Reply from a recent Vigil tech support query.

[lucianaofly 0]

i am not a phisician,and i have 9 vigil units on my rental gears,i am just look to the pressure indicate in each vigil when i begin the skydiving activity...only one unit(vigil 1 ) had a difference of 2 mbar between all the units

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(10 mbar is required to need a maintenance)all unit shows the same temperature (stored in the same locker room).that precision is enough to me.

lucianao

[theonlyski 3]

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As the vigil manual tells us:
"3.5.5. «INFO» Menu (Information)
This menu allows you to display your Vigil
®’s reference parameters (version, date of manufacture and serial number), data of previous jumps, as well as temperature and atmospheric pressure. "

Reading/storing the data is different than using it for purposes of measurement. You're basically measuring the temp of the reserve packtray, which while packed, is quite a bit like a cooler... and those canopies are acting as insulators.

"I may be a dirty pirate hooker...but I'm not about to go stand on the corner." iluvtofly
DPH -7, TDS 578, Muff 5153, SCR 14890
I'm an asshole, and I approve this message

[Abedy 0]

@Riggerpaul: Yeah, you put what I thought pretty much in a nutshell. :-)
Apart from that, I also can go with Dave's thoughts: It's highly important that the AAD keeps track of the pressure at ground level to have a "zero level". If its reading is slightly different from what a calibrated barometer would tell you, that shouldn't be that much a problem since in those altitudes above sea level it's just sort of a parallel shift along the barometric curve that is almost linear in these regions.

When we were at Airtec we were shown several examples of how they measured the air pressure with the first gadgets. One was really nice to look at: About a dozen A4 papers (with a millimeter grid on it) that showed the measured air pressure against the time after leaving the plane. Some 1991 stuff, when Amiga, Atari Falcon etc were still considered high end computers :-)

As for the thoughts about flatflying vs. backflying etc. I asked Helmut Cloth about the problems. He showed me some readings plotted on millimeter grid paper when test jumpers did somersaults etc. Woohoo... that look like the Alps or a bizarre fever curve ;-) Question: How to deal with this problem. I 'membered problems like that when I did my studies in Astronomy (reading out brightness of stars close to the background "noise" of grains and stuff on photographic plates) and asked whether he did FFT and low-pass filtering etc. "Sort of" he answered, but that's not 'nuff, much more sophisticated algorithms needed and you gotta implement them in ASM ;-)

The sky is not the limit. The ground is.

The Society of Skydiving Ducks

[pchapman 261]

Temperature sensors

I'll bet that a temperature sensor inside the case would be useless for adjusting the lapse rate curve. The sensor would only be there to calibrate the pressure sensor or some such thing. (ie, if the voltage output of the pressure sensor varied when the temperature of the chip changed).

To create an accurate lapse rate of pressure vs. altitude, from some standard graph under standard conditions (eg, some form of the International Standard Atmosphere), one needs to know the average air mass temperature from the measuring altitude down to the ground. In practice, just knowing the temperature of the air at the altitude where the temperature sensor is, will give a rough approximation. Pilots may do that when calculating true airspeed for example.

A temperature sensor deep in the reserve tray, inside a box, is probably of little use for adjusting the pressure lapse rate curve. Especially because there will be plenty of times that its temperature will have little relation to outside air temperature -- maybe the rig was in a hot car or in a cool hangar just before the jump.

Pressure lapse rates

There is indeed some error introduced if an altitude "measuring" device doesn't know where its starting altitude is. The pressure change that at sea level gives an 800 ft jump in altitude, would give 930 ft if starting from 5000 ft.

I'll be using 800 ft in my examples as a hypothetical CY-Vigus AAD's firing altitude.

(FWIW, I'm using one particular pressure lapse rate formula, p [inHg] = 29.9212 * (1- (alt [ft]/145442.1)^5.25588. I don't have its source at hand but my old notes show this came from my search of the aeronautical literature as a decent formula.)

But, AAD's do know their starting pressure on the ground, so they have a starting point. They don't need to know the true altitude at the airfield, just the pressure. Whether they calculate a curve or use a lookup table, I would guess (and hope) they don't use a linear relationship between pressure and altitude.

For AADs they only typically need to be extremely accurate over a short distance, such as 800 ft off the ground, so a particular percentage error in a calcuation won't be a big deal.

But absolute pressure errors in the sensor do still count for something. So lets say the sensor in a Cypres is off calibration, or it is off in a Vigil and the owner doesn't actually go and compare the pressure against some calibrated barometer or aircraft altimeter.

(It's a nice to have feature, but in practice you don't hear of anyone doing it. Lucianofly gave a good if rare example of someone with a fleet of Vigils actually comparing the pressures shown on them all, to help detect an anomaly!)

Anyway, say the AAD is at sea level and is supposed to fire at 800 ft, so (by my calculation) it is looking for a .85455 inHg reduction in pressure. But say that the sensor is off by 2% in its accuracy, which (in one direction) would make it think it is starting at 557 ft. When it sees a .85455 inHg loss of pressure, and thinks the AAD is 800 ft over the ground, it turns out to actually be at 813 ft above the ground.

Not too bad actually, for an error coming from a 2% sensor error. (The error in height, 813 vs 800, is only 1.69%).

BUT, if the AAD were more sophisticated, it could do much better:

If the AAD looked for a certain percentage decrease in pressure, rather than a certain absolute decrease in pressure, the error would be much less. (Pressure after all does decay in a non linear way, so a ratio is a much better approximation.) So instead of looking for
.85455 inHg pressure decrease, it would be looking for a pressure that is 97.147% of the original. Then my calculation shows it will fire at 796.9 ft, rather than 800ft. That's an improvement, off by about 3 ft not 13 ft. In that way, even a 2% error in the sensor gives only a 0.4% error in firing altitude.

Who knows if AADs are smart enough to behave this way. An engineer should easily see the principle; but the answer depends on the computing power available.

(These errors will change with altitude, so using the sea level to 800 ft case is just an example.)


Temperature errors

Now there's another confounding variable, the temperature of the air over the height being measured. I'm not sure of the correct calculations off hand, but I can use an approximation.

The error in altitude is approximately 4 ft per 1000ft of height above a known reference point (eg, airfield), per degree Celsius away from the International Standard Atmosphere temperature (for that particular altitude). Colder air produces lower true altitudes than measured from the pressure.

Example 1: Sea level DZ, -5 C in winter, ISA temperature at that altitude is set at 15C, so the temp is 20 C low, with the AAD trying to fire at 800 ft. The error would be 800/1000*20*4 = 64 ft. The AAD will fire about 64 ft low.

Example 2 would be Denver in summer. Say 5000' elevation, where ISA should be 15- 1.98*5000/1000 = 5.1C. If the actual summer temp is 20 more than that, or about 25 C, the approximate calculation will this time give 64 ft high.

Using a pilot's circular slide rule to calculate corrected altitude from pressure altitude, should give a more accurate answer, as it will provide for calcuations that are not linear but ratio based (or logarithmic). The slide rule suggests the error is more like 55 ft.

So were talking about firing 55 ft high for an 800 ft firing altitude, or about 7% high.

All this shows that temperature errors, over ranges that are wide but are encountered in practice, can cause some significant AAD errors, although not huge amounts. I kind of doubt that AADs have any way of compensating for this, but I just don't know.


Conclusions?

Hard to tell since we don't know how AADs actually calculate things. But all this gives some idea of how big errors in firing altitude might be, when an AAD is being fooled by atmospheric conditions other than some idealized situation, or due to errors in the pressure sensor.

It isn't that Cypres' couldn't implement some sort of pressure readout, as Military Cypres' do that. (Just guessing that it is probably the altimeter setting readout, which is what the pressure would be at sea level, rather than the pressure actually seen at the moment.)

AAD's also have to calculate vertical speed, but at least by looking at height alone, we can see some of the issues in AAD errors.



Algorithms

Abedy wrote:

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asked whether he did FFT and low-pass filtering etc. "Sort of" he answered, but that's not 'nuff, much more sophisticated algorithms needed and you gotta implement them in ASM ;-)

Ah, that's all the Airtec secret stuff!

[PiLFy 0]

Kenny,

Yes, you can check the pressure on the unit.

When turning on the vigil, after the unit completes its self test, enter into the " info" section (#2 of the three sections of menu) then look at # 12 inside that section, just after the temperature...

You will see the barometric pressure from the sensor, either in inHg (inches of mercury ) or hPa (hectopascal) . Either because it depends which setting you have the unit set to under your "config" setting , ( US or metric.)

The Vigil 2 manual notes directions to the on page 29:

Switch on the Vigil II to ensure that it is functioning correctly (see § 3.5). (Make sure the atmospheric
pressure shown in the menu is ± equal (up to ± 10 hPa) to the local pressure measured with a
calibrated barometer)

You can also find you local pressure for the day on the web at : http://www.wunderground.com

Hope this helps...

Vladi

OK, I see that I'm a day late, & a dollar short w/the above reply... I think I understand well enough the discussions regarding lapse rates, linear curves, & relative vs. actual pressures. I'm meticulous about this sort of thing. I like to be able to verify that a lifesaving piece of equipment is spot-on. I freely admit that I haven't done my homework on this, yet. I'm not planning on buying either in the near future. I'm dependent on whatever is in the rental rig I use.

I'm not sure if this would make enough of a difference or not. Both Cypres & Vigil calibrate when turned on, unless turned off @some point. That morning calibration is it for the day, right? What happens if a storm front comes through later in the day? Will the low pressure system cause enough of a change to be a problem? I often have to re-zero my alti during takeoff.

You mention more important differences between the two. I know they function a bit differently. Do you see a serious flaw w/one as opposed to the other? Only in certain situations? Please expand on that.

[PiLFy 0]

"I'd rather have a CYPRES than a Vigil" Why?


"I'd rather have an Argus than a Vigil too, but that is a different discussion." An Argus?! Why?

[NovaTTT 1]

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"I'd rather have a CYPRES than a Vigil" Why?


"I'd rather have an Argus than a Vigil too, but that is a different discussion." An Argus?! Why?

Not going to speak for Paul, but I'm thinking that it's because the Vigil has a firing thresh-hold that some, myself included, find intolerable.

In general terms and avoiding & in no reference whatsoever to the Argus Situation, an AAD that doesn't know or isn't sure about what's happening should do nothing whereas in that same situation the Vigil chooses to fire.

CYPRES seems to have the features and parameters most AAD users are comfortable with.

Just my thoughts on this narrow point in this broader converstation.

N

"Even in a world where perfection is unattainable, there's still a difference between excellence and mediocrity." Gary73

[billvon 2,400]

>The sensor would only be there to calibrate the pressure sensor or some
>such thing. (ie, if the voltage output of the pressure sensor varied when
>the temperature of the chip changed).

That's what it is there for. Silicon pressure sensors are temperature dependent and must be compensated by measuring the temperature. Many pressure sensors integrate a temp sensor as well.

[LouDiamond 1]

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It isn't that Cypres' couldn't implement some sort of pressure readout, as Military Cypres' do that. (Just guessing that it is probably the altimeter setting readout, which is what the pressure would be at sea level, rather than the pressure actually seen at the moment.)

I was going to mention this but you beat me to it. The Military Cypres has had this feature for several years(since it was introduced). I won't attempt to explain the reason for it's use here as it's a bit off topic and it does require a good deal of explaining if you are unfamiliar with why it is useful in a military application. For that, I will defer to the Military Cypres owners manual which should be on their website for those who wish to know more.

So, in getting back to the meat of the question/discussion at hand, The (civilian)Cypres does this same step but it just doesn't display it as does the Vigil.

"It's just skydiving..additional drama is not required"
Some people dream about flying, I live my dream
SKYMONKEY PUBLISHING

[Nelyubin 0]

AAD pressure sensors are temperature compensated.
Cypres pressure sensor no has temperature compensation is below 0 degrees Celsius. Cypres pressure sensor no can be stored at temperatures below -40 degrees Celsius. For cold countries, this is a significant factor.

[riggerpaul 1]

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>The sensor would only be there to calibrate the pressure sensor or some
>such thing. (ie, if the voltage output of the pressure sensor varied when
>the temperature of the chip changed).

That's what it is there for. Silicon pressure sensors are temperature dependent and must be compensated by measuring the temperature. Many pressure sensors integrate a temp sensor as well.

Will it matter that, being buried inside the reserve container, it won't always measure the ambient temp?

[pchapman 261]

The point is that the temperature sensor measures the temperature at or near the pressure sensor, as part of the process of measuring pressure accurately. It is about the temperature of the pressure sensor, not of the air outside. That's a separate issue of trying to convert a pressure into an accurate measure of altitude.

=======

Regarding Nelyubin's post:

Cypres 2 manual: storage +71 to -25 C, working +63 to -20 C

Vigil II manual: working temperature +71 to -25 C (with this listed as the range in which the battery pack is designed to operate)

Therefore both are virtually identical. Any differences are likely due to company decisions on how conservative to be in their recommendations, and not any underlying differences in design.

So for neither AAD should you leave the rig in an unheated building in the middle of the Russian winter.

Presumably someone from a cold climate will have asked an AAD company about such events, as it will have happened, and I doubt anyone advised them to throw out their AAD.

(Although you could make the point that at least with the Vigil, you can go check the unit temperature on screen in the morning.)

P.S.:
Argus manual
"Storage temperature: -40° to +80° Celsius Working temperature (inside reserve container): -30° to +70° Celsius "

That's a little wider range. But without any other evidence I'm not convinced it is built fundamentally any differently.

[piisfish 135]

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Cypres 2 manual: storage +71 to -25 C, working +63 to -20 C

Vigil II manual: working temperature +71 to -25 C (with this listed as the range in which the battery pack is designed to operate)

Argus manual
"Storage temperature: -40° to +80° Celsius Working temperature (inside reserve container): -30° to +70° Celsius " .

what was the AAD used in the wingsuit project in Antarctica ?

scissors beat paper, paper beat rock, rock beat wingsuit - KarlM

[billvon 2,400]

>what was the AAD used in the wingsuit project in Antarctica ?

IIRC, the only AAD that I know _worked_ down there was the Michael Kearns save in 1997. Have there been any others?

[Nelyubin 0]

Cypres Pressure Sensors:
http://www.allsensors.com/datasheets/J2/MilliMed/Millivolt%20Output%20H-Grade.pdf

[piisfish 135]

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Cypres Pressure Sensors:
http://www.allsensors.com/datasheets/J2/MilliMed/Millivolt%20Output%20H-Grade.pdf

I guess you just voided the lifetime warranty on your Cypres

Did you find a steelball in it ?

scissors beat paper, paper beat rock, rock beat wingsuit - KarlM

[Nelyubin 0]

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Did you find a steelball in it ?

And should it be?

[USPA 0]

Does that jumper setting change the setting from expert to student etc?

The trouble with skydiving; If you stink at it and continue to jump, you'll die. If you're good at it and continue to jump, you'll see a lot of friends die...

[Nelyubin 0]

Cypres does not offer such opportunities. Producer can do. Free.

[USPA 0]

Let me rephrase the question. Did you find out what the jumper setting does do?

The trouble with skydiving; If you stink at it and continue to jump, you'll die. If you're good at it and continue to jump, you'll see a lot of friends die...

[Nelyubin 0]

I did not set such a goal. need read the program unit.
To hold a series of experiments.

[PiLFy 0]

12.5 year old Birthday Party?

[Nelyubin 0]

Yes