1. Purchasing an Ultrasonic Cleaner
TL;DR: Buy the highest power rating in the tank size range that you can.
Ultrasonic cleaners have three major performance altering characteristics: tank shape, tank volume, and transducer power. Some offer special transducer synchronization circuits and other efficiency improvements, but things on a keyboard scale usually don't require advanced features. Brute ultrasonic force and power line level synchronization is acceptable.
- Tank Shape
Tanks should be cuboid and not overly shallow. It is rare to find tanks that are poorly proportioned.
- Tank Volume
Tank volume and transducer power modify one another significantly. Larger tanks require more total power, but they require less power relative to their volume than smaller tanks. Small tanks dissipate significant amounts of ultrasonic energy, as the waves cannot travel far before meeting metal.
- Transducer Power
The watt per gallon (or liter) rating is a general indicator of ultrasonic cleaner performance. It is by no means the end all in ultrasonic cleaner performance, particularly in larger installations, but for our purposes (less than 5 liter tanks) a higher watt per gallon rating is better. Smaller tanks tend to negate and dissipate ultrasonic waves due to their high surface area relative to water volume.
This PDF contains a chart, located on page three, that lists a rough ideal watt per gallon rating based on tank size:
http://www.ctgclean.com/sites/www.ctgcl ... Volume.pdf
TL;DR: Only clean a few parts, including keycaps (1/4 of a set or less and you MUST stir them), in a single ultrasonic batch (a six minute run time should clean things that aren't very crusty). Use gentle cleaners like denture tablets and warm, not hot, water (which will be heated by the transducers, so be careful).
Ultrasonic cleaning effectiveness is limited by two factors: energy dissipation of materials and shape complexity / volume. Cleaning compounds are used to change the physical characteristics of water, which acts as the primary solution for grime removal. Gentle cleaners like denture tablets and dish soap perform well in most keyboard applications. The cleaner's transducers will gradually heat the water, so keep an eye on it.
- Energy Dissipation of Materials
Rubber and soft plastics can absorb and diminish ultrasonic waves to an extent.
- Item Complexity / Volume
Parts that take up significant space in the bath can reduce overall effectiveness by creating dead zones. Complex objects can create dead zones while simultaneously reflecting, blocking, or otherwise diminishing the ultrasonic waves. Stirring is necessary for workloads like keycaps, even when watt per gallon ratings are high.
3. Mark Recommends a Cleaner
I recommend a Chinese 3.2 liter ultrasonic cleaner that has a greater than 100 watt per gallon rating, even when transducer inefficiency is factored in. Two 60 watt transducers reduce potential dead zones.
http://www.ebay.com/itm/New-Stainless-S ... 2844739182
Most low end units, regardless of their advertising, have a single 60 watt transducer and a less than ideal watt per gallon rating. If their rating ends up high enough, their tank size is often small and minimally useful.