A breathing apparatus normally known as the BA set or the SCBA (Self contained Breathing apparatus) or CABA (Compressed air breathing apparatus) is an equipment used to supply fresh air for human breathing when a person is entering any space where the atmosphere is a suspect and may not support human life. i.e.
- When entering into an enclosed space or
- When entering into a space to fight fire
The Self Contained Breathing Apparatus (SCBA) or Compressed Air Breathing Apparatus (CABA):
The equipment is designed on an open-circuit system. The exhaled air is discharged to the atmosphere and the wearer of this apparatus gets his fresh air from the air cylinder attached to the equipment.
The equipment consists of:
- One cylinder containing air under pressure
- A harness to mount the cylinders on the, back of the person.
- A respiratory system which incorporates a means of reducing the pressure of the air from the cylinder and of supplying the wearer on demand with air according to his requirements, and
- A facemask attached to a demand valve, which maintains a positive pressure inside the mask at all times.
The arrangement of a typical apparatus is as follows:
The cylinders contain approximately 1240 litres of free air compressed to give approximately 31 minutes of full air usage time and 21 minutes of working duration under normal circumstances. Please note that the usage time greatly varies from person to person and the workload the person is performing.
- A reducing valve is connected after the cylinders to reduce the pressure to approximately about 4 bars. This pressure is further reduced by the demand valve, which is attached to the mask of the wearer.
- The demand valve supplies air to the wearer when he inhales and closes when he exhales.
- An Exhalation valve releases the exhaled air from the facemask.
- When approximately 10 minutes of air is left (air cylinder pressure about 60 bar), a warning whistle will sound continuously – warning the user that his air supply would soon be over and he has to move out.
- This warning signal will sound continuously till the air in the cylinder is over.
- The facemask is made of moulded rubber with a series of adjustable rubber straps to secure it to the head of the wearer and fitted with quick release arrangements.
- The user should have a good field of vision so that the wearer does not need to turn his head constantly.
- A gauge is provided to indicate the pressure of the air in the cylinder.
- Spare cylinders should be provided for each set of breathing apparatus and a small air compressor to charge these cylinders is provided.
An approximate Consumption and duration table:
|Degrees of work||Air consumption (ltrs/minute)
|Duration of cylinder having 1800 ltrs capacity(minutes)||
Duration of cylinder having 1800 ltrs capacity(minutes)
|Resting||8 -12||150 -100||225 -150|
|Light Work||12 – 20||100 – 60||150 – 90|
|Moderate Work||20 – 40||60 – 30||90 – 45|
|Heavy Work||40 – 60||30 – 20||45 – 30|
Please note in the above table the total full duration is given. Reduce the safety margin( 10 MIN) from the above table to get the nominal working duration.
- Here we have taken an average consumption of 40 ltrs of air per minute being required by human being when doing heavy work.
- If we have a 1200 ltrs of fully charged cylinder then:
- Nominal work duration= 1200/40= 30 min. – 10 min ( safety margin taken as the alarm whistle will start blowing indicating the wearer to come out to safe atmosphere) so it is 20 min.