NOTE: Make sure you read the first three posts (in order!) before tackling the rest, or it could be confusing: Post 1 is Designing the future, Post 2 is Setting up the problem, and Post 3 is Estimating basic requirements.

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Monday, September 11, 2006

Give yourself a home warranty

Your house won’t be much use to you if the solar panel tracking motor is always failing, or your rainwater catch basin frequently springs leaks, or if deer invade your garden. For a system/community of any kind to be useful, it must be reliable, and reliability must be considered early in the design.

There are many strategies for measuring and achieving reliability. For systems like plumbing and electricity, reliability may be quantified using the mean time between failures (MTBF). Modern appliance manufacturers, for example, typically base their warranty policies on the calculated MTBF – a 5-year warranty on a refrigerator probably means they calculated the MTBF as a standard deviation or more greater than 5 years. The only way to obtain this information at the component level is through testing or operational experience. You can try to estimate MTBF for assembled components using statistics.

The reliability of a system is a direct consequence of its component parts:
Rsys = R1 * R2 * R3… So for a system with 10 sub-components to be 99% reliable, each component must be 99.9% reliable!

Some strategies for reliable design:

1) Redundancy, redundancy, redundancy – Using back-up components in series or in parallel can reduce the chance of system failure. If your anti-deer fence were to blow down, do you have a backup strategy?

2) Contamination/Corrosion – Think about ways to protect machinery from the elements or prevent algal/bacterial growth in your water system.

3) Modularity – Just as for maintainability, using modular components is important.

4) Inspection – Certify each component as you machine or install it. Verify all tolerances.

5) Fail-safes – Allows the system to continue functioning under less than ideal conditions. Examples would be a spillway for an earthen dam or a relief valve on a water heater.

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