If you are just doing your own regs there really isn't a need for a high dollar ultrasound machine, or high dollar cleaning solutions.
The Harbor Freight cleaners are plastic cased, but they have a stainless steel tank that is 2.5 liters in size (although it's a little shallow), a non temperature adjustable 100 watt heater, and 80 watts of ultrasound. They work ok and can be found for around $75. You may need to keep stuff in it longer, but it'll get it done, and if you'e only doing your own regs once a year, it's enough.
However for about twice the money you can get a Kendall commercial grade cleaner in 2L or 3L sizes for $125 and $150 respectively. They are all steel, and the 2 liter model (large enough for a single regulator) has 100 watts of heat and offer 60 watts of ultrasound. The 3 liter model has a 3" longer tank, enough for two regulators, and has 100 watts of heat and 120 watts of ultrasound.
An E-bay search will show several industrial ultrasound machines in 2, 3 6, and 10 liter sizes that all appear to be made by the same company. The specifications look ok, but I don't know how well they actually work.
I have a gallon of the $60 Global solution in the closet and it's been there a few years. You use it straight from the bottle, so folks who use it tend to use it until it's really bad looking, and that's less than good.
I prefer Blue Gold for ultrasonic and O2 cleaning, as well as for O2 cleaning tanks. It is horribly over priced in dive shops and on dive store web sites at about $90 per gallon, however you can get 5 gallons of it for about $150 (shipped) through an aviation supply company. It's used for cleaning jet turbine parts and despite being used for aircraft purposes, they don't rip you off by marking the price up 300%. You dilute it 30 to 1 for ultrasonic cleaning purposes and 15-1 to 20-1 for O2 cleaning purposes, so a gallon goes a long way. Buy five gallons and use it for decades, or do a group buy split it five ways and invest all of $30 into 30 gallons of ultrasound solution - 1/60th the cost of the Global solution. You can afford to change it when it starts looking dirty.
Simple Green Crystal also works great, and it doesn't have the obnoxious smell of Simple Green. I don't buy into the hard on brass argument with Simple Green or Simple Green Crystal as it's largely dependent on how much you dilute the solution. You only need to use it at a 30-1 ratio to be effective for ultrasound use, or for degreasing and O2 cleaning. It goes for about $14 per gallon, so the end cost is about $0.45 per gallon, and once again there's no need to use the stuff until it's black.
Cleaning solutions come in two types - acidic cleaners and alkaline cleaners.
The Global solution, Blue Gold and Simple Green (both regular and Crystal) are all alkaline cleaners. They are the go to choices for degreasing and oil removal.
Acidic cleaners, like acetic acid (white vinegar) or muratic acid (a 30% solution of hydrochloric acid) are useful for removing mineral deposits.
You'd want to dilute the muratic acid 10 or 15 to 1 to create a 2% or 3% HCL solution but even then it's going to remove some chrome if you soak it very long and/or don't neutralize the acid in a baking soda bath after you're done. In stronger concentrations HCL is the method of choice for removing chrome so you'll want to keep that in mind.
A mixture of 50% white vinegar and 50% water is a better option for removing salt, calcium, other mineral deposits, spots of corrosion, or organic growth from the surface, cracks and crevices in your regulator parts.
In short, if you don't to a good job of rinsing your regs, you'll need to use both an acidic cleaner to remove deposits and an alkaline cleaner to clean and degrease the parts.
Some people will whine that an acidic cleaner like acetic acid removed the chrome from parts of their reg, but those are the parts where there was heavy corrosion on the surface and the chrome was already compromised. The ultrasound with the acetic acid cleaner will remove the corrosion - which contains the remains of the chrome plating that used to be under it, exposing the underlying copper plating, or in some cases the brass. (Brass scuba regulator parts are plated with copper, then nickel and then chrome.) Don't let it get that bad in the first place and you won't lose chrome.
As noted in prior posts some plastics don't do well in ultrasound and ultrasonic solutions, so it's best to either avoid them entirely, or limit it to very short periods.
Teflon coated parts should also not be placed in an ultrasonic cleaner. A couple examples of teflon coated part are the piston spring and the diaphragm spring used in Scubapro piston and diaphragm first stages.
