A wood brands ignited by 25 cc of ethyl

alcohol.

15 Class A wood brand on 1000-watt hot plate.

Immediate ignition, flaming fire.

16 Two 12"xl2"x3" (30x30x8cm) pieces of flexible

polyurethane foam ignited by 10 cc of ethyl alcohol.

17A Three 12"xl2"x3" (30x30x8cm) pieces of flexible

polyurethane foam ignited by 10 cc of ethyl alcohol.

19,24 Three 12"xl2"x3" (30x30x8cm) pieces of flexible

polyurethane foam ignited by 10 cc of ethyl alcohol.

20 Raw cotton, 2 pounds (900g) , in pan on 1000-watt

hot plate.

22,23 One Class A wood brand ignited by 10 cc of ethyl

alcohol

25,26 One 12"xl2"x3" (30x30x8cm) piece of flexible polyurethane foam ignited by 10 cc of ethyl alcohol.

7
Table 3

Detector Alarm Response - In Seconds

Test

No.

A Photoelectric J Photoelectric H

Ion

Chamber

Photoelectric C Photoelectric D Photoelectric E Photoelectric G Photoelectric F Photoelectric I

Ion

Chamber K

TGS

Semiconductor

1 30 56 33

32

30

45

45

46

29

30

45

42

27 29 28 34 36 30

27

27

38

35

36

2

3 28 26 28 27 31 32

4

5 41

6 62 44 52 55 72 58 107 57 39

7 63 46 55 76 76 59 132 64 NR

8 44 42 52 42 46 42 41 44 31•J X

9 34 32 30 31 36 30 34 41 NR

10 28 30 28 27 28 28 31 34 28

11 49 52 51 53 58 58 49 43

12 48 49 56 49 46 51 47

13 49 47 49 54 119 52 70 47 44

14 47 53 53 88 * 63 45 49 46

15 139 113

16 156 90 103 162 * 138 134 107

I 17A 154 85 108 163 * 138 170 108

1 19 123 97 102 124 * 124 119 109

i 20 1,656 1,582 1,754 1.762

111

1,690 1,632 L,654 IND

1 21 74 160 72 67 76 103 74 74 73

i 22 103 106 97 103 130 118 103 102 110

23 96 102 92 101 124 102 95 100 101

I 24 138 92 111 141 134 136 138 114

' 25 134 104 109 145 134 ! 132 141 120

26 127 118 137 138 131 i 133 134 113

1 27

i

51

1

50 51 52 55 i

i

54 48

Notes: No alarm or indication

IND Indication but no alarm

* Clock Timer malfunction

NR Not resetable

8
We have found it Is possible to contaminate the crystal structure such

that the device will shift sensitivity. Exposure to such gases as ammonia

and sulphur dioxide cause the surface resistance to decrease for long

periods of time, and, in some cases, it appears to be irreversible.

This condition cannot be electrically supervised so that it would be

impossible to determine if a particular detector was operating properly

unless periodic smoke tests were performed on the device.

6. FALSE ALARM PROBLEMS

The biggest problem which can be forseen in the use of this semiconductor gas sensor as a residential smoke detector is false alarms.

As this sensor is responsive to any oxidizable gas, it will respond to a vast number of ambient conditions normally found within the home. The

use of most aerosol household product near the device as well as alcohol,

perfumes, or even rapid changes in ambient humidity can cause alarms.

These characteristics are especially undesirable considering the

suggested location for a single smoke detector within a home. This

location is in the hallway leading to the bedrooms which would, in a large percentage of homes, place the detector somewhere near a bathroom

door. Since the bathroom is an area where many household aerosol products

are used, a very large number of false alarms could be expected. In some home installations, false alarms have been experienced from various aerosol

products used in the bathroom such as hair sprays, underarm deodorants and

from the use of rubbing alcohol. The steam from hot showers has produced

false alarms upon opening of the bathroom door. Where the units have been

installed near the kitchen, cooking, the slicing of oranges, the pouring

of sherry, and the slow ignition of gas cooking range burners have also

produced false alarms.

Although residential smoke detectors operating on the ionization

chamber principle tend to alarm to cigarette smoking and the particulate

released by cooking, the solid state gas sensor has the same propensity

for alarm to cigarette smoking coupled with its great propensity to

alarm to other oxidizable gases normally present in the home. Since the

ionization detector false alarm problem is considered almost intolerable

by some people, it is felt that the increased false alarm propensity of the

solid state gas sensor will make its use unacceptable.

7. CONCLUSIONS

Based on theoretical and experimental considerations, it appears that

the currently available Taguchi gas sensor has some major drawbacks as a residential fire/smoke sensor. It is almost impossible to produce any

kind of damage-causing fire in a home without producing smoke particulates;

but it is relatively easy to produce a damage-causing fire in the home

which does not produce oxidizable gases at a level detectable by the

Taguchi gas sensors. Based on the test results, we believe that it is

9
likely that a detector employing a Taguchi gas sensor will not detect a certain proportion of residential fires that would be readily detected

by conventional smoke detectors.

In addition to the basic question of the detection capabilities,

the questions of long term sensor stability and propensity to false

alarm provide additional indications which lead to the conclusion that

the sensor is presently unsuitable for residential fire/smoke detection. *

8. GENERAL COMMENTS

Solid state gas sensing semiconductors have a use for the detection

of methane or propane leaks within a home and function quite well for

this purpose. However, from the tests on the current devices it appears

that they should not be relied on as a fire/smoke detection device. The

devices were developed and are manufactured in Japan where they are only

allowed to be sold as gas detectors; and are not accepted as fire detectors

in that country.

Some manufacturers of the Taguchi detectors have stated that the

unresponsiveness of the detectors to full-scale fires and the detector's

false alarm tendency can be corrected by suitable electronic circuitry and

other modifications.

Several U.S. manufacturers are expending time and resources to

optimize the use of the Taguchi gas sensor as a fire/smoke detector for

residential application. In addition, Figaro Engineering of Japan, the

basic manufacturer of the Taguchi gas sensor, is continuing its research

on developing a sensor specifically responsive to low levels of carbon

monoxide. It is possible that this research may result in a fire/smoke

detector at least equal in performance to conventional smoke detectors;

the elimination or at least the reduction of the false alarm problem

inherent in the present Taguchi gas sensor is less certain.

9. REFERENCES

[1] Purt, G., On Taguchi Gas Sensors, 4th International Fire Protection

Seminar, Zurich (Oct. 1973).

[2] Taguchi Gas Sensors, ESL Engineering Report, October 23, 1973

(unpublished)

[3] Gas Sensing Semiconductor, Figaro Engineering, Inc., Osaka, Japan,

March 13, 1974.

[4] Bukowski, R. W. and Bright, R. G. , Results of Full-Scale Fire Tests

of Photoelectric Detectors, Nat. Bur. Stand. Report (to be published).

10
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1. I'UMl l( .ATION OK KI-;i'ORT NO. NBSIR 74-5912. ( lov 't Acce s s ion No.

3. Rci ipicnt'.s Acccs.sion No.

4. T1TI.1-: AND .'^uivrn i.i- Some Problems Noted in the Use of Taguchi Semiconductor Gas

Sensors as Residential Fire/Smoke Detectors

5. I 'ulil id! ion Date October 197^

6. Performing Organization (^ode

7. AUTIIOH(S)

Richard W, Bukowski and Richard G. Bright

8. Performing (^rgan. Report No. NBSIR 74-591

9. PERFORMIN'G ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projecr/Task/Work Unit No. 4908200

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Addres.s (Street, City, State, ZIP)

same as No. 9

13. Type of Report & Period Covered

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14. Spon.soring Agency Code

15. SUPPLEMENTARY NOTES

16. APSTRAC T (A 200-word or less factual summary ol most sign iticant information. If document includes a significant

bibliography or literature survey, mention it here.) This paper examines some aspects of Taguchi semiconductor gas sensors and their

use as residential fire/smoke detectors based on theoretical and experimental considerations. It was found that these sensors have difficulty detecting fires

involving complete combustion along with a greater than normal propensity to false

alarm to other than fire conditions. Both of these problems raise serious questions

as to the suitability of these sensors as residential fire/smoke detectors, at least

at their present state of development.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

False alarm; fire detector; fire test; residential smoke detector; semiconductor

gas sensor; sensor contamination.

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