The following is a copy of the response from Airtec (the manufacturer of CYPRES) to some questions and comments fielded from the "rec.skydiving newsgroup" on Internet, concerning CYPRES accuracy and "misfire rumors". It is reprinted here to provide additional information for all CYPRES owners and riggers.
In regard to a recent Internet letter we want to clarify some issues by first quoting an article which has been published in the September 1992 issue of the U.S. "Skydiving" magazine:
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"...what seems like 750 ft. to a CYPRES (or any AAD that measures altitude by sensing air pressure) may not really be 750 ft.
An altimeter doesn't actually measure altitude, it measures air pressure. It reads air pressure and converts that reading to altitude.
AADs have a tough job because the air pressure around a falling jumper varies from one location on his body to the next. If he's falling face to earth, air pressure is higher on his chest and lower on his back."
Helmut Cloth took this into account when he engineered the CYPRES. It is designed to be mounted inside a piggyback reserve container. In freefall, the air pressure here is about 300 ft. "higher" than ambient. (An altimeter mounted there would read 1300 ft. as the jumper fell through 1000 ft.) The CYPRES compensates for this when it monitors a jumper's descent.
The situation is complicated by the fact jumpers don't always fall in a stable face-to-earth position. If the jumper makes rolls, loops or tumbles uncontrollably, the lower pressure situation on his back can change to the true pressure situation - that corresponding to his altitude - or even to the higher pressure situation on his chest.
The CYPRES' computational powers play a vital role when a jumper is tumbling or rolling. Its software compensates for fluctuating readings to help prevent the device from firing too high or too low.
There are two ways a jumper can outfox the software and make a CYPRES activate high.
The first way is to fall a long time in a stable back-to-earth position. "This will definitely cause a higher activation altitude than 750 ft.," Helmut said. In this situation, the device is fooled into thinking the jumper is lower than he actually is because it is sensing higher pressure than normal.
The second way to trick a CYPRES is to deploy the main at a low altitude while simultaneously rotating to a head-high or head-low position.
This maneuver will cause the CYPRES to think altitude is decreasing rapidly because the low-pressure area on the jumper's back has been cleared.
The CYPRES uses its microprocessor to study the situation and try to determine if the sudden increase in pressure is the result of an attitude change or decreasing altitude.
If it has any doubt as to the jumper's altitude - remember, the CYPRES already knows the jumper is low - it fires the cutter to open the reserve container.
Even when combined with the device's operating accuracy, a "premature" opening created by these circumstances (a low pull while rotating vertically) would still be at about 1050 ft.
All pressure-sensing AADs are affected by where they are mounted and the position of the jumper in freefall. Short of equipping a jumper with some sort of vertical-seeking radar altimeter, there is no solution to the problem..."
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Second we want to share the December 1993 letter that we sent to the skydiver which was originally spoke of:
"...your CYPRES is here at the factory. Thank for the informative letter with it, attention to Cliff Schmucker at SSK.
We have done 40 test flights with the unit. So far it is working correct.
As you mentioned that you don't want this one back, we will take the time for a very intensive investigation. After working with it here, we will mount it in my (Helmut) or Gerard's rig and jump with it.
What we could imagine is that the activation happened at the very end of the deployment sequence of your main, while the vertical speed still just was high enough.
Our experience show that this happens more often than we like it. In a high percentage of these cases the involved skydivers believe it happened too high.
At the beginning that made us unsure and concerned. So we looked at these procedures and units with more than great effort. We found the devices working OK and obviously the altitude being correct.
On the other hand we had skydivers saying: "too high". Difficult for us.
We seriously were in doubt for a while. Later we learned very clear that - of course - everybody relies on his altimeter to detect the altitude.
When looking on the altimeter face and not looking completely vertical it leads to seeing not exactly what the needle shows. The "parallactic effect" makes one to believe something wrong.
Another fact is that an altimeter often doesn't indicate the true altitude. First it is imprecise (doesn't indicate what it should indicate at a certain height) because it is not linear and second it is delayed in changing altitudes. Especially after the vertical deceleration of a canopy opening it is slow to adjust.
In spite of all this skydivers insist on what they supposed to have seen on their altimeter. This is understandable because they know to detect altitude by an altimeter since their first jump, they have never used something else and they never seriously have heard someone saying a skydiver altimeter is not usable to detect the true altitude in skydiving. (I am a skydiver myself, 1800 jumps).
But nevertheless, supposed altitudes sometimes are not correct.
From investigations that we have made during the development phases of the CYPRES (1986 - 1990) we know that typically people believe to be higher than they really are. Those in the air, as well as those who watch it from the ground.
In September, 1993 at the 6th International Safety Congress in Helsinki, Finland, we were asked by the organizers to drop a dummy with the canopy opening initiated by CYPRES.
We did that.
The base was at 650 feet, so the Otter flew forth and back below the clouds for half an hour. Then the pilot pulled up into the clouds above the DZ and the dummy was dispatched from somewhere higher. Approximately 35 world experts in skydiving safety watched the deployment and landing of the dummy. It happened on late afternoon of a Saturday. On Sunday morning at 8 a.m. we asked all these people to write down the opening altitude of the canopy.
They did. We collected their sheets of paper immediately.
The answers: two times 60 meters, 2 times 90 meters, all the rest between 90 and 225 meters. Over all average: 130 meters.
The true altitude of the deployed canopy was 89 meters. So they guessed an average of 46 percent too high.
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On the 6th of July 1993 a guy came into our factory and said:
"I need a new cutter. My CYPRES has fired on the weekend. I looked on my altimeter at 500 meters, saw I was low and pulled immediately. My main opened fast, so I was under canopy at approximately 430 meters. Just after my main was open my reserve container opened. The loop was cut by the CYPRES."
"That was too high. It should work at 200 and something meters, but not above 400. That was too high."
"And don't think I fool you. I wasn't alone. There were 8 other people in the air. They all were open at the same altitude. They all can state that and they do so!"
"And if you shouldn't believe me, in addition there is a video on which you can see the whole procedure very detailed. It was an 8 way with a video. I was the video man. You can convince yourself. Here is the cassette."
And he gave us a video cassette which we immediately looked at together with him. We looked at it twice, a third, a fourth, a fifth time. Very carefully. We took the time.
He was under his reserve for approximately 37 seconds (that corresponds at his weight and reserve with an altitude of approximately 190 meters). Taking into calculation that he had his main open and did cut away etc. the reserve opening has happened at approximately 240 meters.
He also watched it very concentrated. Several times. Then he said:" Yes it was at approximately 240 meters. I was absolutely convinced that it was above 400 meters. And everybody in the 8 way was."
You may have a copy of the video, if you want.
These are impressions about altitude in skydiving. It's really difficult for skydivers to detect the altitude with a true value.
So what could have happened is that it was a bit lower than you thought it was and the cut of loop happened at the latest end of the deceleration phase of your main opening. (Five meters higher and it wouldn't have happened.)..."
Greetings, Gerard R. Fetter
Airtec GmbH - Wunnenberg, Germany
I hope this helps clarify some issues. During the time (since 1992) that we have been doing cutter replacements in the U.S. for Airtec, many have been experienced skydivers simply humming it too low. None have been "misfires".
Hopefully the above quoted article from "Skydiving" helps explain why that, if you dump your main so that you are getting line stretch just above 1050 ft. often enough, eventually your CYPRES will fire, even though you are not yet at 750 ft.
And yes, if you chamber test a CYPRES, it activates higher than 750 ft. (for the above noted reasons).
Concerning other recent questions:
The CYPRES cutter consists of a blade and anvil arrangement. The piston drives the blade into the anvil (the blade is actually permanently embedded into the anvil after activation) cleanly severing whatever was originally between the anvil and blade (the closing loop).
O closing loop | | piston/blade ====> | |== anvil | | | ----- washer & knot
A loop cut by the CYPRES cutter is very clean, squared off, and precise with no ragged edges, unlike a two bladed or scissors type cutter. Thus the concern raised about the loop "hanging up" when the cutter fires from "pinching", etc. is hypothetical at best. When packing rigs with CYPRES it is important to inspect the passage hole of the cutter to make sure that the protective plastic sleeve is intact and in place. This sleeve protects the loop from the blade, and is also severed when the cutter fires. The reason that "soft bodkins" are used with poptop containers is to avoid damage to the cutter or internal sleeve.
About the CYPRES cutter location:
There is no "default" location; for each and every rig Airtec has designed an installation for, there has been individual extensive testing prior to approval. Airtec puts every effort in either designing every "factory" set up itself, or at least checking it for approval. The standard location of the cutter is on the first flap to cover the pilot chute on many non-poptop type rigs (Talon, etc.), and on the bottom of the packing tray on poptop type rigs (Racer, Teardrop, etc.). However there are many exceptions: With the Javelin the cutter is located on the bottom of the packing tray. A Vector-II with a square reserve and a Vector-II pilot chute, and the Sweethog have the cutter located on the first flap to cover the freebag. The cutter location is very important for a number of reasons including pilot chute launch. For additional information about "Retrofit" CYPRES installations (into rigs with out a factory set-up), consult Airtec's "Rigger's Guide to Installation" and accompanying instructional video. (The current revision comes in a large purple loose-leaf binder.) For riggers inspecting and packing rigs with CYPRES already assembled, either factory set-up or retrofit set-up by another rigger, consult Airtec's "Packer's Checklist" to verify the exact details of the required installation. (The "Packer's Checklist" is included as part of the "Rigger's Guide to Installation", and in the CYPRES "Packer's Kit", which also contains the required materials, tools and supplies to make closing loops and pull-ups, as well as the CYPRES temporary pins.) All riggers packing rigs with CYPRES should have one of these two reference documents and kits of rigging supplies.
About the CYPRES batteries:
Airtec currently requires that the batteries be replaced every two years, or every 500 jumps, or if the low battery error code (8998/8999) is encountered during self-test, which ever of these comes first. When stored as instructed, the batteries have a long shelf life, it is a good idea for all riggers maintaining and packing rigs with CYPRES to have a set in stock.
If there are any additional questions or comments please feel free to Call, Fax, or use E-mail. We try to keep track of important questions and concerns from Internet and other sources, but sometimes miss some!
SSK Industries is the U.S. CYPRES Service Representative.
SSK Industries, Inc. - 1008 Monroe Road - Lebanon, Ohio 45036 - USA
Phone: 513-934-3201 - Fax: 513-934-3208 - E-Mail: info@SSKinc.com
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