Information Circular 9449

Proposal for Certification Tests and Standards

for Closed-Circuit Breathing Apparatus

By Nicholas Kyriazi

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

Pittsburgh, PA

August 1999
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction ................................................................................... 2

Proposal for certification tests and standards for CCBA ................................................. 3

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Classification of apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Required components ......................................................................... 3

Test descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Capacity test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Performance test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Wearability test ............................................................................ 4

Quantitative leak test ....................................................................... 4

Gas permeability ........................................................................... 4

High- and low-temperature tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Shock and vibration test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Hypoxia scenario test ....................................................................... 5

Testing procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Continuous monitoring ...................................................................... 5

Monitored stressors and recommended limits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

CO2 and oxygen concentrations ............................................................. 5

Breathing pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

BMS versus human-subject testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Quantitative leak test ....................................................................... 6

Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Reasoning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Test procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

TABLES

1. Work-rate parameters: capacity test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2. Work-rate parameters: performance test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

breaths/min breaths per minute mL/min milliliter(s) per minute

hr hour(s) mm H2O millimeter(s) of water pressure

kg kilogram(s) mm Hg millimeter(s) of mercury

L liter(s) ppm parts per million

L/min liter(s) per minute s second

lb pound(s) °C degree(s) Celsius

min minute(s) % percent
PROPOSAL FOR CERTIFICATION TESTS AND STANDARDS

FOR CLOSED-CIRCUIT BREATHING APPARATUS

By Nicholas Kyriazi1

ABSTRACT

Significant portions of the present Federal regulations for certification of closed-circuit breathing apparatus

(42 CFR 84) are not quantitative. The human-subject testing portion of the regulations specifies activities rather

than metabolic work rates; however, heavy human subjects have higher oxygen consumption, CO2 production,

and ventilation rates than lighter human subjects performing the same activities. This means that apparatus

certified using different human subjects have effectively passed different tests. In addition, apparatus

performance is monitored only during selected intervals of testing, and always during rest, rather than during

the entire performance period. Further, testing is arbitrarily terminated at specific rated durations, leaving

unknown the performance during the remaining duration. Duration itself is misleading to users who believe

that the apparatus will last a certain time no matter how much they weigh or how hard they work. The actual

duration obtained from an apparatus, however, is highly variable, being inversely proportional to the work rate

at which it is used.

In this report, recommendations are made to (1) replace the present regulations for certification of closedcircuit breathing apparatus with quantitative tests specifying metabolic work rate rather than activity,

(2) continuously monitor relevant physiological stressors, (3) continue testing until exhaustion of the oxygen

source, and (4) classify closed-circuit breathing apparatus by quantity of usable oxygen, i.e., capacity, rather

than by duration. Physiology-based stressor levels are suggested as well.

1

Biomedical engineer, Pittsburgh Research Laboratory, National Institute for Occupational Safety and Health, Pittsburgh, PA.
2

INTRODUCTION

The present regulations in the United States for certification

of closed-circuit breathing apparatus (CCBA) (Title 42 of the

Code of Federal Regulations, Part 84) were derived from the

original U.S. Bureau of Mines' (USBM) Schedule 13,

"Procedure for Establishing a List of Permissible Self-Contained

Oxygen Breathing Apparatus," of March 5, 1919. The humansubject tests in Schedule 13 are composed of various activities

associated with mine rescue, such as walking at 3 miles per

hour, carrying a sack of bricks weighing 50 lb over an overcast,

and sawing wood. In both Schedule 13 and the present

42 CFR 84, administered by the Respirator Branch of the

National Institute for Occupational Safety and Health (NIOSH),

Morgantown, WV, these and other activities, performed for

specific time periods, are combined to make tests of 3, 5, 10, 15,

30, and 45 min, and 1, 2, 3, and 4 hr. A manufacturer applying

for certification must request a specific rated duration for its

apparatus so that a time-specific test can be chosen. This is a

reasonable way to qualitatively evaluate mine rescue apparatus,

but it is not quantitative and can be misleading to the user.

Specifying activities rather than work rates results in different work rates being imposed by different human subjects,

varying with weight and physical condition. Using different human subjects for different apparatus certifications means that

different approved breathing apparatus have effectively passed

different tests. This is unfair to both the manufacturers and

the users.

Quantitative performance testing of CCBA ideally requires

specifying the oxygen consumption rate, CO2 production rate,

ventilation rate, and respiratory frequency. This not only provides manufacturers with a complete description of the expected

work rate, but also assures users that all approved apparatus

have passed the same test. This can be done using a breathing

and metabolic simulator (BMS), with control over all of the

aforementioned metabolic parameters, or with a human subject,

with control over only one parameter at a time, usually oxygen

consumption rate.

In addition to specifying work rates instead of activities,

continuous monitoring of the physiological stressors—inhaled

oxygen and CO2 concentrations, breathing pressures, and inhaled wet- and dry-bulb temperatures—is recommended. Further, it is recommended that there be no regulation of components of apparatus, such as specifying activation pressures for

demand and relief valves, flow rates for regulators, etc. The

overall performance of the apparatus, which encompasses the

performance of individual components, is the primary concern.

Additional deficiencies of the present regulations are as

follows:

(1) 42 CFR 84.94 specifies gas flow requirements for

CCBA. Paragraph (c) states that the demand valve for CCBA

is required to supply 30 L of oxygen per minute "in the fully

open position." The test is performed activating the valve

manually (such as with a pencil or finger) to check the flow. In

actual use, however, demand valves are activated either by low

breathing bag volume or low breathing bag pressure. One

approved device delivers 90 L/min when activated manually but

requires large negative breathing pressures to elicit even

30 L/min. However, breathing pressures during human-subject

testing are not presently regulated. Users of this device in an

actual emergency complained that they "couldn't get enough air"

[MSHA 1984] and either breathed around the mouthpiece,

slowed down, or removed the mouthpiece entirely.

(2) Testing is ended at the rated duration requested by the

manufacturer at the time of the submittal. However, the apparatus

is not usually fully expended at the exact time of test termination.

This leaves its remaining performance unknown. It is reasonable

to expect that a user will not necessarily remove an apparatus

exactly at, or before, its rated duration, and that some users will

use the apparatus at a work rate higher than that at which it was

tested, thus experiencing its postrated-duration performance

within the time period of its rated duration. Several approved

apparatus, if used to the full capacity of their stored breathing

gases, will expose the wearer to greater than 10% CO2 (due to an

oxygen supply much greater than the CO2 absorbent capacity) or

lower than 10% oxygen (due to a very large breathing bag, which

is difficult to sufficiently purge of nitrogen).

(3) If donned in a certain way, some CCBA may expose

the user to hypoxic conditions. If sufficient nitrogen is exhaled

into the breathing circuit of a compressed-oxygen apparatus

such that the demand valve is not activated after a full inhalation, and if the user's oxygen consumption rate is higher than

the constant flow rate, the oxygen concentration may fall to

subambient levels before the demand valve can be activated.

(4) Because apparatus are certified for specific durations,

users may expect that the apparatus will last that long regardless

of who is wearing it or what activity is being performed. However, an apparatus used by a heavy person performing hard work

will not last its rated duration and may give the user a false sense

of security, which could be dangerous in an emergency.

Following are recommendations for tests, methods, and

performance criteria for CCBA.
3

PROPOSAL FOR CERTIFICATION TESTS AND STANDARDS FOR CCBA

PREFACE

A user of CCBA should be assured that an approved device

will be safe throughout its use and that all approved devices

have passed the same test. This can be accomplished through

quantitative certification tests and continuous monitoring of

physiological stressors until the stored-gas supply is expended.

In order to fully evaluate an apparatus, there are three major

areas of required scrutiny: capacity, performance, and wearability. Three tests are recommended:

(1) A constant work rate test at a work rate representative

of the intended use, which will be referred to as the "capacity"

test.

(2) A composite test with both high and low work rates,

intended to draw into use all of the components of the apparatus

such as the demand and relief valves, which will be referred to

as the "performance" test.

(3) A human-subject test that includes numerous physical

activities and body orientations, intended to evaluate the apparatus' impact on the wearer, which will be referred to as the

"wearability" test.

The capacity test will determine the quantity of usable oxygen delivered to the user. The quantity of usable oxygen will

be used to classify the apparatus. This test will be at a constant

oxygen consumption rate to be agreed upon by the manufacturer

and NIOSH as representative of the intended use. The duration

attained at that work rate will be noted for the benefit of the

consumer, but will always be linked to the work rate at which it

was obtained. The manufacturer may choose to have an

apparatus tested at several constant work rates to show a range

of usable oxygen quantities varying with work rate.

DEFINITIONS

CCBA - A closed-circuit breathing apparatus is selfcontained and has both a CO2-absorbent component and a compliant breath-storage component (breathing bag).

STPD - Standard temperature and pressure, dry. All volumes in this document are specified at these conditions, which

are 0 °C, 760 mm Hg, and containing no water vapor. (An

example illustrating the importance of specifying conditions is

that 1 L measured at body temperature and ambient pressure,

saturated, (BTPS) at Johannesburg, Republic of South Africa,

where the ambient pressure is 627 mm Hg, becomes 0.83 L

BTPS in Norfolk, VA, and 0.67 L measured at STPD).

VO2 - Volume rate of oxygen consumed in L/min

VCO2 - Volume rate of CO2 produced in L/min

Ve - Volume rate of gas exhaled in L/min

RF - Respiratory frequency in breaths/min

CLASSIFICATION OF APPARATUS

CCBA shall be classified by quantity of oxygen usable by

the subject rather than by duration, which varies inversely with

use rate. The quantity of usable oxygen shall be determined by

the capacity test, described in the "Test Descriptions" section,

at a constant work rate agreed upon by the manufacturer and

NIOSH; the apparatus shall be classified by the quantity of

oxygen used by a human subject or a BMS, described in the

"BMS Versus Human-Subject Testing" section, with the quantity rounded off to the nearest lower increment of 5 L. If more

than one work rate is used, the quantity of usable oxygen at each

shall be noted.

REQUIRED COMPONENTS

(1) Remaining service-life indicator (entry apparatus only).

(2) 25% service-life warning device (entry apparatus only).

TEST DESCRIPTIONS

The capacity and performance tests may be performed on

either a BMS or a calibrated human subject. The wearability

test shall be performed on a human subject. A BMS permits the

selection of not only the oxygen consumption rate but also the

CO2 production rate, the ventilation rate, and the respiratory

frequency. Use of a human subject does not permit this extent

of control. The wide variation between human subjects in the

metabolic parameters other than the oxygen consumption rate

may present a problem. For example, two human subjects

working at the same oxygen consumption rate will probably

have different CO2 production rates, ventilation rates, and

respiratory frequencies. At the least, the other parameters

should be measured and noted for human-subject testing.

The capacity test and the performance test shall be performed on three apparatus each. The wearability test need be

performed on only one apparatus.

All of the tests shall be continuously monitored (as described in the "Testing Procedures" section).

The capacity and performance tests shall be considered concluded either when the stored-gas supply is expended or when

a physiological stressor level has exceeded the recommended

limit (as described in the "Testing Procedures" section). If a

stressor level is exceeded in any test for more than 1 min, the

apparatus fails and is rejected.

Capacity Test

The likely oxygen use rate of an apparatus depends on its

purpose—mine rescue, escape, fire service, toxic chemical
4

cleanup, etc. The capacity test work rate should approximate

the average oxygen use rate for the apparatus' intended use in

order to give the consumer a general idea of its likely duration

for that use. A constant work rate, rather than varying work

rates, is used in order to more easily discern the characteristics

of the apparatus over its entire service life. The apparatus shall

be tested at a work rate agreed upon by its manufacturer and

NIOSH (light, moderate, heavy, or high) until the breathing gas

supply is expended. The quantity of oxygen used at that time

(duration × oxygen use rate) shall determine its classification.

Recommended work-rate parameters are shown in table 1.

Table 1.—Work-rate parameters: capacity test

(all volumes at STPD)

Work rate

VO2,

L/min

VCO2,

L/min

Ve,

L/min

RF,

breaths/

min

Light . . . . . . . . . 1.00 0.80 22.0 17.0

Moderate1 . . . . . 1.35 1.15 30.0 18.0

Heavy . . . . . . . . 2.00 1.80 44.0 20.0

High . . . . . . . . . 2.50 2.50 55.0 22.0

 1

The moderate work rate is the average level at which the

50th-percentile miner (87 kg) performs during the 1-hr

man test 4 in the present regulations [Kamon et al. 1975].

Performance Test

This test will ensure that, in cases of high or low metabolic

work rate, the apparatus will not catastrophically fail in some way.

(It must be assumed that any type of apparatus may be used at

both high and low work rates.) The high work rates used in the

test will likely activate a demand valve, if present, and stress the

CO2-absorbent canister; the low work rate will likely activate a

relief valve, if present. Continuous monitoring will enable evaluation of these components in use. It is recognized that the apparatus may sacrifice some efficiency when used at very high or

low work rates; therefore, its quantity of usable oxygen is not

rated during this test. The test shall consist of repeating the

following three-work-rate cycle until the breathing gas supply is

expended: 3.0 L/min for 5 min, 2.0 L/min for 15 min, and

0.5 L/min (or less for human subject at rest) for 10 min. This cycle requires 50 L of oxygen. If the apparatus contains significantly less than this, as determined by the capacity test, the

alternating times shall be reduced proportionally so that one cycle

of work rates is completed. The performance test shall be started

by the exhalation of two large breaths into the apparatus before

donning it. This will determine the susceptibility of the apparatus

to hypoxia.

The work-rate parameters are shown in table 2.

Table 2.—Work-rate parameters: performance test

(all volumes at STPD)

Work rate VO2,

L/min

VCO2,

L/min

Ve,

L/min

RF,

breaths/min

Very high . . . . 3.00 3.20 65.0 25.0

Heavy . . . . . . . 2.00 1.80 44.0 20.0

Low . . . . . . . . 0.50 0.40 20.0 12.0

Wearability Test

This test will ensure that the apparatus will not be physically harmful to, or hinder, the user significantly in any anticipated activity. At least the following activities shall be

performed, in any order, for sufficient duration to ensure compatibility with the human form: stooped walking, crawling, lying on each side and on back, climbing a laddermill and steps,

crawling on stomach, bending over to touch toes, turning head

from side to side and up and down, and handling anticipated

equipment. Continuous monitoring will be employed to assess

the orientation sensitivity of the apparatus.

The user shall be able to easily control the functioning of

the apparatus through manipulation of readily distinguishable

control mechanisms, if applicable.

Quantitative Leak Test

With a pressure of &300 mm H2O applied to the breathing

circuit of the CCBA, the leak rate into the breathing circuit shall

not exceed 87 mL/min. (See "Testing Procedures" section for

derivation and performance details.)

Gas Permeability

The manufacturer shall provide evidence that the submitted

apparatus does not permit dangerous levels of any gases with

which it is claimed to be compatible, or likely to be encountered

in intended use, to permeate the breathing circuit for the longest

duration possible in use, or on repeated uses if reusable. (The

present regulations at 42 CFR 84.88 require only that the

breathing bag material withstand permeation by gasoline.)

High- and Low-Temperature Tests

The manufacturer shall recommend temperature limits

within which the apparatus must be stored and used. The

CCBA shall pass all performance tests at these limits. The low

and high recommended storage temperatures shall be maintained

for 24 hr, after which the apparatus shall pass the capacity and

performance tests at the corresponding low or high

recommended use temperatures.

Shock and Vibration Test

The CCBA will be subjected to shock and vibration appropriate to its intended use conditions as recommended by

acknowledged industry representatives or the appropriate government agency, after which it must pass the capacity, performance, and wearability tests.

For mine escape apparatus stored on machinery, a shock

and vibration test as described by Stengel et al. [1982] is recommended. For mine escape apparatus that may be belt-worn,

the present Ro-Tap test is appropriate.
5

Hypoxia Scenario Test

Under certain circumstances, it is possible in some CCBA

to create a situation in which oxygen levels fall too low to

sustain human life. For example, on a compressed-oxygen

CCBA with a demand valve, if enough nitrogen is contained in

the breathing circuit to prevent activation of the demand valve

at the end of a full inspiration, and the oxygen-use rate is higher

than the constant flow, oxygen levels may fall to subambient

levels before the demand valve is activated. The potential for

this situation to occur will be ascertained during the first minutes of the performance test. Exhaling into the apparatus twice

before donning it and the immediately following 3.0 L/min

oxygen consumption rate in the performance test together define the hypoxia scenario test.

TESTING PROCEDURES

Continuous Monitoring

The capacity, performance, and wearability tests shall be

continuously monitored, whether using a BMS or a human subject. The stressors monitored shall be, at least, average inhaled

levels of CO2 and oxygen, minimum inhaled CO2 level, end-ofinhalation wet- and dry-bulb temperatures, and peak breathing

pressures. These stressors shall be monitored at the mouth/ apparatus interface with instruments that are capable of breath- bybreath response. The temperature probes shall be located in the

center of the air stream. The pressure tap shall be perpendicular

to the air flow. Each stressor will be discussed separately.

Monitored Stressors and Recommended Limits

Recommended stressor limits are based on physiological

research performed at the Noll Laboratory for Human Performance Research at The Pennsylvania State University and

funded by the former USBM. The contract final reports

describing this research [Kamon et al. 1984a,b; Hodgson 1993]

are available for consultation at the NIOSH Pittsburgh Research

Laboratory.

CO2 and Oxygen Concentrations

The peaks of instantaneous gas concentrations (high oxygen, low CO2) continuously monitored at the mouth during use

are not the actual concentrations inhaled by the user since they

do not take into account apparatus dead space. Dead space is

the apparatus' internal breathing circuit volume containing exhaled air that is directly inhaled, uncleansed of CO2, during

inhalation. The low values of CO2, for example, reveal only the

best performance of the CO2 scrubber. Average inhaled concentrations of both gases can be calculated electronically by

multiplying instantaneous gas concentration values by the

simultaneously occurring inhalation flow rates, summing these

values over the entire inhalation, and then dividing that volume

by the total inspired gas volume [Bernard 1977].

The physiological research conducted at The Pennsylvania

State University showed no disabling physical effects in active

men breathing 5% CO2 for long periods of time [Kamon et al.

1984a]. However, in some test subjects, the decision-making

process was slightly impaired after breathing 4% CO2 for 1 hr.

We therefore recommend that, during use testing, the average

inhaled levels of CO2 remain below 4% and oxygen levels

remain above 16% [Dinman 1978].

Breathing Pressures

During any use test, the peak pressures shall not exceed

200 mm H20 for exhalation and &300 mm H2O for inhalation.

It was found in physiological research [Hodgson 1993] that 80%

of test subjects (n ' 10) could generate these pressures for

approximately 4 min. The population of users who could not

generate these pressures would be limited to work rates that kept

breathing pressures below these levels. This is considered an

acceptable tradeoff for gains in reducing the size and weight of

apparatus.

Classification of apparatus as positive-pressure will be at a

particular level of work: light, moderate, heavy, or high. During the capacity test chosen by the manufacturer, inhalation

pressures must stay above ambient pressure at all times.

It is recommended that the peak pressures generated during

the tests be listed in the approval documentation and available

to the consumer.

Temperature

It has been found that human thermal sensitivity is related

to the wet-bulb temperature. In research at The Pennsylvania

State University [Kamon et al. 1984b], researchers showed that

wet-bulb temperature of the inspired air is a better descriptor of

heat stress to the tissues of the mouth than either dry-bulb

temperature, relative humidity, or both of these combined. For

the same inhaled dry-bulb air temperature, the thermal load of

humid air is higher than that of dry air. The maximum thermal

load tolerated by a human being can be specified by an infinite

number of combinations of dry-bulb temperatures and relative

humidities, or by one wet-bulb temperature. The highest tolerable wet-bulb temperature of inhaled air has been found to be

approximately 50° C [Kamon et al. 1984b]; this is recommended

to be the limit for all respiratory protective devices.

Until recently, breath-by-breath measurement of wet-bulb

temperature was not possible due to the slow response time of

humidity-measuring instruments. However, the Pittsburgh Research Laboratory has developed a fast-response, wet-bulb thermocouple with a response time of <1 s [Kyriazi 1988].
6

It is recommended that the highest inhalation temperature

generated during the tests be listed in the approval documentation and available to the consumer.

BMS Versus Human-Subject Testing

A BMS is a machine that simulates both the breathing and

metabolic functions of a human being. The former USBM

developed four such simulators since the early 1970s [Kyriazi

1986]. Such a machine was desired for the ability to quantify

and control input metabolic levels. A human subject on a treadmill can also be used by varying the speed and/or grade until a

desired oxygen consumption rate is achieved. However, the

CO2-production rate, ventilation rate, and respiratory frequency

will likely vary among human subjects even at the same oxygen

consumption rate. This is a major advantage of the BMS over

human subjects, in addition to the better repeatability and

constancy.

Quantitative Leak Test

Assumptions

(1) CO is the gas most likely to be encountered underground that is dangerous in the lowest concentrations, with a

threshold limit value of 35 ppm.

(2) The highest likely ambient concentration of CO is

10%.

(3) The highest expected realistic oxygen consumption

rate is 3.0 L/min with a corresponding Ve of 80 L/min and peak

inhalation flows of approximately 250 L/min. The in-leakage

is examined at these conditions.

(4) The highest negative pressure permitted during the

proposed tests is &300 mm H2O. This is assumed to occur

during the inhalation peak flows of 250 L/min.

Reasoning

An 87 mL/min in-leakage of 10% CO during an instantaneous inhalation flow rate of 250 L/min results in an inhaled

CO concentration of 35 ppm, occurring at the highest-permitted

negative pressure of &300 mm H2O.

Test Setup

Required components are a rotary vane pump powered

through a voltage regulator, a mass flow meter calibrated for air

or nitrogen, a pressure transducer, and flexible connecting tubing.

Connect the inlet port of the rotary vane pump to the outlet

port of the mass flow meter. Connect the inlet port of the mass

flow meter to a "T" connector branching to the pressure transducer and to the mouthpiece of the breathing apparatus to be

tested.

Test Procedure

(1) Connect the apparatus to the test setup.

(2) Turn on the voltage regulator controlling the

rotary-vane pump. (Note: The flow rate will be high and the

pressure will remain near ambient until the breathing bag is

evacuated of air. After the bag has collapsed, the pressure will

sharply drop.)

(3) Adjust the voltage regulator controlling the pump until

the pressure, as read on the pressure transducer, stabilizes at

&300 mm H2O.

(4) Read the flow rate on the mass flow meter once a

stable number is obtained. This is the apparatus leak rate.

A leak rate of no more than 87 mL/min is permitted. (Note: If

the leak rate continues to slowly fall, this may indicate that

backflow through a relief valve is diminishing as its check valve

seats itself better with continued pressure.)

SUMMARY

In order to determine if a closed-circuit breathing apparatus

is safe to wear and to ascertain its capacity and performance

characteristics, it is necessary to subject it to a known input until

its gas supply is exhausted and to continuously monitor its

output throughout this period. Currently, an apparatus is certified by subjecting it to an unknown and random input,

 monitoring its output only during rest periods and terminating

the test before it is expended, when a user could continue to

wear it. This practice is inadequate to determine the apparatus'

safety, its capacity, or its performance characteristics. The tests

and methods proposed here are an attempt to remedy these

present shortcomings.

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