SCBA is important life saving equipment. The owner / operator should know how to perform an SCBA cylinder breathing duration calculation to ensure safe operation.
We will explain how to perform this SCBA Cylinder Breathing duration calculation in this blog for your SCBA set.
Water Capacity vs Pressure
To calculate the duration, we need to know a number of things about the SCBA cylinder:
Water Capacity
Working Pressure in Bar
The water capacity and service pressure can be found reading the markings label on the cylinder.
The water capacity is normally written in Litres with formats such as “6.8L”, “NWC6.8” or “V6.8L”. Occasionally the water capacity is written in KG on the neck of the cylinder as "WC". One litre of water has a mass of 1 litre.
“PW 300” stipulates the working pressure of the cylinder is 300 Bar.
Calculation
1 - Multiply the water capacity by the pressure to establish Total Volume (TV):
e.g. 6.8ltr Cylinder X 300 Bar = 2,040 ltr (TV)
e.g 10.0ltr Cylinder X 300 Bar = 3000 ltr (TV)
By determining the nominal amount of breathable air available in the cylinder as total possible volume in the cylinder, we can start to work on individual consumption.
How long can an operator breathe from a cylinder? First we must establish how much air an average person will breathe under different circumstances.
This is known as 'respiratory minute volume'. Minute volume is a calculation of the amount of air inhaled or exhaled from a persons lungs in one minute, based on volume and frequency.
A typical adult will breathe between 12-20 breaths per minute, approximately 5-8 litres of air per minute. During moderate exercise, an adult human can breathe between 40-60 litres per minute. Typically, SCBA duration calculations use 40 litres per minute (lpm) as a guide.
To calculate the duration of a SCBA cylinder, we need to do a further calculation.
2 - Divide TV (total volume) by breathing rate to establish 'Total Duration' (TD)
e.g. 6.8ltr Cylinder TV = 2,040 divide by 40 = 51 minutes
e.g 10.0ltr Cylinder TV = 3,000 divide by 40 = 75 minutes
This calculation does not include a safety factor and is based on average breathing rates and cylinder going from completely full to completely empty. Most SCBA equipment has a warning whistle when there is about 50-55 Bar pressure left advising that air supply is about to give out.
A safety factor should be added to SCBA equipment but is not required for emergency escape breathing devices.
3 - Calculation of Safety Factor & Working Duration (WD)
e.g. 6.8L Cylinder = 6.8 (WC) x 50 bar (whistle) / 40 lpm = 8.5mins less 51 (TD) = 42.5mins WD
e.g. 10.0L Cylinder = 10L (WC) x 50 bar (whistle) / 40 lpm = 12.5 mins less 75 (TD) = 62.5 mins WD
You may need to allow more safety factor depending on activity and mission parameters including the walk out time and hazards.
Note: Cylinders with a working duration of less than 15 minutes are classified as 'escape only' (AS1715:2009 5.3.4.2.a.i) and are not suitable for SCBA work.
Calculation Formula
WC (kg/ltr) X Pressure (bar) = TV
TV / 40 (lpm) = TD
Less Safety Factor = TD –(WC (ltr) x Whistle (Bar) / 40 lpm) = WD
Operators of SCBA should be able to calculate this on the run using the pressure guage to establish a running calculation of the expected duration.
Example: an operator is on a SCBA mission, is using a 10x300 cylinder, there is only 100 Bar indicated on the pressure gauge. How long is left in minutes before the whistle?
e.g. 10x100 = 1000ltr (TD) divide by 40 lpm = 25mins, less safety of 10 x 50/40 = 12.5mins, equals 12.5 mins to whistle.