Prism Topaz

The Prism is the ultimate diving tool for the novice and the advanced diver.

**We are pleased to announce that the  Hollis PRISM2’s are available from Hollis’ products page


The PRISM Topaz is a fully closed circuit, constant PO2, mixed gas capable system with digital electronic control and manual backups.

Designed for both the recreational and technical diver, it weighs only 47lbs fully charged and ready to dive (in standard configuration with twin 19cuft cylinders and approx. 6lbs Sodasorb in the radial flow scrubber). This fact misleads many, because with the appropriate user training and diluent, this system may be used to 300ft.

The unit’s integrated buoyancy compenstor and harness assembly includes ditchable weight pockets and an alternative inflation regulator (ScubaPro Air II). Twin over-the-shoulder counterlungs fasten into the harness assembly and offer easy access to manual gas overrides; on the RHS/Inhale lung there is a dual mode automatic/manual diluent addition valve (upper inside of elbow) and on the LHS/Exhale lung there is a manual oxygen addition (lower inside wall).

The unit’s digital electronics are located in a separate, watertight head compartment, outside of the breathing loop, operated by a 9v battery and potted to prevent damage by accidental leaks. Mounted to the outside left of the head compartment is a low-power consumption solenoid assembly.

Inside the electronics, a depth sensor allows the unit to know it’s position relative to the surface, allowing the system to automatically default to a PO2 of 0.7 when the rig is shallower than 18ft. During the pre-dive check the diver calibrates the unit and selects setpoint for the dive (pre-set options are 0.7, 1.0, 1.2, 1.3, 1.4). As soon as 18ft is achieved, the unit automatically switches over to the setpoint selected for the dive. This feature prevents unnecessary off-gassing of the gas supply whilst at the surface or too shallow to achieve an elevated PO2.

The analog secondary display is driven directly by the sensors and requires NO BATTERIES to operate. This display allows the diver to monitor their PO2 and if they chose to increase it above the 0.7 default setting by manually adding additional oxygen into the loop (eg. on ascent).

The primary display may be a sequential LED wrist mounted bar or a mouthpiece mounted HDD (head-down display). They indicate, Alarm, Very Low PO2, Low PO2, Setpoint, High PO2 and Very High PO2. The Alarm is bi-coloured, flashing red to indicate a low battery and green to indicate a sensor/s is out of range.

Principles of Operation

The PRISM Topaz is a digitally controlled, constant PO2, modular, closed circuit diving system.

It’s breathing loop consists of a closable mouthpiece assembly with mushroom/check valves either side, which ensure uni-directional flow. Dual, front mounted counterlungs provide the diver with a flexible reservoir equivalent to the maximum displacement of the diver’s own lung volume. These counterlungs are fitted with both automatic and manual gas addition systems and a variable volume control valve (used upon ascent to vent excess expanding gas volume or to purge the loop). A radial flow scrubber canister is mounted vertically on a backplate attached to the integrated BC, between twin supply cylinders (one each, oxygen and diluent). An optional cowling may be fitted.

The electronics, sensors and batteries are mounted in the electronics head assembly at the top of the scrubber, with supply hoses feeding over the diver’s shoulders into the top of the counterlungs. A lighted primary display offers a quick reference of unit performance with status and alarm indicators. An independent secondary display driven totally by the sensors themselves and requiring no battery, verifies the unit’s performance. With the electronics switched on the secondary also provides a status check for the battery and displays the set point selected for the dive. The electronics vote between the three proprietary galvanic sensors and control the operation of a low wattage solenoid valve on the oxygen supply. Diluent addition is automatically achieved as hydrostatic pressure increases and the counter lung collapses against the valve actuator. Fully charged, in standard configuration, the unit weighs approx. 47lbs and has a recommended scrubber duration of five hours.

A typical inhalation cycle (see diagram below) has the diver inhaling from the right (inhalation) lung, through the mouthpiece assembly into the left (exhalation) lung. The exhaled gas will then pass up over the diver’s left shoulder and into the radial scrubber canister where it will travel down the inner tube and diffuse radially out through the absorbent to the edge of the clear bucket and condensing face. The clean gas will then pass back into the right lung to be rebreathed. During this final step the clean gas passes over the sensing faces of three galvanic sensors that analyze the oxygen content of the gas and report to the electronics.

Breathing Loop Diagram

The electronics take the readings from the three sensors and vote between the readings and the pre-selected set point. This information is then used to control the addition of oxygen as required into the breathing loop. The data is also displayed on the primary display.

Oxygen injection occurs into the expired gas manifold, prior to the scrubber, so that the oxygen is forced through the absorbent before entering the inhalation cycle, thus aiding gas mixing.

A manual oxygen override exists, mounted on the lower inside wall of the left (exhalation) lung and this may be used to fly the system manually, or to rapidly assist the breathing loop to regain set point after an accidental gas loss or mask clear.


Diluent is admitted directly into the loop as a result of increases in hydrostatic pressure. As the ambient pressure on the system increases causing the counterlungs to collapse, a combination automatic/manual diluent addition valve is activated which automatically adds sufficient gas to return the loop volume to ambient pressure. This combination valve is sited at the top of the right (inhalation) lung.

At all times during operation the diver has access to an ON/OFF system switch, the gas supply cylinders, integrated Air II open circuit bailout and manual override features of the unit.

Physical Characteristics

21 inches
17 inches
9 inches
Approx. 47lbs (fully charged)
Scrubber Type
Radial Flow
Scrubber Capacity
approx. 6lbs. of 6-12 Sodasorb
Scrubber Duration
Standard 4 hours (4.5C @ 1.35lt per min CO2, using 6-12/8-12)
4-8 mesh absorbent may be used for training purposes or shallower than 140fsw for a minimum of 2.5 hours or a maximum of 3 hours in cold water and moderate workloads)

Bottle Capacity (standard)
Oxygen:19 cu ft @ 3000 psig
Diluent: 19 cu ft @ 3000 psig
Optional: 13’s, 15’s, 17’s, 19’s, 30’s & 45’s.
Breathing Loop Capacity Approx. 8 litres (minus the diver)
Battery life
Approx. 40 hours @ 70ºF temperatures based upon a standard high quality alkaline such as Duracell Ultra.
3 proprietary galvanic’s – SMS202
Sensor Life:
Approx. 12 months in air or 100 hours dive time
Battery driven sequential LED’sSecondary:
Analogue gauge driven directly by the sensor output, with battery life and setpoint indicator

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