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National Institute for Occupational Safety and Health

National Personal Protective Technology Laboratory 

626 Cochrans Mill Road 

Pittsburgh, PA 15236

Procedure No. RCT-ASR-STP-0121 Revision: 1.3 Date: 10 March 2020

DETERMINATION OF RATED SERVICE TIME - OPEN-CIRCUIT, DEMAND

AND PRESSURE-DEMAND, SELF-CONTAINED BREATHING APPARATUS

STANDARD TESTING PROCEDURE (STP)

1. PURPOSE

This document establishes the procedure for determining rated service time requirements for an

Open-Circuit, Demand and Pressure-Demand, Self-Contained Breathing Apparatus (SCBA) per

performance requirements in 42 CFR, Part 84, Subpart H, Section 84.95.

2. GENERAL

This STP describes the test used for Determination of Rated Service Time - Open-Circuit,

Demand and Pressure-Demand, Self-Contained Breathing Apparatus test in sufficient detail that a

person knowledgeable in the appropriate technical field can select equipment with the necessary

resolution, conduct the test, and determine whether or not the sample passes the test.

3. EQUIPMENT/MATERIALS

3.1. The list of necessary test equipment and materials follows:

3.1.1. National Instruments NI USB-9215A Portable USB-Based DAQ with 

Simultaneous Sampling; LabVIEW 2013; Dell Optiplex 755 Personal Computer, 

SCBA test software

3.1.2. Mechanical breather with 622 Kg m/min. cam as per U.S. BOM Drawings C1748 (3/17/69) Breathing Machine and B-1198 (3/6/69) Breathing Cam

3.1.3. ISI Anthropometric Test heads with tube for measuring breathing resistance and 

air flows - Model SR-085 or equivalent 

3.1.4. Validyne Engineering model DP45-20 transducer used with Validyne 

Engineering model CD-19A carrier demodulator mounted in the Validyne MC1-

333 module case. Pressure range up to 3.5 inches of water - accuracy: ± 0.5% 

F.S.

3.1.5. 3-Liter lung in bottle with plastic tubing, Hans Rudolph Co. part number CM

1435 or equivalent

3.1.6. Electric Timer, calibrated to hundredths of a minute (Precision Scientific

Company) or equivalent 

3.1.7. Dwyer Slant Manometer 0-3", F. W. Dwyer Manufacturing Co., Michigan City, 
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Indiana or Setra Datum 2000 Model 239 digital manometer – accuracy: ± 

0.01%R ± 1 digit, or equivalent.

4. TESTING REQUIREMENTS AND CONDITIONS

4.1. Prior to beginning any testing, confirm that all measuring equipment employed has been 

calibrated in accordance with the testing laboratory’s calibration procedure and schedule. 

All measuring equipment utilized for this testing must have been calibrated using a 

method traceable to recognized international standards when available.

5. PROCEDURE

5.1. Perform pre-test balancing of transducer and recording system.

5.1.1. Connect the transducer to be used during testing in parallel with a manometer. 

Attach the manometer and transducer to a pressure regulated air source. A pinch 

clamp, used for slight pressure changes, is placed in-line with two equal lengths 

of tubing for the manometer and transducer connections. An alternate method to 

generate low pressures for calibration is to use the Dwyer model A-396A 

calibration pump or equivalent. 

5.1.2. Connect the transducer cable to the CD-19A demodulator, and then connect the 

demodulator to the National Instruments DAQ. The DAQ is then connected to 

the PC via USB port. Turn the system on and press the Calibration button. After 

the calibration screen appears, with no load applied to the transducer, press the 

Zero button to set the zero-pressure point. 

5.1.3. Apply a pressure of 0.5 inches of water to the transducer/manometer system. 

Check that the demodulator reads 0.5 inches and adjust if necessary. Then check 

that the waveform displayed is at 0.5 inches and adjust LabVIEW readout if 

necessary.

5.1.4. Repeat steps 5.1.3 with the pressure of 1.0, 1.5, and 2.0 inches of water until each 

pressure point reads correctly on the waveform. No adjustments should be 

necessary at this point.

5.1.5. Verify that the pressures are correct, by applying pressure at 1.5, 0.5, and 0.0 

inches of water in descending order ensuring each pressure point reads correctly 

on the waveform. If adjustments are necessary, then repeat the calibration 

process for all pressures.

5.1.6. After the calibration sequence is complete, remove the pressure source from the 

system.

5.2. Take precautions to mount the pressure transducer in a manner that isolates it from shock 

and vibration, in particular that which is induced by the breathing machine and the 

operation of the SCBA. 
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Procedure No. RCT-ASR-STP-0121 Revision: 1.3 Date: 10 March 2020 Page 3 of 7 

5.3. Fill SCBA cylinder with air to pressure as noted in the instruction manual. Make sure the 

pressure remains within the DOT-certified pressure range. A “+” indicates that the DOT 

pressure may be exceeded by 10%. 

5.4. Assemble respirator. Mount facepiece on anthropometric head, taking care not to block 

resistance port below and left of nose, particularly if a nosecup is used. 

5.5. Connect regulator or breathing tube to facepiece. Do not connect head to breathing 

machine. Turn on breathing machine and use a timer or the built-in tachometer to 

determine that the cam is operating at 24 rpm (24 rpms yields a 40 lpm volume). The 

breathing machine contains a cycle counter for obtaining exact and total cycle counts and 

is used to make precise corrections at end of test. Stop the breathing machine when the 

pistons are at the end of the upstroke and reset counter to zero.

5.6. Check that the waveform reads zero. Then, hit the Data Entry button and enter the task 

number, date and make and model of the unit being tested. (While this is being done the 

transducer should be connected to the recorder, but the transducer should not have any 

pressure load on it). 

5.7. Connect the anthropometric head with the facepiece mounted to the lung-in-bottle 

assembly, using the tubing side that is connected to the breathing bag inside the bottle, 

and then connect the other tube from the lung in bottle to the breathing machine. 

Connect transducer of the PC-based recording system to resistance port of the headform 

with a short length of tubing. Fully open SCBA cylinder valve and, for belt mounted 

regulator type units, fully open main line valve. Make sure any incorporated by-pass 

valve is closed.

5.8. Turn on the breathing machine and timer and hit the Start button on the PC 

simultaneously. 

5.9. When alarm activation is heard, record time and the exact number of cycles indicated by 

the counter on the breathing machine. Continue recording until inhalation portion of the 

breathing curve falls below baseline and record this time. The elapsed time may also be 

determined by viewing the time that appears in the upper part of the display just before 

hitting the ‘Complete’ button when the inhalation pressure goes negative. The time is 

displayed in count down mode from the rated service time, and when the rated time is 

exceeded, the time reads in count–up mode starting from 0, with a negative sign. Add the 

absolute value shown to the rated time for the total elapsed time.

5.9.1. The gauge readings should be from the remote gauge, where provided, but may 

be taken from the cylinder gauge if a remote gauge is not provided. Including the 

data point required to be recorded in the preceding step, record the time observed 

at each of the major gauge pressure increments (minimum of three data 

points). The time interval of the alarm should be included. 

5.10. When tracings are complete - Turn off breathing machine and cylinder valve on SCBA 

and then bleed down high-pressure air trapped in breathing hose by opening the by-pass 

valve, then shut the by-pass off. Disconnect transducer from head, and record counter 
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Procedure No. RCT-ASR-STP-0121 Revision: 1.3 Date: 10 March 2020 Page 4 of 7 

setting on data sheet. 

5.11. Retrieve the tracings for data analysis from the PC –based system which uses a custom 

LabVIEW operating code to display the results. 

5.12. Data Analysis

5.12.1. The PC- based system produces a trace showing the inhalation (negative) and 

exhalation (positive) breathing resistance. The inhalation phase is the component 

for analysis. The PC-based system can be adjusted for sizing, i.e. how many 

peaks will appear on the screen. The spread of the waveform on the PC display 

will not affect the results.

5.12.2. A calibrated electric timer can be used to determine the length of time between 

starting the breathing machine and the point at which the inhalation portion of the 

breathing curve falls below minimum requirements, or the service time is 

determined as stated in section 5.10. 

5.12.3. The corrected end of service time is calculated following the equation below: 

Corrected Time = Final Cycles - (Final Time X 24)

24 + Final Time

5.12.4. The corrected alarm time is calculated following the equation below: 

Corrected Alarm Time = # of Cycles Alarm Activation - (Alarm Time X 24)

24 + Alarm Time

6. PASS\FAIL CRITERIA

6.1. The limits of acceptability for conducting this test are set forth in 42 CFR, Part 84, 

Subpart H, Section 84.95 (a)(b)(c). 

Reference: 

(a) Service time will be measured with a breathing machine as described in 84.88. 

(b) The open-circuit apparatus will be classified according to the length of time it 

supplies air or oxygen to the breathing machine. 

(c) The service time obtained on this test will be used to classify the open-circuit 

apparatus in accordance with 84.53. 
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Procedure No. RCT-ASR-STP-0121 Revision: 1.3 Date: 10 March 2020 Page 5 of 7 

7. RECORDS\TEST SHEETS

7.1. Record test data in a format that shall be stored and retrievable. Data to be reported as 

shown in attached data sheet. 

8. ATTACHMENTS

8.1. Sample Data Sheet
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8.1. Sample Data Sheet 
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Procedure No. RCT-ASR-STP-0121 Revision: 1.3 Date: 10 March 2020 Page 7 of 7 

Revision History

Revision Date Reason for Revision

1.0 23 May 2001 Historic document

1.1 20 September 2005 Update header and format to reflect lab move from Morgantown, 

WV

No changes to method

1.2 5 June 2018 Updated test procedure to reflect new PC based recording system 

using LabVIEW, plus minor editorial changes. Updated header 

with current address, and current NIOSH logo.

1.3 10 March 2020 Updated Section 5, with changes related to the calibration 

sequence and checking for leaks around the face seal.