I use one of the small battery operated scales, just like the one you mentioned. You can buy them on ebay for around $5.

What I have been looking for is the portable device that tells you what the Karat count is on anything tested. You typically use a drop of liquid to start the test. I find a lot of professionally run sales just mismark the jewelry or call something 14K that is just plated. A lot of times a piece of jewelry has been resized and the karat marking is gone and it is nice to know if it is 10, 14 or 18K. Big difference in the value.