Need for Regular Functional Testing of
Portable Gas Detection Equipment
- Evaluation of Field Failure Rates and Interventions in Japan-
Master of Public Health Capstone Project
Haruo Hashimoto
Department of Environmental Health Sciences
Bloomberg School of Public Health
Johns Hopkins University
May 1, 2006
2
Executive Summary
While the early detection of leaks using portable gas detection equipment is a vital measure
to protect workers from hazardous work environment, the practice of regular functional testing of
this equipment has not been established in several countries in Asia and Europe; Japan is the
typical example. The objective of this study is to evaluate the failure rate of portable gas
detection equipment in order to demonstrate the need for regular functional testing. Evaluating
gas detector performance is an important public health issue. When operating properly, gas
detectors provide important engineering controls that help to prevent worker exposure to acutely
toxic or hazardous chemicals. Simple pre-use inspection of detectors is commonly practiced by
workers before their use. However, there can be a certain type of failures which can not be
detected by the pre-use inspection, and those failures are identifiable by functional testing with
challenge gases.
The rates of equipment failures were evaluated, using challenge gases, for 269 hydrogen
sulfide alarm detectors and 206 functional components (of 60 multiple-gas detectors) in three
Japanese refineries. , The identified average failure rates for hydrogen sulfide alarm detectors,
gas detector components, and the total devices were 2.5%, 4.7%, and 3.3% per year respectively.
These failure rates are considered to be unacceptably high. The major causes of the failures were
3
identified as inattentive usage by workers, which meant the failures occurred randomly and
externally. It was demonstrated that increasing functional test frequency, from the typical present
practice of a yearly basis to a monthly basis, was effective in mitigating the risk of a worker
selecting and using a malfunctioning device.
The study result suggests the definite need for regular functional testing of portable gas
detection equipment. It is strongly recommended that all equipment users in local industries, the
equipment manufacturers, and Japanese government (the Ministry of Health, Labor and Welfare),
be informed of the potential failure rate problem, and that frequent functional testing program be
implemented. In addition, since the gas detection equipment is used around the world, the
awareness of this issue should be broadly disseminated.
4
1. Introduction
The research question of this study is whether regular functional testing (or bump testing)
are necessary for portable gas detection equipment. The need for regular functional testing of the
equipment has not been established in Japanese workplaces. The objective of this study is to
evaluate the failure rate of portable gas detection equipment in field use and to demonstrate the
need for the regular functional testing.
Why this is a public health issue? Early leak detection is an important primary prevention
measure. If malfunctions occur, workers are at risk for potentially dangerous over-exposures. It is
important to determine if gas detector malfunctions are due to their inappropriate handling or due
to the intrinsic failure of the devices. If the major causes of failure rest on workers’ inappropriate
handling of the devices (a workplace management and training issue), then a potential promising
intervention for this problem may be to increase the frequency of functional testing. If the major
cause of the device failure is a mechanical problem intrinsic to the instruments, then the solution
would involve better engineering or maintenance of the devices.
Portable gas detection equipment has been used extensively in manufacturing and
construction workplaces, and is considered a critical safety measure that directly protects
workers’ lives. For example, at refineries and chemical plants, gas detection equipment is used to
5
detect oxygen deficient or explosive atmosphere before confined space entry, such as tanks and
reactor vessels. Another purpose is the leak detection function. Workers wearing the toxic gas
leak detector devices are protected if the detectors sound an alarm when elevated concentrations
of toxic gases are detected.
It is common practice for workers to perform pre-use inspection of detectors, that is, he/she
checks the device visually and performs an automated electrical circuit test - so called
“auto-zero-check”. This pre-use practice is of limited utility. Intrusion of water/oils or physical
impact during past use can result in deteriorated response to the actual hazardous atmosphere,
although the device is tested normal during the pre-use inspection. Therefore, it is essential to
test the device against challenge gases every time before use.
However, such functional testing has not been practiced in the five refineries of the
company E in Japan (Table 1). The Japanese manufacturers of gas detection equipment, as well
as the Japanese manufacturers’ professional association, do not recommend their customers
perform functional testing every time before use, and only recommend functional testing or full
calibration once or twice a year1,2,3. The typical practice in most of the major local manufacturing
industries is yearly or biyearly calibration, and the equipment is not tested before its use4,5. It was
even mentioned by a local equipment manufacturer that major proportion of the equipment
6
having been sold in the local market and currently used in the field has not had any functional
testing nor calibration since their first use4.
The use of functional testing seems to be variable depending of each region/country. In the
United States, the practice of frequent functional testing has been widely established; three US
refineries of the company E perform the test every time before use (Table 1). On the other hand,
European refineries perform the test with frequencies varying from one month to 1.5 year, and
one refinery in Asia, outside Japan, holds the frequency of one to three months similarly.
The US OSHA recommends that the accuracy of gas detection equipment be verified
before each day’s use, and ISEA (International Safety Equipment Association, a trade association
for manufacturers of protective equipment and instruments) also provides similar
recommendation6,7. Interviewing ten manufactures of personal gas detection equipment in the
U.S. revealed that all of them recommend at least monthly functional testing, and many of them
recommending daily testing (Table 2)8. The reason of the observed good test practices in the U.
S. may be ascribed to either high failure rates identified in actual workplaces or historical
incidents caused by gas detection equipment failures. However, no report or published paper on
the failure rates of gas detection equipment was identified through an extensive literature search.
The studies on such failure rates might tend not to be published because of perceived low
7
generalizability, because such studies are usually aimed at limited equipment model(s), and also
because failure rates are influenced by the specific handling practices or work environment of the
equipment users. Similarly, no information on past incidents cases caused by gas detection
equipment failures were identified through literature searches, interviews of Japanese
manufacturers, and record search within the company E.
This paper presents an evaluation of the failure rates of the portable gas detection
equipment used in the Japanese workplaces. Based on the failure rate analysis, an evaluation of
the test frequency for appropriately mitigating the risk of potential incidents by equipment
failures is also presented.
2. Methods
The failure rates of portable hydrogen sulfide alarm detectors and portable gas detectors
were evaluated in three manufacturing sites (refinery A, B and C) of the company E in Japan.
2-1. Portable hydrogen sulfide alarm detectors
A portable hydrogen sulfide alarm detector detects hydrogen sulfide gases in the
atmosphere and produces an alarm sound and flashing light when the airborne concentration
exceeds a preset gas level that is usually 10 ppm (Figure 1). 269 hydrogen sulfide alarm
8
detectors made by a Japanese manufacturer (Shin-Cosmos electric Co.) that had been in use in
two manufacturing sites A and B were evaluated. The manufacturer of the alarm detector
guarantees its use for two years without the need for functional testing or calibration, and thus,
the alarm detectors used at Sites A and B had not been tested with standard gas after their start of
use. An initial screening of about 290 alarm detectors by visual inspection and electric functional
check (auto-zero-check) was performed. The visibly-broken or clearly malfunctioning alarm
detectors were excluded, and only the remaining 269 alarm detectors were included in this study.
As a result, this study identified failure rates in a population of apparently normal alarm detectors.
The response of the 269 alarm detectors was tested against the standard gas which was composed
of 25 ppm H2S in nitrogen. The tested personal alarm detectors have a two-step alarm system -
low and high - which beeps as well as flashes at 10 and 15 ppm of hydrogen sulfide levels
respectively. When exposed to the standard gas, the alarm detector typically responds in a
two-step manner at 10 and 15 ppm levels, since the standard gas gradually diffuse into the sensor
part of the alarm detector. Therefore, the alarm detectors that responded to the challenge gas
properly in this two-step manner were defined as having “passed”; other alarm detectors were
classified as “failure”.
2-2. Portable gas detectors
9
A gas detector indicates the atmospheric gas concentration of oxygen, combustibles, or
toxic gases (such as hydrogen sulfide and carbon monoxide), and alarms by sound and flashing
light above preset gas levels. “Multiple gas detectors” that have multiple gas sensors are
commonly used (Figure 2). Sixty gas detectors made of a Japanese manufacturer (Riken
Instruments Co.) that had been in use in two manufacturing sites B and C in Japan were also
evaluated as a part of this investigation. The time duration since the last calibration of the
detectors was one year for site B, and one year or one month, depending on sensors, for site C.
None of the detectors had not been tested with standard calibration gas since their last calibration.
Similar to the hydrogen sulfide alarm detectors, the detectors were first examined by visual
inspection and electric auto-zero-check, and the apparently-abnormal detectors were excluded.
The remaining sixty detectors were included in test pool for functional testing using challenge
gases. Four kinds of standard calibration gases – oxygen, n-pentane, hydrogen sulfide and carbon
monoxide were used. The gas concentrations are shown in Table 3. The gas detectors, when
exposed to the challenge gas, that both displayed a read-out exceeding the preset alarming levels
and generated alarm sound/flashing properly were defined as having “passed”; others were
classified as “failure”. For oxygen sensors, those that failed to display a read-out of less than
20.0% were classified as “failure”.
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A typical gas detector has one pump and one (or more) sensor(s), and each of these
functional “components” can be the cause of failure. There is other type of models, “passive”
sampling devices, that does not equip a built-in pump. A “pump failure” was defined as
insufficient pump suctioning if the sucked air barely reached to the sensor(s) while the pump
motor itself was diagnosed as normal, based on the rotating sound. A “sensor failure” was
defined as deteriorated response of a sensor. The failure rates were analyzed in two ways; per
detector basis, and per functional “component (pump or sensor)” basis. The total number of the
components was 206, comprising 54 pumps and 152 sensors, among 60 detectors tested in total.
3. Result
3-1. Portable hydrogen sulfide alarm detectors
The total number of alarm detectors evaluated was 269; 174 and 95 for Site A and B
respectively. The time duration of use was variable depending on each alarm detector, as shown
in Figure 3. There were total seven failures identified, five for Site A and two for Site B (shown
as “x” in Figure 3). The crude failure rates for Site A, Site B, and combined Sites were 2.9%,
2.1%, and 2.6% respectively (Table 4).
Assuming, based on the reasoning discussed later, that the malfunctioning of the alarm
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detectors originated randomly during their past field use, the observed results were analyzed by
applying the person-time concept from epidemiology. The cumulative time of use for a group of
alarm detectors, “piece-months”, was defined in the same manner as “person-time” in
epidemiology, as follows:
[In epidemiology]
(Incidence rate) = (Number of episodes) / (Total person-years of exposure)
[In this study]
(Failure rate) = (Number of failures) / (Total piece-months of field use)
The result of functional testing for alarm detectors is summarized in Table 4. For Site A, the
cumulative time of use was 2211 piece-months. This number, when divided by 5 (the number of
failures), gives the average “lifetime” of an alarm detector which represents the average time for
unit failure; this number is 442.2 months. The reciprocal number of lifetime, which represents
the “failure rate”, is therefore 0.226% per month. Similarly, the average failure rate of alarm
detectors for Site B is 0.174% per month. There is no statistically significant difference between
the results for Site A and B; the Poisson test was performed using STATA statistics software. The
combined average failure rate for Site A and B is 0.208% per month, which translates into
probabilities for a new alarm detector experiencing a failure before 12 month-age and 24
12
month-age of 2.5% and 5.0% respectively. This means that 5 out of 100 alarm detectors would
experience failures at 24 months time - current end-of-life period, if no functional testing has
been performed during its time of use. This failure rate seems to be very significant and
problematic, in view of the purpose of this equipment that should secure workers’ safety.
3-2. Portable gas detectors
The raw result of functional testing of the portable gas detectors is shown in Table 5. In
Table 6, the analysis result of the testing, per detector basis, is given. The total number of
detectors evaluated was 60; 43 and 17 for Site B and C respectively. There were total 8 failed
detectors identified; each detector had one failed component within it respectively. The numbers
of failure were 7 for Site B and 1 for Site C, which gives crude failure rates of 16.3% for Site B,
and 5.9% for Site C. The combined crude failure rate is 13.3%. The cumulative time after last
test for all detectors at Site B was 516 piece-months, which gives, with 7 failures in total, an
average failure rate of 1.36% per month. Similarly, the average failure rate is 2.00% per month
for Site C. There is no statistically significant difference between the results for Site B and C.
The combined average failure rate for Site B and C is 1.41% per month.
Table 7 shows the analysis result of the testing per component basis. As for 206 components
of 60 detectors tested, there were 3 pump failures and 4 sensor failures (2 for combustible gas
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sensors and 2 for hydrogen sulfide sensors) for Site B, and 1 sensor failure (combustible gas
sensor) for Site C. The crude failure rates for Site B, Site C, and the combined Sites are 4.5%,
2.0%, and 3.9% respectively. The cumulative time after last test for all components at Site B was
1860 piece-months, which gives, with 7 failures in total, an average failure rate of 0.376% per
month. Similarly, the average failure rate is 0.546% per month for Site C. There is no statistically
significant difference between the results for Site B and C. The combined average failure rate for
Site B and C is 0.392% per month, which translates into the 4.70% probability for a detector
component of having failure after one year’s use from the last test. This failure rate seems to be
very significant.
The integrated result of functional testing for the total devices (hydrogen sulfide alarm
detectors and detector components) is shown in Table 8. There is no statistically significant
difference between the results for alarm detectors and detector components. The number of the
total devices evaluated was 475, and there were total 15 failures were identified, which gives the
crude failure rate of 3.2%. The overall average failure rate for the total devices is 0.278% per
month (3.33% per year).
4. Discussion
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The manufacturer examined all of the malfunctioning hydrogen sulfide alarm detectors.
They found that the causes of the failure were either partial corrosion of the circuit board, filter
clogging, or inner mechanical damage9. This suggests that those failures were caused by
rainwater intrusion or incidental physical impact (falling-off etc.) during field use. Therefore, it is
reasonable to interpret that the identified failure rates were not primarily related to the intrinsic
breakdown characteristics of the alarm detector, but to the extrinsic and random failures due to
inattentive usage by workers of the Site A and B.
The gas detector manufacturer reported that the cause of the sensor failures was
deterioration of catalytic combustion or electrochemical sensors10. The manufacturer generally
guarantees that the potential deterioration of the sensors within one year will be insignificant; the
sensor sensitivity typically goes down only gradually. This suggests that the identified rapid
sensor deterioration was most probably caused by sucking overly dense gases of combustibles or
toxics. Also, it was reported that the most probable root cause of the pump failures was the
malfunction of rubber diaphragms of the pumps; the diaphragms were deteriorated by steam or
oil mist sucked erroneously during use10. Thus, it is reasonably interpreted that the identified
failure rates of the gas detectors were also primarily ascribed to the random breakdowns due to
inattentive usage by workers.
15
For workers’ safety, it is essential to reduce the risk of a worker selecting a malfunctioning
device when randomly picking up an alarm detector or a detector from a pool of devices. The
following measures are potentially effective in mitigating this risk, based on the failure causes
mentioned above:
(A) Frequent functional testing by workers,
(B) Communication and training for workers on appropriate/attentive handling of
equipment,
(C) Improvement of equipment durability by manufacturers against inappropriate handling.
Among these, option (A) seems to be the most dependable and effective. However, it is
important to pick the proper functional testing frequency.
Let us define a variable, “selection risk (p)”, which represents the probability of a worker
selecting a malfunctioning device among a pool of devices; those devices have been
function-tested with specified single frequency, but the time duration after the last test for
respective device is randomly distributed. Also, suppose the test frequency being every “m”
months and the common failure rate being “a” % per month. Considering an equipment failure as
analogous to a disease event, the selection risk (the proportion of malfunctioning devices at a
time) represents the point prevalence. There is a following relationship among prevalence,
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incidence rate, and disease duration.
(Prevalence) = (Incidence Rate) x (Duration)
In the case of equipment failure, the “failure rate” represents the “incidence rate”, and “the
average time duration of a device being malfunctioning”, which is “m/2 (month)” represents the
“duration”. Thus, the above equation can be translated into the following relationship. This
equation is also used in the filed of mechanical reliability engineering11.
p = a*m/2
The relationship between “m” and “p” is proportional, and the actual figures are given in
Table 9 for a hydrogen sulfide alarm detector and typical gas detectors. It is demonstrated that
the selection risk, “p”, is significantly reduced in proportion to “m”. While the risk was originally
2.5% for testing in every two years, the selection risk becomes 0.1% with increasing the
functional test frequency (i.e., reducing “m”) to a monthly basis, based on the obtained monthly
failure rate of 0.226% for hydrogen sulfide alarm detectors. The selection risk further goes down
to essentially 0% with daily testing before use.
Similar reasoning holds for gas detectors. For a detector having one functional component, a
sensor, while the risk was originally high - 2.35% - for the current yearly test frequency, the
selection risk becomes 0.2% with monthly testing, based on the obtained failure rate of 0.392%
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per month. Since a typical gas detector has multiple components within it, it might be
worthwhile to consider the selection risk per each detector which represents the probability of
one (or more) of the detector components being malfunctioning. The following equation holds
for a typical detector having one pump and three sensors (four components altogether) within it:
p = (a*m/2) x 4 = 2a*m
As shown in Table 9, while the selection risk is very high – 9.41% - for yearly test frequency, the
risk becomes reasonably low - 0.78% - with monthly frequency. Based on the above
consideration, it is demonstrated that the selection risk can be significantly reduced by only
reducing the testing frequency to monthly from the current 1-2 year interval. It is further
desirable to reduce the frequency to daily, to make the risk minimal.
In order to mitigate the risk for a worker of using malfunctioning devices, it would be
additionally important to educate workers on appropriate usage of the equipment, especially to
avoid physical impact or suction of water/oil/concentrated gases. In addition, the necessity of
immediate ad hoc functional testing should be emphasized in cases where a device has
experienced abnormal conditions or a destructive environment.
It may not be appropriate to generalize the identified failure rates to other gas detection
equipment in field use because of two reasons; one is that this study included a limited selection
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of equipment models, and the other is that the field environment that had been experienced by
the tested devices was also limited to a relatively severe environment in a few refineries.
Many of the overseas manufacturers have developed and are selling gas-supplying adaptors,
and even automated calibration equipment for gas detectors, usually called docking stations12.
On the other hand, Japanese manufacturers do not sell such tools locally, although one of them
actually sells their adaptors only in overseas market4. Japanese manufactures should be
encouraged to provide state of the art functional testing equipment.
5. Conclusion
It has been demonstrated that there are cases where the failure rates of portable gas alarm
detectors and detectors are significantly high. The identified average failure rates for hydrogen
sulfide alarm detectors, gas detector components, and the total devices were 2.5%, 4.7%, and
3.3% per year respectively. This suggests the strong necessity for equipment users to regularly
perform functional testing of these devices. It is strongly recommended for all the local gas
detection equipment users to acknowledge this risk and to initiate regular functional testing, with
at least monthly frequency. It is recommended that the Japanese government, the Ministry of
Health, Labor and Welfare, widely disseminate information on the failure risk of the gas
19
detection equipment as a potentially major occupational safety/health management issues. The
agency should also be encouraged to establish a guideline for implementation of the frequent
functional testing. Currently, the government has no guideline for functional testing for portable
gas detection equipment, while it merely requires functional testing once or twice a year only for
stationary (non-portable) gas detectors attached to the specified high-pressure gas manufacturing
facilities. The manufacturers of gas detection equipment should actively notify their customers of
the necessity for functional testing. It is also recommended that these manufacturers develop and
provide adaptors or equipment for functional testing in the local market. In addition, the
necessity of frequent functional testing should be shared on global basis, to relevant countries in
Asia and Europe.
This study result had been communicated and advocated by the author within the company
E in Japan. As a result, a new program has started to initiate functional testing for hydrogen
sulfide alarm detectors and gas detectors at all operational sites in Japan, with monthly frequency
at first, and with daily frequency within a very near future. Also, the author has communicated
the study finding to the overseas operational sites of the company E, in order to encourage
implementing the similar program. The author also have orally presented this study results at the
annual meetings of the Japan Society for Occupational Hygiene and Engineering in November
20
2004 (Tokyo, Japan) and in November 2005 (Takamatsu, Japan)13, 14.
Acknowledgement
The author acknowledges Messrs. Toshiaki Gotoh (ExxonMobil Japan Co.), Kazuo
Sasaki (Kyokuto Petroleum Industries Co.) and Kazuyoshi Katoh (ditto) for their assistance in
testing the failure rates of gas detection equipment.
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Table 1. Functional testing frequencies of portable gas detection equipment in refineries of company E
Region/Country Site Frequency A 1-2 y B 1-2 y C 1 m -1 y D 3 m -2 y
Japan
E 3 m -2 y F (Malaysia) 1-3 m G (Australia) Every time before use H (Thailand) Every time before use
Asia Pacific
I (Singapore) Every time before use J (Italy) 1 m K (UK) 6 m
L (France) 1 y M (Germany) 1 y
Europe
N (Netherlands) 3 m - 1.5 y O Every time before use P Every time before use USA Q Every time before use
Table 2. Recommended functional testing frequency by US-based gas detector manufacturers8
Name of Manufacturer Recommended Frequency B. D. daily ~ monthly B. W. daily D. I. daily E weekly ~ biweekly I. S. daily I. A. C. daily (for confined space work) M daily R. S. daily (rechargeable), weekly (non- rechargeable) S monthly T daily ~ weekly
Site A Site B Total Number of alarm detectors tested 174 95 269 Number of failures 5 2 7 Crude failure rate (%) 2.9 2.1 2.6
Cumulative time of use (piece-month) 2211 1150 3361
Average time of use (month) 12.7 12.1 12.5
Average time per unit failure (month) 442.2 575 480.14
Failure rate (% /month) 0.226 (*1) 0.174 (*1) 0.208 95% CI (*2) 0.073-0.528 0.021-0.628 0.084-0.429 Failure rate (% / 12 month) 2.71 2.09 2.50 Failure rate (% / 24 month) 5.43 4.17 5.00
*1: No statistically significant difference between Site A and B (p=0.80, Poisson method)
*2: Poisson method
O2
(ppm) Combustible gas
(%LEL*) H2S
(ppm) CO
(ppm)
Gas concentration 19.3 25
(n-pentane, 0.35vol%) 25 103
Pre-set alarming level
18.0 10 10 25
Table 4. Summary result of functional testing for hydrogen sulfide alarm detectors
Table 3. Challenge gas concentrations and the pre-set alarming levels of gas detectors
*LEL: Lowest explosion limit
22
23
Table 5. Result of functional testing for gas detectors
# of Components failed
Site # of
Detectors tested
# of pump per detector
# of Sensors
per detector
# of Components per detector
Duration after last test (month)
Cumulative time after last
test (piece-month) Pump O2
LEL
(*2) H2S CO
Total # of
failures
2 1 1 2 12 48 0 0 0 0 0 0
14 1 2 3 12 04 0 0 1 0 0 15
26 1 3 4 12 48 3 0 1 2 0 612B
1 1 4 5 12 60 0 0 0 0 0 0
(subtotal) 43 60 3 0 2 2 0 718
6 1 1 2 1 12 0 0 0 0 0 0
5 1 2 3 1 15 0 0 0 0 0 0
0 0 3 3 0 0 0 0 0 0 0 0C
6 0 4 4 *1) 56 0 0 1 0 0 16.5 ( 1
(subtotal) 17 83 0 0 1 0 0 11
Total 43 3 0 3 2 0 860 20
*1: Averaged for all components; 1 month for oxygen and combustibles sensors, 1 year for hydrogen sulfide and CO sensors *2: Combustibles sensors which indicate rates (%) against the lower explosion limit (LEL) concentration
24
Table 6. Summary result of functional testing for gas detectors: per detector basis Site B Site C Total
Number of detectors tested 43 17 60 Number of failures 7 1 8 Crude failure rate (%) 16.3 5.88 13.3
Cumulative time after last test (piece-month) 516 50 566
Average time after last test (month) 12.0 2.9 9.4
Average time per unit failure (month) 73.7 50.0 70.8
Failure rate (% /month) 1.36 (*1) 2.00 (*1) 1.41 95% CI (*2) 0.054-2.80 0.051-11.1 0.610-2.79 Failure rate (% / 12 month) 16.3 24.0 17.0
*1: No statistically significant difference between Site B and C (p=0.68, Poisson method)
*2: Poisson method Table 7. Summary result of functional testing for gas detectors: per component basis
Site B Site C Total Number of components tested 155 51 206 Number of failures 7 1 8 Crude failure rate (%) 4.52 1.96 3.88
Cumulative time after last test (piece-month) 1860 183 2043
Average time after last test (month) 12.0 3.6 9.9
Average time per unit failure (month) 265.7 183.0 255.4
Failure rate (% /month) 0.376 (*1) 0.546 (*1) 0.392 95% CI (*2) 0.151-0.775 0.014-3.04 0.169-0.772 Failure rate (% / 12 month) 4.52 6.56 4.70
*1: No statistically significant difference between Site B and C (p=0.68, Poisson method)
*2: Poisson method
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Table 8. Integrated results of functional testing for the total devices (hydrogen sulfide alarm detectors and gas detector components)
Hydrogen sulfide alarm
detectors Gas detector components Total devices
Number of devices tested 269 206 475 Number of failures 7 8 15 Crude failure rate (%) 2.6 3.9 3.2
Cumulative time after last test (piece-month) 3361 2043 5404
Failure rate (% /month) 0.208 (*1) 0.392 (*1) 0.278 95% CI (*2) 0.084-0.429 0.169-0.772 0.155-0.458 Failure rate (% / 12 month) 2.50 4.70 3.33
*1: No statistically significant difference between hydrogen sulfide alarm detectors and gas detector components (p=0.23, Poisson method)
*2: Poisson method
Table 9. Relationship between “selection risk” and functional test frequency
Test Frequency Selection risk (p, %)
month Hydrogen sulfide
alarm detector Gas detector
(*1) Gas detector
(*2) 2 yr 24 2.50 (*3) - - 1yr 12 1.25 2.35 (*3) 9.41 (*3) 6 month 6 0.63 1.18 4.70 3 month 3 0.31 0.56 2.35 1 month 1 0.10 0.20 0.78 Weekly 0.23 0.02 0.05 0.18 Daily 0 0 0 0 p = a*m/2 p: Selection risk (Probability of selecting a malfunctioning device ) a: Failure rate (hydrogen sulfide alarm detectors; 0.226%/m, components of gas
detectors; 0.392%/m) m: Frequency of functional testing (month) *1: In case of a detector with one component (sensor) *2: In case of a typical detector with four components, one pump and three sensors *3: Typical current test frequencies
26
Figure 1. Portable hydrogen sulfide alarm detector.
Figure 2. Typical portable gas detector. (This detector has one pump and three sensors built-in.)
Numb0 20 40
0-12-34-56-78-9
10-1112-1314-1516-1718-1920-2122-23D
urat
ion
of U
se [m
onth
]
Figure 3. Result of functional testing of hyd
xx
x x
x
er of Alarms60 80
A Site B Site
rogen sulfide alarm detectors.
X F il
xx
27
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