Correct, which is why I don't understand the need for redundant altimeter setups. My 0.01% DD failure rate is due to things that a second altimeter would not fix (bad burrito wrap, shock cord mishap, poor battery holders, etc). Do it right, and one altimeter is all you need for basic flights. (I am not counting extreme projects, here.) Commercial ematches are also very reliable. Motor backup is good enough for me. Prepping one altimeter/ebay/payload is enough work as it is. I am not gonna do it twice!
Some issues a 2nd altimeter will alleviate:
Battery connection gone bad, vibration,wire comes loose,[ overzealous charge may cause at apogee], now you got nada for main.
bad ejection starter that passes continuity check
starter bridge wire burns without lighting pyrogen
BP fails to ignite.....it does happen! [to me twice...now that's a weird one]
Failure of one channel on other altimeter.
Switch bounce or disconnect on 1.
This happens to be the flight your altimeter decides to go haywire and cease working.
Some strange anomaly happens to one, but not the other.....[good reason for using 2 brands]
Allows for a larger back-up charge, first one fires NC or chute hangs & second charge pushes them out
Just a few things I have personally witnessed as failures.
Some of these can be handled with just one altimeter by:
using 2 starters per terminal so you have 2 in each charge.
using second set of pyro's for back up when altimeter has 4 pyro hook ups. [Not all altimeters have this capability]
Some you need a totally independent system to do. Wiring problem-failure of unit-battery issues
I agree with JD 100%. All of the DD's I've seen fail as been user error. Take your time, don't get distracted, and double check everything.
I probably have used way more units than most...over 100 by now. Currently 17 altimeters are mounted in various rockets.
I like having 5 rockets ready to fly and that's 8 units right there. 3 flight will use 2 on sled for 3in and above. 2 flight will use 1 @ sled for 38mm & 54mm airframes.
So far I have had these flights which I would not call user error:
3 G-whiz altimeters total fail during flight...landed beeping out ready for flight readings. They were returned and never fixed properly....why you have so many bad opinions about them. Go through here searching for the numerous threads on that.
2 Ravens: 1 fire charge one power up for staging pyro [igniter NOT in motor thank god.] New...never flown.
1 in a 2-stage total failure....beeping ready for flight upon landing. Flown several times before, new one sent, issue never determined
1 Missleworks old version, flown 55 times [used to keep a log] Beeped fine on pad, failed in flight. Sent in for repair, but BP residue had corroded board too badly. I no longer store altimeters on sled in rocket with out removing fired charges and cleaning tubes/bulkplates.
I suppose this one could be called user error. But it failed in flight after testing on ground. A second one would have prevented failure, which is being discussed here.
2 R-Das....tough call here. Both flew fine in different rockets had bad/hard landings, then flew fine for several other flights before failing in flight. Yes they passed the self test mode. So possible damage showing up down the road. 2nd back-up saved the day on both flights. Also returned/repaired and flew fine.
User error....not really, just anomalies that do happen where 2nd altimeter saved the day:
Battery box broke loose from wood sled at apogee from large charge....showing charge pulled -55G's...wood screws pulled from sled. I now use only bolts.
Wire ties holding battery on N-10,000 flight let go, battery clip wires broke.
Switch [key type] lost contact. Use twist & tape exclusively since.
I'm sure others have stories too. On the other hand, many fly just one altimeter and have had no issues.