Quote: (Originally Posted by
silent running)
But remember-and for anybody who doesn't know-the Prism HUD doesn't just tell you everything is OK. It tells you if any sensors have been voted out and if the battery is going, in addition to how high or low the PO2 is off SP. But if you've got the analoge PO2 gage, that info is available through it or not relevent.
It was this point that I was thinking about actually, and I'm not as convinced anymore.
Say I calibrated the Prism electronics in air rather than in 100%. (ie the calibrate button was accidentally pushed with the bucket off) The unit would try to hold .14 at the surface, and if the loop was at .14, the HUD would be indicating 'on setpoint' with no warnings active at all. So whilst the Prism HUD does give you PPO2 indication, it is giving you exactly the same information that it is running the solenoid off. So if there is a problem that means the electronics are acting on erroneous information that they think is correct, the HUD will not alert you to this.
(Example: a MK15.5 I saw recently had a fault where the electronics continually thought the unit was below setpoint. It would show the blue hypoxic light, and fire the solenoid, even though the PPO2 was 2+. The primary display was just displaying the same faulty information that was being used to drive the solenoid)
So I've decided that the 'primary' display used should be one that is completely independant of the electronics that are driving the solenoid. It's really the only way to verify that what the electronics think they are doing is what you actually want them to be doing.
Not sure there is a unit out there that has this though?
Units where both means of reading PPO2 is driven by one set of electronics don't give you a means for verification at all. I can now see why adding a 4th cell is so popular.
On the Prism, I had been running it off the HUD, and just checking the analog guage every now and then. Running with the electronics on and the HUD not working made me realise that hearing the solenoid fire at regular intervals was giving me exactly the same information as the HUD did - ie that the electronics throught that the loop was on setpoint. This made me think about how much I should be trusting the HUD rather than verifying with the analog guage.
(Note: the Prism HUD does give a bunch more info about any problems that occur, but during normal diving all it is displaying is a single green 'on setpoint' light. It's only this component of it's functionality that I'm considering)
So - anyone want to make a unit that has a digital readout of what the electronics think is going on as a secondary, plus a completely independant HUD that gives your PPO2 in analog form and can be used to fly the unit with the primary electronics turned off?
Mike
PS - Dave, I know we've beaten it to death and still disagree. Hope you don't mind that when I see you say that correct dil is essentual, if I counter with an alternate view that correct protocol can work for any dil. Let folks consider the alternatives, rather than accept one idea as gospel.
And if it means I'd not be welcome on your boat... oh well, I'm sure I can cope.
(FWIW, I think correct dil is a good idea, but I think that always using a protocol that will work for any dil is more important. And it saves having to dump dil)