Reliability Engineering can be quite complex and it has, like other professions, its own language to avoid ambiguity. Words such as MTBF, MTTR, failure rate, availability, Weibull curves, chi-square demonstrations etc. can confuse and mislead the average layperson.
I have come across people in my professional life who have mistakenly thought, through no fault of their own, that the term “MTBF” is the same as “service life” (or useful life). I’m not going to give a full definition of these terms as other sites such as http://www.weibull.com/hotwire/issue22/hottopics22.htm do a really good job.
However, just last week, I was attempting to explain MTBF (Mean Time Between Failure) and Useful Life to a colleague when I had the idea of using the famous “bathtub curve” to show that MTBF and Useful Life are not the same. I’m a visual guy, and I love using visuals to convey information in a manner that is simple (stripped to their essence) and not simplistic (stripped of their essence).
Let’s begin. Chart 1 indicates that a certain device has a Useful Life of 4.9 years (this would be typical of an enterprise-level hard drive). But notice that the MTBF is 1,000 years. That does not mean that these devices will last 1,000 years or even close to that figure. Ideally, each device should be replaced every 4-5 years depending on the acceptable failure rate (based on cost, spares availability, etc.).
Now look at Chart 2. The Useful Life of these devices is still 4.9 years, but the MTBF is now 100 years, not 1,000 years. So that should start getting you thinking; same Useful Life but different MTBF.
Finally we have Chart 3. The MTBF is 100 years (the same as the previous device) but the Useful Life is now 49.9 years and not 4.9 years. I think that should convince you that MTBF and Useful Life are not the same.
That’s it. I’m keeping this short and sweet. I hope that this provides a different and useful way of looking at MTBF and Useful Life, understanding that they are not the same, and understanding their relationship within the bathtub curve.
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We headed up into the hills in our van, hoping to come across some remote Padaung villages. The dirt track, however, proved too much for the poor vehicle and we couldn’t proceed much further. There was a road construction crew that we had passed a few minutes earlier and we were able to negotiate the usage of one of their “tractors” that is normally used to haul raw materials.
We jumped into the back of the tractor, and upward and onward we went into the hills, following dirt tracks that should reach some interesting villages. Even the tractor found the going tough, and we had had to dismount several times to give the thing a fighting chance to negotiate the rough terrain. Shake, rattle, and roll; sitting on solid metal was pretty uncomfortable!
After a while we were soon at our target, the tractor chugging its way into the centre of a Padaung village. The villagers informed us that we were the first foreigners they had seen since the British left Burma in 1948. I took that with a pinch of salt though. But the journey was worth it, as the village still practised many traditional techniques. This is a photo of a young school girl from the village.
However, we were filled with dread of the return journey in that truck. But the pain of the return-trip in the truck was short lived as we, instead, soon ditched the truck and hiked through the hills heading in a direction that we thought would lead us to a road. The hike was memorable as it enabled us to take in the wonderful scenery and interact more sociably with the people we met along the way. A good day!
When I first took up photography as a hobby, it was important to me that photos be taken in full-manual mode as I thought that doing so would teach me about exposures. I’m not entirely sure how successful that method was, but it sure did make me a slow photographer! I now shoot almost invariably in aperture-priority mode. A piece of nostalgia for me, here is a photo from my full-manual mode days.
This lady was hard at work in a refuse dump out in the country, several miles from the nearest town. Some of the refuse had been set ablaze and, as I ventured further in, my throat and nose became the first casualties of the onslaught brought on by the thick acrid smoke. There were two benefits though; where there was an abundance of smoke, there was a scarcity of flies, and the rotting stink from the refuse was masked. You pick your poison.
The heat of the sun, coupled with the heat of the fires, did not help. The lady worked on, moving piles of refuse from one area to another, sorting out those that should be burnt, those that should be buried, and those that could be scavenged. This was a hell to me. But these people suffer and endure. She looked over to us, and smiled.
One of the key things I love on my Santa Cruz mountain bike is the threaded bottom bracket. Most other bike manufacturers had moved to press-fit bottom brackets over the years but I, and a few others, were not convinced that press-fit BBs where, as a whole, better than threaded BBs for most people. I really appreciate the easier maintainability of threaded BBs and that they don’t tend to squeak!
A recent BikeRadar article describes how there now seems to be a trend amongst manufacturers to return to threaded BBs. Of course Santa Cruz’s steadfast loyalty to threaded BBs gets a mention :)