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PFD Activation Mechanisms Explained

Posted by Jeff Dusting on 30th Jun 2014

In this short post we explain the 3 common PFD activation mechanisms:

  • Manual activation
  • ProSensor
  • Hammar hydrostatic activation

The Manual Activation System

All inflatable PFDs contain a compressed carbon dioxide (CO2) cylinder that when pierced inflates the PFD bladder.

On a manually inflated jacket, the CO2 cylinder is pierced when the wearer pulse the activation handle or toggle.

All automatic PFDs also include a manual activation over-ride - over-riding the automatic system.   Note also, that all inflatable PFDs include an oral inflation tube, so if the CO2 cylinder fails (or some other problem), you can always inflate the bladder orally.

The ProSensor System Explained

The Pro Sensor Firing System is activated by contact with water. A compressed paper element dissolves to release a sprung needle which pierces the gas cylinder - the Prosensor is a dramatic improvement on the standard MK5 providing the user with a visual status indication, to confirm that the unit has been activated and whether the CO2 cylinder has been pierced -eradicating the possibility of accidentally fitting an empty gas cylinder.

The Pro Sensor is also available in manual only version, affording the user one point of status indication, to confirm if the unit has been manually activated. This unit also provides the added benifit of detecting if the CO2 cylinder has been pierced, eradicating the possibility of accidentally fitting an empty gas cylinder.

Sea, spray, or rainwater running downwards through the unit should not cause activation-only water flowing upwards will activate firing ... take a look at the short video of the 'fire-hose' test where a lucky test subject is fire-hosed with their deckvest on to demonstrate the protection against inadvertent inflation.


  • Very reliable activation when immersed
  • Easy to check and re-arm at sea
  • Easy to convert to ‘manual firing’ only
  • Pro Sensor has an indicator to show armed status
  • Water sensitive activation system
  • Uses a compressed paper capsule that dissolves and releases a spring to puncture the gas cylinder. (Pro Sensor has indictators to show status of cap and cylinder)
  • The cap is designed so that only water flowing upwards through the unit will cause it to activate. Water, spray and rain running down jacket will not cause activation.


  • Can be prone to accidental inflation in heavy breaking sea conditions or if stored in a damp, unventilated location
  • Cylinder can work loose over time so needs checking regularly
  • Cylinder can corrode/leak if stored damp
  • 3 year replacement date printed on firing cap  

The Hammar System Explained

The Hammar System (MA10 Hydrostatic inflation System) is activated by water pressure and will not inflate until the cap is approximately 10cm underwater A valve allows water into the firing head when water pressure exceeds a given spring pressure. This dissolves a compound that then releases a spring to fire the cylinder - a 33g CO2 bottle glued to backing plate inside the bladder.


  • Will not accidently inflate in very wet conditions
  • Cylinder is glued to firing head so will not work loose
  • Cylinder is contained inside the bladder so will not corrode
  • Single indicator clearly shows if unit is armed
  • 5 year expiry date on firing head

  • Considerations
  • More expensive rearming kit (more than double the cost of Pro Sensor kit) but you should not need to do this as often
  • Must use a Spinlock rearming kit as the manual handle is sewn into the head
  • More technical and more critical to rearm and repack
  • For reliable water access at 10cm depth, all air must be removed from the bladder. Air trapped in either the bladder during rearming or in the cover around the firing head can delay, or in extreme cases prevent the head from activating
  • Delayed inflation can also be caused if full immersion of the firing head is delayed by buoyant foul weather gear