The Shearwater HUD (SW) and constant/integrated PO2 Trimix Dive Computer are manufactured by Bruce Partridge in Vancouver BC. Bruce is great to work with and provides superior customer support.
The Shearwater (SW) HUD is calibrated with oxygen. This is easily achieved by filling the loop on a KISS Classic with oxygen (with proper technique of course). Once the loop is at 1.00 atm partial pressure of oxygen then the HUD can be calibrated.
To calibrate the SW HUD, simply turn it on with one button push then push the same button three times in a second. All three red LED lights will stay on for 5 seconds and then the three LEDs will turn amber confirming the calibration. It is important to know that the loop must be at 1.00 atm of oxygen. So if the user is at altitude then it is best to calibrate at sea level before the trip to altitude and hope you do not need to change a cell.
The SW HUD uses three colors, starting with amber meaning a PO2 of 1.0 atm. Each green flash will mean 0.1 atm above 1.0, so two green flashes means a PO2 of 1.2 atm, three green flashes is 1.3 atm and so on. Red flashes work in the opposite direction and subtract a PO2 of 0.1 atm per flash from 1.0 atm. One red flash will mean a PO2 of 0.9 atm, two red flashes will be 0.8atm and so on.
The SW HUD has three LEDs displaying the PO2 of each cell at the same time. This is a functional concept when comparing to a one LED system that scrolls through all three cells. The three LEDs mean much less flashing as compared to the design where one LED is scrolling through all three cells. This allows the diver to become more in-tune with "lots of flashing means a problem", and will draw attention immediately. Also the three LEDs flash each cells PO2 together. So, if one cell is different it will not flash the same amount once again bringing immediate attention to the problem.
In my opinion, this design concept gives the diver a comparison right in front of their eye that is very difficult to miss or “tune out", and will help the diver maintain a more strict control over their PO2.
The SW HUD is to augment PO2 monitoring not replace it. Also the SW HUD can offer redundancy to a single display system in certain applications. This redundancy will depend on how the HUD is installed and what the actual failure in the system is which could require a diver to go to the redundant / backup system. This is best discussed with the CCR manufactures and the SW HUD manufacture.
The SW HUD can be used on most other CCRs. I have seen it used also on a Megalodon.
On a Megalodon the best way to calibrate the HUD is to use the head caps to fill the head only with oxygen. Assuming you do not want to use the head caps then filling the megalodon loop with oxygen would work to, but this is much more wasteful on the oxygen supply and perhaps less accurate (once again this is determined by user technique).
Like on the KISS, it is important to know that the loop must be at 1.00 atm of oxygen. So if the user is at altitude then it is best to calibrate at sea level before the trip to altitude and hope you do not need to change a cell.
A more controversial way to calibrate the HUD at altitude on a COPIS Meg (and I am NOT TELLING you to do this) is to first set the COPIS PO2 displays at the appropriate PO2 for the altitude using conversion charts. Then flow oxygen into the head until the COPIS display reads as high as it will go. Now for the controversial and tricky part. The out flow tube from the head calibration kit needs to be gently squeezed until the COPIS PO2 display reads 1.00 atm, now, you calibrate the SW HUD. The reason for this is the SW HUD needs to be calibrated 1.00 atm of oxygen. At altitude you cannot achieve this PO2 on land. So if you set the COPIS display PO2 correctly according to the tables then you can be fairly certain by increasing the backpressure into the head and bringing the PO2 to a 1.00 atm that the SW HUD will be correctly calibrated. This is a tricky procedure as you need to not only need to keep pressure on the outflow tube (so the PO2 reads 1.00 atm) but also push the SW HUD button 3 times in a row in less than a second! So like I said if at all possible calibrate at sea level before an altitude dive, this is a much safer way to calibrate.
Overall I am extremely happy with my Shearwater HUD:) .
I can honestly say it really does make cell differences and a problem stand out. It also does draw your attention to "lots of flashing" since it does not flash lots when the diver is in a normal PO2 range.
The Shearwater HUD is another great tool the CCR diver has available to them. It can help you maintain a more constant desired PO2 and increase diver safety.