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AD_ward9 · 23rd November 2006, 17:23

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

23rd November 2006, 17:23	#33 (permalink)
AD_ward9 So much more to learn Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Jun 2005 Location: Scotland Posts: 1,862	Re: Cell Linearity 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 Last edited by AD_ward9 : 23rd November 2006 at 18:47.
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