Introduction

The following chapter discusses a number of important issues that affect many classes of experimental hardware. For instance: vibrations can cause problems with optical, cryogenic, and other apparatus, and overheating may afflict electronic instruments, computers, and numerous other items.

Stress derating

The practice of derating (otherwise known as “providing a margin of safety”) is a means of improving the reliability of devices exposed to physical stresses (including power, temperature, electric current, pressure, etc.). If devices are operated continuously at their rated maximum stress levels, they are not unlikely to undergo failure in a relatively short time. Derating consists of operating them at reduced stress levels, at which the probability of failure is comparatively small. The derating factor of a device is usually expressed as a ratio of the stress needed to achieve good reliability to the rated maximum stress. While the term “derating” is normally used in conjunction with electrical and electronic components, the principle is a general one.

Generally, systems comprising a collection of components (e.g. a power supply or a power amplifier) are not derated as a whole, because derating has (or should have been) already been done for each of the constituent components. That is, derating is usually done at the component level, rather than the system (collection of components) level. (An exception is the temperature derating of power supplies – see below.)