So you’re probably wondering why I’ve posted a picture of my son with his first guitar in a blog about instrumented safeguards with a topic of predicting reliability from components… Let me explain. As a former college band rock-star want-to-be I am really enjoying that my son is now playing guitar, and it has been my excuse to visit Guitar Center quite frequently, and also to dust off the old equipment from the college days so that I can jam with my kids. As I was pulling our old equipment out of the attic a found the old Peavey Amplifier / Mixing board the we used – a veteran of several hundred shows. It fired up and executed its assigned task flawlessly even after being mothballed for what is approaching 15 years (yes, even after college I kept the dream alive for a few years).
What makes this so surprising is that it was ultimately a very cheap piece of electronic gear that was “used outside it’s specifications”. Some of its jaded history includes:
- Having at least three pitchers of beer dumped on it (one of them a mega-pitcher, which I believe is a concoction of the fine drinking establishments at Ohio State)
- Being dropped from out of the van more than a dozen times, and one fall from the top of the speaker stack – which was about 7 feet
- Four trips to Florida to play pool decks during Spring Break and associated “wet bodies” dripping on it after leaving the swimming pool
- Hundreds of cigarettes resting and some extinguished on it
- And the ultimate insult of being vomited on by the sound man’s girlfriend
In all of these cases after “air drying” it was back in rocking condition if it went out of service at all.
My experience with this device was so good that I tried to repeat it with other equipment from the same manufacturer, or similar equipment from other manufacturers, but I never obtained the same reliability.
From the SIS perspective, it’s important to know why the performance is so good, and what you can do to get this kind of performance from your SIS. Ultimately, it boils down to beefy, overdesigned components from a design perspective, and proven-in-use equipment from an implementation perspective. After disassembling the amplifier multiple times (and other similar equipment) I noted that all of the components were significantly overdesigned compared to other amplifiers. Does this have an impact on reliability? Absolutely! Will it show up in “predictive” analysis presented by equipment vendors that are trying to sell you their equipment? Not necessarily to doubtful.
When the performance of equipment is predicted, the estimates are based on failure rates of typical components and typical services. While the beneficial effect of over-sizing components can theoretically be included in these predictive calculations, it typically isn’t done very well, because the data sets that these predictive techniques are based upon don’t have different data sets for over-sized components versus tightly sized components. There are simply correlations that gross-up or gross down failure rates depending on the level of “stress” that it is exposed to.
The only true measure of the performance of an instrument is actual field failure data. The standard (IEC 61511/ISA 84) recognized this by requiring you to replace all of the assumptions that you used during the design phase with actual field performance data. This includes replacing the FMEDA data on instruments that you obtained from equipment vendors with the actual performance data obtained from the field.
So if you’re not doing a good job collecting your field data, there’s no better time than the present to start improving your systems. You’re going to need to collect it and use it to meet the requirements of the standard in the future, and it might help you to find your “Peavey Amplifiers” sooner rather than later.