What is an Oxygen Depletion System
(ODS)?
An Oxygen Depletion System (ODS) is
a safety device that activates the flame supervision
device in any situation resulting in the presence of
excessive levels of CO2 (and the deadly carbon
monoxide), which reduce the levels of oxygen causing
the pilot flame to become unstable and lift off the
thermocouple tip. The fire will then be automatically
turned off long before the situation might become
dangerous.
Oxygen Depletion
System works in conjunction with a Flame Failure
Detector (FFD). ODS is a small pilot burner which is
designed to be very sensitive to the amount of air
available; it has an ignition and detection flame
pointing up towards the main burner. The FFD probe is
a device called a thermocouple, and generates a tiny
electrical current when it is heated by the flame. This
current operates a small electromagnet which holds a
gas valve, upstream of the manual control valve, open;
but it is not powerful enough to open the valve by
itself from closed, only to hold it open. So on a gas
appliance with FFD protection, it is always necessary
to hold the control knob pushed in for 5 seconds or so
after the spark, while the thermocouple heats up. If
the flame subsequently blows out, then the
thermocouple will cool down and shut off the flow of
gas to the burner.
ODS is designed so
that if there is not much oxygen available, then the
flame will jump to the electrode, allowing the
thermocouple to cool down and thus shutting off the
gas supply. If the whole thing is working properly,
this will happen quickly enough for there to be no
danger to the user.
Under normal conditions, thermocouple tip is engulfed
holding the flame failure device open, allowing the
fire to operate. When ODS pilot becomes unstable
and clears off from the thermocouple tip as oxygen
levels reduce, thermocouple tip can’t be fully heated
by the flame. Excessive levels of CO2 causes the pilot
flame to fail completely. So the thermocouple tip is
cooled and the flame failure device closes, turning
off the gas supply to the appliance.
----> What is
a Thermocouple?
----> Thermocouple
Standards and Installation
