Alti Lag?

[Geoff 0]

Has any scientific study been done of the phenomenon of mechanical altimeters 'lagging' the true altitude during freefall?
I notice on the way up that if I tap my alti, it jumps upwards by a couple of hundred feet. I presume this is due to the friction in the mechanism slowing the progress of the needle, and the tapping 'breaks' the friction and allows the needle to catch up. But on the way down in freefall, you're moving much faster, so I would expect the 'lag' to be greater - at least a few hundred feet.
An experiment could fairly easily be done by comparing a video of an alti in freefall with data from (say) a Cypres data recorder, as long as you could synchronise the exit time exactly.
Anybody tried this?
Geoff

[indyz 1]

There is a potential problem in that the Cypres' sensor is in the burble while the alti is in cleaner air on the arm. This could possibly be solved by wearing an analog altimeter and a Digitude on the same arm and just videoing them side-by-side. Of course, the true test would be comparing the readings of a mechanical altimeter with another altimeter that is not based on air pressure (GPS?).
--
Brian
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[billvon 3,096]

>Has any scientific study been done of the phenomenon of mechanical altimeters 'lagging' the true altitude during freefall?
Thousands of hours worth, since aircraft altimeters work basically the same way.
>I notice on the way up that if I tap my alti, it jumps upwards by a couple of hundred feet. I presume this is due to the friction in the mechanism slowing the progress of the needle . . .
And also the time it takes air to enter/exit the bellows or Bourdon tube. The size of the hole sets both the lag and how "jumpy" the altimeter is.
>and the tapping 'breaks' the friction and allows the needle to catch up. But on the way down in freefall, you're moving much faster, so I would expect the 'lag' to be greater - at least a few hundred feet.
It's also vibrating in the wind, and that will help with friction. The faster you are descending, the more a small hole causes the alti to lag.
>An experiment could fairly easily be done by comparing a video of an alti in freefall with data from (say) a Cypres
>data recorder, as long as you could synchronise the exit time exactly.
About eight years ago, Scott Smith and I were looking into the next version of Scott's altimeter, with a possible upgrade to an AAD. During the course of the investigation we looked at the Cypres pressure sensor. It's awful - much less accurate than an Alti II, for example. If you wanted to do this right you could just get a calibrated aircraft altimeter, mount it next to the altimeter under test, and jump with them both (with video if desired.)
-bill von

[Geoff 0]

Thanks Bill, that's interesting - I guess the next question is this:
By how much will an alti typically lag during freefall (say at 120mph in the first instance)? and is the lag itself a function of altitude?
Frankly I don't know how much notice to take of people precisely asserting "I pulled my reserve at 900 feet" or "my canopy always takes 600-700ft to deploy", etc... My suspicion is that everyone thinks their alti is a lot more accurate than it actually is.
Anybody know?
Geoff

[nacmacfeegle 0]

Interesting thread folks...
Instrument lag is gonna vary between maunufacturers, models, dust/shit, wear etc inside the instrument, whether it has jewelled bearings, and air temp/pressure etc, If you are worried about your alti, give Doddington's (I think) a call, they used to advertise in the UK mag, but I haven't seen the mag for a while.
Another thing to consider is the fact that there will always be an element of paralax error with any visual analogue device. ie you may not be looking at it from an ideal angle. Take a good look at your alti, can you really accurately discern the difference between say 3200, and 3300 ft? Now tilt it around a bit....
I would estimate that people reading their alti (on a regular jump, quick glance scenario) may mis read it by up to +/- 150 feet, which kind of out weighs all this lag, and accuracy stuff...
Have there ever been any comparaitve tests done under controlled conditions, say with a vaccuum chamber? And are the results published anywhere....
D

[markbaur 0]

Quote

the alti is in cleaner air on the arm

Your hand has a burble as well. It's most obvious with a Digitude: read with the alti on top of your hand, then rotate your hand so the alti is in cleaner air.
The Digitude mount works best with the loop around your thumb, so the instrument is in relatively clean air. Think they're on to something?
Mark

[billvon 3,096]

>By how much will an alti typically lag during freefall (say at 120mph in the first instance)? and is the lag itself a function of altitude?
I don't think it's a function of altitude. My two "worst" altimeters (i.e. the one that read highest at pull time and the one that read lowest) seemed to be about 500 feet apart, but I don't know if that's lag or just error.
>My suspicion is that everyone thinks their alti is a lot more accurate than it actually is.
You got that right. I have to laugh when people are in the plane saying "OK, I'll pull at 3000 and you pull a little below 3500, so we're clear of each other." Every little bit helps, I guess, but when you add up "real" altimeter error (i.e. altimeter in burble,) instrument error, parallax error, reaction time, and difference in canopy opening times - I'd be amazed if both people could really be open within even 400 feet of where they plan to be.
-bill von