The LP-120

Author: Mike Borrello
July 2006 

The LP 120 is an acronym for the Low-Profile, 120 minute rebreather.  The system was developed by engineers Max Kranz, Mike Borrello, and Harold Ellison at Survivair in 1982 which was then a division of US divers Corp. The rebreather was designed and developed under a contract with the US Bureau of Mines.  The purpose of the LP-120 was to provide extended life support in hypoxic or noxious atmospheres for mine rescue workers.  The design called for a very low profile, back mounted unit which contained breathing bags, a carbon dioxide scrubber, a cooling canister, a positive pressure demand regulator, a pressure vessel and pressure reducer.  The back mounted unit connected to the face piece through dual hoses at a face piece T. The T contained directional valves to facilitate unidirectional flow of breathing gas from the face piece.    

By exhaling into the face piece, gas enters the exhalation plenum and immediately expands the exhalation breathing bag against the force of a helical compression spring.  While under pressure, the exhalation breathing bag slowly pushes the exhaled gas through the carbon dioxide scrubber.  The carbon dioxide scrubber removes carbon dioxide from the exhaled gas which then flows through a cooling canister and into the inhalation plenum.  The cooling canister absorbs heat generated by the chemical reaction between the scrubber material and the carbon dioxide.  From the inhalation plenum, gas, now free of carbon dioxide, is inhaled by the user.  Connected to the inhalation plenum is an inhalation bag, also loaded by a helical spring.  Also connected to the inhalation plenum is a positive pressure demand regulator with a set point of 1 to 2 inches of water pressure.  As oxygen is consumed by the user, the positive pressure demand regulator senses a drop in pressure and allows pure oxygen to enter the inhalation plenum until the pressure set point is once again achieved. 

For high-efficiency removal of CO₂, about 600 to 700 g of lithium hydroxide is used as scrubber material.  Although lithium hydroxide is much more efficient than sodasorb or baralyme, one disadvantage is that it produces significantly more heat.  To help meet the NIOSH maximum temperature requirements, a cooling canister is used downstream of the scrubber canister.  This canister was originally fabricated from copperplate and copper tubing for high heat conduction and also contained a special compound sealed within it, lithium nitrate trihydrate.  Later versions of the canister may have been fabricated from aluminum.  Although anhydrous lithium nitrate has a melting point of about 250°C, the trihydrate fuses at 29.6°C and with a heat of fusion greater than ice.  Although other salts were identified that provide higher heats of fusion, lithium nitrate trihydrate is relatively safe and with just the right melting point to regulate temperature as long as the salt is in phase transition.  The cooling canister made the LP-120 unique among other rebreathers, and actually acted as a heat capacitor, reducing the peak operating temperature of the breathing gas within acceptable limits over the two-hour operation. The cooling canister was reusable and was simply replenished by cooling it in a refrigerator until the salt returns to solid form. 

In addition to offering a low-profile so that mine workers can access trapped miners through low overhead passages, the unit also had to be very low weight.  The pressure vessel that contained 100% oxygen is of a fiber wrap design and most other materials in the unit were made from plastics. 

Although the LP 120 met its intended performance requirements, it's doubtful whether the design was ever commercialized by either US Divers or Survivair.

Mike Borrello

Former Design Engineer, US Divers Corp.

stablesystems@ieee.org