TECHNICAL NOTE #2

Stability and Reliability vs. Energy Conservation

Many institutions, especially state supported educational facilities, have recently mandated some kind of energy conservation program to reduce operating expenses. Probably some effort in this direction is being made at your laboratory. I would like to take a moment of your time to discuss some of the consequences of such energy saving programs to the field of microanalysis, although they would probably apply to many other analytical techniques. If you have any comments on the subject I would be happy to hear from you.

There is no argument that turning off lights and other electrically powered equipment saves money. Energy conservation works and can in many situations reduce the operating expenses of large laboratories significantly. What I am concerned about are the effects of such actions on the reliability, stability and accuracy of the analytical microprobe.

When an electrical power supply or device is turned off for a period of time and then turned on again, the restoration of power can produce some interesting consequences which adversely affect the reliability and stability of the microprobe. The three that come to mind are thermal mechanical, thermal electrical effects and the reliability of the electrical device itself. All of these affect the stability of your microprobe and therefore the quality of the analysis.

Taking the last issue first, reliability, an area of considerable concern to me, I just note that at the moment when power is first restored, all electrical equipment looks like a dead short to the power supply. The is due to the fact that many electronic components store electrical energy (an example being the capacitor). When power is restored these un-energized components draw an enormous current until the circuit reaches equilibrium. Although this condition may only last for several milli-seconds, it is probably the most stressful situation that an electrical circuit will ever experience (aside from being hit by lightning).

Many other components such as high voltage transformers (color monitors, gun power supplies) also use transformers and transistors which are greatly stressed at the moment of power on. The heater in a diffusion pump is another example. Until the heater reaches operating temperatures it draws a very large current. This can result in a heater burnout. Have you ever noticed how often a light bulb burns out, just as it is turned on? To eliminate the "power on" stress to these electronic components I suggest that all critical electrical devices be left on all the time, except when for safety reasons they must be turned off during maintenance. I include among these, all vacuum system pumps and circuits, all gun, detector and electronic power supplies, all amplifiers and associated electronics and the computer and monitor. In fact, when I leave for the weekend, I only turn down the filament voltage until the emission drops close to zero. I turn off, only the room lights.

If leaving the monitor on bothers someone, just reduce the brightness adjustment or use any popular screen saver. By the way, all modern color monitors are immune to the so called "screen burn" condition. Screen savers are simply pretty to look at (that's debatable I guess), they provide no functionality except for those packages that include password protection.

Other more insidious stability problems arise from the fact that from the moment when a piece of equipment is energized from an ambient condition, the temperature of it's internal parts begin to increase. This causes, first of all, the actual physical size and shape, and second the electrical characteristics of the various components to change over time. This can affect the electrical stability of the device and is a large contributor of drift in electronic devices.

In addition, the excess heat generated by the losses in the electrical device eventually can warm up other nearby equipment. This can result in measurable changes in mechanical alignment of nearby or associated mechanical devices. For instruments such as wavelength spectrometers, the effect on the x-ray intensities due to changes in temperature can be considerable.

Don't forget, your computer is a mechanical device also. In fact, as far as the computer hard disk is concerned, the worse time of it's life is while the bearing warms up to operating temperatures. If you want to extend the longevity of your hard disk, leave it on all the time. A modern hard disk can run continuously for 5 years without any problems.

For similar reasons, microprobe manufacturers require that the microprobe laboratory be maintained at a constant temperature to reduce mechanical and electrical drift in the instrument. The heat from the various electrical devices in the microprobe itself contribute to this ambient temperature and also need to be maintained at a constant temperature for the most stable and reproducible operation of the microprobe.

If quantitative analysis is important to you, don't sacrifice accuracy for a few dollars a day in operating expenses.