I've never actually used a digital refractometer (or more likely reflectometer in this case); I've only used manual ones. I'm aware the digital reflectometers can be a bit twitchy about polish, cleanliness of the stone and the likes.
Sinokit are a HK-based optoelectronics manufacturer - I'd guess
this is the one you used. At just under $300 it's quite inexpensive for that type of instrument - although I see Presidium have one at a similar level.
If I needed to rely on a device like that it's a poor economy to get a cheap one. Something from Gem-A or GIA (and perhaps the training to use it as part of the identification) would be a safer bet; TBH, I'd stick with a pure optical one rather than electronic.
If you want some more info on the devices -
http://gemologyonline.com/Forum/phpB...p?f=11&t=14336
http://gemologyonline.com/Forum/phpB...p?f=11&t=12951
http://gemologyonline.com/Forum/phpB...p?f=11&t=14515
Added - Quicktest have a nice little read on electronic testers too:
http://www.quicktest.co.uk/gemtesters.htm
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