[Janos]

Quote: (Originally Posted by Genesis)

BTW, this thread went off-topic originally in the first half-dozen pages. Drift is drift, and we seem to have intertwined things but are still making progress - we've established that the OR design does not actually run for 1 billion hours before something breaks that forces the user off the loop!

I guess the difference between the two systems is there's a difference when (say) a cell fails and the unit tells you it has failed and forces you onto bailout, and a unit that doesn't tell you when a cell fails.

The second type relies on a bit of user smartness to work correctly.

I would definte failure as death of the user, not coming off the loop.

Janos

PS - I think that a 1 in a billion rebreather is a great idea. However I don't think it is practical. The worst thing would be a 'breather that claims to be a 1 in a billion, but isn't. At least I know my KISS is trying to kill me!

[Genesis]

Quote: (Originally Posted by Janos)

I guess the difference between the two systems is there's a difference when (say) a cell fails and the unit tells you it has failed and forces you onto bailout, and a unit that doesn't tell you when a cell fails.

The second type relies on a bit of user smartness to work correctly.

I would definte failure as death of the user, not coming off the loop.

Well, if that's the definition of MTBCF being used then not only do I take issue with the claims but also with the twisting around of technical terms with well-defined meanings to suit marketing purposes.

MTBCF (or MTBF) is an already-defined term, and it is often abused or represents fanciful thinking as opposed to a real threat matrix, as I've pointed out in the other thread. If the definition has now been changed so that you can actually have the unit fail and force you off it, and that's not called a "critical failure", then we've left the realm of engineering as it comes to terminology and entered the realm of creative abuse of technical terms to suit marketing goals.

[AD_ward9]

Quote: (Originally Posted by Genesis)

Well, if that's the definition of MTBCF being used then not only do I take issue with the claims but also with the twisting around of technical terms with well-defined meanings to suit marketing purposes.

MTBCF (or MTBF) is an already-defined term, and it is often abused or represents fanciful thinking as opposed to a real threat matrix, as I've pointed out in the other thread. If the definition has now been changed so that you can actually have the unit fail and force you off it, and that's not called a "critical failure", then we've left the realm of engineering as it comes to terminology and entered the realm of creative abuse of technical terms to suit marketing goals.

I am astonished at the lack of basic engineering knowledge from someone developing a rebreather (the K1). I know your background is software, but that is no excuse.

MTBF (Mean Time Between Failure) is different to MTBCF (Mean Time Between Critical Failure). Both terms are defined very exactly by standards and are a complete science. Their accuracy is excellent as huge numbers of components are tested to give a number for every single part.

MTBF is the inverse of the frequency the equipment has a fault, and MTBCF is the inverse of the frequency the equipment has a fault that could kill a user. In practice users often survive critical failures, for rebreathers by about 100:1, so the MTBCF is a pessimistic figure whereas MTBF is quite precise. Remember it is an arithmetic mean.

Every single component has an MTBF figure available from the manufacturer for different levels of stress.

To get the component MTBF, the manufacturer has made huge numbers of components and tested them. He also supplies very sensitive sectors, like automotive, which investigate failures thoroughly and report to the manufacturer. The manufacturer's scientific testing gives a figure, which is checked by the users (at least serious users, such as BMW, Ford, SAAB etc).

For example, an assembly house we use (EMA Ltd) makes about 3 million automotive circuit boards a year, (it was an RF design we did originally in 1992, and they are still making). They had 0 ppm failures every year except in 2003 , when they had one failure with one board shipped to one automotive company. Just for that one failure on a board costing a few dollars, the automaker sent an entire quality team to EMA to investigate. Issue was traced and fixed. That is the seriousness by which big companies deal with quality. There are simply too many components in a modern car to do otherwise: if every component had a 1 ppm failure rate, then there are so many parts in a modern car, that a lot of cars would fail.

So, back to how we get an MTBF figure: we contact every component manufacturer and get his MTBF data for the component we design in. We look at it and if it is better than the industry average for that component (these figures are published), with the stress applied to it in the particular application, then we use the worse figure. This means we always take worst case figures. For important components like sensors and injectors, we check the performance of the component and its failure modes using a test plan, specific to rebreather use, to double check the manufacturer's stress data. We have published some of these test plans, such as the one we use for O2 sensors. Some of our competitors help, reviewing our test plans, as do our customers' quality teams, to make sure they do not miss anything. We use a formal (maths) environment to ensure we model the stresses correctly (also published).

We then use a method defined in a MIL spec to add the numbers up. There is another method, in a BSI standard but the BSI method is for more light use and not suited to stressed environments.

For every single component, we published the MTBF figure we used and where it comes from. We can trace it to specific tests carried out by the manufacturer, or when their figure is better than normal, to an industry average for that component type under that level of stress.
Every manufacturer we use is ISO 9000 certified, so we can trace their figures back as far as we like. All the companies we use have entire departments dedicated to this MTBF and quality issue.

We also published our calculation, even in nice graphical form so anyone can follow it. We also published the safety review of the O.R. electronics.

I think you just made a very good point, albeit obliquely: it is very dangerous for amateurs to be let loose designing life critical systems - leave it to the professionals, but demand to see all their calculations and methods. If you insist on pursuing this route, I can recommend some introductory texts on reliability analysis to determine MTBF figures.

I note you are still posting on Rick and your "teddy out of pram" thread, this same material. It should go in the right place. Andy gave you a hint. Worth listening from time to time .

Alex

[Sponge-Bob]

Hi guys,

I'm new to the forum but have been lurking a while and following this thread with great interest.

Alex has done a great deal of work on the Deep Life rebreather it won a Smart Scotland award a few years back for design, technology and innovation.

Many of Alex's idea's are now part of other companies upgrades (that many divers have paid extra for) that Alex provided to companies 'free gratis'. Most companies would have done a re-call! When it comes to electronics he knows his stuff.

Testing your product on a consumer is bad form. Why is it acceptable that a unit should kill you?

Testing the electronics of a unit by leaving it turned on for 24 hours then stamping and shipping all those that are still working in the morning as tested was the way it used to be in the bad old days before Alex.

Love him or loathe him the guy is try to do some good and save lives. Lots of positive input and lively debate with lots of fresh ideas will hopefully make the Deep Life rebreather a reality.

Cheers

[Genesis]

Quote: (Originally Posted by AD_ward9)

I am astonished at the lack of basic engineering knowledge from someone developing a rebreather (the K1). I know your background is software, but that is no excuse.

Actually, my background is in engineering. You're the one who is astonishing me.

Since you choose to respond here rather than in the other thread, I'll do it in both places.

Quote:

MTBF (Mean Time Between Failure) is different to MTBCF (Mean Time Between Critical Failure). Both terms are defined very exactly by standards and are a complete science. Their accuracy is excellent as huge numbers of components are tested to give a number for every single part.

Really?

Then you can explain why in the case I cited in the other thread, we saw actual failures of anywhere from ten to fifty times more prevalent than predicted, and we were operating in a more tightly controlled environment than that demanded by the manufacturers to meet those specs!

This is not a matter of fanciful reliance on manufacturer data - this is real world experience. Investigation showed that up to half of all the components in these devices had "published" MTBFs that were derived by statistical extrapolation from stress conditons away from the normal and expected operating regime.

The problem with this Alex is that when one does accelerated life testing the shape of the curve is not scientifically extrapolable to the normal operating environment with exacting confidence - only with high confidence. In the language of statistics, we can provide some number of sigmas worth of confidence that our number is correct, which sounds good until you consider the number of individual components in the device! Error rates are additive and as such as component count goes up so does uncertainty. Yet that uncertainty is not stated in the MTBF (or MTBCF) numbers!

One of the basic foundational principles of science is that you cannot state a measurement without its uncertainty. A raw number is meaningless. This is first semester college level science stuff, and yet you won't find an uncertainty value on ANY published MTBF or MTBCF figure - including yours! Why not? Because neither you or they know what that number is.

Quote:

MTBF is the inverse of the frequency the equipment has a fault, and MTBCF is the inverse of the frequency the equipment has a fault that could kill a user. In practice users often survive critical failures, for rebreathers by about 100:1, so the MTBCF is a pessimistic figure whereas MTBF is quite precise. Remember it is an arithmetic mean.

But you just redefined it Alex.

You claimed that:

Quote:

3. The Open Revolution safety case demands bail out. The equipment forces the user onto bail out. So tell me how can the system operate if it has no bail out? To take your brakes analogy, try it with the brake lining removed.

But in the same breath you claim the unit has more than a one billion hour mean time before a critical fault occurs. Which is it Alex?

As I pointed out (and which you have intentionally ignored) if you were to ship 100,000 of these units to people who each dove them 10,000 hours over their lifetime, you'd expect (arithmatically) to have one critical failure on them over that entire population and lifecycle!

That means the risk of death if you dive this unit alpine (one in 270,000 assuming 10,000 lifetime diving hours) is lower than the risk of death by lightning strike (1 in 83,000). Indeed, its roughly the same risk as that of being killed by asteroid impact (1 in 200,000 to 1 in 500,000)

Isn't that good enough for your company to advocate that one can dive the unit without bailout?

Quote:

To get the component MTBF, the manufacturer has made huge numbers of components and tested them.

Yes, but not under realistic conditions. That's because he can't - the amount of time to detect the first failure at a component level is measured in hundreds of thousands of years, even across very large populations of components. So instead what is done is accelerated life testing, where components are exposed to intentional overstress and the failure rate under "normal" conditions is extrapolated. This is an inexact process, which is why you do not see uncertainties on published MTBFs!

Quote:

So, back to how we get an MTBF figure: we contact every component manufacturer and get his MTBF data for the component we design in. We look at it and if it is better than the industry average for that component (these figures are published), with the stress applied to it in the particular application, then we use the worse figure. This means we always take worst case figures.

So what? Garbage in, garbage out. No uncertainty figures on the MTBFs mean that you're accurately computing a number that has no tolerance, and thus do not know what the tolerance is on the result!

Quote:

We then use a MIL spec method to add the numbers up. There is another method, in a BSI standard but the BSI method is for more light use and not suited to stressed environments.

I understand that Alex. But again, if you have numbers as input that are without uncertainties then you are not stating a scientifically-derived value.

If I tell you that the diameter of this piece is 1.5", I have told you nothing of value at all, because I have not specified a tolerance.

A proper scientific measurement must include a tolerance. Therefore, if I am going to tell you that I want to specify that piece's diameter, I must say something like "1.5 inches +/- 0.001". THAT is a defensible specification.

An MTBCF of "2.7 billion hours" is worth exactly nothing without a tolerance.

As I pointed out in the other thread, I have had large populations of electronic equipment in controlled environments WELL within manufacturer specifications fail at anywhere from ten to FIFTY times rates their MTBFs would imply.

As I pointed out, the problem was that the MTBF was specified in an incomplete manner, just as yours is.

Compute the uncertainty on your "2.7 billion hour" claim and I suspect you'll be quite disturbed. I know we were, when we started doing this sort of analysis.

In fact, we only had to do it once to realize that the "common usage" of MTBFs in electronic components are really quite fanciful, and thus you cannot use said comparisons as a meaningful tool.

Most people would think that if they had two disk drives available for them to purchase on their computer, one with a 500,000 hour MTBF and one with a 750,000 hour MTBF, that the 750,000 hour one should, over large populations of units, be the more reliable device.

That is not necessarily an accurate statement.

What if I told you that the 500,000 hour MTBF was +/- 5,000 hours (a 1% tolerance) while the 750,000 hour MTBF was +/- 325,000 hours - a 50% tolerance!

Now which one do you buy? I buy the 500,000 hour one, because I can more accurately plan life-cycle costs; I have absolutely no idea where in the 750,000 hour range the device that I have in my hand falls, and two of them may be on polar opposites of their expected service life, with one as little as 325,000 hours and the other at nearly 1.1 million!

When you demand from suppliers (and publish) MTBFs that contain uncertainties then you're putting forward a figure that can be understood to actually mean something.

Quote:

I think you just made a very good point, albeit obliquely: it is very dangerous for amateurs to be let loose designing life critical systems - leave it to the professionals, but demand to see all their calculations and methods.

Yet another personal attack - yet you won't even stand in front of your own design and its claimed MTBCF!

If you truly believe that your device will not fail, on average, for 2.7 billion hours, then certainly you can safely advocate that someone dive it alpine, when if your specified MTBCF is accurate, even to within only 50%, their risk of getting killed by doing so approximates that of being killed by lightning strike or asteroid impact!

[Faceless]

Yet another "basic scholar stuff" :
Monte Carlo method - Wikipedia, the free encyclopedia

Does Monte Carlo method sounds like a junk to you as well ?

I suspect you just want to "win" in this discussion no matter what.

Anyway, i don't know about Alex, but I'm outta here.
It's boring.

[AD_ward9]

Quote: (Originally Posted by Genesis)

Then you can explain why in the case I cited in the other thread, we saw actual failures of anywhere from ten to fifty times more prevalent than predicted, and we were operating in a more tightly controlled environment than that demanded by the manufacturers to meet those specs!

This is not a matter of fanciful reliance on manufacturer data - this is real world experience. Investigation showed that up to half of all the components in these devices had "published" MTBFs that were derived by statistical extrapolation from stress conditons away from the normal and expected operating regime.

The problem with this Alex is that when one does accelerated life testing the shape of the curve is not scientifically extrapolable to the normal
If you truly believe that your device will not fail, on average, for 2.7 billion hours, then certainly you can safely advocate that someone dive it alpine, when if your specified MTBCF is accurate, even to within only 50%, their risk of getting killed by doing so approximates that of being killed by lightning strike or asteroid impact!

We are going in circles.

The reasons your numbers were out is because you did not do your homework properly.

That is why we have 5 year studies of oxygen sensors, not accelerated test plans.

Always trace the MTBF figure back to a non-accelerated number, unless it is totally traceable and verified to non-accelerated numbers. Do not design with the latest component just out in a safety critical system: safety design takes time, like good wine and whisky. If your supplier cannot give you actual MTBF data for huge numbers of the parts, then do not enage in a folly of using that part.

However, judging by your previous posts, I cannot understand how you did an MTBF calculation before. Please publish it with something that allows me to trace when you did it. You need to get real experience, not just quote what you believe are the results of others, unless you post a link to their work.

Yes, 1 billion hours was picked by the EN61508 directorate to mean that the chance of equipment killing you would be about the same as other background risks, such as lightning etc. It is not 1 billion hours just from picking a number.

Again, if you want to argue about MTBF calculations, please do it in the right place. There are lots of books and courses on this sort of stuff which you could try first.

I am out of here too: my time is too valuable to waste like this. Thanks Faceless for trying to explain things to Genesis: we have never met but if you are ever in St Petersburg, it would be a pleasure - your answers affirm the very high opinion I hold of Russian education. Also thanks to Andy, for letting us know Genesis' background. Green posted to those who did their best to explain things.

Alex

[Sponge-Bob]

"In an age of learning, the learned find themselves beautifully equipped to deal with a world that no longer exists"

[Genesis]

Faceless, did you read the link you posted before you posted it?

Off one of the links in that wiki is the key to this:

Quote:

For engineering purposes, reliability is defined as:

the probability that a system will perform its intended function during a specified period of time under stated conditions.

and

First, reliability is a probability. This means that there is always some chance for failure. Reliability engineering is concerned with meeting the specified probability of success, at a specified statistical confidence level.

The problem that quoting a bare "MTBF" or "MTBCF" presents is that the uncertainty in the quoted number is undefined, and unfortunately, the uncertainty in most electronic component MTBFs, which make up the data from which a systemic value is computed, tends to be quite high.

Adding to this is the usual failure mode for electronic devices, more commonly known as a "bathtub"; that is, it is expected that a significant percentage of failures that will ever occur will happen early on, while the remainder (statistically speaking) will happen somewhat close to the expected service life of the device. This does not mean you can't have a failure any time during the totality of service life, but it does speak to the probability.

This is not limited to rebreathers. Just recently I saw a presentation on marine engines where the manufacturer was claiming "10,000 hours before overhaul" as the expected service life. That sounds pretty good! But - ask them to put that in writing in the form of a warranty, and suddenly you get presented with a warranty that specifies 3 years with no more than 400 hours of operation annually! That's 1200 hours if you use the boat exactly 400 hours every year. In reality since most recreational vessels spend one hundred engine hours underway a year, the warranty is good for three hundred hours - or three percent of the claimed "service life".

Now why is this important? Only because Alex has spent his life on this forum, including right here in this thread, slagging off every other eCCR design on the market.

Well, if indeed the DeepLife design is so vastly superior that I should not expect one I purchase to fail in a way that will force me off the loop for 2.6 billion hours, then by God, DeepLife should be willing to advocate that I dive that unit alpine! After all, the odds of me drawing the short straw in the gene pool via the unit are roughly equal to that of being struck by an asteroid.

If the computed value, however, is just that - a computed number, and there's no actual belief that the unit will in fact continue to function properly from the date of its construction until well past the time of my demise (from other causes, natch) then throwing this figure around is being done for marketing or political (e.g. to claim complience with an EN directive) purposes, and is not actually a statement of expected reliability.

My question is simple - is it smoke and mirrors like the Marine Engine folks, or is it real?

If its real then its worth a very significant premium in the marketplace in terms of cost. But if its smoke and mirrors, well, then you're really just paying for bragging rights, aren't you?

Anyway, I've said it here on this thread enough times that I believe thinking people can understand. Those who wish to argue for purchasing products based on marketing-speak are welcome to do so - just don't pollute the discussion by claiming that you're doing based on scientific evidence.

You're not.

[Andy M]

Quote: (Originally Posted by Faceless)

I suspect you just want to "win" in this discussion no matter what.

He didnt get voted Usenet Kook of the month for nothing. He has been doing this for 20 years all over the Internet.

Quote: (Originally Posted by Faceless)

It's boring.

Seconded.