How inaccurate is a ProTrack when mounted in a helmet?

[grue 1]

I know the manual says it's less accurate when in a helmet/on the head vs on a leg, but how much less accurate are we talking, when it comes to recording speeds and such?

cavete terrae.

[jakee 1,254]

The max speed that comes up on the display and the peak highs and lows on a jump track graph will be pretty much useless, but the average speed readouts should still be ok.

That at least is my understanding.

Do you want to have an ideagasm?

[peek 20]

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I know the manual says it's less accurate when in a helmet/on the head vs on a leg, but how much less accurate are we talking, when it comes to recording speeds and such?

I hope you are not putting too much trust in any of the numbers!

Get the software that allows you to download and graph the data from the jump. The human mind is much better at figuring out what is going on than trusting the software in the unit, which makes assumptions.

Also, make sure you know what type of airspeed is being referred to, true vs. any other.

[pchapman 261]

Short answer: What Gary Peek said.

Long answer:

If you only work off the Protrack screen:
1. Don't use the maximum speed. It often picks up unrealistic spikes.
2. Use the First Half average speed, not the Overall average speed because the latter tends to include too much of the bottom end of the dive, which may include unrealistic spikes.
3. Use SAS speeds to better compare to others' data, and to one's own data from other days and dropzones.

To delve more into the data, get the Jumptrack software so one can see the graph to better understand what is likely real, and what isn't.

But there's still a big problem with the Jumptrack graphs: data averaging. The manual doesn't tell you that the data is smoothed by averaging over the previous six seconds. Some averaging is OK; the problem is that it uses the previous 6 seconds, rather than 3 previous, and 3 future. That means the speed graph result tends to be shifted about 3 seconds later than reality, which makes it very confusing to correlate any speed with an event that took place at a particular altitude.

To get around this averaging problem, you need to look at the raw data, and then average as you see fit. I prefer a 3 second centered average as the best compromize between jagged peaks in the quarter second data points, versus over smoothing from using too long a time period.

To see the original data and average as you see fit, either

a) buy the Paralog software which I understand gives one a lot more choice in how data is presented, or

b) export to a text file from Jumptrack, then load the text file into Excel to create one's own graphs.

Regarding spikes in speed:

I find that the Protrack, mounted on the outside of my Protec helmet, often records a many-second long acceleration to a very high speed, like 155 mph, when starting to turn and track away from a belly fly formation.

Or, body position during a freefly jump can also cause spikes. For example, during head down practice where I was doing simple turns, and transitions to and from a sit, the Protrack recorded a couple big swings in speed. Supposedly down to 95 mph, and 7 seconds later, 185 mph SAS. I think not.

When calculating the average speed of the dive (or the first or second half of that), the first 15 seconds of the dive is excluded. That's great, as it pretty much excludes the period before terminal is reached.

But at the bottom end, only 7 seconds is excluded. That's far too little these days with high breakoffs, long tracks, and long snivels before one is slow enough to have the Protrack record that one has deployed. So the Protrack's overall average speed tends to include stuff at the bottom end one isn't trying to measure, including any unrealistic speed spike on breakoff.

(I find the Protrack is fine for long sustained tracks, just not for fast transitions into a track from other activity.)

Even the SAS airspeed won't quite be right unless one gets into dealing with air mass temperatures and thus density. Temperatures can be entered into Jumptrack, but that's excessive for most applications. Just accept that the Protrack has to make some reasonable and not perfectly accurate assumptions about the atmosphere, to translate pressure changes into one's vertical speed.

So in some areas the Protrack on a helmet has its issues. Not all the issues will necessarily go away if mounted elsewhere.

In other areas the Protrack is pretty good. I certainly see the difference in speeds between tandems with light, medium, and heavy people. Or the difference between an RW jump that felt fast, average, or floaty. I can see these differences even when just using the First Half speed, rather then checking Jumptrack graphs after the weekend.

[LemmingBoy 0]

So what is more accurate, an altimaster III or a protrac inside a factory diver?

The reason I ask is because my altimaster constantly reads high (between 500-750 ft) when my protrac is going off. To make it more confusing, the needle always reads low at anything above 10,000 AGL (which in CO is ~15,500 MSL) and I have not been able to figure out where it begins to measure right again (but I trust it under canopy).

[IanHarrop 37]

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So what is more accurate, an altimaster III or a protrac inside a factory diver?

The reason I ask is because my altimaster constantly reads high (between 500-750 ft) when my protrac is going off. To make it more confusing, the needle always reads low at anything above 10,000 AGL (which in CO is ~15,500 MSL) and I have not been able to figure out where it begins to measure right again (but I trust it under canopy).

With an altimeter that constantly reads high one of the things you can do to correct the problem is to send it into Alti-2 to be re-calibrated. Here's a link to the page thats tells you how. In my experience Alti-2 service is excellent.

http://www.alti-2.com/repairs.htm

"Where troubles melt like lemon drops, away above the chimney tops, that's where you'll find me" Dorothy

[tetra316 0]

I had mine say I did 330mph on a tandem once

[LemmingBoy 0]

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With an altimeter that constantly reads high one of the things you can do to correct the problem is to send it into Alti-2 to be re-calibrated. Here's a link to the page thats tells you how. In my experience Alti-2 service is excellent.

http://www.alti-2.com/repairs.htm

I guess I should have been more specific, the altimaster always reads high compared to the protrac. I haven't been able to compare them to any other altimeters outside the plane. It boils down to the "man with two watches never knows exactly what time it is" syndrome- never knowing which one to trust. I was wondering if there was a general rule about how accurate one would be over the other.

Thanks for the tip though on the repairs. I guess there's a reason for the warranty.

[jurgencamps 0]

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I know the manual says it's less accurate when in a helmet/on the head vs on a leg, but how much less accurate are we talking, when it comes to recording speeds and such?

Optima, protrack, neptune, viso ... are accurate in measuring altitude, even when mounted in your helmet.
If you want to measure your speed on the other hand (for speeddiving), whatever device you use, don't put it in your helmet. See your manual for the explanation.

[labrys 0]

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Optima, protrack, neptune, viso ... are accurate in measuring altitude, even when mounted in your helmet.
If you want to measure your speed on the other hand (for speeddiving), whatever device you use, don't put it in your helmet. See your manual for the explanation.

Can you explain this in more detail? I don't see anything in my Neptune manual that explains why it can't measure speed correctly when it's in a helmet.

If speed is measured as a change in altitude over time, then the only things that could cause inaccurate speed measurements are incorrect altitude readings or incorrect time measurement. Assuming that having the thing in a helmet won't affect it's timing, the only thing left that could cause incorrect speed measurements is incorrect altitude measurements.....

Owned by Remi #?

[jurgencamps 0]

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Can you explain this in more detail? I don't see anything in my Neptune manual that explains why it can't measure speed correctly when it's in a helmet.

http://www.l-and-b.dk/pdf/english_protrack.pdf, see pg 15.

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If speed is measured as a change in altitude over time, then the only things that could cause inaccurate speed measurements are incorrect altitude readings or incorrect time measurement. Assuming that having the thing in a helmet won't affect it's timing, the only thing left that could cause incorrect speed measurements is incorrect altitude measurements.....

Your reasoning about speed is wrong. My english is not good enough to explain this, but just read the remarks in this thread about the "spikes".
If you put the audible in your helmet and you turn your head, you disturb the airflow around your head and you will get those inaccurate "speedreadings" because the air pressure changes (whatever device you use). And the only thing those devices measure = air pressure.

Maybe it is not in the manual of your neptune, because nobody uses a neptune to measure his max speed during a speeddive.

Jurgen