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Stretching lineset back to trim

 

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Martini  (D 23756)

Dec 30, 2012, 3:44 PM
Post #51 of 73 (1465 views)
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Re: [julio_gyn] Stretching lineset back to trim [In reply to] Can't Post

I think I get what you're thinking, that the slider starts with zero velocity/zero heat at the top and builds heat as it travels downward. The first part is somewhat correct but max velocity is not necessarily at the bottom of the lines because the slider may slow down considerably before hitting the risers. Also the slider may reach its max velocity very near the starting point depending on how the canopy opens.

As far as stretching goes, all of the stretching should take place in the shrunken areas because those areas are shrunken due to "curling" of the heated fibers. The unaffected fibers are straight and won't significantly stretch. Spectra lines are considered non-stretch just like HMA or Vectran. The problem is that Spectra isn't non-shrink like those fibers.


Martini  (D 23756)

Dec 30, 2012, 4:11 PM
Post #52 of 73 (1457 views)
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Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

Too bad, you often post good stuff. I'm not sure why you, as well as others, have chosen an adversarial position on this topic. And I disagree, people can be influenced to change what they do. If John LeBlanc or Bill Booth supported this technique it would be the norm. If they came out against it the discussion would be over. I don't expect that either of them will publicize an opinion so it is up to us to discuss the subject and if possible study it to reach a sensible conclusion. But if you really don't care then that's OK too, I expect at this point most people don't care either. But this isn't like discussing an RDS that will only ever affect a few, if stretching was a generally accepted tool it could affect many.

I didn't invent this process and I don't mean to deliberately promote it but I think that it merits discussion. I also believe that since I have used the technique and have given it some consideration that I have something to contribute here.


AggieDave  (D License)

Dec 30, 2012, 4:25 PM
Post #53 of 73 (1450 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
I didn't invent this process and I don't mean to deliberately promote it but I think that it merits discussion. I also believe that since I have used the technique and have given it some consideration that I have something to contribute here.

The short version is that line design has changed and what we expect out of our canopies has changed. For a good number of people in the sport, gone are the days of dacron and even spectre lines. What you're left with are line designs that not only don't respond to attempts to stretch the lines, but lines that could even be damaged by the forces their unknowing owners applied due to reading a handful of posts.

Also, the large majority of people reading this thread have zero idea how to measure line trim and could only line up the line groups and see that some don't match. Just because the outside A-lines don't match the inside A-lines doesn't mean that any of them are wrong, it depends on the canopy. Then you're dealing with the cascades and even some sections that are mixed line types (like on control lines).

Basically in the hands of 80-90% of the modern fun jumper population, trying to "stretch" lines will cause nothing but grief. The other 10-20% will either not read this or will already know the answer.

With those newer design line types, jumpers are much better off replacing their linesets a little before they notice wildly bad openings or have lines break. Marat gave us an extreme example of that when his canopy loaded from a thermal after rounding out of his dive, obviously his case is beyond what the average jumper would accomplish; however, how many "average" jumpers have you personally seen at a DZ jumping X-braced canopies with HMA lines? Pulling the spectre lines on your Sabre1 210 didn't really do much in either direction besides maybe temporarily clean up an opening. Back when folks were jumping in the neighborhood of 1:1 it really didn't effect much either way. Now in the world of 1.5+:1 for a weekend jumper, things could be exaggerated to the point of injuring someone (through a violent opening, for example).

Your response would be that this technique is for line types that would respond to such methods, how many of your average jumpers in the world, scanning through DZ.com can even name more than a couple line types? How many of them know exactly what is on their canopy?

That is why I'm opposed to a general speculation thread like this and I'm generally opposed to this technique.


Martini  (D 23756)

Dec 30, 2012, 5:37 PM
Post #54 of 73 (1437 views)
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Re: [AggieDave] Stretching lineset back to trim [In reply to] Can't Post

Thanks for the thoughtful and well written post.

In my first post I wrote that HMA and Vectran do not respond to stretching but you're right, there are many who can't distinguish Spectra from string cheese.

The stretching I referred to was on a Sabre 120 (not 210) loaded at 1.6-1.7, it was deliberately done to clean up twisting openings. The lineset was not only out of trim lengthwise but more significantly was badly asymmetrical. The stretching adjustment had the desired effect and lasted about 100 jumps, I did it again to good effect but after that I had it relined.

While use of this method probably should be something that is either taught or performed by a competent rigger some people seeking do it on their own would be good with a clear set of instructions and video specific to their canopy type. While I appreciate your point of view regarding potential pitfalls I would prefer not to disregard a tool that I, as well as others, have found to be useful. I have never attempted to give detailed instruction on this technique and wouldn't except with qualified people and on the same canopy type. I'll always be willing to discuss this topic though, thanks for contributing more depth to the subject.


sundevil777  (D License)

Dec 30, 2012, 6:19 PM
Post #55 of 73 (1429 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

Another thing that should be kept in mind is that the stretching that has been applied, to my understanding - how it has been described, is well under the load that a line should easily withstand. So, why should applying a load so much under that rated load cause anyone to assume bad consequences would result?


(This post was edited by sundevil777 on Dec 30, 2012, 6:19 PM)


Martini  (D 23756)

Dec 30, 2012, 10:33 PM
Post #56 of 73 (1409 views)
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Re: [sundevil777] Stretching lineset back to trim [In reply to] Can't Post

Two reasons.

Stretching could result in breaking or at least thinning some of the fibers, in fact that seems very likely. Not all fibers are at the same stress level, the line is not perfectly homogenous to begin with and heat/shrinkage will most certainly not be uniform.

Some degree of makeup may be a factor similar to steel with grains, crystallization, work hardening, micro tears etc. In other words shrinking and stretching might end up with material that has physical properties that differ from the original.


BrianSGermain  (D 11154)

Dec 31, 2012, 10:47 AM
Post #57 of 73 (1355 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

PD has not tested this in any way, therefore their oppinion is as uselful as someone you meet on the bus. It is unwise to advise beyond your data.

I have developed this method over many years, and I have data to support the conclusion that this is a hugely valuable technique. It does not last forever, but the shrinking is a long slow process, and it takes as long to back out of trim as it took to get that way in the fear place.

Heat shrinking is only part of the dimensional change on a spectra lineset. My original article in parachutist magazine on this topic is what PD is basing their information on, but I have learned more since them. It appears that heat shrinking is not sufficient to cause the magnitude of the change that we are seeing. Lack of loading must therefore be a significant aspect of the difference between the outboard and inboar lines. It is a combination of these two forces that causes a canopy to become "out of trim". Therefore, the direct loading of the "unloaded" lines, including the brakes, changes the parachute significantly, and for a useful duration.

I find that most canopies with 500 jumps on it has a differential of 3 inches or so. This can be completely irradicated with this technique. That is a singnificant difference both in statistics and in useful practice.

Regarding all the guessing about stressing the lines I have no evidence to support the conclusion that stretching the lines weakens them in any significant way. Controlled test results are coming shortly.

-Brian


(This post was edited by BrianSGermain on Dec 31, 2012, 1:03 PM)


Martini  (D 23756)

Dec 31, 2012, 10:53 AM
Post #58 of 73 (1349 views)
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Re: [BrianSGermain] Stretching lineset back to trim [In reply to] Can't Post

Quote:
This can be completely irradiated with this technique.

Then I'm never using the technique again, radioactivity scares me. Wink


wolfriverjoe  (A 50013)

Dec 31, 2012, 11:07 AM
Post #59 of 73 (1343 views)
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Re: [BrianSGermain] Stretching lineset back to trim [In reply to] Can't Post

Thanks for the input, Brian.

Has anyone done any strength testing of the stretched lines?

Has anyone taken lines that were stretched several times before replacement and then pulled them until they broke to see if there is any significant loss of strength?

I really can't see doing this to a material without losing some of the strength, but I'd like to know if it's enough to cause concern.


BrianSGermain  (D 11154)

Dec 31, 2012, 1:04 PM
Post #60 of 73 (1319 views)
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Re: [wolfriverjoe] Stretching lineset back to trim [In reply to] Can't Post

As I mentioned, there is no evidence of a significant reduction in the tensile strength of the line post stretching.


julio_gyn  (A 705)

Dec 31, 2012, 2:50 PM
Post #61 of 73 (1298 views)
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Re: [sundevil777] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
the lines will build up heat from the top down on the lines,

How do you make this conclusion?

well, I assume the slider will accelerate as it start its descent...

In reply to:
whenever you stretch it again you can't control where it will happen,

"Control" may not be possible in the strict sense, but it might be that the areas that shrunk are where the stretching will occur. It depends on the mechanism of how the fibers shrink, and what happens during stretching.

The lack of interest by some to want to know is, interesting.
hum... I'm not against any way of doing it, just curious about the thread....


julio_gyn  (A 705)

Dec 31, 2012, 2:59 PM
Post #62 of 73 (1294 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
I think...max velocity is not necessarily at the bottom of the lines because the slider may slow down considerably before hitting the risers. Also the slider may reach its max velocity very near the starting point depending on how the canopy opens.

ok, i got it.. so the slider hits its vmax and then (maybe by the friction generated) it slows...

In reply to:
As far as stretching goes, all of the stretching should take place in the shrunken areas...

I think the fiber is a "plastic" and the place it shrink will get "harder" and deform or restrict the stretching to more "softer" areas...

Sorry if I'm having a hard time to understand all this, and causing any trouble to the thread..


masterrigger1  (D 14167)

Dec 31, 2012, 4:07 PM
Post #63 of 73 (1275 views)
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Re: [BrianSGermain] Stretching lineset back to trim [In reply to] Can't Post

Brian,
I hate to disagree with you, but I do on several points.
Quote:
....
but the shrinking is a long slow process,...

It is not always slow.
Spectra can start into a transformation state with a temperature as low as 190 degrees.
As the the rate of heat rises, so does the shrink rate.

If you have a traditionally slow opening canopy, the rate may be as low as an inch per 175 jumps. This would be an ideal rate of shrinkage.

But then again, if you have a few fast openings where the heat was let's say ten fold the nominal number, then you would have increased and rapid shrinkage.

This does happen BTW. I have seen canopies with just a few jumps have as much shrinkage as a canopy with hundreds of jumps. It just depends.

This is one of the main reasons that we should all move away from Spectra as a main stream line material. It is a moving target of sorts.

Quote:
Heat shrinking is only part of the dimensional change on a spectra lineset. My original article in parachutist magazine on this topic is what PD is basing their information on, but I have learned more since them. It appears that heat shrinking is not sufficient to cause the magnitude of the change that we are seeing. Lack of loading must therefore be a significant aspect of the difference between the outboard and inboar lines. It is a combination of these two forces that causes a canopy to become "out of trim". Therefore, the direct loading of the "unloaded" lines, including the brakes, changes the parachute significantly, and for a useful duration.

As I have probably written more on this subject (line materials) in these forums than anyone else (including yourself), I would have to say that I could not disagree more.
First Spectra does stretch, but it will return to it's original length when unloaded or the weight removed.
You can verify this over and over again on the line table....

The dimension changes that we see are due to the slider /line friction. The heat is the only culprit, plain and simple.

I absolutely do not agree with stretching the lines.

This is why:
The lines vary in shrink rate along the length of the line. This is due to the friction increases and decreases during the opening.

If you look at the carriers (twisted bundles) of the line weave, you will notice size and shape differences along the length of a used line.

Also some carriers will be locked down with one another and others will be free.
Stretching the line with these carriers locked down is a really bad idea.

Plain and simple... just get a line set that does not have these issues to begin with!!

For those of you that do not know me, I have for years been working on different line mediums to replace Spectra used on parachutes. We have had great success over the years.
In that time, we brought out the first competition swoop lines, coated Technora, coated Vectran, and enhanced braiding of both materials.

The result is that most manufacturers (including PD) have started to follow in both Icarus' and Skyworks footsteps with regard to these line materials.

This is a good thing IMHO...better flying canopies for everyone!

Lastly, modifying the trim is well.....a modification.
...and who can modify parachute things....legally?Wink

Cheers,
MEL


popsjumper  (D 999999999)

Dec 31, 2012, 4:38 PM
Post #64 of 73 (1265 views)
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Re: [BrianSGermain] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
As I mentioned, there is no evidence of a significant reduction in the tensile strength of the line post stretching.

Are you willing to post your data, testing methodology and conclusions? As yet, with no back-up to your statements, we just met on a bus.

Why has this not been passed on to canopy manufacturers?


(This post was edited by popsjumper on Dec 31, 2012, 4:43 PM)


skyjumpenfool  (Student)

Dec 31, 2012, 7:22 PM
Post #65 of 73 (1235 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
Do you have something useful to say or just want to stir the pot?

I do love to stir the pot...

When the lines become out of trim enough to affect the canopy's performance, I like to call that a "wear indicator". Time for new lines. Streaching may or may not work? Fact of the matter is, I don't care! Lines are cheap when my ass is on the line (punn intended). Cool

You do whatever you like. Shocked


Martini  (D 23756)

Jan 1, 2013, 12:12 AM
Post #66 of 73 (1215 views)
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Re: [skyjumpenfool] Stretching lineset back to trim [In reply to] Can't Post

Your approach isn't stirring the pot, it's pretty much the norm. Things aren't really that black and white though. Lines going out of trim are like boiling the frog, it usually happens so slowly that you only realize the trim issue when the performance degrades to a clearly unacceptable level. By occasionally stretching the lines you may enjoy a much better overall performance. You may not care, that's fine it's your canopy. I believe stretching lines is a good plan so I do that. It's not just a case of taking horribly out of trim lines and resurrecting them to save a few bucks. But as you say, do whatever you like.


popsjumper  (D 999999999)

Jan 1, 2013, 2:02 AM
Post #67 of 73 (1208 views)
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Re: [skyjumpenfool] Stretching lineset back to trim [In reply to] Can't Post

In reply to:
In reply to:
Do you have something useful to say or just want to stir the pot?

I do love to stir the pot...

When the lines become out of trim enough to affect the canopy's performance, I like to call that a "wear indicator". Time for new lines. Streaching may or may not work? Fact of the matter is, I don't care! Lines are cheap when my ass is on the line (punn intended). Cool

You do whatever you like. Shocked

THIS^^^^


craigbey  (C 31991)

Jan 1, 2013, 7:34 AM
Post #68 of 73 (1150 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

The first person who responded to your OP described the technique of stretching lines back into trim as "kung-foo". That was an appropriate choice of words.

Spend some more time with different experienced riggers and they might share with you some of the voodoo magic they have used on their own gear.

These techniques may be tricks that such experienced riggers can employ on their own gear or the gear of others jumpers if they are fully aware of the nature of such modifications. So I guess it's good to see some technical discussion of what could happen when you stretch Spectra lines back into trim.

I can't say whether stretching Spectra is good or bad. But I do know of a couple of other types of materials that work a lot better.

Wink

This sport is not cheap. Don't try to make it so.


Martini  (D 23756)

Jan 1, 2013, 9:57 AM
Post #69 of 73 (1114 views)
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Re: [masterrigger1] Stretching lineset back to trim [In reply to] Can't Post

I glad you're posting on this subject, right now it appears that you are the only one with scientific knowledge. I'd like to ask a few questions, I really have no agenda, naturally I'd like confirmation that the technique I've been using is sound but you have already stated that you are against line stretching and I'm quite open minded.

Can you describe the amount of tensile loss as a function of stretching heat-shrunk lines? I know there are several variables but a general idea would be useful if you haven't graphed your testing.

I've never subscribed to the theory that unloaded lines shrink, that would mean that they shrink just sitting on the shelf. Since paraglider lines also shrink but aren't subject to frictional heat, do you think that exposure to sunlight is causing shrinkage? I'm aware of the increased complexity of PG lines (sheathing, mixed materials etc).

I have three Spectra-lined Sabres, one with nearly new lines, the others with decent but slightly out of trim lines. I don't know if non-Spectra linesets are available but in any case I'll be jumping the current linesets for a while. Do you believe that I'm putting myself at risk by stretching these lines to some degree? BTW these canopies have consistently butter-soft openings.

I do own one HMA-lined canopy, it's my preferred line material. Unfortunately it's a smallish crossbraced canopy and not my favorite for wingsuiting which is what I generally do. No perfect solutions here barring purchase of another canopy, I like the Sabres too much to replace them, never mind the cost.

Thanks for contributing here, we all benefit from more reason and less speculation.


masterrigger1  (D 14167)

Jan 1, 2013, 11:00 AM
Post #70 of 73 (1090 views)
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Re: [Martini] Stretching lineset back to trim [In reply to] Can't Post

Quote:
I'd like to ask a few questions, I really have no agenda, naturally I'd like confirmation that the technique I've been using is sound but you have already stated that you are against line stretching and I'm quite open minded.

There are a couple of reason that I am against stretching the lines.
As someone that has been in the Non-Destructive Testing field for 30 some odd years, It is my belief that doing this procedure is taking the material into the plastic stage for sure,
Taking load bearing material, that is still to be used or placed back into service, into this state of deformation is not good.

Line material can be subject to five states.
1. non-loaded state
2. Loaded state
3. Elastic
4. Plastic
5. Compromised, broken or breached state.

To get Spectra, that has gone through a shrink cycle, back to a longer dimension than it currently has, there is only one state that will do it. That would be the Plastic phase or state.

As you can see from the list above, you are right next door to the Compromised state.

How close is the question at hand.
If you have 550 Spectra, you can rest assured that it probably is closer than say if you had 725 Spectra.

Quote:
Can you describe the amount of tensile loss as a function of stretching heat-shrunk lines? I know there are several variables but a general idea would be useful if you haven't graphed your testing.

It would vary so much that I would not even know where to start.

As I stated before, there are areas along the line length that have more shrinkage and damage than other areas of the same line.

When stretching the line, the areas where the bundles are locked down will tend to go Plastic first, causing further loss of load capabilities.

Think salt water taffy here....
The more plastic you go, the less tensile strength you will have.

Quote:
Since paraglider lines also shrink but aren't subject to frictional heat, do you think that exposure to sunlight is causing shrinkage? I'm aware of the increased complexity of PG lines (sheathing, mixed materials etc).

Remember that Spectra can start shrinking at about 190 degrees Fahrenheit.
I have seen a canopy that was left sweltering in a hot car for a good length of time have this type of shrinkage. It was a canopy with less that 15 jumps on it BTW.

Quote:
I have three Spectra-lined Sabres, one with nearly new lines, the others with decent but slightly out of trim lines. I don't know if non-Spectra linesets are available but in any case I'll be jumping the current linesets for a while.

PD does have alternate line types available and we also have them here.

Quote:

Do you believe that I'm putting myself at risk by stretching these lines to some degree?

I think I have this covered just above a couple of paragraphs, but yes to some degree.

The problem is that the A-B measurement is pretty critical to one another along with the brake set measurement. Get anyone of these two wrong and you could have serious issues.
Just saying....

Happy New Year,
MEL


Martini  (D 23756)

Jan 1, 2013, 2:18 PM
Post #71 of 73 (1055 views)
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Re: [masterrigger1] Stretching lineset back to trim [In reply to] Can't Post

Thanks Mark,

It's great to get some insight into this subject. I didn't really expect that you had done a series of tensile tests on stretched lines but having an expert opinion of the mechanics is helpful. I would have expected that loading the shrunken lines into the elastic state would have been sufficient to "uncurl" them into extension. I have also seen and heard of canopies with serious trim issues after relatively few jumps, it's a great indicator of how heat sensitive Spectra is. Also nice to hear that alternative line types are available even for the Sabre, I'll look into it if I need new linesets.

Happy New Year to you too.

Paul


base283  (D 15343)

Jan 1, 2013, 8:29 PM
Post #72 of 73 (1008 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

FWIW, I am a parachute rigger, I work on paragliders also. and line stretching works IMHO on the polymids as opposed to aramids like kevlar, Dacron seems to be the most receptive. It has to do with the molecular structure. Aramid molecules connect head on whereas Polymids are angled giving it an elongation property. I had an asymmetrical opening on a BASE Canopy that knocked the lines out of spanwise trim by over an inch and stretched them back out. But this was a basic 7 cell design based to something similar to a Django Pegasus canopy. If one is dealing with a critical profile I would reline it. Remember the Nova canopy? The profile was very sensitive to trim variations.
Some of the class 3 (expert competition paragliders) are relined after only a few flights. more to do with line wear (0.6mm upper gallerie cascades) though but it negates the need to stretch them out. A lightly loaded 7 cell wont be affected much but a highly loaded swoop canopy could fold on ya more easily. The effect is exponential due to the speed/wingloading in the equation. Ian does have a point but a student Manta wont care but sometimes you have to trim them due to the regulations of rigging in some countries. I hope this helps.
take care though.
space


jonathan.newman  (D 30644)

Jan 3, 2013, 9:43 AM
Post #73 of 73 (880 views)
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Re: [EOCS] Stretching lineset back to trim [In reply to] Can't Post

I just got done "stretching" a stilleto 135 back into trim. It involved measuring line trim, picking out the bartacks of the offending lines, and moving the larksheads to the right length. It's back in service with smooth, on-heading, and non-spinning openings. Flies like new.

As an aside to this thread, I don't think the lengths will change much more as the worst lines have already shrunk by 3 - 5 inches. I'm just glad that PD or whoever cut the last lineset left enough line in the fingertrap for me to do this .

I just want to throw this out as a labor-intensive alternative to physical stretching. I believe as MEL and other engineer types said that deforming a line by restretching will either be temporary or damaging. But we're talking about 20+ lines rated at 550 to 1000 pounds each. You can degrade that a lot before you start seeing breakage.

There is room in this argument for all of us to be right.

What I'd like to see is someone at a rigging loft cut a line set from pre-shrunk spectra (bake it).


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