Same idea as the industrial version that I use to clean injectors.
This type is much better though
http://cgi.ebay.co.uk/INDUSTRIAL-6-5-LI ... dZViewItem
I use a proper ultrasonic cleaner solution from RS and clean them with 12v applied to the coils to open the valve so the seats get a good clean.
I test them using a small pressure tank full of the ultrasonic cleaner and pressurised with compressed air to 3 bar, you can tell by the spray pattern when they are right. A nice cone shaped mist is perfect, most I've cleaned have started with a weak jet but after half an hour or so in the tank give a fine mist.
Cheers,
Iain T