Here is the logic for the Shearwater HUD:
First, there are "bad" alarms and "good" alarms. For example a fire bell is a bad alarm.
It is bad because the absence of a ringing bell doesn't mean there is no fire.
It just means the alarm isn't ringing. The fire bell may not be ringing because the battery is dead,
the smoke detector isn't in the right place, the installer screwed up the installation,
there is a foreign object stuck in the ringer ........
It doesn't mean that everything is ok.
A good alarm is one where there is an obvious difference between the lack of function and the lack of an alarm.
A solid green light doesn't do that.
There are integrated and redundant HUDs too.
Integrated HUDs can alarm you for things like deco ceilings and distance from setpoint.
But they can't do that and be redundant too.
If you want redundant, then they have to calibrated separately and they can't display "deco"
information unless they have a separate deco computer with its own set of tissues, gases, etc.
And of course there is the HUD that just displays the setpoint.
This type is very useful for scootering, low visibility, filming, and manually maintaining setpoint.
The HUD that we did for the Meg was a good ppo2 display.
But it didn't do the alarm function. There was very little difference in the visual characteristics
of the light between a good situation and a bad situation.
The lights flashed at the same brightness and same period regardless of the value.
So for a 1.6 ppo2, it just did more flashes.
The spaces between flashes stays the same, it just takes longer to display them.
For higher or lower ppo2s it can take over 10 seconds for a sequence.
With the Shearwater HUD, we tried to find the best of all worlds. It displays the ppo2.
Since it uses three LEDs, it can display them faster. A typical 1.3 takes about 2 seconds to read.
But instead of a continuously varying display time, it's always 5 seconds.
It displays nothing in between. So as the ppo2 gets farther from 1.0 the light DENSITY gets higher.
So 0.80 is RR________ RR________ RR________
but 0.20 is RRRRRRRR__RRRRRRRR __RRRRRRRR__
And, if you are more than 0.50 away from 1.00, it turns up the power to the high intensity LEDs. So they get brighter.
At 0.20 you have three very bright red LEDs flashing just about continuously in the corner of your eye.
And there were a couple of unexpected benefits.
You don't have to count.
For me at least, if I engage my eyes to look, I don't need to actually engage my brain to count to 3.
It's hard to explain, because obviously some part of my brain is counting.
And a stray sensor jumps out at you.
All three flash three times, then one flashes once more.
It catches your attention when they don't all flash the same number of times.
So the Shearwater HUD is a ppo2 meter that is redundant and also has some use as an alarm. | 
