pan y agua
Blackburn Nuero 6 vs Garmin
Put a Blackburn Nuero 6 on our new tandem. It's wireless and had about all the features I could imagine, short of power. (which short of 2 SRMs, there doesn't seem to be a workable tandem answer.)
So far it works great. No problem picking up from the back of the tandem.
The one major issue is the accuracy of the altimeter function. On rides around Jackonsivlle, the total climbing seems too high. (such as 1000 vertical in 65 miles.)
So I tested it against a Garmin vista HCX. For the same ride the Garmin gave total ascent at 176 feet,and the Blackburn at 900.
Believe it or not, Jacksonville is not totally flat, and there are areas where you're going up and down on repeated 30-40 foot rises. My theory is that the Garmin is smoothing out and not even registering small elevation changes, while the Blackburn is accumulating a bunch of small elevation changes.
What's the sampling rate for each device?
Not familiar with the Garmin Vista but the 305 registers every single foot or fraction thereof... I would guess the average elevation gain going over an overpass is about 30' so... even for Jacksonville to only gain 176' in 65 miles seems very low to me.
Going from memory, I believe my CicloSport HAC4 has something like a 15 second sampling rate and 10' elevation sensitivity, whereas Debbie's Polar S720i could be user-set to something as low as 2 seconds with the same elevation threshold. We also have an Avocet Vertech which updates continuously with perhaps 5' or even 1' elevation changes being recorded.
As you'd expect, the Avocet was the most accurate, followed closely the the Polar when it was set to a 5 second sampling rate, and the HAC4 would always be 10% - 15% off what was being recorded by the Avocet. Also, because all three devices used barometric pressure, if a weather front came through during a ride that too would throw off all three units. However, even more interesting was the additional differences you'd find when you compared the HAC4's real-time watch displayed data to what you ended up with after downloading to your computer. Interestingly enough, the downloaded data was different and again, going from memory, I believe I read somewhere that the software on the computer looked at more data points that were simply stored instead of processed by the watch as a memory saving feature.
Anyway, it would be interesting to know how the sampling rates and sensitivity compare as I suspect the answer to your question may be found there.
The Garmin 705 defaults to "smart" sampling, which means it stores a reading when "data" changes. Not sure if that's just when HR numbers change or speed or elevation or what. My 705 seems to take a reading every 3 or 4 seconds. You can also set it to record data every second. I don't think it would make much difference to the total elevation numbers. I've never compared the data from it to any other device.
I use the free Sporttracks software to analyze my rides and it has a nice feature where you can classify climbing zones. This lets you track the total elevation gain when you were truly climbing (i.e. more than 2% grade or whatever) rather than taking credit for every 2 foot bobble in the pavement.
pan y agua
I'm trying to figure out the increments that register on each device. The Nuero's owner's manual doesn't tell you its sensitivity, or whether it can be user programed (but the manual does come in Slovensky,as well as Suomi, so I guess you can't cover everything in 13 seperate languages)
Watching the total ascent increase as you ride it appears to update in increments as small as a few feet, so it would appear that it registers changes smaller than 10 feet.
I believe the Vista HCX takes a change of 10 feet to register, if I'm reading the manual correctly.